Carbon Oxyanion Self-Transformation on NiFe Oxalates Enables Long-Term Ampere-Level Current Density Seawater Oxidation

Seawater electrolysis is an attractive way of making H2 in coastal areas, and NiFe-based materials are among the top options for alkaline seawater oxidation (ASO). However, ample Cl- in seawater can severely corrode catalytic sites and lead to limited lifespans. Herein, we report that in situ carbon oxyanion self-transformation (COST) from oxalate to carbonate on a monolithic NiFe oxalate micropillar electrode allows safeguard of high-valence metal reaction sites in ASO. In situ/ex situ studies show that spontaneous, timely, and appropriate COST safeguards active sites against Cl- attack during ASO even at an ampere-level current density (j). Our NiFe catalyst shows efficient and stable ASO performance, which requires an overpotential as low as 349 mV to attain a j of 1 A cm-2. Moreover, the NiFe catalyst with protective surface CO32- exhibits a slight activity degradation after 600 h of electrolysis under 1 A cm-2 in alkaline seawater. This work reports effective catalyst surface design concepts at the level of oxyanion self-transformation, acting as a momentous step toward defending active sites in seawater-to-H2 conversion systems. In situ carbon transformation (from C2O42- to CO32-) not only facilitates the generation of active sites but also defends the key active oxyhydroxides against surface chlorine chemistry during oxygen evolution reaction in alkaline seawater. Moreover, NiFe oxalates on Ni foam ((NiFe)C2O4/NF) outperforms reported catalysts by achieving 600-h operation at 1 A cm-2 with minimal potential losses.+image

Automated summary

This study reports an efficient corrosion of NiFe-based catalyst in alkaline seawater and the protection of active sites through in situ carbon oxyanion self-transformation. The catalyst shows high efficiency and stability in alkaline seawater with minimal activity degradation. This research provides important design concepts for protecting active sites in seawater-to-H2 conversion systems.