Investigation of Transfer and Transport Kinetics on the Cobalt-Borate Catalyzed BiVO4 Photoanode.
Authors: Satpati Ashis Kumar 
Published: February 2020

BiVO4 is considered as promising photoanode material for solar water splitting [1]. Its performance is limited by slow water oxidation kinetics and poor charge separation [2]. To enhance water oxidation kinetics of BiVO¬4, Oxygen evolution catalyst (OEC) has been incorporated on the surface of photoanode. Cobalt borate (Co-Bi) is known as an excellent catalyst for water oxidation [3, 4]. In this work, we have incorporated Co-Bi as OEC on the surface of BiVO4 to enhance charge transfer and transport efficiency and hence overall enhancement of water oxidation kinetics. Mo-doped BiVO4 was coated on FTO by spin coating technique using precursor solution followed by calcination at 500oC for 3 hours. After formation of BiVO4, Co-Bi has been incorporated on photoanode surface by photoelectrochemical deposition technique. Photoanode material were named as S1: BiVO4 and S2: BiVO4/Co-Bi
Photoelectrochemical activity was performed by chopped light voltammetry. Photocurrent of S2 was found to increase 36.61% of S1 at 0.95V which indicated that Co-Bi increases the surface catalytic activity of BiVO4. The onset potential was also found to shift cathodically on incorporation of Co-Bi. Charge transfer and transport efficiency were found to increase from 67% to 70% and 32% to 50% respectively upon incorporation of Co-Bi. These data shows that OEC facilitates separation of photogenerated charge and enhancement of interfacial charge transfer efficiency. Enhancement of Charge transport efficiency indicated that Co-Bi works as hole collector and hence mobility of photogenerated hole increases many folds. Further interfacial charge transfer efficiency was measured by electrical impedance spectroscopy (EIS). The Rct value was found to decrease from 2677Ω to 334Ω and relaxation frequency was found to increase from 52Hz to 185Hz which suggests faster charge transfer at the interface. Further, kinetics was measured from scanning electrochemical microscopy (SECM) technique. In this study probe was approached to substrate by probe approach technique (PAC) with different bias potential at the substrate and measure feedback current at the probe to study the hole transfer rate from substrate to the analyte. In this study 0.1m ferricyanide solution was used as probe redox. The interfacial charge rate constant was found 3-7×10-3 cm/sec with increasing positive bias on the substrate which suggest greater band bending at more positive potential and hence higher rate constant of hole transfer. Further localized kinetics was measured by SECM imaging on the substrate in feedback mode. Higher cathodic current shows better catalytic site for hole transfer. Co-Bi incorporation on the BiVO4 surface enhances overall efficiency of water spitting.

References

1. Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S., Solar Water Splitting Cells. Chem. Rev. 2010, 110 (11), 6446-6473.
2. Kuang, Y.; Jia, Q.; Nishiyama, H.; Yamada, T.; Kudo, A.; Domen, K., A Front Illuminated Nanostructured Transparent BiVO4 Photoanode for >2% Efficient Water Splitting.
Adv. Energy Mater. 2016, 6 (2), 1501645.
3. Turhan E. A., Nune S. V. K., Ülker E., Şahin U., Dede Y., Karadas F., Water Oxidation Electrocatalysis with a Cobalt‐Borate‐Based Hybrid System under Neutral Conditions, Chem. Eur. J. 2018, 24 (41), 10372-10382.
4. Libin Yang, Danni Liu, Shuai Hao, Rongmei Kong, Abdullah M. Asiri, Chengxiao Zhang and Xuping Sun , A cobalt-borate nanosheet array: an efficient and durable non-noble-metal electrocatalyst for water oxidation at near neutral pH, J. Mater. Chem. A, 2017,5, 7305-7308.