# Electrocatalysis for the Oxygen Evolution Reaction in Acidic Media: Progress and Challenges
## 1 Introduction  
Searching for sustainable, clean, and highly efficient energy is the main method for 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 based on carbon-based fuels [1; 2; 3; 4]. Human actions have led to the carbon dioxide content in the atmosphere rising rapidly and exceeding 400 ppm currently, mostly originating from the burning of coal, oil, and gas [2; 5]. The application of wind and solar power and other types of renewable electricity generation technologies seems to be an efficient way to fulfill the requirement of an energy revolution [6; 7; 8]. However, they are strongly intermittent in nature due to diurnal or seasonal variations [9]. Converting their energy to a zero-emission chemical energy carrier such as hydrogen is an alternative that can achieve versatile utilization, such as clean heating or electricity at a later stage, on account of the high energy density of hydrogen [5; 10; 11; 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; 14; 15]. Proton exchange membrane water electrolysis (PEMWE) operating in acidic environments has offered an effective way to produce sustainable, high-purity hydrogen through targeted electrochemical reactions since the 1960s [16] (Figure 1). PEMWE has the advantages of a faster dynamic response, a higher current density, and lower crossover of gases and is considered to be the basis of a hydrogen society in the future [17; 18; 19]. Electrochemical water splitting involves two heterogeneous multi-step half-reactions, which are referred to as the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER) [20, 21]. Owing to the inherent energy barrier, the practical operating voltage of commercial water electrolyzers is higher than the theoretical 1.23 V (versus a reversible hydrogen electrode) under the standard conditions (298 K and 1 atm) [22, 23]. For example, industrial electrolytic water generally maintains the external voltage at 1.8\(\sim\)2.0 V [16]. Typically, the descriptor of overpotential is used to show the difference between the thermodynamic potential and the practical potential required to drive the electrochemical reaction [24]. The overpotential mainly comes from the electrochemical polarization on the anode side (\(\eta_{d}\)) and cathode side (\(\eta_{c}\)) and the ohmic polarization caused by other resistors (\(\eta_{other}\)) [25]. Comparing \(\eta_{a}\) and \(\eta_{other}\), the intrinsically sluggish kinetics of the OER involving a four electron-proton coupled reaction (Equation (1)) hampers the overall water-splitting process [16, 26]. \[2\mathrm{H_{2}O(l)}\rightarrow\mathrm{O_{2}}+4\mathrm{H^{+}}+4\mathrm{e^{-}} \tag{1}\]  
One solution to this conundrum is to develop suitable catalysts with high efficiency and low overpotential [27]. However, most of the excellent OER catalysts with high activity and durability are not stable in acidic solutions [28]. They are easily oxidized and decomposed in a strong acid system, which is one of the indispensable working conditions for PEMWE [29]. Currently, the iridium (Ir) and ruthenium (Ru)-based electro-catalysts are regarded as the state-of-the-art commercial electrocatalysts for the OER [30, 31]. Compared with other catalysts, they exhibit excellent OER catalytic activity due to their inherent promising activity, even if severe corrosion still exists under strong acid working conditions [32]. This provides a driving force for the vast majority of studies on the modifications of these electrocatalysts, including composition, structure, and morphology optimizations [23, 33, 34, 35]. Outstanding OER electrocatalysts should have excellent intrinsic activity and sufficient active sites [18], and these requirements are generally combined with simplicity and controllability.