# Electrocatalysis for the Oxygen Evolution Reaction in Acidic Media: Progress and Challenges
## 4 Summary and Outlook  
The rational design of a catalyst to improve its activity and stability in acidic systems is an important prerequisite and basis for the realization of PEMWE. This review not only surveyed our fundamental understanding of the OER, but also summarized the optimization strategies to optimize the catalyst's performance under acidic systems. Through the investigation of the available reports, some general principles for the design and preparation of potential catalysts were reviewed. As more and more effort is devoted in the development of OER catalysts under acidic conditions, the work in this area has made rapid progress. The performance of OER catalysts is getting closer to practicality. However, more and more problems have come along with the progress. Greater efforts should be made on the basis of the following aspects. Research on precious-metal-free OER catalysts has not made a substantial breakthrough owing to the harsh working conditions. Research on the OER in acid media is mostly focused on noble-metal-based electrocatalysts, such as Ir/Ru-based materials. Even RuO\({}_{2}\) will dissolve during the catalysis process due to phase inversion. Perovskite oxides seem to be promising candidates to replace the noble-metal-based electrocatalysts. However, some questions remain to be solved. For example, most of the perovskite catalysts with high OER activity in acidic solutions are not really precious metal free, as their B-sites require the participation of precious metals. Furthermore, in depth study on overcoming the continuous leaching and dissolution problems of these kinds of catalysts under harsh working conditions is still required. All in all, tremendous efforts should be devoted to  
Figure 6: (**a**) TEM images and a schematic illustration of Ir nanowire intermediates obtained at different reaction times from 5 min to 20 h [62]. (**b**) Fine-structure characterization of a Ru@IrO\({}_{\text{x}}\) catalyst and investigation of its OER electrocatalytic activity [99]. (**c**) Fine-structure characterizations of ultra-thin NaRuO\({}_{2}\) nanosheets including the crystal structure, an atomic force microscopy (AFM) image, a TEM image, and selected area electron diffraction (SAED) patterns [100]. improving the performance of nonprecious-metal-based catalysts, including the intrinsic activity as well as corrosion resistance in an acid environment at high oxidation potentials. Accurate detection of reaction intermediates is an important prerequisite for the establishment of mechanisms. However, limited by the temporal and spatial resolution of the current characterization equipment, direct experimental evidence for the proposed OER mechanisms and active sites in acid media is still limited despite the many different proposed reaction mechanisms. Recently, more and more evidence has revealed that the reaction mechanism of the OER varies with different catalysts in an acidic environment. Moreover, the structure of catalytic centers seems to change during the electrocatalytic process, which greatly increases the difficulty of identifying the mechanism. At present, most of the studies on the evolution of reaction intermediates and reaction centers are realized through density functional theory simulations accompanied by experimental results. However, we need to find correlations between different reaction mechanisms, so as to provide a strong foundation for the rational design of catalysts. Driven by this goal, more in situ characterization techniques, such as in situ XAS, in situ diffuse reflectance infrared Fourier transform (DRIFT) spectra, and femtosecond spectra, should be developed to enable us to understand the reaction mechanism more deeply. In addition, isotope-labeled electrochemical mass spectroscopy (DEMS) is another powerful tool to explore the mechanism of the reaction. Only in this way can great progress be made to promote a deeper understanding of the mechanism. We believe that opportunities and challenges coexist.