BiVO₄ Photoanode Surface Modification with Metal Borate Decorated Ti₃C₂T_x MXenes OER Catalyst

Among n-type semiconductors suitable as photoanode absorber materials for photoelectrochemical (PEC) water splitting, BiVO₄ has been widely studied for its relatively narrow bandgap (2.4 eV) and good stability under water oxidation conditions. Anyways, sluggish oxygen evolution reaction (OER) kinetics at BiVO₄ surface is a severe limit to overcome for the achievement of efficient photoelectrodes, requiring proper catalyst integration. (1) Transition metal borates are emerging as OER catalyst both for direct electrolysis and PEC water splitting due to their high activity and low cost. (2-4) Also, decoration of conducting nanostructures, such as carbon nanotubes, graphene and MXenes with metal borates has been demonstrated to combine efficiently charge transfer and catalytic activity of the two materials for improved oxygen production performances. (5-7)<br/>In this work, a BiVO₄-based photoanode is produced with a typical synthesis route consisting in BiOI electrodeposition and thermal conversion with vanadium source. The surface of BiVO₄ absorber material is then modified by drop casting an ethanol dispersion of Ti₃C₂T_x MXenes decorated by room temperature precipitation of cobalt (Co-B_i), nickel (Ni-B_i) or iron borate (Fe-B_i). The effect of the relative amount of MXenes and borates in the catalyst hybrid is investigated, as well as the effect of the catalyst loading on the photoelectrode surface. The presence of the catalyst is proved to increase the generated photocurrent under illumination thanks to an enhanced charge transfer at the photoanode/electrolyte interface is proved by linear scan voltammetry (LSV) and electrochemical impedance spectroscopy (EIS). Photanodes with hybrid catalyst show an improved performance with respect to the bare borates, with MXenes/Co-B_ihybrid showing a charge extraction efficiency overcoming 70%.<br/>1. Kim, J. H. & Lee, J. S. Elaborately Modified BiVO 4 Photoanodes for Solar Water Splitting. <i>Adv. Mater.</i> <b>31</b>, 1806938 (2019).<br/>2. Dastafkan, K., Li, Y., Zeng, Y., Han, L. & Zhao, C. Enhanced surface wettability and innate activity of an iron borate catalyst for efficient oxygen evolution and gas bubble detachment. <i>J. Mater. Chem. A</i> <b>7</b>, 15252–15261 (2019).<br/>3. Choi, S. K., Choi, W. & Park, H. Solar water oxidation using nickel-borate coupled BiVO4 photoelectrodes. <i>Phys. Chem. Chem. Phys.</i> <b>15</b>, 6499–6507 (2013).<br/>4. Xie, C., Wang, Y., Yan, D., Tao, L. & Wang, S. In situ growth of cobalt@cobalt-borate core-shell nanosheets as highly-efficient electrocatalysts for oxygen evolution reaction in alkaline/neutral medium. <i>Nanoscale</i> <b>9</b>, 16059–16065 (2017).<br/>5. Liu, J. <i>et al.</i> Hierarchical Cobalt Borate/MXenes Hybrid with Extraordinary Electrocatalytic Performance in Oxygen Evolution Reaction. <i>ChemSusChem</i> <b>11</b>, 3758–3765 (2018).<br/>6. Chen, P. <i>et al.</i> Strong-Coupled Cobalt Borate Nanosheets/Graphene Hybrid as Electrocatalyst for Water Oxidation Under Both Alkaline and Neutral Conditions. <i>Angew. Chemie Int. Ed.</i> <b>55</b>, 2488–2492 (2016).<br/>7. Turhan, E. A. <i>et al.</i> Water Oxidation Electrocatalysis with a Cobalt-Borate-Based Hybrid System under Neutral Conditions. <i>Chem. - A Eur. J.</i> <b>24</b>, 10372–10382 (2018).