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OER Catalysis at Activated and Codeposited NiFe-Oxo/Hydroxide Thin Films Is Due to Postdeposition Surface-Fe and Is Not Sustainable without Fe in Solution

Cite this: ACS Catal. 2020, 10, 1, 20–35
Publication Date (Web):November 4, 2019
https://doi.org/10.1021/acscatal.9b02580
Copyright © 2019 American Chemical Society
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Abstract

This work examines by electrochemical measurements a hypothesis that low-coordination Fe on the surface (surface-Fe) of NiFe-oxo/hydroxide promotes catalysis for the oxygen evolution reaction (OER) rather than Fe in the bulk structure (bulk-Fe) even in ultrathin films that are mostly surface. The effect of method of incorporation of Fe in Ni-oxo/hydroxide on the electrochemical behavior and OER activity is interrogated, and the sustainability of OER catalysis at NiFe-oxo/hydroxide is examined in the absence of Fe in solution. Ni(Fe)-oxo/hydroxide ultrathin films of a few monolayers and thicker films of tens of monolayers of Ni(OH)2 were deposited at anodic bias from potassium borate buffer containing Ni nitrate or Ni and Fe nitrates at a 6:4 Ni:Fe ratio and were conditioned and studied in 1 M KOH containing Fe or purified from Fe. Fe was incorporated in NiFe-oxo/hydroxide during codeposition but removed from solution during conditioning and catalysis, was included postdeposition during conditioning and catalysis in Fe-containing solution, or was incorporated postdeposition by conditioning in Fe-containing solution and then removed from solution during catalysis. Ultrathin and thicker NiOxHy and Ni0.6Fe0.4OxHy films exhibited high OER currents and low Tafel slopes in the range of 40 mV/dec in 1 M KOH after activation that included Fe from solution. However, ultrathin and thicker codeposited Ni0.6Fe0.4OxHy films exhibited low OER currents in Fe-purified KOH, which further decreased with the application of anodic bias, and exhibited high Tafel slopes of ca. 100 mV/dec or higher, in a behavior similar to that of NiOxHy in Fe-free KOH. Fe included postdeposition or surface-Fe is therefore indicated to be responsible for high OER catalysis in ultrathin and thicker NiFe-oxo/hydroxide films. The sustainability of OER catalysis at postdeposition activated Ni(Fe)-oxo/hydroxide still required the presence of Fe in solution. NiOxHy films activated for OER postdeposition in Fe-containing electrolyte did not sustain their high OER catalysis in Fe-free KOH but were deactivated with potential cycling. An exchange that causes surface-Fe to move into higher coordination bulk-Fe is proposed to cause the loss of OER activity of activated NiFe-oxo/hydroxide in Fe-free electrolyte.

Introduction

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The sun is a tremendous source of energy on earth. However, even when solar energy becomes an economically viable alternative to fossil fuel, its dependence on diurnal cycle, season, and weather will require this harvested energy to be stored for later use on demand. Electrolysis of water to produce hydrogen using solar power promises a solution for energy storage, mimicking what nature does in photosynthesis, storing energy in high-energy-density chemical bonds. Significant advances have been made toward solar water splitting since it was first reported at a semiconductor in 1972. (1) It can be achieved in a photoelectrochemical cell, by photocatalysis, or by coupling photovoltaic devices to electrolytic cells. (1−4) In all of these configurations, multielectron catalysts are essential components to reduce activation overpotentials.
The oxygen evolution reaction (OER), one of the two half-reactions of water splitting (2,3) as 4OH → O2 + 4e + 2H2O in alkaline medium, is the more kinetically demanding and has been catalyzed most effectively by the rare-metal oxides IrO2 and RuO2. (3,5) Searching for efficient and stable non-noble-metal catalysts and understanding the governing structure–reactivity relations for rational catalyst design have been the subject of extensive recent research. (5−19) Ni and its bimetallic and trimetallic oxides, particularly with Fe, have been reported as some of the most efficient non-noble OER catalysts in alkaline medium; (5−19) a Ni0.9Fe0.1Ox OER activity was reported to surpass that of IrO2. (16) Early studies by Corrigan (6) and more recent ones by Boettcher and co-workers (20,21) show that Fe impurities in the electrolyte enhance Ni-oxide OER activity, but the role that Fe plays remains elusive. Different hypotheses have been proposed regarding the active phase and regarding the active site as a Ni catalytic site with promoting effect by Fe or an Fe catalytic site. (19,21−26) In studies supporting the Fe catalytic site, DFT calculations showed that an Fe site surrounded by Ni nearest neighbors in γ-NiOOH has near-optimum adsorption energy for OER intermediates, resulting from Fe–O bond contraction in Ni1–xFexOOH. (22) In another report, Fe in β-NiOOH exhibited the lowest OER overpotential by DFT. (24) Goldsmith et al. showed that the onset of catalysis in NiFe oxyhydroxide coincided with the formation of Fe4+, proposing that the oxidation of Fe3+ to Fe4+ is facilitated by the NiOOH lattice and that catalytically active sites are likely to be located in defect sites, edges, and corners and the electrophilicity of the Fe-oxide there will be suitable to water oxidation. (27) Recently Martirez and Carter showed that an Fe pathway reduces the overpotential by 0.34 V in comparison to a Ni pathway and that a path involving an Fe(IV)-oxo species is stable under a low coordination environment of Fe in β-NiOOH. (28) Their calculations show that four-coordinated Fe on the surface of β-NiOOH—freeing it to form two additional bonds—is involved in catalysis and results in surpassing the formation of high-energy intermediates. (28)
Thin Ni-(oxo)/hydroxo films deposited at low anodic bias in borate (Ni-Bi; Bi = borate) (29−31) have been reported to reach maximum OER catalysis after the application of anodic bias for hours in borate buffer. (29) The increase in OER activity was initially ascribed to γ-NiOOH formed by overcharging and was accompanied by an increase in Ni charge to IV, (29) but Boettcher and co-workers later showed that Fe gets included in Ni-hydroxide films from traces of Fe in borate, (20) causing promotion of the OER, in agreement with the effect of Fe traces in KOH on promoting OER activity in Ni-oxide. (6,21) Nocera and co-workers later reported that Fe inclusion by incubation in solution or by codeposition of Ni0.9Fe0.1-Bi and then anodization in Fe-free solutions increased the Ni charge to IV, (32) which was ascribed to Fe making the transformation β to γ easier, (29) and proposed that greater Ni(IV) generation caused by the presence of Fe leads to greater Ni–O covalency, increasing the oxyl character and leading to higher OER catalysis. (32)
In an earlier study in our laboratory, the electrochemical behavior of codeposited NixFe1–x-Bi (x = 0.4, 0.6, 0.9, 1) was examined by interrogating the Ni(OH)2/NiOOH redox potentials, peak separations, Tafel plots, and turnover frequency (TOF) per Ni center for films as-deposited or anodized in 1 M KBi that contained traces of Fe. (7) The electrochemical characteristics when Fe was coincluded during deposition of thin Ni-Bi were found to be different from those when it was included during anodization postdeposition. (7) To reconcile the observed electrochemical differences based on the mode of incorporation of Fe, our group hypothesized that modification of a surface site with the inclusion of Fe rather than the bulk inclusion of Fe in NiFe-Bi is what caused the increase in OER activity with anodic conditioning in Fe-containing KBi. (7) Possibly supporting this, spiking KBi with Al3+ suppressed the OER while the redox peaks still shifted anodically with potential cycling similar to the observed anodic shift when the solution was spiked with Fe3+. (7) The Boettcher group reached a similar conclusion by spiking KOH with Fe, relating electrochemical behavior and conductivity to OER activity and comparing to codeposited films, and proposing that the electronic structure of the bulk oxy(hydroxide) is not the major influencer of OER activity. (33) Their cathodically codeposited NiFeOxHy films exhibited OER activity similar to that of Fe-spiked NiOxHy with the same 25–30% Fe content. (33)
Fe has been included in Ni-oxides by codeposition electrochemically at anodic or cathodic bias, via annealing of the mixed salt solutions, or postdeposition by aging NiOx films by incubation with and without bias in solutions that contain Fe. (6,7,14,20,21) The OER activity was mainly measured, except recently, (20,21) in electrolytes that had not been (indicated to be) purified from Fe. (29,30) Upon codeposition of Fe in NiOxHy, Fe3+ will substitute for Ni2+ during film formation, resulting in Fe in the Ni(OH)2 lattice. (34) This Fe is coordinated to six lattice oxygens, (28) and the excess charge is compensated by anion bonding between sheets (35,36) and will be termed “bulk-Fe”. When Fe is included postdeposition from the electrolyte by aging or under bias, Fe can be thought to become precipitated—initially—at the surface, occupying different sites including edges and corners. This Fe will be termed “surface-Fe”. This surface-Fe will have low coordination to lattice O. (28) This work examines the hypothesis proposed to explain earlier work (7) that surface site modification by Fe rather than its bulk incorporation in NiFe-oxo/hydroxide films is what causes promotion of OER catalysis even in ultrathin films of only a few monolayers that are mostly surface. In that case, a second question we ask is if the active site could form by an exchange from bulk-Fe to surface-Fe in NiFe-oxo/hydroxide, and another question that would follow is if an active site can sustain OER catalysis in NiFe-oxo/hydroxide in the absence of Fe in solution.
To test the hypothesis and address these questions, we examined the effect of mode of inclusion of Fe in Ni oxo(hydroxide) films deposited at anodic bias in KBi as ultrathin (2–7 nm) and thicker films (up to ca. 70 nm) on the electrochemistry and OER activity in KOH purified of Fe or containing traces of Fe. There are two main electrochemical experiments in this work: the first will coinclude Fe in Ni oxo(hydroxide) ultrathin films and multilayered films at a 40:60 Fe:Ni ratio but will remove Fe from solution during anodization and catalysis; the second will include Fe postdeposition from an electrolyte containing Fe to activate the films for OER and then remove Fe from solution to test the sustainability of OER active sites. We present electrochemical evidence supporting the hypothesis that bulk-Fe is not promoting catalysis but postdeposition surface-Fe is, in ultrathin and thicker films, and we communicate that high OER catalysis is not sustainable at both codeposited NiFe-oxo/hydroxide and postdeposition activated NiFe-oxo/hydroxide in the absence of Fe in KOH.

Results and Discussion

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Dependence of OER Catalysis at NiOxHy on Solution Fe vs Codeposited Fe

Ni- and NiFe-oxo/hydroxide films were electrodeposited at anodic bias on FTO with low Fe content from 0.4 mM Ni(NO3)2 or mixed Ni(NO3)2:Fe(NO3)3 at a 6:4 ratio in pH ∼9.2 0.1 M KBi buffer. (7,20,29,32) The deposition charge was set at 1, 10, or 100 mC/cm2 to vary loading. Films were rinsed and moved to a nonglass cell with 1 M KOH purified from Fe (21) (Fe-free KOH) or to a cell with 1 M KOH unpurified from traces of Fe (KOH). Films are termed relative to the Ni:Fe ratio in the deposition solution and to the deposition charge, for example as NiOxHy)1mC or Ni0.6Fe0.4OxHy)100mC.
Figure 1A,B shows CVs of NiOxHy)1mC (A) and Ni0.6Fe0.4OxHy)1mC (B) at 10 mV/s in Fe-free KOH: a first scan (a, as-deposited), after anodic conditioning (b, conditioned), and after Tafel plot measurements (c, after-Tafel). Films deposited at 1 mC/cm2 are ultrathin (2–7 nm) consisting of a few monolayers of electrochemically active Ni(OH)2. The amount of electrochemically active Ni was determined by integration of the NiOOH/Ni(OH)2 cathodic peak charge from CVs at 10 mV/s which equaled 0.68 ± 0.24 mC/cm2 for NiOxHy)1mC and 0.50 ± 0.10 mC/cm2 for Ni0.6Fe0.4OxHy)1mC. The Ni(OH)2 monolayers and thickness assumed a film density of 1.25 g/cm3 and 8 Å per monolayer. (35) In measurements in Fe-free KOH assuming 1.2 e/Ni, nNi = 5.85 ± 2.04 nmol/cm2 in as-deposited NiOxHy)1mC (N = 5; range 3.4–8.1 nmol/cm2, 4.34 ± 1.51 nm, 5.42 ± 1.89 monolayers), and 4.29 ± 0.86 nmol/cm2 in as-deposited Ni0.6Fe0.4OxHy)1mC (N = 7, 2.8–5.3 nmol/cm2, “Ni(OH)2 equivalent” thickness of 3.18 ± 0.64 nm and 3.98 ± 0.80 monolayers). The average was 73 ± 29% Ni in as-deposited Ni0.6Fe0.4OxHy)1mC relative to NiOxHy)1mC with a wide range.

Figure 1

Figure 1. (A, B) Cyclic voltammograms in Fe-free 1 M KOH solution at NiOxHy)1mC (A) and Ni0.6Fe0.4OxHy)1mC (B) as a first scan (a, blue), after anodic conditioning in the same solution (b, red), and after Tafel plot measurements (c, green). (C, D) CVs in unpurified 1 M KOH at NiOxHy)1mC (C) and Ni0.6Fe0.4OxHy)1mC (D) as a first scan (a, blue), after anodic conditioning in the same solution (b, red), and after Tafel plot measurements (c, green). The insets show the corresponding redox peaks and the onset of oxygen evolution for the three scans shown at each film. The scan rate is 10 mV/s.

CVs of as-deposited NiOxHy)1mC and Ni0.6Fe0.4OxHy)1mC in Fe-free 1 M KOH show the major Ni(OH)2/NiOOH redox peaks, small cathodic and anodic shoulders prior to the major peaks, water oxidation current at higher potential, and a reverse scan (positive to negative) that did not follow the forward scan. After anodization in Fe-free KOH, OER currents decreased for both films but the OER onset did not change, an anodic wave appeared at around 0.7–0.8 V, and the CVs exhibited hysteresis with the oxidation current on the reverse scan lower than that on the forward scan. CVs after-Tafel show a further decrease in OER activity without a change in onset and persistence of the wave at 0.7–0.8 V and of hysteresis. The anodic peak potential Ep,a of NiOxHy)1mC shifted cathodically by 6 mV after conditioning and by 3 mV after-Tafel and was unchanged for Ni0.6Fe0.4OxHy)1mC after conditioning and shifted anodically by 3 mV after-Tafel. The cathodic peak potential Ep,c of NiOxHy)1mC remained almost in the same position, but an overall 8 mV anodic shift was observed for Ni0.6Fe0.4OxHy)1mC after-Tafel (3 mV upon anodization, then 5 mV). There were variations between films but with similar main features. Figure S1 presents other examples of CVs of NiOxHy)1mC and Ni0.6Fe0.4OxHy)1mC in Fe-free KOH: OER currents of NiOxHy)1mC increased slightly after anodization and decreased after-Tafel with the same OER onset, a second wave at ca. 0.75 V and hysteresis were observed, and Ep,a shifted slightly anodically after anodization and returned to the same position after-Tafel; the CVs for Ni0.6Fe0.4OxHy)1mC show behavior similar to that in Figure 1B after anodization with only 3 mV shifts in Ep,c and Ep,a (opposite direction) but with larger anodic shift in Ep,c (by 22 mV) and Ep,a (by 13 mV) after-Tafel. The effect of cycling the potential in Fe-free KOH was also examined. Figure S2A shows CVs of NiOxHy)1mC whose potential was scanned in 10 cycles at 100 mV/s before acquiring a CV at 10 mV/s in Fe-free KOH. This as-deposited film CV did not exhibit hysteresis, but potential cycling caused the OER current to decrease and the wave at ∼0.75 V and hysteresis to appear. Ep,a shifted slightly cathodically in the second scan relative to the first scan (which is generally observed), and then remained essentially unchanged (except for the last CV), and Ep,c was almost unchanged. As the electrochemistry of this system is a function of many variables, (35,36) variations can be attributed possibly to differences in an initial mixed phase deposition and in remains of traces of Fe in purified KOH. The three general features, however, are that with anodic bias or cycling the potential at codeposited Ni0.6Fe0.4OxHy)1mC and NiOxHy)1mC in Fe-free KOH (1) OER currents decreased and were lower on the reverse scan, (2) an anodic wave appeared at ca. 0.75 V, and (3) the redox peaks can be concluded not to be systematically affected by anodic bias or potential scanning, as there is no trend in the small shifts. The decrease in OER current with anodic bias is not caused by film dissolution, as is evident from the magnitudes of the cathodic peak. The amount of Ni in NiOxHy)1mC measured for four films studied as-deposited until after-Tafel equaled 6.46 ± 1.73 nmol/cm2 as-deposited, 6.34 ± 1.85 nmol/cm2 after 3 h anodization in Fe-free KOH, and 5.91 ± 1.11 nmol/cm2 after-Tafel, therefore with an average decrease of only 8.5 ± 2.8%. Ni0.6Fe0.4OxHy)1mC showed similar stability with 4.46 ± 1.11 nmol/cm2 for as-deposited (N = 4), 4.36 ± 0.93 nmol/cm2 for anodized, and 4.13 ± 1.04 nmol/cm2 after-Tafel, thus a 7.4 ± 2.6% average decrease.
Figure 2 presents Tafel plots (overpotential η versus log J) of films—with CVs in Figure 1—anodized in Fe-free KOH and measured in the same solution. The Tafel slopes of Ni0.6Fe0.4OxHy)1mC and NiOxHy)1mC equaled 104 and 84 mV/dec, respectively. The Tafel slopes of the two films in Figure S1 equaled 165 mV/dec for Ni0.6Fe0.4OxHy)1mC and 120 mV/dec for NiOxHy)1mC (Figure S3). Differences in Tafel slopes are also possibly related to ranges of α/β and differences in inclusions of Fe from any remains in Fe-free KOH and are affected by OER currents that are decreasing with bias (referring to the CV post-Tafel in comparison to CV after anodization). The high Tafel slopes and the CVs show that codeposition of Fe in ultrathin NiFeOxHy did not promote OER activity in the absence of Fe in solution and that anodic bias and potential scanning in Fe-free solution resulted in a less active catalyst.

Figure 2

Figure 2. Tafel plots, overpotential η (mV) versus log(J (A/cm2)), for NiOxHy)1mC (red, ▲) and Ni0.6Fe0.4OxHy)1mC (green, ■) anodized in Fe-free KOH and measured in the same solution and for NiOxHy)1mC (orange, ◆) and Ni0.6Fe0.4OxHy)1mC (blue, ●) anodized in unpurified 1 M KOH and measured in the same solution, with the corresponding slopes for the best fit linear plot.

The slow OER at codeposited Ni0.6Fe0.4OxHy)1mC in Fe-free KOH and the electrochemical signature are contrasted with the behavior of NiOxHy)1mC and Ni0.6Fe0.4OxHy)1mC in unpurified 1 M KOH, as reported in the literature (6,21) and shown here for comparison. Figure 1C,D shows CVs of NiOxHy)1mC (C) and Ni0.6Fe0.4OxHy)1mC (D) in 1 M KOH, as a first scan (a), after anodic conditioning (b), and after-Tafel (c). As observed for Ni-Bi films in KBi, (7,29,30) OER currents increased and Ep,a and Ep,c shifted anodically, shifting E1/2 by 29 and 22 mV for NiOxHy)1mC and Ni0.6Fe0.4OxHy)1mC films, respectively, after anodization in unpurified KOH (insets). The anodization it traces show an increase in OER current with anodization in KOH, in contrast to the decrease in current in the it curves in Fe-free KOH (vide infra). The OER activity remained high after-Tafel; the currents either did not decrease (Figure 1C,D) or decreased slightly only at high potentials (Figure S4) for different films. For films in Figure 1C,D, E1/2 remained almost the same after-Tafel for NiOxHy)1mC (shifted anodically by 4 mV) or shifted further positively by 20 mV for Ni0.6Fe0.4OxHy)1mC. For both NiOxHy)1mC and Ni0.6Fe0.4OxHy)1mC in Figure S4, E1/2 shifted positively with conditioning and further positively after-Tafel. The CVs after conditioning and after-Tafel do not show the second wave plateau or hysteresis; the reverse scan almost traces the forward scan. Cycling the potential of NiOxHy)1mC in KOH similarly resulted in reduction in the OER overpotential, loss of hysteresis, and anodic shifts in Ep,a and Ep,c of Ni(OH)2/NiOOH (Figure S2B). After anodization in KOH a cathodic peak/shoulder (marked as A′ in the inset) appeared positive of the major cathodic peak (denoted A), this peak A′ increased after-Tafel and is seen more clearly at codeposited Ni0.6Fe0.4OxHy)1mC in Figure 1D and Figure S4B insets.
Tafel plots in unpurified KOH are presented in Figure 2 and Figure S3 with slopes of 37 and 45 mV/dec, respectively, for NiOxHy)1mC and Ni0.6Fe0.4OxHy)1mC films in Figure 1C,D (Figure 2) and slopes of 44 and 43 mV/dec, respectively, for films in Figure S4 (Figure S3), in the 40 mV/dec range reported for thin Ni-Bi in KBi and Ni(OH)2 in KOH containing Fe traces. (6,7,20,29,30) For these ultrathin films, currents divided by the amount of Ni calculated from after-Tafel CV (since there is a decrease in charge after-Tafel) show a proportionality of OER activity to the Ni centers over a moderate overpotential range, in both Fe-free and Fe-containing electrolytes. Normalized Tafel plots are shown in Figure S5 (using 1.2 e/Ni in Fe-free KOH and using 1.2 or 1.6 e/Ni in KOH).
The addition of Fe during anodic codeposition of Ni(Fe)OxHy did not shift E1/2 and did not decrease peak separation; ΔEp = 78 ± 6 mV for Ni0.6Fe0.4OxHy)1mC (N = 7) and 76 ± 3 mV for NiOxHy)1mC (N = 5) in Fe-free KOH and 76 ± 5 mV (N = 9), and 79 ± 3 mV (N = 10), respectively, in KOH. An earlier study of these films in KBi showed increased reversibility with codeposition of Fe but ΔEp was larger in KBi (112 ± 7 mV for Ni0.6Fe0.4-Bi and 134 ± 8 mV for Ni-Bi), and there was also no measurable shift in E1/2. (7) Fe inclusion in Ni- or Co-oxides has been reported in many studies to cause an anodic shift in the redox peaks. (6,20,21,23,26,37,38) A study of the literature, however, shows that this was not observed in every case (7) and may depend on the deposition method or method of incorporation. (37,39,40) For instance, Figure 1 in ref (39) in work by Swierk et al. (39) showed smaller peak separation but no anodic shift for Ni(OH)2 peaks with Fe, (39) and Figure 1B in ref (40) in work by Scherson and co-workers showed possibly a small cathodic shift. (40) Boettcher and co-workers showed that, while the redox peaks were anodically shifted in cathodically codeposited Co(Fe)OxHy, the incorporation of Fe from solution into cathodically deposited CoOxHy did not shift the peaks. (37) Including Fe from solution in NiOxHy shifted the peaks anodically, but the initial inclusion of Fe which increased OER activity did not. (33) It may be that the anodic codeposition of Ni(Fe)OxHy does not result in Fe in a Ni(OH)2 phase with strong interactions or that Fe is distributed in domains so as to cause an anodic shift. An anodic shift was observed with applying anodic bias postdeposition in Fe-containing electrolyte.
Thicker Ni0.6Fe0.4OxHy)100mC and NiOxHy)100mC with greater loading were also examined. The anodic deposition takes about 30 min in borate unpurified from traces of Fe. The Ni cathodic peak charge (CVs at 10 mV/s) in Fe-free KOH equaled 4.9 ± 2.1 mC/cm2 for as-deposited Ni0.6Fe0.4OxHy)100mC (N = 7) and 8.1 ± 2.7 mC/cm2 for NiOxHy)100mC (N = 4), showing 60% ± 33% Ni in Ni0.6Fe0.4OxHy)100mC to NiOxHy)100mC but also with a wide range. In KOH, the charges were similar, equaling 5.2 ± 1.3 mC/cm2 for Ni0.6Fe0.4OxHy)100mC (N = 4 films) and 10 ± 3.8 mC/cm2 for NiOxHy)100mC (N = 2), with 52% ± 24% Ni on average in the former. As the Ni0.6Fe0.4OxHy)100mC and NiOxHy)100mC films grow, they become more OER active, the latter possibly due to traces of Fe in borate, and the bulk of the 100 mC/cm2 charge will be for the OER leading to films only ca. 10–12 times thicker than those deposited at 1 mC/cm2.
An SEM image of as-deposited Ni0.6Fe0.4OxHy)100mC in Figure 3 shows nanoflake-like structures, and the EDX spectrum identifies the Ni and Fe peaks. XRD spectra of NiOxHy)100mC and Ni0.6Fe0.4OxHy)100mC as-deposited and anodized in KOH and Fe-free KOH showed only peaks corresponding to FTO, confirming the amorphous nature of the anodically deposited films (Figure S6). This is consistent with the work of Dinča et al. for thicker Ni-Bi films prepared at 1 C/cm2 on ITO that only showed ITO diffraction peaks. (31) Ni-Bi films thus deposited have been reported by Nocera and co-workers as amorphous containing ca. 2 nm nanocrystalline domains. (29)Figure 4A,B shows CVs of Ni0.6Fe0.4OxHy)100mC in Fe-free KOH (A) and in unpurified KOH (B) at 10 mV/s: as-deposited (a), conditioned (b), and after-Tafel (c). Figure 4C shows only the potential region of the peaks between 0.15 and 0.6 V for as-deposited and after-Tafel NiOxHy)100mC in Fe-free and KOH (the full CV scans are given in Figure S7).

Figure 3

Figure 3. SEM image (A) and EDX spectrum (B) of Ni0.6Fe0.4OxHy)100 mC as-deposited film.

Figure 4

Figure 4. CVs acquired at Ni0.6Fe0.4OxHy)100mC in Fe-free 1 M KOH (A) and Ni0.6Fe0.4OxHy)100mC in unpurified 1 M KOH (B), as a first scan (a, blue), after anodic conditioning in the same solution (b, red), and after Tafel plot measurements (c, green). The insets show the anodic and cathodic redox peaks/shoulders from the same scans labeled A, A′, and B. The scan rate is 10 mV/s. (C) CVs acquired at 10 mV/s, between the same scan limits as in (A) and (B), showing the region of the redox peaks and onset of oxygen evolution for Ni.OxHy)100mC in unpurified 1 M KOH (i) and in Fe-free 1 M KOH (ii) as-deposited (top, blue scans) and after-Tafel plot measurements (bottom, green scans). (D) Tafel plots, overpotential η (mV) versus log(J (A/cm2)), for NiOxHy)100mC (red, ▲) and Ni0.6Fe0.4OxHy)100mC (green, ■) anodized in Fe-free KOH and measured in the same solution and for NiOxHy)100mC (orange, ◆) and Ni0.6Fe0.4OxHy)100mC (blue, ●) anodized in unpurified 1 M KOH and measured in the same solution, with the corresponding slopes for the best fit linear plot.

As-deposited Ni0.6Fe0.4OxHy)100mC is not concluded to be more OER active than NiOxHy)100mC. CVs and currents normalized to nmol of Ni vs overpotential plots calculated from CVs (at 10 mV/s) in KOH and Fe-free KOH (Figure S8) show similar onsets in the CVs; the normalized currents vs η plots show better activity for Ni0.6Fe0.4OxHy)100mC in Fe-free KOH, but the activity is almost the same at both films in KOH, and there are variations between films. This behavior can be understood if anodic codeposition of Fe with Ni does not place Fe primarily in sites favorable for the OER in Ni0.6Fe0.4OxHy and if at anodic bias (50 mV more positive than the anodization of these films in borate) Fe is also included in NiOxHy in active sites from traces of Fe in KBi. Ni0.6Fe0.4OxHy)100mC (0.16 mM Fe3+ during 30 min at anodic bias), like thinner films, still required activation postdeposition in Fe-containing KOH. The CV of Ni0.6Fe0.4OxHy)100mC anodized in Fe-free KOH showed almost no change in the OER onset (inset of Figure 4A) and some increase in the OER current at high potential. After-Tafel, there was no change in OER onset, the OER current decreased at higher potential, and hysteresis increased, mimicking the general behavior of ultrathin films. A similar behavior was observed for NiOxHy)100mC in Fe-free KOH (Figure S7A). Cycling the potential of Ni0.6Fe0.4OxHy)100mC in Fe-free KOH decreased OER currents, shifted the onset to more positive potential, and increased hysteresis (Figure S9), indicating that the thicker Ni0.6Fe0.4OxHy)100mC is also reverting into a less active structure. In KOH, OER currents increased for both films after anodization, and the redox peaks shifted slightly anodically for NiFeOxHy)100mC, but only the cathodic peak shifted anodically for NiOxHy)100mC. The anodization it trace of Ni0.6Fe0.4OxHy)100mC in Fe-free KOH shows a decrease and then a small increase in current, while in KOH the current increased significantly, consistent with Ni0.6Fe0.4OxHy)1mCit curves that showed a decrease in current in Fe-free KOH and an increase in KOH (Figure S10). The difference in behavior between Ni0.6Fe0.4OxHy)1mC and Ni0.6Fe0.4OxHy)100mC in Fe-free KOH, where a more significant decrease in OER activity is observed after-Tafel for the thinner films and a small increase is observed after anodization for the thicker films, can possibly be caused by the larger amount of Fe if some Fe leaching out is reactivating sites in the picture presented below, with slower deactivation in thicker films because of the larger area. Similarly for NiOxHy)100mC in Fe-free KOH, there was a small increase in current after anodization—possibly because of some activation of the included Fe from traces in borate—but then the current decreased post-Tafel. After-Tafel, OER currents at Ni0.6Fe0.4OxHy)100mC decreased in 1 M KOH and a wave appeared at higher potential, requiring further study.
Tafel plots in Fe-free KOH of anodized (in same solution) Ni0.6Fe0.4OxHy)100mC (51 nmol/cm2, 38 nm “thick equivalent” based on Ni(OH)2 content post-Tafel; as-deposited 8.0 mC/cm2, 69 nmol/cm2) and NiOxHy)100mC (72 nmol/cm2, 54 nm thick, from post-Tafel CV; as-deposited 11.9 mC/cm2, 103 nmol/cm2) are presented in Figure 4D, with slopes of 106 and 152 mV/dec, respectively. For comparison, Tafel slopes of thick films anodized in KOH (and measured in KOH) equaled 47 and 38 mV/dec, respectively, for Ni0.6Fe0.4OxHy)100mC (43 nmol/cm2, 32 nm “thick equivalent”, post-Tafel; as-deposited 6.2 mC/cm2, 53 nmol/cm2) and NiOxHy)100mC (68 nmol/cm2, 50 nm, post-Tafel; as-deposited 12.6 mC/cm2, 109 nmol/cm2) similarly to thin films. This confirms a slower OER in the absence of Fe in solution also when Fe is present during codeposition of thicker films with higher loading.
In KOH, steady-state currents normalized to nmol of Ni (and assumed total metal) show lower activity for Ni0.6Fe0.4OxHy)100mC relative to NiOxHy)100mC (for Figure 4D; presented in Figure S11B). The normalized Tafel plots for three Ni0.6Fe0.4OxHy)100mC are presented in Figure S11B, in comparison to two NiOxHy)100mC films, showing lower activity for all three Ni0.6Fe0.4OxHy)100mC. On the other hand, normalized Tafel plots for thinner Ni0.6Fe0.4OxHy and NiOxHy (at 1 and 10 mC/cm2 in Figure S12) showed OER activity per Ni similar to that of NiOxHy of different thicknesses (1, 10, and 100 mC/cm2) in KOH (Figure S11). The variation of TOF with loading will depend on electronic and mass transport resistances in films of different thickness, which vary with morphology and deposition method. (41) Batchellor et al. showed that the TOF for Ni(Fe)OxHy prepared by continuous cathodic deposition decreased with increasing loading, while the TOF increased with loading for films prepared by pulsed deposition, (41) which was attributed to denser, more conductive films and more uniform distribution of Fe using the latter method, with the increase in TOF with loading ascribed to changes in nanostructure/morphology. (41) The same factors can be causing a lower TOF with increasing loading for Ni0.6Fe0.4OxHy, if Fe at higher concentration in the thicker films and transformations during anodization with uptake of Fe from solution will affect the morphology and Fe distribution, leading to electronic and mass transport resistances. The lower activity of Ni0.6Fe0.4OxHy)100mC occurring only in KOH, therefore, is likely affected by transformations in this electrolyte, rather than the initial deposition. The proportionality of the activity to loading for other films can be attributed to the same nanostructuring being maintained with increasing thickness, and shows no dependence on the addition of Fe during codeposition. Figure 4D shows Ni0.6Fe0.4OxHy)100mC to be more active than NiOxHy)100mC in Fe-free KOH, but we note that this is not always observed. There was a greater variability for Ni0.6Fe0.4OxHy and NiOxHy in Fe-free KOH in the Tafel slope range and the proportionality of activity to electroactive Ni; this can possibly be due to differences in trace Fe after cleaning and to transformation during Tafel plot measurements (difference between CVs after anodization and post-Tafel). For Ni0.6Fe0.4OxHy and NiOxHy at 1 and 100 mC/cm2 in Fe-free KOH, except for the thickest NiOxHy, the activity is proportional over a small overpotential range to Ni loading (Figures S5 and S13).
CVs of as-deposited NiOxHy)100mC (Figure 4C) show minor redox peaks denoted B, at 0.32 and 0.56 V in Fe-free KOH and at 0.32 and 0.55 V in KOH right before the onset of oxygen evolution, possibly for a quasi-reversible process thought to correspond to one couple, as the peaks disappear together with potential scanning or aging (Figure S14). This process occurs at potentials more positive than the major cathodic/anodic redox peaks denoted A at 0.242 V/0.410 V in KOH and 0.241 V/0.402 V in Fe-free KOH. The B redox process is still seen in the fourth scan at NiOxHy)100mC at 100 mV/s in Fe-free KOH (Figure S15). The oxidation B appears as a shoulder as it merges with the onset of the OER for as-deposited NiOxHy)100mC in KOH and as-deposited Ni0.6Fe0.4OxHy)100mC in both solutions. The small cathodic shoulder in CVs of as-deposited ultrathin films in both solutions is attributed to this process. A small shoulder was also seen at times at the foot of the anodic wave A, at ca. 0.430 or 0.435 V for as-deposited NiOxHy)100mC (Figure 4C) in Fe-free KOH and for as-deposited Ni0.6Fe0.4OxHy)100mC in Fe-free KOH; it is unclear if this is related to the B redox state. Figure S14 shows scans of NiOxHy)100mC in Fe-free KOH without overcharging up to only 0.6 V, taken at 15 min intervals: the cathodic peak B disappeared immediately and the anodic peak/shoulder disappeared gradually. Soaking Ni0.6Fe0.4OxHy)100mC and NiOxHy)100mC for 3 h in Fe-free KOH also resulted in peaks B disappearing.
Ni(OH)2 can precipitate as a hydrated α-Ni(OH)2 or a crystalline dehydrated β-Ni(OH)2 depending on the preparation conditions. (35,36) In a process known as aging, α-Ni(OH)2 will transform in water or alkaline medium to the more stable β-Ni(OH)2, proposed to occur by mixed dissolution/redeposition and “zipper” mechanisms. (36) The corresponding redox couples are α-Ni(OH)2/γ-NiOOH and β-Ni(OH)2/β-NiOOH, (35,36,42) and a range of α/β mixed phases is possible with a range of electrochemical properties. (36) The β-NiOOH can overcharge to γ-NiOOH, and the latter can be reduced back directly to β-Ni(OH)2. (35) The stability of the redox couples can be affected by guest metals; (35,36) for instance, Fe3+ inclusion was reported to stabilize the α-Ni(OH)2/γ-NiOOH couple. (35,36) The Bode diagram for the transformations of Ni(OH)2/NiOOH is presented in Scheme 1. (35,42) Since earlier studies leading to this diagram have likely taken place in electrolytes with traces of Fe, Klaus et al. have examined recently the transformations of Ni-oxy(hydroxide) (30 nm) in Fe-free or unpurified KOH and reported that the Bode diagram was followed in Fe-free KOH and that incorporation of Fe from KOH results in a structure akin to the Ni-Fe layered double (oxy)hydroxide (LDH) which blocks aging to the β phase. (19)

Scheme 1

Scheme 1. Bode Diagram Showing the Transformations of Ni(OH)2/NiOOH (35,42)
The minor redox peaks B of as-deposited thick films disappeared not only after conditioning or post-Tafel but also with time in solution without overcharging. They would be for an unstable phase or sites unlikely to depend on a high amount of Fe during deposition, as they are seen for NiOxHy)100mC—and are possibly better defined and of greater magnitude for as-deposited NiOxHy)100mC than for Ni0.6Fe0.4OxHy)100mC. Klaus et al. observed a peak at 0.60 V vs Hg/HgO in Fe-free 1 M KOH which disappeared with aging and reappeared with overcharging attributed to γ-NiOOH. (19) The peaks disappeared with incubation in solution, which would be in line with the α/γ couple as the α phase ages to the β phase, but peaks B did not reappear after the potential was scanned to positive values that overcharge β-NiOOH to γ-NiOOH. Assumed for one redox couple, they are more irreversible than the A process and are at more positive potential, inconsistent with the α/γ couple reported to be more reversible than β(II)/β(III) at more negative potential. (35,43) The B peaks could correspond to sites, for instance Ni edges and corners in α-Ni(OH)2/γ-NiOOH, that get smoothed out quickly upon dissolution/redeposition and that do not re-form on potential scanning. Boettcher and co-workers attributed an oxidation at 0.6 V vs Hg/HgO to Ni(IV) possibly at edges and corners or other sites in a small amount. (21)
The wave at 0.75 V in Fe-free KOH can correspond to β-NiOOH overcharging to γ-NiOOH, consistent with the anodic peak observed by Klaus et al. at 0.7 V vs Hg/HgO in Fe-free KOH. (19) In their work, aging in Fe-free KOH lowered the OER activity and anodically shifted the anodic peak. (19) Upon a return of the potential, the CVs post-Tafel and after potential scanning of NiOH)1mC revealed a decrease in OER current and hysteresis, but the A peaks did not systematically shift anodically or cathodically in Fe-free KOH. Either the γ-NiOOH phase—without Fe or that contains Fe in the bulk—is less active for the OER or some initial active defect sites or sites with Fe (as the borate contains traces of Fe) are blocked. (19,33) In earlier studies the β phase has been reported to be more OER active, (35,36) but the effect of Fe in the electrolyte is not evident. The decrease in OER activity with aging in Fe-free KOH was attributed by Boettcher and co-workers to a decrease in defect sites in the β phase or reduced OH/O2 transport. (21) Since the A peaks did not exhibit a significant consistent shift in Fe-free KOH, γ-NiOOH formed by overcharging may be reducing directly to β-Ni(OH)2 at a potential similar to that for β-NiOOH or the peaks correspond to a mixed α/β → γ/β occurring without a differentiation for thin films in Fe-free KOH.

Sustainability of the Active Site

We asked whether films activated for OER postdeposition by conditioning in the presence of Fe can sustain OER catalysis in Fe-free solution. Ni-oxo/hydroxo films deposited at 1 and 10 mC/cm2 were thus anodized in 1 M KOH and then tested in Fe-free KOH. Figure 5 shows CVs of NiOxHy)1mC (A) and NiOxHy)10mC (B) as follows: (i) a CV scan was acquired in Fe-free 1 M KOH, (ii) the film was moved to 1 M KOH, a CV was acquired, the film was anodized for ca. 3 h, and a CV was collected, and (iii) the film was moved back to Fe-free KOH, a CV was acquired, all at 10 mV/s, and then the potential was cycled in 10 CVs at 100 mV/s, a CV was acquired at 10 mV/s, and this was repeated n times (n between 6 and 25).

Figure 5

Figure 5. CVs for NiOxHy)1mC (A) and NiOxHy)10mC (B): (a) a first CV for the as-deposited film in Fe-free KOH, (b) a second CV after the film was moved to 1 M KOH, (c) CV after anodization in 1 M KOH, (d) CV of the anodized film moved back to Fe-free 1 M KOH, and (e) CVs acquired in Fe-free KOH after increments of 10 CVs were taken at 100 mV/s in this solution. This was repeated n times between 6 and 25, and some of the CVs are shown. The scan rate is 10 mV/s.

The first CV of as-deposited NiOxHy)1mC (Figure 5A) in Fe-free KOH shows a lower OER current than the first CV in KOH. After anodization in KOH, the first CV in Fe-free KOH shows the same OER catalytic activity as in KOH, with a difference that E1/2 for Ni(OH)2/NiOOH is cathodically shifted by 9 mV in the latter. Upon potential cycling in Fe-free KOH, the high OER activity was gradually lost, a second oxidation wave appeared, and hysteresis increased. The decrease in OER activity is not caused by net dissolution of Ni: from the first CV in purified KOH nNi = 4.94 nmol/cm2, and after 60 CVs nNi = 4.46 nmol/cm2. From this CV to that after six cycles there is an anodic shift of 8 mV in E1/2 resulting only from the cathodic peak, and closing in of the peaks from ΔEp = 93 mV to ΔEp = 82 mV.
Figure 5B shows a similar result for the thicker NiOxHy)10mC. The film anodized in KOH has the same OER activity initially in Fe-free KOH as in KOH, but with potential scanning in Fe-free KOH, OER currents decreased. We continued scanning the potential until it was shown that the OER activity dropped below that of the as-deposited film and a wave at 0.75 mV and hysteresis appeared. No decrease in charge was observed after 60 CVs; 21 nmol/cm2 was calculated for the first anodized-film CV in Fe-free KOH and 22 nmol/cm2 after 60 CVs. After 250 CVs the loading decreased to ca. 15 nmol/cm2, but this does not explain the reduced OER activity (compare to anodized Ni–Bi)1mC with 4.94 nmol/cm2 in Fe-free KOH immediately after activation, Figure 5A). An anodic shift in both Ep,a and Ep,c resulted from cycling the potential, with E1/2 shifting 10 mV from 0.326 V (first CV after activation, Fe-free KOH) to 0.336 V after 60 CVs and by another 12 mV to 0.348 V after 250 CVs, and ΔEp changed from 103 to 92 mV after 60 CVs and to 94 mV after 250 CVs. Thus, the electrochemical signatures indicate that the active structure is reverting back to the original inactive structure in the absence of a sufficient Fe source, and the dynamics of the system did not allow Fe in the active film to sustain OER activity or prevent this transformation in Fe-free solution. The same behavior was observed for codeposited Ni0.6Fe0.4OxHy)10mC (Figure S16). OER currents increased after anodization in KOH, and the first CV of the anodized film in Fe-free KOH reflects this activation, but upon continued potential cycling in Fe-free KOH, the OER currents decreased and hysteresis increased. After the first 10 CVs, ΔEp decreased by 19 mV without a change in E1/2, then E1/2 shifted anodically from 343 to 356 mV. After the first 10 CV cycles, the anodic peak shifted cathodically before it started shifting anodically, which could indicate Fe leaching out initially—but E1/2 did not shift cathodically as might be expected from Fe leaching and the greater reversibility has been attributed to Fe in Ni(OH)2.

OER Activity at Ni(Fe)-Oxo/Hydroxide Is Thus Conditional on Fe Source in Solution

Fe can replace a Ni site in the bulk of Ni(OH)2 or can deposit on its surface. The starting hypothesis from earlier work was that Fe in bulk sites does not promote OER catalysis but Fe in surface sites does. (7,33) A theoretical study by Carter et al. showed that a path involving low-coordination Fe in β-NiOOH lowers the OER overpotential. (28) The hypothesis is supported in this study by the observations that for ultrathin and multilayered codeposited NiFe-oxo/hydroxo films the OER activity is not promoted by Fe codeposition and is not sustained for both postdeposition Fe-activated Ni-oxo/hydroxo and NiFe-oxo/hydroxo films in the absence of Fe in solution: Tafel slopes of codeposited NiFe-oxo/hydroxide ultrathin and thicker films anodized in Fe-free KOH are in the range of 100 mV/dec or higher, and continued application of anodic bias in Fe-free KOH decreases the OER activity. Codeposited NiFe-hydroxide films deposited at anodic bias at 6:4 Ni:Fe in borate also needed activation in Fe-containing electrolyte. Applying anodic bias to cause NiFe-hydroxide to go into a γ-like structure with increased Ni(IV) (29,32) cannot explain that the NiFe-oxo/hydroxide films still needed Fe in solution during anodization or catalysis. When Fe is incorporated postdeposition from solution, Fe is proposed to initially occupy a surface site that is not fully coordinated to lattice O, (28) and the films are highly active for the OER with Tafel slopes of 40 mV/dec. The active site cannot be sustained, however, in the absence of Fe in solution, even once it is fully OER activated, taking Fe out of solution results in the structure reverting to the OER inactive/less active form. One can propose that Fe leaches out, inactivating the film, but if traces are sufficient to activate Ni(OH)2 films, then if this Fe redeposits it must be doing it differently: that is, any Fe leaching out is likely redepositing in an inactive site with redeposition of Ni that dissolved in KOH. Otherwise, there would be observed a cathodic shift or no change in E1/2, or increased irreversibility with Fe leaching, but E1/2 of the redox process A shifts anodically and the peaks close in with deactivation of Fe-activated films upon potential cycling in Fe-free KOH. The picture presented in Scheme 2 is proposed to explain the results.

Scheme 2

Scheme 2. Proposed Model for Activated Ni(Fe)OxHy by Included Surface Fe and the Inactivation Mechanism in Fe-Free KOH Solution
Surface-Fe is proposed to promote activity, not bulk-Fe. Active surface-Fe sites are proposed to become bulk sites as Ni dissolves and redeposits during catalysis in Fe-free solution. Any leached Fe is not redepositing as a surface-Fe active site or activating a surface site but must be codepositing with dissolved Ni as bulk-Fe. The presence of codeposited Fe is by itself not sufficient to ensure OER activity by a process of exchange, given the dynamic nature of Ni(OH)2 electrodes, to move from being an inactive bulk-Fe site to being an active surface-Fe site. As this deactivation takes place in Fe-free KOH, an anodic shift of the peaks still occurs in subsequent CVs, just at it occurs during anodization in KOH. The anodic shift of the major redox peaks is thus again not correlated with the increase in OER activity (7,33) and has been attributed in the literature to Fe going in the bulk of NiOxHy. (33) Fe moving to becoming bulk-Fe is proposed to explain the loss of activity in Fe-free KOH that happens with the anodic shift. The anodic shift may be due to a Fe-Ni structure (not itself the one causing the OER activity), but it requires with Fe inclusion (postdeposition) application of an anodic bias, as an anodic shift with initial inclusion by codeposition of Fe in Ni-Bi was not observed in earlier work (7) or in this work. We also attribute the closing of the peaks with deactivation to Fe moving from surface sites to bulk sites in this structure. A greater reversibility is in line with more reversible α/γ and Fe stabilizing this couple. It was observed by Corrigan with Fe inclusion (6) and earlier for NiFe-Bi ultrathin films in KBi (7) and in this work for Ni0.6Fe0.4OxHy)100mCEp = 121 ± 22 mV (N = 5) in Fe-free KOH and 127 ± 4 mV (N = 3) in KOH) in comparison to NiOxHy)100mC (145 ± 14 mV (N = 3) in Fe-free KOH and 157 ± 16 mV (N = 2) in KOH), though this also depends on thickness.
The anodic shift in the redox peaks observed upon anodization in KOH or upon deactivation of the Fe-activated films in Fe-free KOH did not similarly occur for NiOxHy)1mC in Fe-free KOH (Figure 1 and Figures S1A and S2A) upon anodization (ca. 3 h), post-Tafel (ca. 2 h), or potential cycling and is therefore not caused by a phase change to β-Ni(OH)2, whose redox peaks are reported at a potential more positive than for α/γ. (35,43,19) Boettcher and co-workers observed that in Fe-free KOH the major anodic and cathodic peaks of Ni(OH)2 shifted anodically by only 5 and 2 mV, respectively, after 13 CVs acquired every 5 min, with a small peak appearing at positive potential attributed to β-Ni(OH)2; in KOH with traces of Fe the redox peaks shifted anodically between 20 and 30 mV, indicating that the anodic shift is caused by the presence of Fe in solution. (21) This is consistent with our observations, except that we did not observe a second peak in Fe-free KOH with anodic bias (anodization or post-Tafel) or multiple CV scans, possibly as a result of the disorder in the anodically deposited films and mixed α/β phase.
An alternative mechanism for deactivation in Fe-free KOH could be Fe leaching out and not redepositing because of very low Fe in solution, or that remaining dissolved Ni(OH)2 or colloids from the Fe purification that uses Ni(OH)2 absorb Fe, preventing redeposition. Klaus et al. reported that Ni was at >400 ppb in Fe-purified 1 M KOH but as it settled for 1 h Ni dropped below the ∼0.2 ppb detection limit. (44) Boettcher reported that Ni-Bi peaks (Figure S5 of ref (20)) increased because of remaining Ni in Fe-purified borate (20)—which we also observed in KBi. However, the Ni(OH)2 peaks did not increase in multiple CVs in Fe-free KOH. Roger and Symes reported that, with 17 nM Ni, OER currents at FTO increased due to deposition of ca. 0.34 nmol/cm2 of catalyst, (45) and Ni removal with a resin decreased the current. (45) A resin was not used here post-purification, (46) but Fe-free KOH was passed through a filter after centrifugation, and multiple CVs at FTO showed low OER currents and a decrease with potential cycling (Figure S17). It is possible there are traces of Ni not sufficient for a catalytic film to deposit on FTO or to increase the peaks. Ni must have also dissolved during measurements at the nmol level, as indicated from the changes in peak charges. We therefore tested the effect of spiking Fe-purified KOH with Fe at levels comparable to what can be supposed to be leached on the OER. A 1.2 nmol amount of Fe3+, equivalent to 25% of Ni in NiOxHy)1mC (4.78 nmol/cm2), was added to 20 mL of Fe-free KOH (ca. 3 ppb or 60 nM), and this caused OER currents to increase with anodization (Figure S18). We remark, however, that this amount does not sustain the OER, and cycling the potential after anodization lowered OER currents. Multiple CVs acquired with addition of 1.2 nmol in another experiment also decreased the OER activity at NiOxHy)1mC. After spiking with 2.4 nmol of Fe (equivalent to about 40% Fe), OER currents at NiOxHy)1mC (5.65 nmol/cm2) increased with potential cycling and hysteresis decreased, and the redox peaks shifted positively, and with 21 nmol of Fe added in total, the OER currents increased significantly and hysteresis disappeared after the first 10 CVs and then multiple cycles had a minimal effect on the current but the peaks continued to shift anodically (Figure S19). This was also tested for a thicker NiOxHy)100mC (nNi = 69.5 nmol/cm2) after spiking Fe-free KOH with 8 nmol of Fe (ca. 10% of the Ni) (Figure S20). Potential cycling did not increase the OER, but the OER current increased significantly after anodization for 1 h, though again the activity is not sustained at this low level for this film with potential cycling (Figure S20). The experiments show that Fe at levels comparable to amounts that can leach, when it is added to Fe-free KOH, increase the OER activity with anodization, but a minimum amount must be necessary to sustain the OER. Hence, the picture proposed is that deactivation is due to Fe moving from an active site (surface-Fe) to an inactive site (bulk-Fe), rather than the inability of low Fe levels in this Fe-purified solution to activate sites, and this picture is consistent with the anodic shift and increased reversibility with deactivation. It may be deduced also that activation (surface-Fe) is favored at low anodic bias, and deactivation (Fe moving to bulk site) is favored with potential cycling or high bias (post-Tafel).
Furthermore, and requiring further study, we observed that the major redox process for the thicker as-deposited films consisted of two merged peaks A and A′. The existence of two redox processes is seen in the shoulder on the rising oxidation current in the CV of as-deposited Ni0.6Fe0.4OxHy)100mC in unpurified KOH (Figure 4B inset), and in the shape of the anodic peak after conditioning or post-Tafel for NiOxHy)100mC and Ni0.6Fe0.4OxHy)100mC in unpurified KOH (Figure 4B,C). They can be well differentiated at a higher scan rate (Figure S15, referring to the major anodic peak for NiOxHy)100mC at 50 and 100 mV/s). After anodization and post-Tafel, comparing CVs of NiFeOxHy)100mC in unpurified and Fe-free KOH (Figure 4A,B) shows a clearer A′ cathodic shoulder and two merged anodic peaks in KOH. The CV of NiOxHy)100mC post-Tafel in KOH (Figure 4C) also shows the cathodic shoulder A′ and the anodic peak as two merging peaks. For ultrathin films, the cathodic process A′ was observed after anodization in KOH and became more prominent post-Tafel (Figure S4 and Figure 1), and appears better differentiated for films with codeposited Fe. In Fe-free KOH, the A cathodic peaks are generally more symmetrical. The A′ cathodic peak was also observed at NiOxHy)1mC with potential cycling when 0.16 mM Fe3+ was added to Fe-free KOH (Figure S21), while it was not observed with cycling the potential of Ni0.4Fe0.6OxHy)100mC in Fe-free KOH; thus, it is not caused by Fe in a bulk phase undergoing an anodization-caused transformation. It must be determined if the A′ redox process is related to Ni sites that are catalytically active with Fe inclusion or that are nearing an Fe active site in the active phase. This is stated because the A′ peak is the one most dependent on Fe in solution, and because high OER activity is shown to depend on Fe in solution and not in the bulk. It is however emphasized that a claim is not made that the splitting is related in a causal way to the increase in OER activity but would be a consequence of Fe inclusion, as it was also observed with addition of Al3+ ions to Fe-free KOH in ongoing work. For ultrathin films the splitting A,A′ first accompanied the increase of OER activity in unpurified KOH, but currents remained the same or slightly decreased post-Tafel despite the greater splitting; thus, the activity could have been maximized at a certain surface-Fe loading. During deposition of thicker films with anodic bias at longer time, Fe will be included as both bulk-Fe and surface-Fe with beginning anodization; hence, A′ was detected in CVs of as-deposited thicker films.
There are reports of codeposited NiFe examined in Fe-free electrolyte that do not show the same deactivation as in this work. (6,21,47,48) Trotochaud et al. showed that OER activity did not decrease for 40 nm thick of Ni0.95Fe0.05(OH)2 and Ni0.75Fe0.25(OH)2 cathodically deposited on Au in Fe-free 1 M KOH in 13 CVs at 5 min intervals during 1 h. (21) This is not in contradiction with this work, as the OER onset for Ni0.6Fe0.4OxHy)100mC of similar thickness was the same after anodization (low bias, ca. 3 h) and post-Tafel (ca. 2 h), while OER currents at high potentials increased slightly and then decreased slightly. Multiple potential scans were needed to deactivate Ni0.6Fe0.4OxHy)100mC (Figure S9), while ultrathin films are deactivated post-Tafel. The Tafel slopes of thin and thick Ni0.6Fe0.4OxHy were, however, all high in Fe-free KOH, while Enman et al. reported a Tafel slope of 40 mV/dec for Ni0.9Fe0.1OxHy (10–20 nm, spin-cast with annealing on Au/Ti or Pt/Ti) in a 1 h Tafel measurement in Fe-free 1 M KOH and no decrease in TOF after 50 cycles. (47) It may be that a different deposition method, with possible Au substrate effect, results in a more active initial catalyst. A NiFeOxHy film on Au has been reported by Boettcher and co-workers to be more active than on GC, (21) and the OER at Au with Fe was higher than at Ni(OH)2/Au in the absence of Fe. (33) In the same work by Enman et al. an increase in overpotential for 0.5 mA/cm2 from 277 to 299 mV after cycle 50 was reported for Ni0.9Fe0.1OxHy (Table S2 of ref (47)), and the TOF of Ni0.7Fe0.3OxHy is shown to decrease after 50 cycles (Figure 3 of ref (47)). (47) In other work, Klaus et al. showed that electrochemical cycling of sputtered NiFe-oxide lowered the Fe:Ni ratio and slightly lowered the OER activity, but not for a cathodically deposited film on Au after 38 h in Fe-free KOH. (48) It is not evident how to explain this stability within the picture presented in this work, except to note experimental variations in the deposition method and the electrolyte (0.1 M KOH from Fe-free 1 M KOH). (48) In earlier work, Corrigan also reported a low Tafel slope of 25 mV/dec in purified (5.5 M) KOH at 10% Fe in cathodically deposited NiFe-OH on a Ni strip. (6) The films were thick, containing 0.29 mg of Fe and 3.0 mg of Ni or 5.19 μmol and 51 μmol, and leaching of 10% of the Fe into the (31 mL) KOH solution, supposedly, would lead to 9.3 ppm of Fe. (6) The dynamics of Ni(Fe)OxHy films and active site formation and its sustainability must depend on loading, electrolyte composition, pH and ionic strength, initial structure, and possibly substrate for very thin films. Methods of catalyst preparation included cathodic deposition which precipitates the hydroxide by increasing the local pH due to nitrate reduction, (37,49) anodic deposition, oxide formation, and spin coating with low-temperature annealing, and will lead to different crystallinities/nanocrystalline sizes (36,37,49−52) and different initial sites of Fe. Anodic and cathodic deposition can result in different crystalline domain sizes, (21,29,31,37,46,52,53) with amorphous films reported with anodic deposition. (29,31,52) It might be also that transformations will change the dynamics and sustainability of the active site. Boettcher and co-workers showed that nanocrystalline NiOx rock-salt thin films transform to the active Ni(OH)2/NiOOH with Fe absorbed from the electrolyte. (14) It is speculative, and requires studying, but a picture could be drawn that, as larger crystalline domains containing Fe transform into the active NiFe-hydroxide layer, active sites are formed and are replenished on the surface. Also, speculatively, if leaching of Fe from thicker films takes place, without significant dissolution of Ni, it is possible that films can maintain activity. A critical condition is seen that Fe can deposit postdeposition of Ni(OH)2 at anodic bias and not along with it, as multilayered films at anodic bias for 30 min at Fe:Ni 4:6 still required activation, while with Fe traces, most of the activation takes place in a shorter time at an anodic bias. For a full picture, further studies are needed for films at different deposition methods and loadings, substrates, and concentration of the electrolyte, along with structural characterization.
Boettcher and co-workers studied the effect of mode of Fe incorporation in Co(Fe)OxHy on the OER in Fe-free KOH at anodically and cathodically deposited CoOxHy. (37) Their results are seen to be consistent with this work regarding the dynamic nature of the active site and show dependence on the deposition method, but with differences as noted between Ni and Co. In their work, CoOxHy activated from solution Fe lost OER activity with potential cycling in Fe-free KOH; this was attributed to Fe leaching from surface and edge sites showing about a 60% decrease in Fe. (37) On the other hand, the OER activity was stable for cathodically codeposited Co(Fe)OxHy for 20 cycles in Fe-free KOH with 10% loss of Fe. (37) The redox peaks of cathodically deposited CoOxHy did not shift with Fe inclusion from solution or as OER activity decreased in Fe-free KOH, which was attributed to this Fe not strongly interacting with Co. (37) For anodically deposited CoOxHy with Fe added from solution, two peaks were observed after spiking and potential cycling; the peak that was anodically shifted from CoOxHy was attributed to Fe getting into sites strongly interacting with Co and this Fe is not easily removed by cycling in Fe-free KOH and does not contribute to the decrease in OER activity. (37) Upon loss of OER activity in Fe-free KOH, the redox peaks of anodically deposited CoOxHy did not shift further anodically—a main difference from Ni(Fe)OxHy upon potential cycling in Fe-free KOH in this study. Inclusion of Fe from solution in cathodically deposited NiOxHy in another study by Boettcher and co-workers shifted the peaks anodically, (21) and the differences were attributed to a stronger Co–O bond making bulk incorporation of Fe in CoOxHy slower. (37)
Effects of other impurities—other than Fe—in the electrochemical cell from other traces of metals in KOH and from the counter electrode should also be considered. A Pt counter electrode is used in studies of OER catalysis, since it supports a cathodic current flow and thus will not be anodically biased to form a Pt-oxide that would dissolve into Pt ions, which could redeposit on the working electrode, and since Pt is not a good OER catalyst. (46) The study of the transformations of Ni(OH)2 in Fe-free KOH which showed conformity to the Bode diagram (19) and other OER studies used a Pt counter electrode, (17,19,54) and Pt has been employed as a substrate for OER catalysts. (54,55) However, it must also be considered whether any counter electrode material could block or modify active sites, affecting OER currents or other electrochemical features. The potential scans ensured that the large currents are only anodic at the working electrode, under OER operating conditions, and only when the NiOOH is reduced (cathodic peak) did a cathodic current flow. A CV of an anodized NiOxHy)1mC film taken to more negative potentials into the HER region for testing showed no indication of Pt deposition that would have manifested in oxygen reduction (ORR) in aerated solution and in Pt oxide reduction and HER/HOR currents similar to that of a Pt electrode (Figures S22 and S23). But furthermore, to understand the effects that any traces of Pt would have had, we spiked KOH solutions with K2PtCl4(aq) and we monitored the effects on the redox peaks and OER in comparison to the behavior before the addition in response to potential cycling. A CV of NiOxHy)1mC after spiking aerated 1 M KOH with 4 μL of 50 mM K2PtCl4(aq, aged) featured cathodic currents from the ORR and Pt-oxide reduction and the HER/HOR currents (Figure S24), but despite the indication of Pt deposition with spiking of KOH, the OER currents remained unchanged and the splitting of the cathodic NiOOH peak and the anodic shift were still maintained. Similarly, spiking Fe-free KOH with Pt2+ (Figure S25) was monitored to neither cause a different decrease nor an increase of OER activity, and it caused neither a splitting of the cathodic peak nor an anodic shift in the peaks, and hysteresis was still observed. The use of a Pt counter electrode is therefore concluded not to affect these electrochemical features or OER currents in both solutions. To also be considered is that KOH contains impurities other than Fe. KOH in this work was specified to include Fe ≤0.0005%, Al ≤0.001%, Ni ≤0.0005%, and heavy metals ≤0.0005% specified as Ag/As/Bi/Cd/Cu/Hg/Pb/Sb/Sn/Mo. Fe can also be added from glass components etched in 1 M KOH, (21) and contamination from the glass with other elements (e.g, Al, Pb, B, Si) has also been reported. (56,57) Glass components are the substrate, though low Fe content FTO was used, and the reference electrode enclosure. The cell was made of quartz for measurements in KOH, but the solution containers were made of glass, while the cell and all containers were plastic for measurements in Fe-purified KOH. Subbaraman et al., studying the anomalous activity of electrocatalysts in alkaline electrolyte, analyzed KOH from different sources (including that used in this study) by ICPMS and found traces of the transition metals Co, Cu, Cr, and Ni in addition to Fe, in smaller amounts (Fe at ca. 11–23-fold more on average). (58) KOH therefore contains Al, which can be at a concentration greater than that of Fe, and other metals in lower amounts, and these metals can be included in Ni(OH)2 replacing Ni or Fe sites; their individual roles in affecting catalysis, whether a promoting effect on Ni-oxide as observed for Co (13) or an effect on morphology as reported for Cr (59) or a poisoning effect as reported for Al3+ in KBi, (7) and their combined effects at these levels have not been examined and would require more investigation. The purification process that used Ni(OH)2 to remove Fe may have also removed some of these metals. In ongoing studies on the effect of Al on the OER in KOH solutions, when Al3+ was added to purified 1 M KOH, the cathodic peak split into two peaks (A, A′) as observed with addition of Fe3+, while the OER currents still decreased with potential cycling, which may indicate that the purification process possibly removed Al, since the redox peak is not observed to split in purified KOH.

Conclusions

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The main observations in this study are (1) incorporation of Fe during deposition of a Ni0.4Fe0.6OxHy film does not lead to high OER activity when Fe is not present in solution and (2) activation of Ni(Fe)OxHy films for OER postdeposition in Fe-containing electrolyte does not lead to sustainable high OER activity in the absence of Fe in solution during catalysis. The electrochemical results support the hypothesis that surface site activation by Fe is the cause of high OER activity and not Fe inclusion in the bulk. A picture is presented to explain the electrochemical results and consists of the presence of inactive bulk-Fe with high coordination and active surface-Fe with low coordination, where a certain amount of Fe in solution is necessary to maintain the activity of surface-Fe; otherwise, it moves toward increased coordination and Fe is not instead exchanged from bulk sites to surface sites to sustain OER activity. We do not make a claim that only an Fe active site, despite being a clear possibility, can explain the observations. Therefore, we conclude that the work shows that Fe can activate OER catalysis in Ni-oxo/hydroxide thin films only on occupying a surface site which is thus not fully coordinated, that Fe in solution would still be needed to sustain high catalytic turnover without a blocking of the active site occurring by the dynamic nature of the electrodes, and that codeposition of Fe in these films does not sustain the active site. Further studies are needed and are ongoing to examine any leaching effect on structure and the mechanism of dissolution and redeposition in the presence and absence of Fe in solution, to differentiate between surface and bulk phases, and to examine the stability and conditions for sustainability of high OER catalysis and should there be an optimum or only a minimum concentration of Fe in solution required to sustain maximum OER at Ni(Fe)-oxide films with varying deposition conditions, surface area, and crystallinity.

Experimental Methods

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Materials

Nickel (II) nitrate hexahydrate (Ni(NO3)2·6H2O, 99.999%, trace metal analysis, Aldrich) and iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O, 99.95%, trace metal basis, Aldrich) were deposited on fluorine-doped tin oxide coated glass (FTO, R = 7 Ω sq–1, low iron content, Solaronix). Boric acid (H3BO3, 99.5%, Aldrich), potassium hydroxide (KOH, puriss, Aldrich, specified by the manufacturer to contain Fe ≤0.0005%, Al ≤0.001% Ni ≤0.0005%, and heavy metals ≤0.0005%, indicated as Ag/As/Bi/Cd/Cu/Hg/Pb/Sb/Sn/Mo at ≤5 ppm total without exact determination of distribution as per communication with Merck), and deionized water (resistivity 18 μΩ cm, Nanopure Diamond) were used for electrolyte preparation. Potassium tetrachloroplatinate(II) (K2PtCl4, 99.99%, Aldrich) was used to test any effects of platinum.

Ni0.4Fe0.6OxHy and NiOxHy Catalyst Film Deposition

FTO electrodes were cleaned prior to film deposition for 15 min in both isopropyl alcohol and deionized water by ultrasonication followed by air drying. Ni-oxo/hydroxo films in borate (or Ni-Bi) and NiFe- oxo/hydroxo films in borate (Ni0.6Fe0.4-Bi) were electrodeposited on FTO electrodes in a three-electrode electrochemical cell with Ag/AgCl (in 3 M NaCl; Bioanalytical Systems (BAS)) as the reference electrode and 1 mm diameter Pt wire as the counter electrode. Deposition was from 0.4 mM Ni(NO3)2 or 0.4 mM total concentration of mixed Ni(NO3)2 and Fe(NO3)3 with an Ni:Fe ratio of 6:4 in 0.1 M potassium borate buffer (KBi) pH ∼9.2 electrolyte at a constant anodic potential of 0.953 V vs Ag/AgCl until a specified charge was reached. Loading was varied by setting the charge at 1, 10, and 100 mC cm–2. The electrode geometric area was set at 1 cm2, defined using insulating epoxy resin. The same epoxy resin (epoxy resin and hardener, Alteco F-05) was used to set the surface area prior to film deposition for all measurements in KOH and Fe-free KOH.

Preparation of Fe-Free KOH Electrolyte

KOH electrolyte was purified following a reported procedure. (21) A 2 g mass of nickel nitrate (99.999%) was added to ∼20 mL of deionized water and ∼2 mL of 1 M KOH to form a pure nickel hydroxide precipitate. (21) The solution was mixed and centrifuged at 4000 rpm for 2 min. The supernatant was then decanted. Consecutive additions of ∼20 mL of deionized water and ∼2 mL of 1 M KOH to the precipitate were done followed by shaking, centrifugation, and decanting. This was repeated three times. Finally, ∼50 mL of 1 M KOH was added to the precipitated solid and the solution was allowed to rest for ∼3 h. It was then centrifuged at 4000 rpm for 10 min, and the supernatant was collected by filtration through a 0.45 μm pore size syringe (32) filter (part 431220, surfactant-free cellulose acetate SFCA membrane, 28 mm, Corning Incorporated) using a 60 mL plastic Luer-Lok syringe (Becton, Dickinson and Company) and used as Fe-free KOH. All containers used were made of plastic.

Electrochemical Measurements

Electrochemical measurements were conducted using CHI630A and CHI450C (CH Instruments, Austin, TX) electrochemical workstations in a three-electrode electrochemical cell. The cell setup consisted of the film on FTO glass as the working electrode, an Ag/AgCl (BAS) reference electrode (in all but one measurement that used Hg/HgO (BAS) reference as indicated), and a 1 mm Pt-wire counter electrode. The electrochemical cell was made of either quartz glass for unpurified KOH and KBi experiments or of polypropylene for purified Fe-free KOH experiments. Cyclic voltammograms (CVs), anodic conditioning, and Tafel measurements were done in 1 M KOH, unpurified or purified from Fe impurities, of pH ∼13.6. Cyclic voltammograms were performed by applying a potential ranging between 0 and 1 V vs Ag/AgCl, scanning first in the positive direction. After electrodeposition, CVs at each of 10, 20, 50, and 100 mV s–1 were first acquired; this was followed by anodic conditioning for 10400 s at 0.50 or 0.51 V in 1 M unpurified KOH (referred to also as KOH) or in 1 M Fe-free KOH with stirring at 600 rpm. The conditioned film was then tested again by acquiring CVs at 10, 20, 50, and 100 mV s–1. The CVs were not iR corrected. iR correction was applied only for calculations of overpotentials for Tafel plots and current vs overpotential plots. For Tafel measurements, using amperometric it curves, steady-state currents were measured with stirring at 600 rpm using a magnetic stirrer at different potentials. Tafel plot measurements were performed by measuring the current for 600 s to reach a steady state at different potentials in the range of ∼0.46–0.72 V vs Ag/AgCl with 20 mV increments from high to low potential. The duration of the measurement of a Tafel plot was about 2 h. A final CV was acquired after Tafel measurements at 10 mV s–1, referred to as after-Tafel CV. For Tafel plots, for calculation of the overpotential, the solution resistance was measured to correct for ohmic potential losses using the iR compensation test (45) of the CH instrument. The open circuit potential (Voc) was measured, and when it was reproduced within three measurements, the average was entered into the iR compensation test in the CH instrument software and R was measured. This R value was used for iR correction for overpotential calculations. R was generally in the range of 20 Ω; the average was 24 ± 8 Ω (22 ± 3 Ω for 50 experiments, and in 2 experiments R was 52 and 67 Ω and still yielded linear Tafel plots, with 1, 10, and 100 mC films in cleaned and uncleaned KOH). A value of 36 Ω was reported for iR correction in Tafel plots at Ni-Bi in 0.2 M KBi. (31) Overpotential calculations were performed using the following equation: η = EEeq(O2/H2O) – iR, where Eeq(O2/H2O) was determined at the specific pH of the solution measured using a Mettler Toledo SevenCompact pH/ion meter, which was in the range of 13.6 ± 0.2 for 1 M KOH using the equation Eeq (V) = 1.23 – (0.059 × pH) + (0.059 × log(0.209)/4) – 0.197.
To determine the amount of electrochemically active Ni, the cathodic peak charges were determined via the Gaussian peak shape mode using the CHI450C software. Current density was calculated using the electrode geometric area.

Effect of Potential Cycling

To test the effect of potential cycling, Ni-oxo/hydroxide and Ni0.6Fe0.4-oxo/hydroxide films were electrodeposited from borate solutions on FTO at 0.953 V vs Ag/AgCl by passing a charge of 1 or 100 mC cm–2 and were then moved to a 1 M Fe-free or unpurified KOH cell. A CV was acquired at 10 mV s–1, and then 10 CVs were acquired at 100 mV s–1, followed by 1 CV at 10 mV s–1. This was repeated up to seven times.
In the experiment to study the sustainability of the active site in the absence of Fe in solution, Ni- and NiFe-oxo/hydroxide films were similarly deposited from borate solution on FTO by passing a charge of 1 or 10 mC cm–2. The films were then moved to 1 M Fe-free KOH, and a first CV was acquired at 10 mV s–1. Then, the films were rinsed and moved to 1 M unpurified KOH electrolyte and a second CV at 10 mV s–1 was acquired. The films were anodically conditioned (at 0.5 V) for ca. 3 h with stirring at 600 rpm, and then a CV was acquired after anodization at 10 mV s–1. The film was then moved back to Fe-free 1 M KOH solution, and a CV was acquired at 10 mV s–1 in 1 M Fe-free KOH followed by 10 CVs at 100 mV s–1. This was repeated 6 times for the 1 mC cm–2 film and 25 times for the 10 mC cm–2 film.

Effect of Addition of Fe3+ to Fe-Purified KOH Electrolyte

A Ni-oxo/hydroxide film was similarly electrodeposited at 1 mC cm–2 and rinsed and moved to 1 M Fe-free KOH, where one CV at 10 mV s–1 was acquired. A 0.16 mM amount of Fe3+ was added to 20 mL of electrolyte followed by a CV at 10 mV s–1. Ten CVs at 100 mV s–1 followed by a CV at 10 mV s–1 were acquired, and this was repeated six times. Spiking the Fe-free KOH with lower Fe amounts was also tested. A NiOxHy)1mC film was rinsed and moved to 1 M Fe-free KOH, where two CVs were acquired at 10 mV s–1 to test in Fe-free KOH, and then 2.4 nmol Fe3+ was added and a CV was acquired at 10 mV s–1, 10 CVs at 100 mV s–1, and 1 CV at 10 mV s–1; this was repeated 6 times. In the same experiment, 18.6 nmol of Fe3+ (total 21 nmol) was further added and a CV at 10 mV s–1 was acquired followed by 10 CVs at 100 mV s–1 and a CV at 10 mV s–1; this was repeated 6 times. To test spiking with lower amounts, 1.2 nmol of Fe3+ was added to Fe-free KOH, three CVs were measured at 10 mV s–1, and the film was then anodized at 0.5 V for 3 h with stirring at 600 rpm. To test the sustainability of activity after anodization, 3 CVs at 10 mV s–1 were acquired, followed by 10 CVs at 100 mV s–1 and 1 CV at 10 mV s–1; this was repeated twice.
A similar experiment was conducted for thicker NiOxHy)100mC. Two CVs were acquired at 10 mV s–1 in Fe-free KOH, and then 2.4 nmol of Fe3+ was added, and a CV was acquired at 10 mV s–1, followed by 10 CVs at 100 mV s–1, and 1 CV at 10 mV s–1; this was repeated 6 times. A 5.6 nmol amount of Fe3+ (total 8 nmol) was then added, and a CV at 10 mV s–1, 10 CVs at 100 mV s–1, and a CV at 10 mV s–1 were acquired; this was repeated 6 times. The film was then anodized at 0.5 V for 1 h with stirring at 600 rpm, and a CV was acquired at 10 mV/s, 20 CVs were then acquired at 100 mV s–1 followed by a CV at 10 mV s–1; this was repeated twice.

Electrode Aging in Purified Fe-Free KOH

A Ni-oxo/hydroxide film was electrodeposited in borate on FTO with a charge of 100 mC cm–2. The film was then rinsed and moved to 1 M Fe-free KOH, where a CV at 10 mV s–1 was first acquired between 0.18 and 0.6 V in the region of the redox peaks without overcharging followed by aging in the same solution for 15 min. This was repeated four times with a total time of 1 h.

Testing Effect of Addition of Pt to Electrolyte

Ni-oxo/hydroxide films were electrodeposited at 1 mC cm–2 and then moved to 1 M KOH or 1 M Fe-free KOH. CVs were acquired between 0 and 1 V in the positive direction unless indicated otherwise. Two CVs at 10 mV s–1 were acquired followed by five CVs at 100 mV s–1 and a CV at 10 mV s–1, repeated twice. A CV from −1.5 to 1.0 V scanned in the positive direction was acquired at 10 mV s–1 to test the film in the negative range followed by a CV at 10 mV s–1 from 0 to 1 V. Ten CVs at 100 mV s–1 followed by a CV at 10 mV s–1 were acquired; this was repeated twice. After this initial testing, 8 μM of K2PtCl4(aq, aged 1 day) (4 μL of 50 mM added to 25 mL of 1 M KOH) were added to the electrolyte followed by a CV at 10 mV s–1, and then 10 CVs at 100 mV s–1 were acquired followed by a CV at 10 mV s–1; this was repeated twice. A CV from −1.5 to 1 V at 10 mV s–1 was acquired at the end to test this range. In addition, the potential of another similarly electrodeposited film was scanned before and after the addition of 8 μM of K2PtCl4, in aerated and deaerated KOH by purging with N2(g) for 35 min. Another film was anodically conditioned, and then its potential was scanned from −1.5 to 1.0 V to test in this range in 1 M KOH. For comparison, CVs were acquired at a Pt-disk electrode (d = 2 mm, CH Instrument) in aerated and deaerated 1 M KOH.

SEM Imaging and EDX

SEM images were taken for Ni0.6Fe0.4-Bi films electrodeposited at 100 mC cm–2 on FTO using a Tescan MIRA 3 LMU FEG SEM, equipped with a SE detector and an IN-Beam SE detector, and EDX spectra were obtained using an Oxford Instruments X-Max 20 EDX detector running with Oxford INCA software.

XRD

XRD spectra were acquired for films deposited at 100 mC cm–2 on FTO, as-deposited and after anodization, using a Bruker D8 Advance X-ray diffractometer.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acscatal.9b02580.

  • Cyclic voltammograms in 1 M Fe-free KOH at NiOxHy)1mC and Ni0.6Fe0.4OxHy)1mC as a first scan, after anodic conditioning in the same solution and after Tafel plot measurements; cyclic voltammograms of a NiOxHy)1mC film as a first scan in 1 M Fe-free KOH and in 1 M KOH, and then after multiple CVs showing a decrease in OER activity in 1 M Fe-free KOH and increase in OER activity in 1 M KOH; Tafel plots, overpotential η (mV) versus log(J (A/cm2)), for NiOxHy)1mC and Ni0.6Fe0.4OxHy)1mC films anodized in 1 M Fe-free KOH and 1 M KOH and measured in the same solution; cyclic voltammograms in 1 M KOH at NiOxHy)1mC and Ni0.6Fe0.4OxHy)1mC as a first scan, after anodic conditioning in the same solution and after Tafel plot measurements; normalized Tafel plots, overpotential η (mV) versus log (I/n (A/nmol)), for NiOxHy)1mC, and Ni0.6Fe0.4OxHy)1mC films anodized in 1 M Fe-free KOH and 1 M KOH and measured in the same solution for Figure 2 using 1.2e/Ni and 1.6e/Ni as indicated; XRD diffraction patterns for Ni0.6Fe0.4OxHy)100mC on FTO, as-deposited and anodized in 1 M Fe-free KOH, and of FTO electrodes; cyclic voltammograms of NiOxHy)100mC in 1 M Fe-free KOH and 1 M KOH as a first scan, after anodic conditioning in the same solution and after Tafel plot measurements; first scan CVs and normalized CVs (I/n vs η) at NiOxHy)100mC and Ni0.6Fe0.4OxHy)100mC in 1 M Fe-free KOH and 1 M KOH; CVs of Ni0.6Fe0.4OxHy)100mC as a first scan and after multiple CVs in 1 M Fe-free KOH; anodization plots of Ni0.6Fe0.4OxHy)1mC and Ni0.6Fe0.4OxHy)100mC films performed at 0.5 V vs Ag/AgCl in 1 M Fe-free KOH or 1 M KOH; Tafel plots and normalized Tafel plots, overpotential η (mV) versus log (I/n (A/nmol)), for NiOxHy)100mC and Ni0.6Fe0.4OxHy)100mC films anodized in 1 M Fe-free KOH and 1 M KOH and measured in the same solution using 1.2e/Ni; Tafel plots and normalized Tafel plots for NiOxHy)10mC and Ni0.6Fe0.4OxHy)10mC anodized and measured in 1 M KOH; cyclic voltammograms up to 0.6 V of a NiOxHy)100mC film as a first scan in 1 M Fe-free KOH followed by CVs after aging for 15, 30, 45, and 60 min in the same solution; cyclic voltammograms of a NiOxHy)100mC film as a first scan in 1 M Fe-free KOH at 10 mV/s followed by CVs at 20, 50, and 100 mV/s; multiple CVs of Ni0.6Fe0.4OxHy)100mC in Fe-free KOH; CVs of Ni0.6Fe0.4OxHy)10mC anodized in KOH after being moved to Fe-free KOH in response to multiple potential cycles; CVs of FTO as a first scan and after potential cycling in 1 M KOH and 1 M Fe-free KOH; cyclic voltammograms at NiOxHy)1mC after adding 1.2 nmol of Fe3+ to Fe-free KOH before and after anodization and potential cycling; cyclic voltammograms at NiOxHy)1mC after adding 2.4 nmol and then a total of 21 nmol of Fe3+ to Fe-free KOH before and after anodization and potential cycling; cyclic voltammograms at NiOxHy)100mC after adding a total of 8 nmol of Fe3+ to Fe-free KOH, before and after anodization and potential cycling; cyclic voltammograms of a NiOxHy)1mC film as a first scan in 1 M Fe-free KOH before and immediately after addition of 0.16 mM Fe3+ and then multiple CVs acquired showing an increase in OER activity; cyclic voltammograms of an anodically conditioned NiOxHy)1mC film scanned to −1.5 V in aerated 1 M KOH and CVs of another NiOxHy)1mC film before and after the addition of 8 μM K2PtCl4 in aerated 1 M KOH and then after bubbling nitrogen; cyclic voltammograms of a Pt-disk electrode in aerated and deaerated 1 M KOH; cyclic voltammograms of NiOxHy)1mC in 1 M KOH before and after spiking with 4 μL of 0.05 M K2PtCl4; cyclic voltammograms of NiOxHy)1mC in 1 M Fe-free KOH before and after spiking with 4 μL of 0.05 M K2PtCl4 (PDF)

  • Crystallographic data from ref (28) for the structure drawn in Scheme 2 (CIF)

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Author Information

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  • Corresponding Author
  • Authors
    • Rida Farhat - Department of Chemistry, American University of Beirut, Beirut, Lebanon 110236
    • Jihan Dhainy - Department of Chemistry, American University of Beirut, Beirut, Lebanon 110236
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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We thank the University Research Board (URB) (Awards 103186 (Project 23239), 2016-17 and 103603 (Project 24689), 2018-2019) and the K. Shair CRSL endowed research fund (Award 103365 (Project 23855), 2017-18) of the American University of Beirut for financial support of this research. We acknowledge the Central Research Science Laboratory (CRSL, AUB) for access to SEM and EDX instruments and thank Mr. Joan Younes at CRSL for assistance he provided while acquiring SEM images and EDX analysis.

References

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  • Abstract

    Figure 1

    Figure 1. (A, B) Cyclic voltammograms in Fe-free 1 M KOH solution at NiOxHy)1mC (A) and Ni0.6Fe0.4OxHy)1mC (B) as a first scan (a, blue), after anodic conditioning in the same solution (b, red), and after Tafel plot measurements (c, green). (C, D) CVs in unpurified 1 M KOH at NiOxHy)1mC (C) and Ni0.6Fe0.4OxHy)1mC (D) as a first scan (a, blue), after anodic conditioning in the same solution (b, red), and after Tafel plot measurements (c, green). The insets show the corresponding redox peaks and the onset of oxygen evolution for the three scans shown at each film. The scan rate is 10 mV/s.

    Figure 2

    Figure 2. Tafel plots, overpotential η (mV) versus log(J (A/cm2)), for NiOxHy)1mC (red, ▲) and Ni0.6Fe0.4OxHy)1mC (green, ■) anodized in Fe-free KOH and measured in the same solution and for NiOxHy)1mC (orange, ◆) and Ni0.6Fe0.4OxHy)1mC (blue, ●) anodized in unpurified 1 M KOH and measured in the same solution, with the corresponding slopes for the best fit linear plot.

    Figure 3

    Figure 3. SEM image (A) and EDX spectrum (B) of Ni0.6Fe0.4OxHy)100 mC as-deposited film.

    Figure 4

    Figure 4. CVs acquired at Ni0.6Fe0.4OxHy)100mC in Fe-free 1 M KOH (A) and Ni0.6Fe0.4OxHy)100mC in unpurified 1 M KOH (B), as a first scan (a, blue), after anodic conditioning in the same solution (b, red), and after Tafel plot measurements (c, green). The insets show the anodic and cathodic redox peaks/shoulders from the same scans labeled A, A′, and B. The scan rate is 10 mV/s. (C) CVs acquired at 10 mV/s, between the same scan limits as in (A) and (B), showing the region of the redox peaks and onset of oxygen evolution for Ni.OxHy)100mC in unpurified 1 M KOH (i) and in Fe-free 1 M KOH (ii) as-deposited (top, blue scans) and after-Tafel plot measurements (bottom, green scans). (D) Tafel plots, overpotential η (mV) versus log(J (A/cm2)), for NiOxHy)100mC (red, ▲) and Ni0.6Fe0.4OxHy)100mC (green, ■) anodized in Fe-free KOH and measured in the same solution and for NiOxHy)100mC (orange, ◆) and Ni0.6Fe0.4OxHy)100mC (blue, ●) anodized in unpurified 1 M KOH and measured in the same solution, with the corresponding slopes for the best fit linear plot.

    Scheme 1

    Scheme 1. Bode Diagram Showing the Transformations of Ni(OH)2/NiOOH (35,42)

    Figure 5

    Figure 5. CVs for NiOxHy)1mC (A) and NiOxHy)10mC (B): (a) a first CV for the as-deposited film in Fe-free KOH, (b) a second CV after the film was moved to 1 M KOH, (c) CV after anodization in 1 M KOH, (d) CV of the anodized film moved back to Fe-free 1 M KOH, and (e) CVs acquired in Fe-free KOH after increments of 10 CVs were taken at 100 mV/s in this solution. This was repeated n times between 6 and 25, and some of the CVs are shown. The scan rate is 10 mV/s.

    Scheme 2

    Scheme 2. Proposed Model for Activated Ni(Fe)OxHy by Included Surface Fe and the Inactivation Mechanism in Fe-Free KOH Solution
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    • Cyclic voltammograms in 1 M Fe-free KOH at NiOxHy)1mC and Ni0.6Fe0.4OxHy)1mC as a first scan, after anodic conditioning in the same solution and after Tafel plot measurements; cyclic voltammograms of a NiOxHy)1mC film as a first scan in 1 M Fe-free KOH and in 1 M KOH, and then after multiple CVs showing a decrease in OER activity in 1 M Fe-free KOH and increase in OER activity in 1 M KOH; Tafel plots, overpotential η (mV) versus log(J (A/cm2)), for NiOxHy)1mC and Ni0.6Fe0.4OxHy)1mC films anodized in 1 M Fe-free KOH and 1 M KOH and measured in the same solution; cyclic voltammograms in 1 M KOH at NiOxHy)1mC and Ni0.6Fe0.4OxHy)1mC as a first scan, after anodic conditioning in the same solution and after Tafel plot measurements; normalized Tafel plots, overpotential η (mV) versus log (I/n (A/nmol)), for NiOxHy)1mC, and Ni0.6Fe0.4OxHy)1mC films anodized in 1 M Fe-free KOH and 1 M KOH and measured in the same solution for Figure 2 using 1.2e/Ni and 1.6e/Ni as indicated; XRD diffraction patterns for Ni0.6Fe0.4OxHy)100mC on FTO, as-deposited and anodized in 1 M Fe-free KOH, and of FTO electrodes; cyclic voltammograms of NiOxHy)100mC in 1 M Fe-free KOH and 1 M KOH as a first scan, after anodic conditioning in the same solution and after Tafel plot measurements; first scan CVs and normalized CVs (I/n vs η) at NiOxHy)100mC and Ni0.6Fe0.4OxHy)100mC in 1 M Fe-free KOH and 1 M KOH; CVs of Ni0.6Fe0.4OxHy)100mC as a first scan and after multiple CVs in 1 M Fe-free KOH; anodization plots of Ni0.6Fe0.4OxHy)1mC and Ni0.6Fe0.4OxHy)100mC films performed at 0.5 V vs Ag/AgCl in 1 M Fe-free KOH or 1 M KOH; Tafel plots and normalized Tafel plots, overpotential η (mV) versus log (I/n (A/nmol)), for NiOxHy)100mC and Ni0.6Fe0.4OxHy)100mC films anodized in 1 M Fe-free KOH and 1 M KOH and measured in the same solution using 1.2e/Ni; Tafel plots and normalized Tafel plots for NiOxHy)10mC and Ni0.6Fe0.4OxHy)10mC anodized and measured in 1 M KOH; cyclic voltammograms up to 0.6 V of a NiOxHy)100mC film as a first scan in 1 M Fe-free KOH followed by CVs after aging for 15, 30, 45, and 60 min in the same solution; cyclic voltammograms of a NiOxHy)100mC film as a first scan in 1 M Fe-free KOH at 10 mV/s followed by CVs at 20, 50, and 100 mV/s; multiple CVs of Ni0.6Fe0.4OxHy)100mC in Fe-free KOH; CVs of Ni0.6Fe0.4OxHy)10mC anodized in KOH after being moved to Fe-free KOH in response to multiple potential cycles; CVs of FTO as a first scan and after potential cycling in 1 M KOH and 1 M Fe-free KOH; cyclic voltammograms at NiOxHy)1mC after adding 1.2 nmol of Fe3+ to Fe-free KOH before and after anodization and potential cycling; cyclic voltammograms at NiOxHy)1mC after adding 2.4 nmol and then a total of 21 nmol of Fe3+ to Fe-free KOH before and after anodization and potential cycling; cyclic voltammograms at NiOxHy)100mC after adding a total of 8 nmol of Fe3+ to Fe-free KOH, before and after anodization and potential cycling; cyclic voltammograms of a NiOxHy)1mC film as a first scan in 1 M Fe-free KOH before and immediately after addition of 0.16 mM Fe3+ and then multiple CVs acquired showing an increase in OER activity; cyclic voltammograms of an anodically conditioned NiOxHy)1mC film scanned to −1.5 V in aerated 1 M KOH and CVs of another NiOxHy)1mC film before and after the addition of 8 μM K2PtCl4 in aerated 1 M KOH and then after bubbling nitrogen; cyclic voltammograms of a Pt-disk electrode in aerated and deaerated 1 M KOH; cyclic voltammograms of NiOxHy)1mC in 1 M KOH before and after spiking with 4 μL of 0.05 M K2PtCl4; cyclic voltammograms of NiOxHy)1mC in 1 M Fe-free KOH before and after spiking with 4 μL of 0.05 M K2PtCl4 (PDF)

    • Crystallographic data from ref (28) for the structure drawn in Scheme 2 (CIF)


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