CO2 electroreduction activity and dynamic structural evolution of in situ reduced nickel-indium mixed oxides

In the field of CO2 electroreduction (CO2ER), tuning the selectivity among diverse products remains a major challenge. Mixed metal catalysts offer possible synergetic effects which can be exploited for tuning product selectivity. We present a simple wet chemical approach to synthesize a range of nickel-indium mixed oxide (Ni(A)In(B)Ox) thin films with homogeneous metal distribution. CO2 electroreduction results indicate that the Ni(A)In(B)Ox mixed oxide thin films can achieve high CO selectivity (>70%) in contrast with the single metal oxides NiO (H-2 >90%) and In2O3 (formate >80%). The relative composition Ni(40)In(60)Ox attained the best CO selectivity of 71% at moderate cathodic bias of -0.8 V-RHE, while a higher cathodic bias (E < -0.9 V) promoted a decrease of CO in favor of formate. A detailed investigation of the Ni(40)In(60)Ox thin films following progressive stages of reduction during CO2ER revealed dynamic structural transformations strongly dependent on applied bias and electrolysis time. For the CO-selective catalyst composition, at moderate cathodic bias (E < -0.8 V) and short electrolysis times (1 h), the catalyst is composed of nickel-indium alloy grains embedded in amorphous Ni-In mixed oxide as observed by electron microscopy. Extending electrolysis time at -0.8 V for 10 h, or increasing the applied reductive bias to -1.0 V, result in a complete reduction of the residual oxide film into an interconnected array of multicomponent (In, Ni, Ni3In7) nanoparticles which display significantly lower CO selectivity (<50%). Our results indicate that the persistent amorphous NiInOx oxide/alloy composite material preserved in the early stages of reduction at -0.8 V plays a key role in CO selectivity. The highly dynamic structure observed in this catalytic system demonstrates the importance of conducting detailed structural characterization at various applied potentials to understand the impact of structural changes on the observed CO2ER selectivity trends; and thus be able to distinguish structural effects from mechanistic effects triggered by increasing the reductive bias.