In this work, we study the impact of Fe incorporation into nickel (Ni) catalysts for AOR, building upon previous literature that elucidated the role Fe plays in Ni catalysts for OER. We employ traditional techniques such as rotating disk electrode (RDE) along with more novel in situ and operando techniques including electrochemical mass spectrometry (EC-MS) and on-line inductively coupled plasma mass spectrometry (ICP-MS) to simultaneously understand activity, stability, and selectivity to OER versus AOR. Specifically, we investigate Ni catalysts in Fe-free electrolyte, Ni catalysts in unpurified electrolyte, and codeposited NiFe catalysts.
Our RDE results reveal that trace Fe surface incorporation into Ni (~7 at% according to x-ray photoelectron spectroscopy) has a minimal effect on ethanol oxidation performance, while codeposited Fe (20 at%) significantly reduces ethanol oxidation current density (by 50% at 1.5 V vs. RHE). Additionally, EC-MS experiments reveal that the onset of OER on our Ni-based catalysts is not significantly impacted by simultaneous ethanol oxidation in 0.1 M KOH electrolytes with and without 0.1 M ethanol. Taken together, our results introduce the hypothesis that OER and AOR occur on different active sites in this case—we suggest that while OER is catalyzed on NiFe edge/defect sites, AOR is catalyzed on “bulk” Ni sites. Overall, our work provides insights relevant for the design and understanding of both AOR and OER electrocatalysts.