Pt Surfaces Modified with Mo Species as an Improved Electrocatalyst for Efficient Water Splitting
To understand the redox chemistry at the cocatalyst interface, electrochemical methods were utilized to model the electrocatalytic reactions present at the surface of photocatalysts. Particularly, a model platinum-disk electrode and homogeneously dispersed Pt nanoparticles on carbon black (Pt/C) were electrochemically coated with Mo materials. A systematic investigation by varying the loading of Mo was performed on the electrocatalytic activity for HER as well as for ORR. The Mo species were characterized by XPS, FT-IR, in-situ UV-Vis electrochemical spectroscopy and Raman spectroscopy. Electro-kinetic studies, cyclic voltammetry analysis and hydrodynamic techniques were used. When Mo species were electrodeposited in the Pt surfaces with a Mo:Pt ratio of 400:1 (estimated from electrodeposition charges), the Mo-modified Pt exhibited HER activity but inhibited ORR activity. Interestingly, the presence of Mo produces an almost insensitive surface towards ORR. The results indicate that the O2 adsorption step may be the rate-determining step estimated via the Koutecky-Levich equation and Tafel analysis. Moreover, hydrogen oxidation reaction (HOR) was hindered under the explored conditions and no diffusion effects were observed for the Pt-Mo catalysts. It appears to exist an optimum value of electrodeposited Mo species in which the creation of a selective catalyst was achieved. When the Mo/Pt ratio is less than 400 the active sites follow the Pt electrode behaviour with high activity for HER and ORR; yet when the ratio was larger than 400 both HER and ORR were completely inhibited. The possible mechanisms responsible of the selectivity towards HER and ORR are discussed based on the nature of the active sites associated with the Pt-Mo synergetic effect. The stability of the Mo materials was improved in the order alkaline < neutral < acidic conditions. This study provides an insight into the design of future cocatalysts required to drive overall water splitting in a single photocatalyst surface and the challenges ahead for future water splitting systems
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