Oxygen Electrocatalysis on Mixed-Metal Oxides/Oxyhydroxides: From Fundamentals to Membrane Electrolyzer Technology

  • Raina A. Krivina
    Raina A. Krivina
    Department of Chemistry and Biochemistry and the Oregon Center for Electrochemistry, University of Oregon, Eugene, Oregon97403, United States
  • Yingqing Ou
    Yingqing Ou
    Department of Chemistry and Biochemistry and the Oregon Center for Electrochemistry, University of Oregon, Eugene, Oregon97403, United States
    College of Chemistry and Chemical Engineering, Chongqing University, Chongqing, China
    More by Yingqing Ou
  • Qiucheng Xu
    Qiucheng Xu
    Department of Chemistry and Biochemistry and the Oregon Center for Electrochemistry, University of Oregon, Eugene, Oregon97403, United States
    Department of Materials Science, East China University of Science and Technology, Shanghai, China
    More by Qiucheng Xu
  • Liam P. Twight
    Liam P. Twight
    Department of Chemistry and Biochemistry and the Oregon Center for Electrochemistry, University of Oregon, Eugene, Oregon97403, United States
  • T. Nathan Stovall
    T. Nathan Stovall
    Department of Chemistry and Biochemistry and the Oregon Center for Electrochemistry, University of Oregon, Eugene, Oregon97403, United States
  • , and 
  • Shannon W. Boettcher*
    Shannon W. Boettcher
    Department of Chemistry and Biochemistry and the Oregon Center for Electrochemistry, University of Oregon, Eugene, Oregon97403, United States
    *Email: [email protected]
Cite this: Acc. Mater. Res. 2021, 2, 7, 548–558
Publication Date (Web):July 13, 2021
https://doi.org/10.1021/accountsmr.1c00087
Copyright © 2021 Accounts of Materials Research. Co-published by ShanghaiTech University and American Chemical Society. All rights reserved.
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Abstract

Conspectus

Catalyzing the oxygen evolution reaction (OER) is important for key energy-storage technologies, particularly water electrolysis and photoelectrolysis for hydrogen fuel production. Under neutral-to-alkaline conditions, first-row transition-metal oxides/(oxy)hydroxides are the fastest-known OER catalysts and have been the subject of intense study for the past decade. Critical to their high performance is the intentional or accidental addition of Fe to Ni/Co oxides that convert to layered (oxy)hydroxide structures during the OER. Unraveling the role that Fe plays in the catalysis and the molecular identity of the true “active site” has proved challenging, however, due to the dynamics of the host structure and absorbed Fe sites as well as the diversity of local structures in these disordered active phases.

In this Account, we highlight our work to understand the role of Fe in Ni/Co (oxy)hydroxide OER catalysts. We first discuss how we characterize the intrinsic activity of the first-row transition-metal (oxy)hydroxide catalysts as thin films by accounting for the contributions of the catalyst-layer thickness (mass loading) and electrical conductivity as well as the underlying substrate’s chemical interactions with the catalyst and the presence of Fe species in the electrolyte. We show how Fe-doped Ni/Co (oxy)hydroxides restructure during catalysis, absorb/desorb Fe, and in some cases degrade or regenerate their activity during electrochemical testing. We highlight the relevant techniques and procedures that allowed us to better understand the role of Fe in activating other first-row transition metals for OER. We find several modes of Fe incorporation in Ni/Co (oxy)hydroxides and show how those modes correlate with activity and durability. We also discuss how this understanding informs the incorporation of earth-abundant transition-metal OER catalysts in anion-exchange-membrane water electrolyzers (AEMWE) that provide a locally basic anode environment but run on pure water and have advantages over the more-developed proton-exchange-membrane water electrolyzers (PEMWE) that use platinum-group-metal (PGM) catalysts. We outline the key issues of introducing Fe-doped Ni/Co (oxy)hydroxide catalysts at the anode of the AEMWE, such as the oxidative processes triggered by Fe species traveling through the polymer membrane, pH-gradient effects on the catalyst stability, and possibly limited catalyst utilization in the compressed stack configuration. We also suggest possible mitigation strategies for these issues. Finally, we summarize remaining challenges including the long-term stability of Fe-doped Ni/Co (oxy)hydroxides under OER conditions and the lack of accurate models of the dynamic active surface that hinder our understanding of, and thus ability to design, these catalysts.

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1. Introduction

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The ability of hydrogen (H2) to serve as a medium between chemical and electrical energy makes it an attractive alternative to carbon-based fuels for powering the transportation of people and goods, heating buildings, producing fertilizers, refining metals, and making electricity with fuel cells. (1,2) Today most hydrogen is produced by the steam reforming of fossil fuels. (1) An alternative is water electrolysis. Water electrolysis/splitting involves two half-reactions: the oxygen-evolution reaction at the anode (OER) (in base, 4OH → 2H2O + 4e + O2; in acid, 2H2O → 4H+ + O2 + 4e) and the hydrogen-evolution reaction (HER) at the cathode (in base, 2H2O + 2e → H2 + 2OH; in acid, 2H+ + 2e → H2). Driving the OER results in a large overpotential (excess driving force beyond the thermodynamic requirement) that decreases the efficiency of electrolyzers. Electrocatalysts capable of lowering the overpotential have been under investigation for nearly a century.
The oldest electrolyzer technology is alkaline water electrolysis (AWE). AWE is typically operated at ∼80 °C in a concentrated basic liquid electrolyte (KOH or NaOH). Although the current densities are typically <500 mA·cm–2, the low materials cost from the inexpensive (steel) cell components and non-PGM catalysts leads to a relatively low overall cost for hydrogen. (1) A newer technology, PEMWE, replaces liquid electrolyte with a solid, locally acidic ionomer membrane. (3) Pure water is fed to a compressed stack, allowing operation at >2 A·cm–2 and yielding high-purity output hydrogen. (1,3) The solid–polymer electrolyte reduces the crossover of hydrogen and oxygen gas, allowing thinner electrolytes with lower resistive losses and the direct electrochemical compression of the output hydrogen. (3) The acidic membrane, however, requires the use of PGM catalysts and expensive stack materials (e.g., Ti) because non-PGM catalysts corrode and dissolve.
AEMWE, in principle, can combine the advantages of AWE and PEMWE. The system uses the same compressed stack design as PEMWE but replaces the acidic membrane with a basic one enabling the use of non-PGM catalysts and cheaper stack materials. (1,4) AEMWE is, however, in its early stage and has challenges to overcome before it can compete in efficiency and durability with PEMWE. The design of active and stable OER and HER catalysts is imperative for the development and commercialization of AEMWE systems.
Iron-containing Ni- and Co-based catalysts have superior OER performance in base and AWE compared to other catalysts, including PGMs. (5−7) The origin of their high performance has been subject to much discussion. Friebel et al. reported Fe3+ as an active site in Ni1–xFexOOH using a combination of operando X-ray absorption spectroscopy (XAS) and DFT calculations. (8) Gorlin et al. found that for Ni1–xFexOOH with x > 0.09, Ni remains largely in the +2 oxidation state during OER. (9) In contrast, Li et al. proposed that Fe3+ promotes the formation of Ni4+ that is responsible for the enhanced OER activity. (10) Smith et al. studied Fe100–yCoyOx and observed Co oxidation under OER conditions. (11) Gong et al. concluded that undercoordinated Fe3+ was an active site, despite Fe not being oxidized. (12) Hunter found evidence for Fe6+ and ferrate-like species under nonaqueous conditions and assigned these as the active sites in OER. (13) Chen et al. found Fe4+ under active conditions with Mossbauer spectroscopy but demonstrated that this species persisted after the applied potential was removed. (14) The Fe cation sites are also dynamic, dissolving, and redepositing depending on the concentration of solution Fe3+. (15)
Understanding the mechanism of OER under basic conditions and the identity of active sites is important for designing better catalysts for both AWE and AEMWE. Additionally, the effects of Fe adsorption/desorption, catalyst restructuring, and its electrical conductivity should be accounted for when incorporating Ni(Co)1–xFexOOH into traditional AWE or membrane-electrode-assembly (MEA) electrolyzer systems.
Here we describe our efforts to identify and understand the active sites in Fe-doped Ni and Co (oxy)hydroxides. We aim to understand the mechanisms of Fe incorporation into the catalysts, the role played by Fe during OER, and use this understanding to elucidate design principles for OER catalysts in electrolyzer technologies. Finally, we discuss research directions related to OER catalysts and electrolyzers relevant for high-performance non-PGM AEMWE.

2. Intrinsic Activity Trends for Metal (Oxy)Hydroxides

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The use of thick catalyst layers and poorly defined active surface areas, as well as differences in electrical conductivity and local surface structure, has historically complicated the direct comparisons of intrinsic OER activity. (6,8) A better comparison can be made if the catalysts are prepared as thin films (2 to 3 nm thick) supported on conductive supports that minimize ohmic and mass-transfer overpotentials and the amount of inactive (bulk) catalyst. (16) This thin-film configuration also allows in situ mass monitoring using a quartz-crystal microbalance (QCM) and the calculation of lower-limit turnover frequencies (TOF, the number of O2 molecules evolved per metal active site per second) based on the total number of metal cations. The use of TOFs is a good metric for the comparison of intrinsic catalytic activity as long as the assumptions underlying the calculation are made clear. In alkaline media, for example, IrOx is ∼10-fold less active than Ni0.9Fe0.1Ox (TOFs = 0.009 ± 0.005 s–1 and 0.21 ± 0.03 s–1 at η = 300 mV, respectively). (16) We compared the activities of many first-row transition-metal oxides (NiOx, CoOx, NiyCo1–yOx, Ni0.9Fe0.1Ox, IrOx, MnOx, and FeOx). We also analyzed if synergetic effects existed with two or more different metals. In the case of Co and Ni, we observed no synergy, in contradiction to previous trends reported. (17,18) The addition of Fe to Co or Ni improved the performance to different degrees, suggesting different interactions of Fe with the matrices. (16,18) X-ray photoelectron spectroscopy (XPS) showed that the catalysts were converted to (oxy)hydroxide phases during operation, in agreement with thermodynamics-based Pourbaix diagrams. (16,19) CoOxHy and NiOxHy both form layered structures with octahedrally coordinated metal cations and nanoscale ordered domain sizes where the individual layers are noncovalently bonded and intercalated with ions and water. (19,20)
The electrical conductivity of the catalyst phases is important in designing high-performance electrodes. For (oxy)hydroxides, the oxidation and protonation states of the material depend on the applied potential; therefore, the conductivity is different than that measured from pressed pellets of the catalyst powder. We measured thin-film conductivity as a function of applied potential using microfabricated interdigitated array (IDA) electrodes. (19,21,22) We discovered that the electrical conductivity varied with composition and applied potential. CoOxHy and NiOxHy were insulating when the cations were +2 and conductive when oxidized to nominally +3 or +4, a conversion that happens under OER conditions and that is (usually) reversible. The redox behavior of CoOxHy and NiOxHy provides another mechanism to measure the number of electrochemically accessible metal cations, which can be done by integrating the pre-OER redox feature visible in the voltammetry. (23) It is important to note, however, that exactly which of these electrochemically accessible metal sites are catalytic sites remains an open question. (24−27)
XPS analysis of thin-film CoOxHy and NiOxHy catalysts revealed Fe impurities absorbed from the electrolyte even without intentional addition of Fe salts. (16,19,21) To observe the intrinsic activity, Fe impurities had to be removed. We produced Fe-free KOH by absorbing Fe species with Ni(OH)2 and Co(OH)2. (19) We note that these Fe-free electrolytes do contain residual Co(OH)2 or Ni(OH)2 that can be removed by filtration with a 0.1 μm poly(ether sulfone) filter. The catalysts tested in Fe-free KOH did not show any traces of Fe incorporation by XPS. (19) We then reported the first intrinsic activity trend of first-row transition metals with the rigorous exclusion of Fe impurities. We accounted for the influence of the film thickness, conductivity, and underlying substrate interactions (Figure 1). (28−30) We discovered that, in the absence of Fe, NiOxHy was a terrible OER catalyst—3 orders of magnitude less active than Ni(Fe)OxHy. (19) CoOxHy shows higher activity than NiOxHy in the absence of Fe, but when Fe is added, Ni(Fe)OxHy has a higher TOF than Co(Fe)OxHy at all overpotentials. (21) The activity of FeOxHy was found to be strongly dependent on the film thickness and the support (see below). (28,30,31) At this point, we proposed that Fe was an essential component of the active site in all of the first-row transition-metal catalysts under alkaline conditions (with reasonably high activity) whether incorporated intentionally or from the electrolyte impurities. (28)

Figure 1

Figure 1. (a) Steady-state activity trends as a function of potential for first-row transition-metal (oxy)hydroxides on Au (solid) and Pt (open) quartz-crystal microbalance electrodes. The TOFs reported are based on the total mass measured with the QCM assuming that every metal cation is a possible active site. (b) Data plotted at η = 450 mV (top) and at η = 350 mV (bottom). Compositions in (b) are ordered according to the atomic number of the host/primary cation. Lines and shading guide the eye. All the films had a loading of 8 to 12 μg·cm–2 except for the thin FeOxHy which was 0.5–1.0 μg·cm–2. Adapted from ref (30) with permission. Copyright 2015 American Chemical Society.

3. Understanding Fe-Based Active Sites

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To better understand how Fe-based OER catalysts function, we studied pure FeOxHy. (28) To do this, the instability of FeOxHy under alkaline OER conditions and its low electrical conductivity had to be addressed. FeOxHy films were prepared by three methods—electrodeposition, (19,32) thermal decomposition of spin-cast metal-nitrate films, (16) and thermal evaporation of Fe metal—and were probed by XPS after annealing in air and immersion in 1 M KOH. (30) Fe was oxidized to nominally FeOOH under these conditions regardless of the preparation method. Using electrodeposited FeOxHy films, the dependence of OER activity on the substrate type (Au and Pt) and film thickness was investigated (correcting for dissolution via QCM mass measurements). At η = 350 mV, the OER current was insensitive to mass loading but depended on the substrate identity with FeOxHy on Au being much more active than on Pt (likely due to interaction of Fe and Au oxides under OER conditions). (31,33) At η = 450 mV, the OER current was proportional to an FeOxHy loading of up to ∼3 μg·cm–2 before increasing more slowly between 3 and 7.5 μg·cm–2 and remaining constant after 7.5 μg·cm–2. We concluded that only a thin layer of FeOxHy closest to the electrode surface is active at η = 350 mV due to the low electrical conductivity of FeOxHy. At η = 450 mV, the thickness of the active layer increases because the electrical conductivity increases as the Fe accesses higher oxidation states. (30)
The comparison of the intrinsic activity of Ni(Fe)OxHy, Co(Fe)OxHy, and FeOOH was made by calculating the TOF per “electrochemically active” Fe site (TOFFe) at η = 350 and 450 mV. To account for limited conductivity, we approximated the amount of “active” FeOxHy as that within a loading range such that roughly <1 mV of ohmic drop occurs at 1 mA·cm–2 (0.1 μg·cm–2 at 350 mV and 5 μg·cm–2 at 450 mV). It was observed that at η = 350 and 450 mV, Ni0.75Fe0.25OOH had a higher TOFFe than Co0.54Fe0.46OOH and FeOxHy/Au. This data suggested that the “host” and therefore the local chemical environment of Fe enhance the OER activity. (28)
We investigated how Fe was involved in OER using operando XAS. For this, we studied Co(Fe)OxHy, which shares key features with Ni(Fe)OxHy. The OER activity of Co(Fe)OxHy increases 100-fold with 40–60% codeposited Fe relative to Fe-free CoOxHy. Voltammetric analysis shows an anodic shift in the Co2+/3+ redox wave with increasing Fe content, suggesting strong electronic coupling between Co and Fe. (21) The nominal Co2+/3+ wave is, however, more negative than the nominal Ni2+/3+ wave, even in the presence of large amounts of Fe. This is important because it allowed us to resolve the local structure and oxidation states at both Fe and Co sites by XAS at potentials more positive than the Co wave (i.e., when the catalyst is in the relevant oxidized active state but not yet evolving oxygen) and compare to more-positive potentials where substantial OER current is passing. Compared to previous studies, (8,13,14) our work was important because we engineered porous catalyst structures as thin films where essentially all of the Co and Fe sites were exposed to electrolyte. (7) The porous structure was achieved by electrodeposition/precipitation from a precursor salt solution via the cathodic reduction of NO3 that raises the pH at the electrode surface. (7) The XAS thus reported on changes to surface-active Co and Fe sites without a large background signal from sites buried and not accessible to the electrolyte. In Co(Fe)OxHy, we found a 6% decrease in the Fe–O bond length and a small absorption-edge shift at OER potentials indicating the partial oxidation of Fe relative to the pre-OER resting state (likely 3+). (7) The oxidation of Co was observed only in Fe-free CoOxHy, consistent with Fe being a key species at the active site involved in stabilizing OER intermediates. (7)
Although NiOxHy is intrinsically less active than CoOxHy, Ni(Fe)OxHy is more active than Co(Fe)OxHy. In NiOxHy a lower OER onset potential upon Fe incorporation is accompanied by a positive shift of the Ni2+/3+ wave. (19) Others correlated the increased OER activity to this shift (34,35) and hypothesized that charge transfer from Ni2+ to the Lewis acid Fe3+ makes the Ni site more oxidizing and facilitates the oxidation of water into O2.
We doped NiOxHy with Fe, Ti, Mn, La, and Ce cations and studied the correlation between the Ni2+/3+ potential and OER activity. (36) Solution spin-casting and electrodeposition were used to prepare films with well-mixed cations and similar mass loading and morphology (Figure 2a,b). (36) Ce was the only cation besides Fe which enhanced the activity of NiOxHy (Figure 2c). During the initial voltammetry in rigorously Fe-free KOH solution, Ni0.85Ce0.15OxHy and Ni0.65Ce0.35OxHy exhibited activities ∼8 and 4 times higher than for pure NiOxHy, respectively. The increased activity of Ce-incorporated films declined during cycling. The O 1s XPS spectra of Ni(Ce)OxHy collected before and after extended cycling showed a change from a metal hydroxide to oxide. We interpreted this as phase-segregated cerium oxides forming that eliminated the more-active Ni–O(H)-Ce motifs.

Figure 2

Figure 2. Cyclic voltammetry (cycle 5) of spin-cast Ni1–zMzOxHy films, where M is the metal cation (Ce, La, Mn, Ti, and Fe), at approximately (a) z ≈ 0.1 and (b) z ≈ 0.3 in Fe-free 1 M KOH at 20 mV·s–1. (c) OER TOF of spin-cast films at a 400 mV overpotential at cycle 5 (solid) and cycle 50 (pattern) in Fe-free 1.0 M KOH from voltammetry data collected at 20 mV·s–1. TOFtm values are calculated by assuming that all metal cations are active (and thus are lower limits); the OER current is taken as the average of the forward and reverse scans. Adapted from ref (36) with permission. Copyright 2016 American Chemical Society.

The incorporation of Ti, Mn, or La cations into NiOxHy did not improve the activity. (36) Ti and La shifted the Ni redox wave positively, but the OER activity remained the same as in the pure NiOxHy. The lack of activity enhancement accompanying the shift in the Ni2+/3+ redox potential indicates that there is no strong relationship between the two. In retrospect, this is perhaps not surprising because after the oxidation of Ni/Co the films are conductive with relatively delocalized electronic structures and additional applied potential will increase the oxidative driving force regardless of the initial Ni2+/3+ wave position. (37,38) The unique role of Fe in enhancing the OER activity also supports the idea that Fe serves a critical role, likely bonding to intermediates, as a part of the active site.

4. Modes of Fe Incorporation in Ni/Co (Oxy)Hydroxides

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We explored three ways of incorporating Fe into Ni/Co (oxy)hydroxides: codeposition, adsorption from electrolyte impurities, and adsorption from an Fe-spiked electrolyte. We found that the activity enhancement relative to how much Fe is incorporated depended on the incorporation method and that some Fe sites appear more active than others. (39) We cycled a freshly electrodeposited NiOxHy film in Fe-spiked KOH (1 mM Fe(NO3)3). After the initial cycles, a dramatic increase in activity was observed, as evidenced by the >100 mV decrease in the onset potential (Figure 3a). Meanwhile, the Ni redox peak potential and average e per Ni in the redox wave were largely unchanged. (39) After 100 cycles, the anodic peak potential (Ep,a) of NiOxHy shifted positively by ∼30 mV. In contrast to the change in the Ep,a, the OER activity was only marginally improved after 100 cycles, even as Fe was incorporated up to 24%. This suggested that Fe species might initially be absorbed on the edge/defect sites where they drive fast water oxidation while having little influence on the Ni redox features, whereas further Fe incorporation by cycling leads to (inactive) Fe sitting on internal sites within the 2D (oxy)hydroxide structure.

Figure 3

Figure 3. (a) CVs showing that spiking 1 mM Fe3+ leads to a 130–150 mV decrease in overpotential for NiOxHy in the first 2 cycles with a much smaller decrease over the next 98 cycles. (c) Fe (24% ) is incorporated into NiOOH compared to only ∼9% in CoOxHy after 50 cycles. The extended cycling studies suggest that (b) Fe permeates the bulk of NiOxHy but (d) accesses only the surface of CoOxHy. Adapted from refs (39) and (15).

We compared the results to a coelectrodeposited Ni(Fe)OxHy film and found for similar Fe content that the codeposited Ni(Fe)OxHy shows a more-prominent positive shift of the Ni redox peak. We hypothesize that codeposited Ni(Fe)OxHy has Fe homogeneously distributed throughout the structure that strongly interacts with Ni, substantially affecting the average Ni redox properties. The fact that the OER activity of Ni(Fe)OxHy appears not to depend on the bulk electronic structure or the Ni2+/3+ redox peak potential suggests that it is the surface-bound, under-coordinated Fe sites that are responsible for the high OER activity. Chen et al. also proposed that due to the disordered structure of the deposited Ni(Fe)OxHy there exist edge/defect sites which host Fe species and that these Ni–O–Fe motifs are responsible for efficient water oxidation. (14) This proposal was based on the observation of long-lived Fe4+ sites in Mössbauer experiments that are evidently not highly OER-active.
Similar to Ni(Fe)OxHy, the incorporation of Fe into CoOxHy via cycling in Fe-spiked KOH results in enhanced OER activity. (15) However, Fe is difficult to incorporate into the bulk structure of CoOxHy, as evidenced by the almost unchanged Ep,a after extended cycling (Figure 3b). This result can be explained by the stronger Co–O, compared to Ni–O, bond. (40) After cycling CoOxHy in Fe-spiked KOH, we transferred the Co(Fe)OxHy to Fe-free KOH. With further cycling, a fast deactivation process was observed. Post-mortem XPS characterization showed a 60% decrease in Fe content. (15) In comparison, codeposited Co(Fe)OxHy showed little activity loss when cycled in Fe-free KOH, and XPS showed only an ∼10% Fe loss. These results suggest that dynamic Fe species are incorporated during cycling in Fe-spiked KOH. These Fe atoms likely reside at surface edge/defect sites and thus easily dissolve in Fe-free electrolyte during OER when solution Fe is absent and hence cannot replenish the surface Fe sites. Fe in the codeposited Co(Fe)OxHy film is distributed throughout the film, which prevents its rapid loss in Fe-free KOH.
We further applied electrochemical atomic-force microscopy (EC-AFM) to study morphology dynamics during OER. We began with single-layer Ni(OH)2 (SL-Ni(OH)2) nanosheets synthesized hydrothermally and exfoliated. (41) In the absence of Fe, the SL-Ni(OH)2 preserved its original hexagonal shape and smooth surface at the open-circuit voltage (OCV) in 0.1 M KOH (Figure 4a). (42) With an increase in potential to 1.41 V vs RHE, which corresponds to the onset of Ni oxidation, the SL-Ni(OH)2 roughened (Figure 4b). Further increases in potential lead to restructuring of the single nanosheets into nanoparticles (Figure 4c,d). The volume and surface area of the nanoparticles increased with cycling. Once the growth of the Ni(OH)2 nanoparticles halted, we introduced 3 ppm Fe(NO3)3 into the electrolyte (Figure 4g). The Fe spike caused a ∼19% volume expansion after three CVs, and Fe was observed to deposit inhomogeneously. After Fe in KOH was increased to 12 ppm, segregated FeOOH appeared to deposit (Figure 4h). The restructuring of the Ni(OH)2 nanoparticles during Fe incorporation might explain how Fe moves into the bulk during extended CV cycling. The morphology changes seem to be the result of a Ni dissolution/redeposition process. (42,43)

Figure 4

Figure 4. AFM images of single-layer Ni(OH)2 in Fe-free KOH (a) at the open-circuit voltage and then after the chronopotentiometry measurement (for 500 s) at (b) 1.41 V vs RHE, (c) 1.46 V vs RHE, and (d) 1.61 V vs RHE (scale bars = 200 nm), (e) after one LSV in 0.1 M KOH, (f) after 252 CV cycles, and in the presence of (g) 3 ppm Fe(NO3)2 and (h) 12 ppm Fe(NO3)2 after three CV cycles (scale bars = 200 nm). AFM topographical images of Ni0.8Co0.2OxHy nanosheets (i) at the beginning and (j) after 200 CV cycles between 1 and 1.7 V vs RHE in 0.1 M KOH (scale bars = 100 nm). Single-layer Co(OH)2 nanosheet images under identical conditions (k) at the beginning and (l) after the electrochemical testing. Adapted from refs (42) and (43).

Co(OH)2 nanosheets undergo fewer morphological changes under electrochemical conditions. (43) EC-AFM showed that Co-rich Ni1−δCoδOxHy and CoOxHy initially had a porous morphology and the higher porosity was likely able to reduce the mechanical stress that originated from redox processes (Figure 4i–l). As a result, Co-rich Ni1−δCoδOxHy exhibited higher electrochemical and mechanical stability consistent with our hypothesis that Fe absorbed from the electrolyte is difficult to incorporate into the bulk of CoOxHy. (15,43)
Markovic and co-workers used scanning tunneling microscopy (STM) and ICP–MS techniques combined with isotope labeling to investigate the dynamic stability of Fe sites in transition-metal (oxy)hydroxide clusters. (44) STM revealed that the cluster height of Ni (oxy)hydroxide on Pt(111) slightly increased when the electrode was immersed in Fe-spiked KOH solution, suggesting that Fe was adsorbed on the surface. The dynamic Fe exchange at the catalyst–electrolyte interface was monitored by isotopic labeling starting with 56Fe on the electrode and 57Fe in the electrolyte. 56Fe dissolved during chronoamperometry, while 57Fe in electrolyte deposited onto the electrode surface. The system eventually reaches a dissolution–redeposition steady state. During activity tests in Fe-spiked electrolyte, the OER performance improved. When measured in Fe-free KOH, the OER performance degraded, consistent with our results above. The sum of these studies indicates that the high OER activity of Fe–MOxHy can be maintained only in the presence of Fe in the electrolyte, supporting the view that the dynamic Fe exchange underlies the stability/performance of these catalysts.
We further studied how more-complex compositions behave in an effort to increase the intrinsic activity/stability as well as better understand the mechanism. For example, others reported that mixed Ni–Co–Fe (oxy)hydroxides are more active than Ni-Fe systems, (45,46) although explanations for the activity trends differed and the measurements were not performed in a way in which reliable intrinsic activities could be confirmed. We considered several reasons why the addition of Co might enhance the activity of Ni(Fe)OxHy: (1) the onset of CoOxHy electrical conductivity is at lower overpotentials, so active sites might become electrochemically accessible at more negative potentials; (2) CoOxHy may take up more Fe than NiOxHy before Fe phase segregates, so the addition of Co might allow the formation of more Fe active sites; and (3) the electronic interaction of Fe, Ni, and Co might yield more-optimal intermediate binding energies. For some more-complex compositions, morphological changes can increase the active Fe-site density due to leaching during OER, as likely observed for Cr-containing Ni–Fe (oxy)hydroxides or other compositions with base-soluble cations. (47,48)
We first evaluated the OER activity of Ni(Co)OxHy with the rigorous exclusion of Fe impurities. (18) Generally, a single redox wave was observed for the mixed-metal system that shifted negatively with increasing Co. This data suggests a strong electronic interaction between homogeneously mixed Ni and Co cations. The most-active binary composition was only 2-fold more active compared to the parent compounds. We then examined the ternary Ni(Co,Fe)OxHy and found that the best composition was only ∼1.5 times more active than Ni(Fe)OxHy on a per metal cation TOF basis. (18) The OER activity for pure and Fe-containing binary Ni/Co compounds does not correlate with the position or size of the redox waves, which instead correlates with the bulk composition of the system.

5. Toward Advanced Electrolysis: Ni- and Co-Based Catalysts with Dynamic Fe Sites in AEMWE

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AEMWEs in principle allow for the use of earth-abundant OER catalysts over IrO2 required for commercial PEMWE. (1,49,50) While the AEM provides a locally basic environment, the feed is often pure water, and a detailed chemical understanding of the activity and durability of OER catalysts under these conditions is missing.
We compared the performance of first-row transition-metal (oxy)hydroxide/oxide catalysts in three-electrode alkaline electrolytes and AEMWE in pure water. (49) The best OER catalyst in alkaline electrolyte, NiFeOxHy, performed poorly in AEMWE. However, we found a correlation between the electrical conductivity of the catalyst, measured ex situ, and the performance of the electrolyzers (Figure 5). NiCoOx, NiCoOx:Fe, and NiCoFeOx with high intrinsic electronic conductivity have smaller cell voltages at 0.2 A·cm–2, better than that of IrOx. NiFeOxHy shows the lowest ex situ conductivity (6 × 10–9 S·cm–1) and the highest cell voltage. (49) In the absence of the liquid electrolyte, we hypothesize that the bulk of NiFeOxHy cannot convert to a conductive oxidized form which prevents high activity.

Figure 5

Figure 5. Electrical conductivity, activity, and degradation rate in pure-water AEMWE at 0.2 A·cm–2 for a series of the first-row transition-metal (oxy)hydroxide/oxide catalyst powders. Adapted from ref (49).

Work in liquid electrolyte shows that absorbed Fe participates in a dissolution–redeposition equilibrium and is an essential component of the active site. (44) In a typical AEMWE system with pure-water feed, dissolved Fe would enter the neutral water flow where its solubility is higher than in an alkaline electrolyte. This phenomenon might explain why the NiCoOx:Fe (with Fe mainly adsorbed on the surface) degraded more quickly than the NiCoFeOx (prepared by the coprecipitation of Ni, Co, and Fe) during our AEMWE stability test.
In three-electrode experiments, a thin catalyst layer is deposited on conductive electrodes in 1.0 M KOH with rapid stirring to enhance mass transport. The KOH permeates the porous catalyst assembly in ways that the solid ionomer cannot. Three-electrode conditions thus do not account for the bottlenecks of the MEA including catalyst utilization, mass-transport limitations (water and gas), and ionic (OH) and electrical conductivity. Thus, laboratory-scale results obtained via alkaline electrolyte cells can be more promising than those measured in an AEMWE. (51)
Kim et al. showed high performance for a Ni–Fe-based nanofoam OER catalyst in pure water (2.7 A·cm–2 at 1.8 V, 85 °C) using a highly basic ammonium-enriched ionomer, but the performance decayed rapidly (1.3 mV·h–1 at 200 mA·cm–2 and 60 °C), which was attributed to the oxidation of the ionomer but is also likely due to the flushing of residual KOH from the system (Figure 6, blue trace). (50) Yan and co-workers demonstrated that by growing FexNiyOOH directly on a compressed Ni foam, one avoids the use of Ti or stainless steel gas diffusion layers (GDL) altogether, improves access to the active sites through the pores in the foam, and reduces the catalyst loss during operation in pure water (1020 mA·cm–2 at 1.8 V, 80 °C, 0.56 mV·h–1 for >160 h at 200 mA·cm–2) (Figure 6, red trace). (52) Good initial AEMWE performance is evidently achievable with non-PGM catalysts in pure water, but stability remains a serious issue that can be solved likely by understanding and controlling the ionomer and catalyst chemistry during OER.

Figure 6

Figure 6. Summary of AEMWE performance in pure water at 200 mA·cm–2 (except the orange trace, 500 mA·cm–2). Catalysts, membrane materials, and degradation rates are identified for each trace. Adapted from refs (50), (52), (54), and, (55).

We used a reference electrode with an AEMWE stack to measure the impedance and polarization responses of the anode and cathode separately. (53) This technique is valuable in pinpointing the source of performance loss, particularly when using impure water feeds. (2) We also developed easily adoptable procedures for MEA fabrication and a baseline for AEMWE performance in pure water with all commercial components (0.67 mV·h–1 after break-in at 500 mA·cm–2, 55 °C): a PiperION PAP-TP-85 ionomer/membrane, steel GDL, an IrO2 anode, and a Pt cathode (Figure 6, orange trace). (54) Soni et al. further showed promising durability with a trimethylammonium-modified poly(fluorene-alttetrafluorophenylene) membrane and ionomer (Figure 6, green trace). (55)

5.1. Possible Effects of Dynamic Fe Sites on Membrane Stability

Most AEMWE tests use PGM catalysts or basic liquid electrolytes that minimize catalyst instability. (56,57) Substantial in situ and postoperation chemical analysis is usually lacking. Fe enhances the OER activity of Co- and Ni-based catalysts when absorbed on the surface or substituting for Ni or Co sites. (15,44,58) The maintenance of high activity is contingent upon sufficient Fe in the electrolyte (44,58) such that the rate of Fe deposition equals that of dissolution. (44) The presence of Fe cations in the water feed will likely have a detrimental effect on durability, for example, due to Fenton processes with hydrogen peroxide. (2) Hydrogen peroxide can be formed by gas mixing in the stack. (3) Degradation can be avoided by preventing gas crossover which requires more-selective AEMs. Fe cations traveling through the membrane from the anode will also deposit on the cathode, causing HER activity loss, or may accumulate in the membrane and precipitate. Postoperation chemical analysis of the membrane (EDX, XPS, and NMR) and catalyst structure at the anode and cathode (XRD, SEM, and XPS) and in situ ICP–MS analysis of the outgoing water flow are needed to detect catalyst leaching and redistribution.
One strategy to reduce Fe leaching may be to incorporate it within the catalysts’ bulk structures. However, the preferential leaching of Fe compared to Ni under highly basic conditions and elevated temperatures in such devices will have to be addressed. (59,60) Improved stability might be achieved through novel synthesis methods and taking advantage of the catalyst/support interactions. (61) The MEA configuration, though, might help to prevent the loss of dissolved species and favor their redeposition at the anode, while a more-selective AEM will slow the crossover of the those species to the cathode. (2) At the cathode, adjusting the water flow and current densities along with selecting an HER catalyst with a crystal structure different from that of the precipitating/depositing species may minimize deactivation. (2)

5.2. Catalyst Utilization

Unlike a catalyst immersed in KOH, the catalyst embedded in an ionomer pressed against the solid-ionomer membrane has a limited supply of OH that must travel through the membrane and ionomer/catalyst network. (49,51) The catalyst powders are usually spray-coated on a porous transport layer (PTL). Good electrical contact with the PTL is crucial for Ni- and Co-based catalysts that have lower intrinsic electrical conductivity than IrO2. Hegge et al. demonstrated that the utilization can be improved by maximizing the catalyst porosity and electrical contact using nanofibers. (62) The phase transformation and surface reconstructions observed during OER for Ni-based transition-metal (oxy)hydroxides also present challenges. In an MEA, the ionomer and the membrane are responsible for carrying OH. Dynamically changing catalyst/membrane and catalyst/ionomer interfaces might cause interruptions in the OH transport chain, which, along with limited electrical conductivity in poorly integrated catalyst/PTL configurations, might result in high ohmic losses.

5.3. pH Changes at the Anode/Membrane Interface

Operating AEMWE with a pure water feed at high current densities will affect the pH at the anode. Holdcroft et al. visualized the transport of OH through an AEM from cathode to anode during electrolysis, demonstrating that the interface between the anode and the membrane remains less basic than the rest of the membrane due to the fast consumption of OH. (4) Non-PGM transition-metal oxides dissolve faster at lower pH, which presents a durability challenge for PGM-free AEMWE. (3) Another source of acidification at the anode is ionomer oxidation. Kim et al. studied the acidification of phenyl-containing AEMs by phenol formation and showed how it leads to performance loss in AEMFCs. (63) Membrane acidification might be decreased by replacing the phenyl groups in the backbone with more oxidatively stable components and minimizing their adsorption onto the catalyst surface (50) by tuning the phenyl adsorption energy. (63)
However, it is still not clear what the pH is at the catalyst surface during operating AEMWE in pure water or with a supporting electrolyte. The means of measuring the pH at these nanoscale interfaces would be useful, as would non-PGM OER catalysts with wider pH stability. Co- and Ni-based catalysts corrode and dissolve at acidic pH, (64) although Co oxides are more pH-stable than Ni oxides. Fe, Ni, and Co might be paired with more-acid-stable elements such as Sb and Ti to improve durabilty. (65)

6. Conclusions and Outlook

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Despite decades of work, there remains a substantial need to further understand and control the OER reaction under alkaline conditions, particularly in the context of the dynamic Fe sites in AWE and AEMWE systems. On the basis of our work and that of many others around the world, we have learned the following lessons: (1) crystalline metal oxides that are capable of driving OER at relatively low overpotentials are structurally dynamic under alkaline oxidizing conditions, favoring hydrated disordered (oxy)hydroxide phases during operation; (2) minimizing electrical-conductivity and mass-transport limitations is essential for fundamental activity measurements and for engineering high performance in practical electrodes; (3) the dynamic adsorption or incorporation of Fe is central for the highest-activity catalysts, and Fe is a key component of the OER active site and very likely directly bonding with intermediates; (4) bulk material properties, either of oxides themselves or even of the active (oxy)hydroxide phases (such as the Co/Ni redox potential, eg orbital filling, covalency, etc.), are not simply correlated with the OER activity. Other electronic and structural descriptors are probably needed to rationalize the OER activity.
We suspect that these lessons are universally applicable to OER catalysts under alkaline conditions. It is established that phosphide- and sulfide-based OER catalysts serve only as precursors to the (oxy)hydroxide phases discussed above, (66) and we have not been able to observe high catalytic OER activity from any non-precious-metal material in the rigorous absence of Fe. Furthermore, Binninger and co-workers argue that all metal oxides are unstable under OER conditions. (67) This appears to be particularly true for high-activity perovskite oxides, such as Ba0.5Sr0.5Co0.8Fe0.2O3 (BSCF), which was identified as a material with high OER activity on the basis of molecular orbital principles (68) (assuming retention of the as-prepared crystalline morphology during OER). Later, it was discovered that surface reconstruction and cation leaching lead to Co(Fe)OOH shells. (69,70) Epitaxial [001]-oriented LaNiO3 single crystals rearrange to (oxy)hydroxides when they are Ni-terminated (but not when they are La-terminated and largely OER-inactive). (71) La1–xSrxCoO3 formed a thin CoOOH layer that absorbs Fe, which was most pronounced for the higher Sr-substituted species. (72) The chemical tunability, high activity, and structural dynamicity of perovskite oxides make them a rich system for the further study of surface-absorbed Fe. All of these dynamic processes should be accounted for in AWE and AEMWE systems in order to design for optimal performance and durability. Much work remains.

Author Information

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  • Corresponding Author
  • Authors
    • Raina A. Krivina - Department of Chemistry and Biochemistry and the Oregon Center for Electrochemistry, University of Oregon, Eugene, Oregon97403, United States
    • Yingqing Ou - Department of Chemistry and Biochemistry and the Oregon Center for Electrochemistry, University of Oregon, Eugene, Oregon97403, United StatesCollege of Chemistry and Chemical Engineering, Chongqing University, Chongqing, China
    • Qiucheng Xu - Department of Chemistry and Biochemistry and the Oregon Center for Electrochemistry, University of Oregon, Eugene, Oregon97403, United StatesDepartment of Materials Science, East China University of Science and Technology, Shanghai, China
    • Liam P. Twight - Department of Chemistry and Biochemistry and the Oregon Center for Electrochemistry, University of Oregon, Eugene, Oregon97403, United States
    • T. Nathan Stovall - Department of Chemistry and Biochemistry and the Oregon Center for Electrochemistry, University of Oregon, Eugene, Oregon97403, United States
  • Notes
    The authors declare no competing financial interest.

Biographies

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Raina A. Krivina

Raina A. Krivina received her B.S. in chemistry at the Richard Stockton University of New Jersey. She is currently a fourth-year Ph.D. candidate at the University of Oregon. Her research focuses on understanding degradation pathways in AEMWE, designing new catalyst materials for PEMWE, and nanomaterials synthesis.

Yingqing Ou

Yingqing Ou received his B.S. in chemistry from Sichuan University in 2009. He is a Ph.D. candidate in chemistry and chemical engineering at the Chongqing University, China. He is currently a visiting scholar at the University of Oregon. His research interests focus on the synthesis of nanomaterials and their applications in energy conversion.

Qiucheng Xu

Qiucheng Xu is a joint Ph.D. student at East China University of Science and Technology and the University of Oregon in materials science and engineering. His research interests include electrocatalyst design and membrane-based electrolysis devices.

Liam P. Twight

Liam P. Twight received his B.S. in chemistry from California State University, Long Beach and is now a doctoral student in chemistry at the University of Oregon. He is studying the fundamental electrochemical aspects of the oxygen evolution reaction on transition-metal oxides.

T. Nathan Stovall

T. Nathan Stovall is a third-year undergraduate student pursuing a B.S. in chemistry. His research at the University of Oregon is focused on developing novel OER catalysts for PEMWE and studying the modes of degradation in AEMWE. After the completion of his undergraduate studies, he plans to pursue a Ph.D. in chemistry investigating the fundamentals of renewable energy devices.

Shannon W. Boettcher

Shannon W. Boettcher is a professor in the Department of Chemistry and Biochemistry at the University of Oregon and the founding director of the Oregon Center for Electrochemistry. His research interests center on electrochemistry and understanding and developing inorganic materials for energy conversion and storage.

Acknowledgments

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This work was funded by National Science Foundation grant 1955106 and DOE EERE grant DE-EE0008841.

References

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Cited By


This article is cited by 1 publications.

  1. Zhengguang Zou, Qian Wang. Synergistic Effect of Bimetallic Sulfide Synthesized by a Simple Solvothermal Method for High-Efficiency Oxygen Evolution Reaction. Energy & Fuels 2021, 35 (21) , 17869-17875. https://doi.org/10.1021/acs.energyfuels.1c02810
  • Abstract

    Figure 1

    Figure 1. (a) Steady-state activity trends as a function of potential for first-row transition-metal (oxy)hydroxides on Au (solid) and Pt (open) quartz-crystal microbalance electrodes. The TOFs reported are based on the total mass measured with the QCM assuming that every metal cation is a possible active site. (b) Data plotted at η = 450 mV (top) and at η = 350 mV (bottom). Compositions in (b) are ordered according to the atomic number of the host/primary cation. Lines and shading guide the eye. All the films had a loading of 8 to 12 μg·cm–2 except for the thin FeOxHy which was 0.5–1.0 μg·cm–2. Adapted from ref (30) with permission. Copyright 2015 American Chemical Society.

    Figure 2

    Figure 2. Cyclic voltammetry (cycle 5) of spin-cast Ni1–zMzOxHy films, where M is the metal cation (Ce, La, Mn, Ti, and Fe), at approximately (a) z ≈ 0.1 and (b) z ≈ 0.3 in Fe-free 1 M KOH at 20 mV·s–1. (c) OER TOF of spin-cast films at a 400 mV overpotential at cycle 5 (solid) and cycle 50 (pattern) in Fe-free 1.0 M KOH from voltammetry data collected at 20 mV·s–1. TOFtm values are calculated by assuming that all metal cations are active (and thus are lower limits); the OER current is taken as the average of the forward and reverse scans. Adapted from ref (36) with permission. Copyright 2016 American Chemical Society.

    Figure 3

    Figure 3. (a) CVs showing that spiking 1 mM Fe3+ leads to a 130–150 mV decrease in overpotential for NiOxHy in the first 2 cycles with a much smaller decrease over the next 98 cycles. (c) Fe (24% ) is incorporated into NiOOH compared to only ∼9% in CoOxHy after 50 cycles. The extended cycling studies suggest that (b) Fe permeates the bulk of NiOxHy but (d) accesses only the surface of CoOxHy. Adapted from refs (39) and (15).

    Figure 4

    Figure 4. AFM images of single-layer Ni(OH)2 in Fe-free KOH (a) at the open-circuit voltage and then after the chronopotentiometry measurement (for 500 s) at (b) 1.41 V vs RHE, (c) 1.46 V vs RHE, and (d) 1.61 V vs RHE (scale bars = 200 nm), (e) after one LSV in 0.1 M KOH, (f) after 252 CV cycles, and in the presence of (g) 3 ppm Fe(NO3)2 and (h) 12 ppm Fe(NO3)2 after three CV cycles (scale bars = 200 nm). AFM topographical images of Ni0.8Co0.2OxHy nanosheets (i) at the beginning and (j) after 200 CV cycles between 1 and 1.7 V vs RHE in 0.1 M KOH (scale bars = 100 nm). Single-layer Co(OH)2 nanosheet images under identical conditions (k) at the beginning and (l) after the electrochemical testing. Adapted from refs (42) and (43).

    Figure 5

    Figure 5. Electrical conductivity, activity, and degradation rate in pure-water AEMWE at 0.2 A·cm–2 for a series of the first-row transition-metal (oxy)hydroxide/oxide catalyst powders. Adapted from ref (49).

    Figure 6

    Figure 6. Summary of AEMWE performance in pure water at 200 mA·cm–2 (except the orange trace, 500 mA·cm–2). Catalysts, membrane materials, and degradation rates are identified for each trace. Adapted from refs (50), (52), (54), and, (55).

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