Pt Surfaces Modified with Mo Species as an Improved Electrocatalyst for Efficient Water Splitting

Increasing energy demand along with the environmental concerns associated with our absolute dependence on fossil fuels has attracted huge research efforts to utilize renewable energy. Using solar energy, the photocatalytic water splitting reaction represents one of the most efficient systems to produce stoichiometric amounts of hydrogen (and oxygen) without any carbon footprint.^[1] The cocatalyst role in the water splitting reaction is of paramount importance. Noble metals (Pt, Rh, Pd, Ru, etc.) are efficient electrocatalysts to reduce protons/water and form hydrogen gas (hydrogen evolution reaction, HER). Pt is also the most common used cocatalyst in the photocatalytic field.^[2] However, it is well known that Pt is highly active on the oxygen reduction reaction (ORR), which can be described in a photocatalytic system as the reaction of water formation (back reaction of the water electrolysis).^[3] If Rh cocatalyst was loaded in the surface of an advanced photocatalytic systems such as (Ga_1−xZn_x)(N_1−xO_x) stable production of hydrogen and oxygen gases were not detected. Only when a thin layer of Cr oxide was present around the hydrogen evolution site overall water splitting was achieved.^[4] Based on electrochemical studies of model electrodes, it was suggested that the oxide layer selectively permeates protons and hydrogen molecules but not oxygen molecules, thus suppressing the back reaction catalyzed by noble metals (selective HER without introducing ORR).^[5]

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

^[1] K. Takanabe, K. Domen, Green, 2011, 1, 313.

^[2] K. Takanabe, K. Domen, ChemCatChem 2012, 4, 1485.

^[3] A. T. Garcia-Esparza, D. Cha, Y. Ou, J. Kubota, K. Domen, K. Takanabe, ChemSusChem 2013, 6, 168.

^[4] K. Maeda, K. Teramura, D. Lu, T. Takata, N. Saito, Y. Inoue and K. Domen, Nature 2006, 440, 295.

^[5] M. Yoshida, K. Takanabe, K. Maeda, A. Ishikawa, J. Kubota, Y. Sakata, Y. Ikezawa, K. Domen, J. Phys. Chem. C, 2009, 113 10151.