Understanding the Degradation Mechanism for NiMo Composite and Effect of Ionomer on the Activity Towards HER

Abstract

Hydrogen is an essential commodity for achieving cleaner and greener energy.1,2 Due to its high gravimetric energy density, hydrogen is favored as a carbon-neutral fuel.2,3 Moreover, the electrochemical accessibility of hydrogen evolution and oxidation reactions enable hydrogen to be used as a reversible fuel, as in hydrogen battery anodes and unitized reversible fuel cells.4,5 Platinum group metals (PGMs) are excellent catalyst for hydrogen chemistry, as they exhibit negligible activation barrier for HER and HOR, but they are costly and the upstream processes for mining and purification are unsustainable.6 A promising alternative to PGM catalysts would be nonprecious transition metal alloys/composites in form of nanoparticles. These materials are more readily available and lower in cost. Nonetheless, non-precious catalysts developed to date have never met the benchmarks set by PGMs in terms of catalytic activity and stability.Our lab is continuing a long-running effort to develop and improve nickel-molybdenum (Ni–Mo) composites, which stand as one of the strongest candidates for non-PGM HER and the HOR. In prior work, we developed a method to synthesize Ni–Mo nanoparticles supported on oxidized carbon, which increases catalyst dispersion and significantly reduces losses due to interfacial resistivity.7 The catalyst composite has a core@shell structure, where in the core is mainly nickel-rich alloy and the shell is primarily molybdenum-rich oxide.Furthermore, DFT calculations suggest the addition of Mo in the catalyst subsurface weakens hydrogen binding to surface Ni sites, thus increasing the overall activity toward HER/HOR.8 In ongoing work to further develop Ni–Mo/C composites, we are focusing on catalyst degradation under storage and operating conditions. To examine catalyst durability upon extended storage, we measured activity toward hydrogen evolution progressively for a single batch of catalyst powdered retained at room temperature in an aerobic environment over 850 hours. We observed a monotonic decrease in catalytic activity culminating in a >90% decrease in activity. Post-mortem analysis via XRD showed the increased peak intensities corresponding to NiO and MoO3. We further observed that the catalyst remains active over extended periods when stored under a dry, oxygen-free atmosphere. Finally, exposing the degraded catalyst to thermal reduction increased catalytic activity. Together, these results strongly suggest aerobic oxidation as the primary degradation mechanism under shelf-storage, with straightforward mitigations steps readily available by excluding air and water. This finding further suggests that oxide component of the synthesized catalyst is not an active site for hydrogen chemistry, although we speculate that some surface oxide is beneficial for enhancing the rate of water dissociation.9 We also studied the impact of the ionomer chemistry on the catalytic activity of Ni–Mo composites for alkaline anion exchange membrane (AEM) electrolyzer applications. Surprisingly, we observed an ~80% reduction in catalytic activity for Ni–Mo composites formulated into thin film HER catalysts using poly(aryl piperidinium) (PAP) carbonate AEM ionomers relative to control samples based on Na-exchanged Nafion ionomer binders. Further work showed this result was specific to the carbonate form of the ionomer, as HER activity was fully recovered when we used halide forms of the PAP ionomer. Work is ongoing to establish the physicochemical basis of this poisoning effect.Based on these findings, we developed functional AEM electrolyzer assemblies with IrOx anodes and Ni–Mo/C cathodes, where the cathode composition was formulated based on our best results from RDE studies. These MEAs yielded electrolyzer performance properties that were indistinguishable from MEAs we tested using Pt–Ru/C cathodes that were otherwise identical in composition.8 This leads us to conclude that advanced Ni–Mo/C catalyst composites can indeed achieve performance parity with PGM catalysts on the basis of initial catalytic activity, whereas significant work remains to characterize durability under extended operating conditions.