Unraveling Molybdenum Dissolution-Induced Surface Reconstruction of Cobalt Molybdenum Oxide Electrocatalysts during the Oxygen Evolution Reaction

Understanding and controlling the evolution of electrocatalyst structure, chemistry, and active sites under catalytically relevant conditions is critical in developing electrocatalysts with stable and durable activity toward the oxygen evolution reaction (OER). Molybdenum (Mo) is routinely combined with cobalt hydroxide electrocatalysts to tune the electronic environment of the active sites and modulate their activity, contributing to improved OER activity. However, the phenomena that underpin the enhanced OER activity of the resulting materials largely stem from Mo dissolution, and their extent on the structure and activity remains elusive. In this work, we investigate cobalt-molybdenum oxide electrocatalysts, CoMoOx, synthesized via the electrodeposition method across a range of Co/Mo compositional ratios and CoMoO4(HT) synthesized hydrothermally, to establish how composition and synthesis route govern Co-site evolution and the ensuing OER activity. Electrochemical benchmarking revealed that the high Mo content of electrodeposited CoMoOx(1:4) produces a Mo-rich surface with fewer accessible Co sites and slower charge-transfer kinetics, as evidenced by a Tafel slope of 73.1 ± 8.1 mV dec−1 and a high charge-transfer resistance of 5.74 Ω (over 4 times higher than CoMoOx(6:1)). Operando Raman spectroscopy, ex situ X-ray diffraction, X-ray photoelectron spectroscopy, and inductively coupled plasma−optical emission spectroscopy collectively demonstrate that MoO42− dissolution begins on contact with 1 M KOH in the absence of applied potential and continues under anodic polarization, driving irreversible reconstruction of as-synthesized materials to Co(OH)2/Co3O4 sites and subsequently to CoOOH. These materials therefore function as pre-catalysts, and the working surface is determined by the starting composition rather than being the as-synthesized phase. Crucially, the reconstruction pathway is synthesis-dependent; crystalline hydrated CoMoO4(HT) proceeds via CoOOH to further-oxidized, partially deprotonated CoOOHx, at relevant OER conditions, whereas electrodeposited films form predominantly β-CoOOH as the surface species, with progression beyond CoOOH developing only partially and to an extent that scales with the initial Mo content. These findings provide a mechanistic blueprint for leveraging controlled cation dissolution to engineer defect-rich, high-performance electrocatalytic interfaces.