Recently, we published our original research in Nature Catalysis reporting that the stability of γ-MnO₂ for acid oxygen evolution reaction (OER) can be enhanced through a strategy of modulating the lattice oxygen structure. By increasing the concentration of planar oxygen, we extended the lifetime of γ-MnO₂ at 200 mA cm^-2 by 40 times. Importantly, this strategy proved effective in a proton exchange membrane (PEM) electrolysis environment, ultimately demonstrating 1000 hours of water splitting in a PEM electrolyzer at 200 mA cm^-2 using only pure MnO₂ as an anode catalyst. This is significantly higher performance compared to the first paper from our group in 2019 (0.1 mA cm^-2 for 10 hours, R. Soc. Open Sci., 2019).

    The motivation for this work is our recognition of the significance of catalyst stability, particularly in real applications. In the past few decades, electrochemical catalysis research has largely focused on improving catalyst activity, but did not focus as much on stability. The lack of stability is especially pronounced for the OER catalysts used in PEM water electrolysis due to the harsh acid environment. To address sustainability concerns surrounding the supply of materials, scaling up PEM electrolyzers necessitates the development of alternative non-noble materials to replace iridium as anode catalysts. However, non-noble materials often suffer from rapid dissolution. The lack of understanding of the structure factors governing stability further imposes a challenge in developing robust non-noble catalysts. Recognizing that this scientific gap was an opportunity to make a breakthrough, I initiated a research project in 2021 with the goal of identifying universal descriptors for highly stable water electrolysis catalysts. This project commenced with γ-MnO₂ as the primary material of interest. The benefit of γ-MnO₂ is its unique intergrowth structure of pyrolusite and ramsdellite unit and their ratios are controllable through electrodeposition conditions and post-thermal treatment. Additionally, the dissolution species of MnO₄^- exhibits a sensitive UV-vis signal which enables us to track the dissolution process (Angew. Chem. Int. Ed.，2019).

    To explore the correlation between structure and stability, we sought techniques for a comprehensive structure analysis of γ-MnO_2, especially quantitative analysis. Given the disordered structure of MnO₂ resulting from the random intergrowth of pyrolusite and ramsdellite unit, we conducted X-ray total scattering experiments and pair distribution function (PDF) analysis. This effort revealed a close correlation between lattice planar oxygen and stability. Moreover, this work also marks the starting point of our research using the Spring-8 Synchrotron facility. We have since explored more advanced functions, including operando electrochemical X-ray absorption fine structure (XAFS) and time-resolved XAFS to advance ongoing and future projects.

    For the stability test, we conducted the evaluations in both the conventional three-electrode system and the PEM electrolyzer cell. While results from the conventional three-electrode system are informative, they alone cannot reliably assess applicability in practical applications due to the significant environmental differences with PEM electrolyzers. We anticipate that evaluating stability in PEM cells will be an indispensable step in the forthcoming research endeavors. We can conveniently assemble our electrode with pure MnO₂ inside the PEM cell because the catalysts were deposited on the Pt/Ti mesh, which serves as a porous transport layer (PTL) within the PEM cell. This fabrication method also holds promise for simplifying the steps and reducing costs in the construction of PEM electrolyzers, compared to the commonly used spray-coating method for powder samples.

Outlook

Considering the typical operation lifetimes of 5 to 10 years, and the required current densities of ampere levels in PEM electrolyzers, the best scenario is having both robust and highly active catalysts. In this work, we have successfully enhanced the stability of γ-MnO₂ by modulating the lattice oxygen structure. What we find particularly exciting is that this enhancement in stability did not come at the expense of the catalysts' activity. This suggests that developing catalysts which possess both high activity and stability should be possible. However, there remains a critical knowledge gap in the understanding of the dissolution process which is essential for exploring novel strategies in developing highly stable water electrolysis catalysts. This process is intricately linked with the oxygen evolution reaction and represents a complex reaction cycle. To advance in this field, a collaborative effort that spans across disciplines such as materials science, spectroscopic chemistry, crystallography, and even theoretical science is demanded. Furthermore, it is essential to consider not only steady-state conditions but also dynamic operational scenarios, as real-world operation conditions are often dynamic due to the intermittent nature of renewable energy sources. Advanced time-resolved techniques may be indispensable in this regard.

    Finally, we firmly believe that there is a promising future ahead, one that brings us closer to practical applications and bridges the gap between academic research and industrial requirements. With this in mind, we welcome potential collaborations and experts to join us in unraveling the secret of catalysis, contributing to a sustainable future.