Tuning the Ni/Co Ratios and Surface Concentration of Reduced Molybdenum States for Enhanced Electrocatalytic Performance in Trimetallic Molybdates: OER, HER, and MOR Activity

The heterogeneous nature of trimetallic catalyst systems comes with beneficial synergy for catalytic applications. Nonetheless, the challenges associated to validate and control the critical characteristics for enhanced electrocatalytic efficiencies are yet to be known. Herein, a genre of trimetallic catalysts were synthesized with Ni, Co, and Mo metals, whose morphological, structural, electronic, and interface properties depend on Ni/Co and Mo+4/Mo+5 ratios and the dominant element on the surface of the catalyst. The results suggest that introduction of a third metal, that is, Mo, is only beneficial when the Ni/Co ratio is optimally maintained. By combining the surface electronic and structural analyses such as electron energy loss spectroscopy, X-ray photoelectron spectroscopy, line scan, Raman spectra, and electrochemical data, it was realized that Mo and metallic Ni on the surface favor oxygen evolution reaction (OER), and methanol oxidation reaction (MOR) activity, respectively. Interestingly, the adsorbed water molecules were found to be vital for higher performance in both OER and MOR processes. The chronopotentiometry tests performed with the optimized catalyst Ni56Co7Mo37 for OER showed an overpotential around ca. 309 mV at ca. 720 mA·cm–2 even after 17 h (and also a Tafel slope of 60 mV·dec–1 at 10 mA·cm–2). The peak current density (from the cyclic voltammetry tests) in the optimized catalyst, that is, Ni66Co31Mo3 (for MOR), showed almost 300-fold higher activity in 1 M KOH + 1 M CH3OH solution when compared with only 1 M KOH. Further, comparative studies were also conducted with bimetallic catalysts of Ni, Co, and Mo to understand the better combinations for promoting OER, hydrogen evolution reaction, and MOR efficiencies. This work highlights the importance of maintaining the elemental composition ratios at the bulk and surface that lead to an active environment for OER and MOR processes and thereby opening gateways for a rational design of trimetallic electrocatalysts.