10.1016_j.nanoen.2020.105392.pdf

Nano Energy 78 (2020) 105392Available online 17 September 20202211-2855/ 2020 Elsevier Ltd. All rights reserved.Electrocatalysts for acidic oxygen evolution reaction: Achievements and perspectives Zhijie Chena, Xiaoguang Duanb, Wei Weia, Shaobin Wangb, Bing-Jie Nia,* aCentre for Technology in Water and Wastewater, School of Civil and Environmental Engineering, University of Technology Sydney, NSW, 2007, Australia bSchool of Chemical Engineering and Advanced Materials, The University of Adelaide, South Australia, 5005, Australia A R T I C L E I N F O Keywords: Oxygen evolution reaction Acidic media Electrocatalysts Water splitting Stability A B S T R A C T Developing efficient electrocatalysts toward acidic oxygen evolution reaction (AOER) is of vital significance in proton exchange membrane (PEM) water electrolysis, which is a promising technique to tackle the approaching energy crisis by supplying high-purity hydrogen. In this work, we first present a general introduction to the AOER mechanism as well as the most important parameters in evaluation of the catalytic performances of catalysts. Fruitful achievements of noble metal-based catalysts (e.g., metals, alloys, and oxides) and noble metal-free catalysts (e.g., transition metal oxides, chalcogenides, and metal-free materials) are fully described, with an emphasis on advanced strategies of catalyst modification/engineering, structure-catalysis correlations, and evolution of catalyst structures and surface chemistry under operational conditions. The representative elec-trocatalysts are benchmarked based on their catalytic performances. Finally, the challenges are summarized and future opportunities are directed for the rational design of AOER catalysts toward sustainable fuel production. 1. Introduction Hydrogen, with zero carbon footprint, has been recognized as a green energy carrier to tackle the approaching energy crisis 1,2. However, the production of molecular hydrogen (H2) heavily relies on non-renewable fossil fuels such as natural gas and crude oil. Addition-ally, the usage of fossil fuels will produce low-purity hydrogen with a large amount of gaseous pollutants 3. Thus, it is urgent to develop green and sustainable technologies for hydrogen production. Alterna-tively, electrochemical water splitting is a simple process to decompose water molecules (H2O) into oxygen gas (O2) and H2 4. This approach can produce high-purity hydrogen with negligible impurities. Based on the properties of electrolytes, the water splitting reactions can be roughly classified into three categories including alkaline water elec-trolysis (AWE), solid oxide electrolysis (SOE), and proton exchange membrane (PEM) water electrolysis. The PEM technique has gain soaring interests because of its merits over other technologies, such as compact system, high current density, low gas crossover, high efficiency, fast response, and small footprint 3,57 (Table 1). Therefore, it is of great significance to develop high-performance PEM electrolyzers to produce hydrogen in an efficient manner. Notwithstanding, the sluggish kinetics of four-electron acidic oxygen evolution reaction (AOER) remains the bottleneck in the PEM water electrolysis process. Since the efficiency of AOER highly depends on the electrode materials (i.e., electrocatalysts), developing advanced catalysts for AOER is a perquisite to realize an efficient PEM water electrolysis process. Intensive efforts have been devoted to developing highly efficient electrocatalysts for AOER which greatly push the boundaries of AOER speedily during the past few years, in terms of both performances and catalytic mechanisms. Precious metal-based materials, especially Ir and Ru-based metals and oxides are state-of-the-art catalysts for AOER, because of their impressive activity and stability in acidic solutions. Diverse Ir/Ru metals are developed into well-defined nanostructures, e. g., nanosheets (NSs) 11, nanotubes 12, and nanoparticles (NPs) 13, 14) and substrates (e.g., graphene 15,16, graphite foam 17, carbon nanobowl 18, and montmorillonite 19), which all exhibit favorable AOER performances. Alloying Ir/Ru with other metal elements is a proven strategy to further improve the catalytic activity and durability in OER. For Ir/Ru oxides, the catalytic performance not only depends on the nanostructures but also on the chemical compositions 20,21. For example, the catalytic properties of rutile IrO2, amorphous IrOx, perovskite (e.g., SrIrO3), pyrochlore (e.g., Y2Ir2O7), and hollandite (e.g., KxIrO2) were investigated and a structure-performance relationship was unveiled 22. The rich valence states and variable coordination * Corresponding author. E-mail address: (B.-J. Ni). Contents lists available at ScienceDirect Nano Energy journal homepage: http:/ https:/doi.org/10.1016/j.nanoen.2020.105392 Received 7 August 2020; Received in revised form 9 September 2020; Accepted 13 September 2020 Nano Energy 78 (2020) 1053922environments (i.e., octahedral and tetrahedral sites) of the Ir sites in these oxides result in the tunable OER performances 23. Apart from the noble metal catalysts, many low-cost transition metals (TMs) (e.g., Mn, Co, Fe, Ni, Ti) and metal-free materials (e.g., C, N) also exhibit promising performances toward AOER. For example, ternary membrane hybrids of MoSe2/MoO2/carbon nanotube (nano-sheet/nanobelt/CNT) showed a better AOER activity than RuO2 in 0.5 M H2SO4, with a much lower overpotential (20: 162 vs 273 mV) to reach a current density of 20 mA cm-2 24. More recently, some carbonaceous catalysts also displayed satisfactory OER performances in acidic elec-trolytes, such as amino-rich hierarchical-network carbon 25, porous fluorographdiyne 26, and nitrogen-doped holey carbongraphene 27. To date, a handful of reviews have been delivered to summarize the advances of OER catalysts, which primarily focus on the new materials, strategies of catalyst design, and reaction mechanisms 8,9,23,2838. Most of the reviews are limited to neutral and alkaline media and low-cost TM-based catalysts are highlighted, especially Fe/Co/Ni-based oxides/hydroxides and chalcogenides. For AOER, there are only a few reviews that intended to propose the reaction mechanisms and outline some specific categories of catalysts. In 2017, Reier et al. summarized the AOER mechanisms on heterogeneous and homogenous catalysts, with limited information of the metal oxides 29. More recently, Sun et al. 9, Siwal et al. 8, Wang et al. 37, and Lei et al. 38 encap-sulated the catalysts for AOER with an emphasis on different aspects, such as PEM design, Ir-based catalysts, NM-free catalysts, and NM-based catalysts. Different from OER in alkaline and neutral solutions, AOER still heavily relies on Ir and Ru, because most of the TM-based catalysts are vulnerable in the acidic and oxidative conditions. In addition, the evolution of catalyst structures and surface properties in acidic elec-trolytes is quite important for the investigation of AOER mechanisms, which attracts little attention in previous reviews. Therefore, it is necessary to summarize the rapid progress in NM-based AOER catalysts and outline the structure-performance relationship for the development of next-generation catalysts. Also, the development of low-cost catalysts is requested which is booming in this field. In this context, this review aims to not only offer a comprehensive overview of the latest progresses in AOER catalysts grouped by the nature of elements (i.e., NM-based, TM-based, and metal free catalysts) but also identify the advanced strategies to precisely regulate the internal and external characteristics of the electrocatalysts toward efficient AOER. Herein, we present a comprehensive review of the recent progress of the electrocatalysts (mainly heterogeneous catalysts) toward the AOER. First, we will discuss the electrochemistry of AOER and several popular parameters/indexes for evaluating the properties of catalysts. Second, fruitful achievements of noble metal (Ir and Ru)-based catalysts will be described, including metals, alloys, and oxides. Then, non-noble metal catalysts based on TMs and metal-free materials will be introduced, with an emphasis on TM oxides. Especially, an emphasis is laid on the state- of-the-art strategies of catalyst modification/engineering, structure- catalysis correlations, and evolution of catalyst structures and surface chemistry under operational conditions. Subsequently, the electro-catalysts are benchmarked based on their performances to identify both the most promising catalysts for AOER and the opportunities and chal-lenges in this field. Finally, we will provide a summary of the studies and offer future perspectives in this booming field. This timely and comprehensive review will inspire future studies to advance the devel-opment of AOER catalysts with respectable performances for energy sustainability. 2. Electrochemistry of AOER 2.1. Mechanisms of AOER Hitherto, many studies have proposed possible mechanisms of AOER based on experimental evidence and density functional theory (DFT) calculations. Two most plausible AOER mechanisms, adsorbates evolu-tion mechanism (AEM) and lattice-oxygen participation mechanism (LOM), are extensively investigated based on the origin of oxygen in the dioxygen product. AEM hypothesizes that the generated oxygen mole-cules are derived from the water in the electrolytes, which is different from LOM that postulates the oxygen are partially from the lattice ox-ygen in the metal oxides 4,23. Fig. 1 depicts the typical AEM and LOM pathways for AOER. For the Table 1 Comparison of different water electrolysis technologies 3,810. Electrolysis process Technical parameters Advantages Disadvantages Typical electrolyte Current density/A cm-2 Cell voltage/ V Operating temperature/C Energy consumption/kW h kg-1-H2 System efficiency/% PEM Electrolysis H2SO4 solution 1.02.2 1.82.2 5084 4773 4571 Low ohmic losses Compact system design High voltage efficiency High gas purity High current density Quick system response High dynamic operation High cost of the components Limited choices of stable electrocatalysts for the OER Alkaline Electrolysis KOH solution 0.20.7 1.82.4 6080 5073 5060 Well-established technology Relative low cost Vast materials available Commercialized Low gas purity Limited current density Gas cross-diffusing problem High ohmic losses Low operational pressure Corrosive liquid electrolyte Solid Oxide Electrolysis Y2O3/ZrO2 1.02.0 1.31.4 500850 8590 High efficiency High working pressure Non-noble electrocatalysts Poor durability Laboratory stage Bulky system design Z. Chen et al. Nano Energy 78 (2020) 1053923AEM mechanism, water molecules first adsorbed on the surface of oxygen-coordinated metal (M) sites by a one-electron process, gener-ating an adsorbed *OH at the M sites. Then, the *OH intermediate is further oxidized to form the *O species. Afterwards, an intermediate species of *OOH is produced via adsorbing another water molecule. Eventually, the *OOH species adsorbed on M site is oxidized by a one- electron transfer process to release one molecular oxygen and recover the initial M active site. In the LOM pathway, the first two steps are the same as the AEM mechanism by forming the *O species. Subsequently, the *O species can be coupled with the lattice oxygen ions to release oxygen molecules, simultaneously forming vacancies in the catalyst crystal structure. After that, the as-formed vacancies can be occupied by a water molecule via generating the *OH species. Finally, the proton can be removed by a one-electron oxidation step 39. The AOER mechanisms are highly dependent on the nature of cata-lysts, and it is still a great challenge to put forward a universal mecha-nism for different types of electrocatalysts. For example, Reier et al. compared the AOER mechanism for the oxides of Pt, Au, Ru and Ir based on the results of in situ studies, such as differential electrochemical mass spectroscopy (DEMS) using isotope-labeled electrolytes, X-ray photo-electron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS) 29. For Pt oxides, the surface of two-dimensional (2D) Pt oxides appear to be the active phase for catalysis, while the origin of evolved O2 re-mains ambiguous due to the conflict results reported by different groups. During an OER process, AuOOH (-oxides) will discompose to Au2O3 (-oxides) and the initially evolved O2 arises exclusively from lattice OH. The mechanisms for electrochemical deposited and thermally prepared Ru oxides are quite different, for the former one, part of the evolved O2 arises exclusively from the lattice OH, and RuO4 is detected during OER. For the latter one, the oxygen atom in the evolved O2 is partially from the lattice oxygen. At last, for thermally prepared Ir oxides, the evolved O2 arises to some extent from lattice OH and OOH is transiently observed during OER. Generally, the oxygen evolution process is quite sensitive to the surface properties of the catalysts, and the dramatic dynamic reorgani-zation of catalyst structures and surface chemistry under operational conditions profoundly governs the catalysis mechanism. To gain a better understanding of the AOER mechanisms, advanced in situ techniques are necessary to monitor the surface evolution (e.g., leaching, amorphous oxides formation) of catalysts and the intermediates (e.g., OOH, OH) during the AOER process. 2.2. Activity evaluation of catalysts The catalytic activity of electrode materials in AOER can be evalu-ated by several important parameters, including the overpotential (), Tafel slope, turnover frequency (TOF) and exchange current density (j0), which can be further classified into two subclasses representing the thermodynamic and kinetic properties 40,41. Thermodynamic activ-ity parameters, i.e., the areal, mass and specific activities, are obtained by normalizing current to geometrical area, catalyst mass, and electro-chemical active surface area (ECSA) (or Brunauer-Emmett-Teller (BET) surface area), respectively. , which is the extra potential beyond the theoretical voltage of 1.23 V, is required to initiate the electrochemical reactions due to the reaction hindrances in a real system. During the assessment of the three parameters, at a fixed current (e.g., 10 mA) is measured after normalization with different current normalization methodologies and recorded as one of the principal screening parame-ters, like 10 mA cm-2geo, 10 mA mg-1 or 10 mA cm-2ECSA. The widely accepted areal and mass activities are just regarded as the engineering perspectives of the electrode performance, and they cannot reflect the intrinsic activities of the catalysts due to the dramatic variations in catalyst loading and engineered nanostructure. In comparison, the specific activity, especially with the ECSA normalization method, is more accurate and it is recommended for the activity comparison among different academic reports. Hence,