applied sciences Review Electrocatalysis for the Oxygen Evolution Reaction in Acidic Media: Progress and Challenges Hui-Ying Qu 1,* , Xiwen He 1,2, Yibo Wang 2 and Shuai Hou 2,* 1 Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, Guangxi University, Nanning 530004, China; [email protected] 2 Laboratory of Advanced Power Sources, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China; [email protected] * Correspondence: [email protected] (H.-Y.Q.); [email protected] (S.H.) Abstract: The oxygen evolution reaction (OER) is the efficiency-determining half-reaction process of high-demand, electricity-driven water splitting due to its sluggish four-electron transfer reaction. Tremendous effects on developing OER catalysts with high activity and strong acid-tolerance at high oxidation potentials have been made for proton-conducting polymer electrolyte membrane water electrolysis (PEMWE), which is one of the most promising future hydrogen-fuel-generating technolo- gies. This review presents recent progress in understanding OER mechanisms in PEMWE, including the adsorbate evolution mechanism (AEM) and the lattice-oxygen-mediated mechanism (LOM). We further summarize the latest strategies to improve catalytic performance, such as surface/interface modification, catalytic site coordination construction, and electronic structure regulation of catalytic centers. Finally, challenges and prospective solutions for improving OER performance are proposed. Keywords: water electrolysis; acidic oxygen evolution reaction; electrocatalyst; OER activity; metal–support interaction; electronic effect; coordination environment Citation: Qu, H.-Y.; He, X.; Wang, Y.; Hou, S. Electrocatalysis for the Oxygen Evolution Reaction in Acidic Media: Progress and Challenges. 1. Introduction Appl. Sci. 2021, 11, 4320. https://doi. Searching for sustainable, clean, and highly efficient energy is the main method for org/10.3390/app11104320 solving the energy crisis and environmental pollution problems brought about by the first and second industrial revolutions, which have built a modern and prosperous society Academic Editor: Alfio Dario Grasso based on carbon-based fuels [1–4]. Human actions have led to the carbon dioxide content in the atmosphere rising rapidly and exceeding 400 ppm currently, mostly originating Received: 14 April 2021 Accepted: 7 May 2021 from the burning of coal, oil, and gas [2,5]. The application of wind and solar power Published: 11 May 2021 and other types of renewable electricity generation technologies seems to be an efficient way to fulfill the requirement of an energy revolution [6–8]. However, they are strongly Publisher’s Note: MDPI stays neutral intermittent in nature due to diurnal or seasonal variations [9]. Converting their energy to with regard to jurisdictional claims in a zero-emission chemical energy carrier such as hydrogen is an alternative that can achieve published maps and institutional affil- versatile utilization, such as clean heating or electricity at a later stage, on account of the iations. high energy density of hydrogen [5,10–12]. Therefore, an increasing number of sustainable pathways for energy conversion and storage technologies, including water electrolysis, batteries, and fuel cells, have been proposed and extensively investigated [5,13–15]. Proton exchange membrane water electrolysis (PEMWE) operating in acidic environments has offered an effective way to produce sustainable, high-purity hydrogen through targeted Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. electrochemical reactions since the 1960s [16] (Figure1). PEMWE has the advantages of This article is an open access article a faster dynamic response, a higher current density, and lower crossover of gases and is distributed under the terms and considered to be the basis of a hydrogen society in the future [17–19]. conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Appl. Sci. 2021, 11, 4320. https://doi.org/10.3390/app11104320 https://www.mdpi.com/journal/applsci Appl.Appl. Sci. Sci. 20212021, ,1111,, x 4320 FOR PEER REVIEW 22 of of 15 15 FigureFigure 1. 1. SchematicSchematic of sustainablesustainable pathways pathways for for energy energy conversion conversion and and storage storage based based on electrocatalysis. on electroca- talysis. Electrochemical water splitting involves two heterogeneous multi-step half-reactions, whichElectrochemical are referred to water as the splitting cathodic involves hydrogen two evolution heterogeneous reaction (HER)multi-step and thehalf-reac- anodic tions,oxygen which evolution are referred reaction to (OER)as the [20cathodic,21]. Owing hydrogen to the evolution inherent energyreaction barrier, (HER) theand prac- the anodictical operating oxygen evolution voltage of reaction commercial (OER) water [20,21]. electrolyzers Owing to the is inherent higher than energy the barrier, theoretical the practical1.23 V (versus operating a reversible voltage of hydrogen commercial electrode) water electrolyzers under the standard is higher conditions than the theoretical (298 K and 1.231 atm) V (versus [22,23]. a For reversible example, hydrogen industrial electrode) electrolytic under water the generallystandard maintainsconditions the(298 external K and 1voltage atm) [22,23]. at 1.8~2.0 For Vexample, [16]. Typically, industrial the electrolytic descriptor ofwater overpotential generally ismaintains used to showthe external the dif- voltageference at between 1.8~2.0 the V thermodynamic[16]. Typically, the potential descri andptor the of practicaloverpotential potential is used required to show to drive the differencethe electrochemical between the reaction thermodynamic [24]. The overpotential potential and mainly the practical comes from potential the electrochemical required to h h drivepolarization the electrochemical on the anode reaction side ( a) [24]. and cathodeThe overpotential side ( c) and mainly the ohmic comes polarization from the electro- caused by other resistors (hother)[25]. Comparing ha and hother, the intrinsically sluggish kinetics chemical polarization on the anode side (ηa) and cathode side (ηc) and the ohmic polariza- of the OER involving a four electron–proton coupled reaction (Equation (1)) hampers the tion caused by other resistors (ηother) [25]. Comparing ηa and ηother, the intrinsically sluggish overall water-splitting process [16,26]. kinetics of the OER involving a four electron–proton coupled reaction (Equation (1)) ham- pers the overall water-splitting process [16,26]. + − 2H2O(l) ! O2 + 4H + 4e (1) 2H Ol →O +4H +4e (1) One solution to this conundrum is to develop suitable catalysts with high efficiency andOne low solution overpotential to this [conundrum27]. However, is to most develop of the suitable excellent catalysts OER with catalysts high withefficiency high andactivity low overpotential and durability [27]. are However, not stable most in acidic of the solutions excellent [OER28]. catalysts They are with easily high oxidized activ- ityand and decomposed durability are in anot strong stable acid in acidic system, solutions which [28]. is one They of theare easily indispensable oxidized workingand de- composedconditions in for a PEMWEstrong acid [29 system,]. Currently, which the is iridiumone of the (Ir) indispensable and ruthenium working (Ru)-based conditions electro- forcatalysts PEMWE are [29]. regarded Currently, as the the state-of-the-art iridium (Ir) an commerciald ruthenium electrocatalysts (Ru)-based electrocatalysts for the OER [30 ,are31]. regardedCompared as withthe state-of-the-art other catalysts, commercial they exhibit electrocatalysts excellent OER for catalytic the OER activity [30,31]. dueCompared to their withinherent other promising catalysts, activity,they exhibit even excellent if severe corrosionOER catalytic still existsactivity under due strongto their acid inherent work- promisinging conditions activity, [32]. even This if provides severe corrosion a driving still force exists for theunder vast strong majority acid of working studies condi- on the tionsmodifications [32]. This ofprovides these electrocatalysts, a driving force including for the vast composition, majority of structure, studies on and the morphology modifica- tionsoptimizations of these electrocatalysts, [23,33–35]. Outstanding including OER comp electrocatalystsosition, structure, should and have morphology excellent intrinsicoptimi- zationsactivity [23,33–35]. and sufficient Outstanding active sites OER [18], electrocatalysts and these requirements should have are generallyexcellent combinedintrinsic activ- with itysimplicity and sufficient and controllability. active sites [18], In this and regard, these optimizationsrequirements are ofthe generally reaction combined energy barrier, with simplicityelectronic and conductivity, controllability. and reaction In this regard, surface optimizations area of the OER of the electrocatalysts reaction energy are barrier, of great electronicimportance conductivity, [18,36,37]. Theand transportreaction surface efficiencies area ofof electrons,the OER electrocatalysts ions, and produced are of oxygen great importanceare directly [18,36,37]. related to theThe number transport of channels,efficiencies which of electrons, depend onions, rational and produced surface/interface oxygen areengineering directly related through to the nanostructural number of channels modifications,, which depend