# Effects of Metal Combinations on the Electrocatalytic Properties of Transition-Metal-Based Layered Double Hydroxides for Water Oxidation: A Perspective with Insights

• Zheng Wang
Zheng Wang
Guangdong Key Lab of Nano-Micro Material Research, School of Chemical Biology and Biotechnology, Peking University Shenzhen Graduate School, Shenzhen 518055, China
More by Zheng Wang
• Xia Long*
Xia Long
Guangdong Key Lab of Nano-Micro Material Research, School of Chemical Biology and Biotechnology, Peking University Shenzhen Graduate School, Shenzhen 518055, China
*E-mail: [email protected] (X.L.).
More by Xia Long
• , and
• Shihe Yang*
Shihe Yang
Guangdong Key Lab of Nano-Micro Material Research, School of Chemical Biology and Biotechnology, Peking University Shenzhen Graduate School, Shenzhen 518055, China
Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China
*E-mail: [email protected] (S.Y.).
More by Shihe Yang
Cite this: ACS Omega 2018, 3, 12, 16529–16541
Publication Date (Web):December 4, 2018
https://doi.org/10.1021/acsomega.8b02565
ACS AuthorChoice
1720
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## Abstract

Transition-metal-based layered double hydroxides (TM LDHs) have emerged as highly efficient water oxidation catalysts. They are promising and have the potential to replace the rare and expensive precious metal-based ones such as RuO2 and IrO2, which have been well established. In this perspective, we will summarize the current development of TM LDHs as oxygen evolution reaction (OER) catalysts toward electrochemical water splitting. Particular emphasis will be placed on the roles of the transition-metal cations and the effects of their combination on their catalytic performance for the OER. It is hoped that this perspective will provide fundamental guidelines for future researches in this booming area.

## 1. Introduction

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The hydrogen and oxygen generation from water splitting is of great importance since it can simultaneously tackle the issues of energy crisis and environmental pollution problems. Indeed, it has aroused extensive research interest ever since the concept of water splitting was demonstrated, which has intensified in the recent few decades. (1) Electrochemical (EC) water splitting separates the whole process into two half-reactions, hydrogen evolution reactions (HERs) and oxygen evolution reactions (OERs). As is well known, compared to HER, OER is much more kinetically sluggish and requires a very high overpotential, thus limiting the water splitting efficiency and hindering the development of the hydrogen production industry based on water splitting.
The state-of-the-art catalysts of RuO2 and IrO2 show effective water oxidation activity. (2) However, the high cost of Ir ($16 181 kg–1) and Ru ($2000 kg–1) significantly limits their large-scale utilization. (3) Fortunately, the first-row transition-metal-based compounds with controlled chemical composition and microstructures have been found to have comparable water splitting performance, with much more bountiful resources and lower prices. Among these materials, transition-metal-based layered double hydroxides (TM LDHs) with the general formula of [M1–x2+M3+x(OH)2]x+(An)x/n·mH2O, consisting of edge-sharing hydroxyl coordinated octahedrons and intercalated anions between hydroxyl layers, (4) have been extensively studied, which surprisingly have exhibited even higher OER activities with overpotentials as low as ∼200 mV, which are smaller than that of RuO2 and IrO2 (∼250 mV). In spite of the tremendous research progress that has been made in developing TM LDH as efficient OER catalysts, the understanding of the underlying reasons for the high water oxidation performance of TM LDH is still limited. For instance, the synergistic effect between the divalent and trivalent cations and their effects on OER remain elusive. We notice that some excellent reviews have summarized the recent developments of TM LDH catalysts, but dedicated discussions on the effects of metal combinations in TM LDH on their catalytic performance are still missing despite the significance of such combinations. (2,5)
In this perspective, we survey the recent developments of TM LDHs with unary, binary, and ternary transition-metal ions and their OER catalytic performances, focusing on the effects of metal combinations that have strong influences on the OER activity. First, the basic criteria for evaluation of water oxidation catalysts including overpotential (η), Tafel slope (b), exchanged current density (i0), geometric current density (jg), and specific current density (js) will be introduced, followed by the discussion of the OER mechanism with which the TM LDH catalysts operate. Then, the specific TM LDHs with unary, binary, and ternary transition metals will be presented and the relationship between their chemical compositions and OER performance discussed. Explanations will be given on the active sites of NiFe LDH and the reactivity difference between different binary metal LDHs with binary transition metals. Finally, a summary and prospect will be provided encompassing the challenges and opportunities in this exciting research area.

## 2. Basic Criteria for OER Catalysts and Mechanisms of OER

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The basic criteria to evaluate OER catalysis will be introduced in this section. Overpotential (η), calculated by the difference between the applied potential (E) to reach a certain current density and the equilibrium potential (Eeq) (as shown in eq 1), is the critical and most frequently used descriptor to evaluate the performance of an OER catalyst. Eeq is the half reaction’s thermodynamically determined reduction potential, and E is the potential at which the redox event is experimentally observed. Generally, η is usually given at the onset of OER and at the current density of 10 mA cm–2, which corresponds to a 10% solar to hydrogen efficiency under 1 sun illumination. The existence of overpotential implies that the cell requires more energy than thermodynamically expected to drive a reaction. Therefore, a lower overpotential suggested a better OER activity. (2)(1)Tafel slope (b), which describes the influence of potential/overpotential on steady-state current density, is another key descriptor to evaluate OER kinetics. The value of b could be calculated by eq 2, where R, T, and F are ideal gas constant, temperature, and Faradaic constant, respectively. α is the transfer coefficient that is highly related to Tafel slope. It has been reported that if b = 120 mV dec–1, the rate-determining step is dominated by the single-electron transfer step. If b = 60 mV dec–1, it hints that the chemical reaction after one-electron transfer reactions is the rate-determining step. If b = 30 mV dec–1, the rate-determining step is the third electron transfer step. Therefore, from the value of Tafel slope, we can roughly determine the rate-determining step of OER. Generally, small Tafel slope indicates fast reaction kinetics, and the rate-determining step is supposed to be at the ending part of the reaction. (2) Therefore, catalysts with small Tafel slopes often show good catalytic activity for OER. However, it should be noted that the Tafel slope is often overestimated if geometric current density is used because of the fact that the geometric current density is usually smaller than the specific current density. Moreover, Tafel slope is not accurate to describe the performance of OER catalysts because of its oversimplified assumptions. (6)(2)Turnover frequencies (TOFs) refer to the turnover per unit time, representing the total number of moles transformed into the desired product by one mole of active site per time. Therefore, the number of TOFs determines the level of activity of the catalysts, which is given by eq 3, where j is the current density at a specified overpotential, A is the area of the electrode, and m is the number of moles of active materials deposited onto the electrodes. (7) Moreover, it has been suggested that TOFs at different overpotentials could be different; therefore, the applied overpotential should be provided when presenting TOF. (8)(3)Exchange current density (i0) is defined as the current density at η = 0 (j0) divides surface area (A); the magnitude of i0 reflects the intrinsic charge transfer between reactant and catalyst (eq 4). A higher i0 hints at better catalytic performance. i0 can be described by eq 5, in which k° is the rate constant, and ρ and ω are the reaction orders of Red and Ox, respectively. (9)(4)(5)Different from exchange current density, geometric current density (jg) is given by the current density normalized by geometric surface area at a certain overpotential. jg has practical meaning in developing water splitting devices; however, it usually overestimates the electrochemical performance of a catalyst due to the larger actual surface area than the geometric surface area. Figure 1 shows the Tafel slopes of Co3O4 and IrO2 by using the current density calculated by Brunauer–Emmett–Teller (BET), electrochemical (EC), and disk surface area, respectively. From Figure 1, we can also see that the Tafel slope of Co3O4 is smaller than that of IrO2 with respect to the geometric surface area. One may rush to conclude that the OER performance of Co3O4 is better than IrO2. However, if the BET or EC surface area is used, the performance of Co3O4 is poorer than that of IrO2, (10) suggesting that the applied active surface area is quite important to determine the performance of a catalyst. (10) Generally, the more accurate the surface area used, the more accurate the current density that is obtained, leading to a more precise evaluation of the catalyst.
Understanding the mechanism of OER has a fundamental importance in designing new OER catalysts; therefore, let us first discuss the general mechanisms of OERs before presenting the detailed reactivity about LDH. The catalytic cycle is shown in Figure 2; association mechanism and oxy–oxy coupling mechanism are generally proposed. There are four elementary steps for the association mechanism (eqs 69): association of hydroxide anions to form absorbed OH* accompanied by losing one electron, generation of reactive oxy intermediate O* from OH* with loss of one electron and generation one molecule of water, nucleophilic attack of absorbed oxy O* by the hydroxide anion with release of one electron to form O–O bond giving OOH*, and formation of one molecule of oxygen with release of an electron and one molecule of water to regenerate the catalyst and complete the catalytic cycle. For the oxy–oxy coupling mechanism (eqs 6, 7, and 10), one molecule of oxygen will be generated accompanied by the regeneration of catalyst after generating the oxy intermediate O*. For the association mechanism, the formation of OOH* is generally regarded as the rate-determining step due to the large energy barrier according to density functional theory (DFT) calculations. (12) For the oxy–oxy coupling mechanism (given by eqs 6, 7, and 10), the coupling between two oxy is supposed to withstand a very high kinetic barrier and thus is the rate-determining step. (13)
In the above-mentioned processes, formation of OOH* involves oxidation of oxygen from O* to OOH*, which is usually regarded as the rate-determining step. Therefore, LDH with high oxidation ability would facilitate the formation of OOH*. In addition, OER involves formation and cleavage of metal–oxygen bonds; in principle, catalysts with superior OER activity should possess a suitable oxygen bonding strength, neither too strong nor too weak. As oxidation ability and oxygen binding energy of LDH vary with the change of transition metals, they have a critical influence on the OER activity of LDH. Therefore, in this perspective, we will focus on the effects of chemical composition on the oxygen binding energy and oxidation ability of LDH. Unary, binary, and ternary transition-metal-based LDHs toward OER will be discussed.(6)(7)(8)(9)(10)

## 3. Unary Metal-Based LDH

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Unary metal-based LDHs exhibited limited OER activity, but they provide an ideal platform for us to understand the intrinsic OER activity of LDHs due to their structural simplicity. In this part, we will first introduce Ni-based LDH, followed by Fe-based LDH, Co-based LDH, and the recently reported V-based LDH. As transition-metal hydroxide and transition-metal-oxy/hydroxide can interconvert via Bode’s Diagram, (14) we also treated transition-metal oxyhydroxide as LDH for simplicity. The performance evaluation factors such as overpotential at 10 mA cm–2, Tafel slope, and TOF of unary metal LDHs are given in Table 1.
Table 1. Overpotential at 10 mA cm–2, Tafel Slope, and TOF in 1 M KOH of Unary-Transition-Metal-Based LDHs
catalystsoverpotential at 10 mA cm–2 (mV)Tafel slope (mV dec–1)TOF (s–1)references
Ni(OH)2/NiOOH297a290.17Boettcher (16)
γ-NiOOH660b  Friebel and Bell (17)
γ-FeOOH550b  Friebel and Bell (17)
α-Co(OH)2400b440.070Wang (19)
β-Co(OH)2463b390.021Wang (19)
β-CoOOH426b360.042Wang (19)
Co LDH393590.001Hu (25)
Co LDH340560.801Kang and Yao (21)
monolayer Co LDH350450.003Hu (25)
VOOH27068 Liang and Wang (24)
a

Overpotential at 1 mA cm–2.

b

In 0.1 M KOH solution.

### 3.1. VIII Group Single Transition-Metal Hydroxides/Oxyhydroxides

Ni-based compounds are the most widely used OER catalysts; actually, NiOx was employed for OER early in the 1980s. (15) However, it did not arouse research interests until 2012 when Boettcher in situ generated nickel layered hydroxide/oxyhydroxide from NiOx through an electrochemical conditioning process (Figure 3). (16) The as-in situ-generated Ni hydroxide/oxyhydroxide exhibited an outstanding OER performance, with a low overpotential of 297 mV at 1 mA cm–2, an extremely small Tafel slope of 29 mv dec–1, and a considerably large TOF of 0.17 s–1 at η = 300 mV in 1 mol L–1 (M) KOH, better than that of the state-of-the-art catalyst of IrOx catalysts (η = 378 mV at 1 mA cm–2, b = 49 mv dec–1, TOF = 0.0089 s–1 at η = 300 mV in 1 M KOH).
NiFe LDH is the most effective for OER (will be discussed in Section 4.1), but NiOOH and Ni(OH)2 are not so effective toward OER. Therefore, studying the OER activity of Fe-based LDH is important to understand the superior activity of NiFe LDH and has aroused much attention. Friebel and Bell studied the intrinsic OER activity of γ-FeOOH and found that the overpotential at 10 mA cm–2 of γ-FeOOH is 550 mV in 0.1 M KOH, which is smaller than that of Fe-free γ-NiOOH (η = 660 mV at 10 mA cm–2 in 0.1 M KOH). However, it is much higher than that of (Ni,Fe)OOH (η = 360 mV at 10 mA cm–2 in 0.1 M KOH). (17) Moreover, the calculations also indicated that the overpotential of γ-FeOOH is 520 mV, in good agreement with experiments. (17) Boettcher also studied the OER activity of FeOOH and suggested that FeOOH had high OER activity, but was limited by its poor conductivity, which has a measurable conductivity of 2.2 × 10–2 mS cm–1 only when the overpotential is larger than 400 mV. (18)
Similar to Ni and Fe, the remaining first-row group VIII transition metal, cobalt, can also form a layered double hydroxide structure and, of course, has received much interest. Wang compared the OER activities of α-Co(OH)2, β-Co(OH)2, and β-CoOOH (19) and found that α-Co(OH)2 will transform to γ-CoOOH before the OER, and the resulting γ-CoOOH inherits a large basal distance of α-Co(OH)2. It has an overpotential of 400 mV at 10 mA cm–2 in 0.1 M KOH, interestingly, a Tafel slope of 44 mV dec–1 when η is smaller than 350 mV, and 130 mV dec–1 when η is larger than 350 mV. Moreover, α-Co(OH)2 is more active than β-Co(OH)2, which might be due to the large interlayer space in the α-Co(OH)2.
Materials with ultrathin structures usually have high specific and exposed surface area and are abundant in vacancies, leading to a higher number of active sites and thus higher activities. Pan and Wei synthesized an atomically thin γ-CoOOH, with a thickness of only 1.4 nm. Expectedly, the as-prepared γ-CoOOH has very high mass activities and abundant-active sites, thus leading to a sharp increase of OER activity with η = 300 mV at 10 mA cm–2 and a Tafel slope of 38 mV dec–1 in 1 M KOH. Interestingly, the as-prepared γ-CoOOH is half-metallic in contrast to its bulk, which was proposed to be related to the presence of dangling bonds in the CoO6–x octahedron as supported by DFT calculations. (20) Kang and Yao also prepared Co-based LDH with atomic thickness for OER. Owing to its ultrathin structure, the OER activity of the Co-based LDH can have an overpotential of 340 mV at 10 mA cm–2, a Tafel slope of 56 mV dec–1, and a TOF of 0.801 s–1 at η = 350 mV in 1 M KOH. (21) Besides ultrathin structures, a larger interlayer spacing of LDH can also lead to a higher number of active sites. Sun and Chen reported benzoate anion interacted with CoOOH with an interlayer spacing as large as 14.72 Å, which allows the easy permeation of water and hydroxide, resulting in a higher number of active sites. (22) And the overpotential at 50 mA cm–2 is only 291 mV in 1 M KOH.

### 3.2. V-Hydroxides/Oxyhydroxides

In addition to the intensively studied VIII transition-metal-based LDH, V and Mn compounds have also been investigated. In 2012, Markovic studied the trends for OER on 3d transition-metal hydr(oxy)oxide catalysts (M2+δOδ(OH)2−δ/Pt(111)) and discovered that the reactivity toward OER is in the order Mn < Fe < Co < Ni, which is governed by the OH–M2+δ bond strength (Ni < Co < Fe < Mn). (23) According to the Sabatier principle, too weak or too strong M–OH bonds would retard the OER reactivity. Copper and zinc have too many d electrons in the d orbitals, which cause strong repulsions between the d electrons and 2p electrons of oxygen. Expectedly, Cu(OH)2 and Zn(OH)2 exhibited poor OER activities. Early transition metals, such as titanium, have few d electrons in the d orbitals, which lead to very strong OH–M bonds, and are supposed to be ineffective toward OER. (23)
Despite that the early transition-metal hydroxide has a strong M–OH bond, which is proposed to be unfavorable for OER, VOOH hollow nanospheres, structurally resembling lepidocrocite γ-FeOOH as evidenced by their similar X-ray diffraction (XRD) pattern (Figure 4a), have been employed as an efficient OER catalyst by Liang and Wang, (24) exhibiting an overpotential of 270 mV for OER at 10 mA cm–2 and a Tafel slope of 68 mV dec–1 in 1 M KOH. It has been well established that V is an early transition metal, favoring its high oxidation states, +5 and +4, whereas V has an oxidation state of +3 in VOOH. It may be of concern that VOOH is unstable during OER; however, the stability test given in Figure 4b shows that VOOH is quite stable during OER. And the reactivity of VOOH does not decrease even after 5000 cycles, as shown in the inset of Figure 4b. Moreover, the prepared VOOH can be used as HER catalyst, with an overpotential of 164 mV at 10 mA cm–2 and a Tafel slope of 104 mV dec–1. The advanced water splitting performance of VOOH in this work was contributed to the large surface area from the hollow sphere morphology.

## 4. Binary Metal-Based LDH

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Unary-transition-metal-based LDHs are limited by their low intrinsic OER activity or conductivity. Fortunately, by doping the second metal ions into the unary-TM LDH, the as-formed binary LDHs such as NiFe LDH, NiCo LDH, and CoFe LDH showed a much higher OER performance, which will be summarized and discussed in this section. The performance evaluation factors such as overpotential at 10 mA cm–2, Tafel slope, and TOF of binary metal LDHs are given in Table 2.
Table 2. Overpotential at 10 mA cm–2, Tafel Slope, and TOF in 1 M KOH of Binary Metal-Based LDHs
catalystsoverpotential at 10 mA cm–2 (mV)Tafel slope (mV dec–1)TOF (s–1)references
(Ni,Fe)OOH33630 Boettcher (16)
NiFe LDH208480.028Yang