Since 2017, I have been working to find alternative catalysts for iridium used in PEM electrolyzer. In 2019, I and coworkers reported how manganese oxide shows exceptional stability among earth-abundant materials for oxygen evolution reaction (OER) in acid (Angew. Chem. Int. Ed.，2019). At the time, non-precious metal catalysts had difficulty maintaining activity for more than a week at 10 mA cm-2, while manganese oxide (γ-MnO2) was able to stably electrolyze water for over 11 months at 10 mA cm-2. This was made possible by the identification of a stable potential window for γ-MnO2 in which the OER can be catalyzed efficiently while simultaneously suppressing deactivation pathways.
In mid-2019, a new catalyst (Co2MnO4) was synthesized by mixing cobalt, which has excellent activity, with manganese, which has excellent stability, through a thermal decomposition method. The obtained material shows both high activity and stability, as the catalyst was able to electrolyze water continuously for 1500 hours at 200 mA/cm2. In particular, the stability of the Co2MnO4 electrocatalyst was improved by a factor of 60 relative to that of Co3O4 (Nature Catalysis, 2022), and the current density is 20-times higher than that of MnO2. The high intrinsic activity and stability of Co2MnO4 are comprehensively analyzed by state-of-the-art crystallography and spectroscopy methods and DFT calculations. More details on this work can be found here: “Enhancing the Stability of Cobalt Spinel Oxide Towards Sustainable Oxygen Evolution in Acid” in Nature Catalysis.
In this study, we performed water electrolysis at different current densities up to 1000 mA/cm2 to evaluate the stability of the catalyst. At higher current densities, catalyst detachment from the substrate becomes more severe, which suggests more complex deactivation mechanisms including catalyst dissolution, detachment, and physical disruption due to intense bubble generation must be considered to rationalize the stability. Such complex phenomena are rarely the focus of recent studies. However, even the Tafel slope at 1000 mA cm-2 is 10-times higher than that at 10 mA cm-2, suggesting different factors determine the activity at ampere-level current densities. Significant improvement in the activity and stability should be possible, once we know the mechanism of the catalyst at ampere-level current densities. However, intense bubble generation at ampere-level current densities makes in-situ spectroscopy difficult due to reasons such as inhomogeneities and light scattering. In addition, the optimal binding energy at higher catalytic rates may also deviate from the conventional understanding obtained at near-equilibrium states (J. Phys. Chem. Lett. 2019; ACS Catal. 2021). Therefore, technological advances and comprehensive theoretical models are needed to reveal the complexity of the reaction at ampere-level current densities.
So far, the majority of earth-abundant electrocatalysts are evaluated at a current density of 10 mA cm-2 for both activity and stability. The basis for this criterion comes from the current density expected from a solar-to-fuel efficiency of 10%. However, both PEM-type and alkaline-type electrolyzers are highly integrated systems that require current densities in the order of hundreds to thousands of milliamperes per square centimeter. The possibility of operating at higher current density provides direct benefits when considering integration with nuclear, hydro, solar and wind power for green hydrogen production. However, it also has indirect benefits, because high current densities will allow for smaller electrolyzers, thus minimizing the carbon footprint of the manufacturing process. We would like to take this opportunity to call for further research aimed towards achieving water electrolysis at ampere-level current densities (Figure 1).
Figure 1. It takes about 298, 000 hours (over 34 years) to split 1 liter of water at a current of 10 mA, assuming a Faraday efficiency of 100%. The hydrogen produced (0.11 kg) can only satisfy the energy consumption of one person for 1.4 hours, assuming a per capita energy consumption of 2500 W. In order to match the time span, we need 200 A even assuming continuous operation during night.