Development of a Simple Ink Coating Procedure for BiVO4 Photoanodes for Oxygen Evolution
Authors: Henke Kira Viktoria , Weinrich Henning , Tempel Hermann , Eichel Rüdiger-A. 
Published: February 2024

Photoelectrodes can be implemented in electrochemical cells to accelerate the reaction kinetics of the intended reactions, utilizing the support of visible light. Especially, photoanodes for the oxygen evolution reaction (OER) are of great interest for energy storage and conversion, since the OER is the anodic half-cell reaction of many Power-to-X technologies such as water splitting, CO2 electroreduction and metal-air battery recharge. However, the OER still limits their potential by being a kinetically hindered 4-electron transfer process. In previous research, bismuth vanadate (BiVO4) has shown significant advances for the onset potential and increase in current density of the OER under illumination. Additionally, BiVO4 is a low-cost material, making it favorable over commonly used noble metal electrocatalysts, leading to its rise as one of the most suitable photoelectrocatalysts for OER photoanodes. However, BiVO4 still does not match practical requirements, demanding for further research and development.
In this work, we focus on the implementation of a simple ink coating procedure for BiVO4 photoanodes. Ink coating demonstrates a versatile and adaptable preparation procedure for photoelectrodes, since photoelectrocatalyst synthesis and catalyst deposition are decoupled, providing a large degree of preparative freedom. On the one hand, the catalyst synthesis can be modified and adjusted separately. On the other hand, the catalyst deposition can be performed via various techniques using an ink. Thus, the photoelectrode preparation is not restricted to a certain substrate and specific synthesis conditions.
In the proposed ink coating procedure, first neat and (Mo or W)-doped BiVO4 photoelectrocatalysts are prepared via hydrothermal synthesis also including Co-Pi as a post-deposited co-catalyst. Afterwards, the resulting catalysts are deposited onto FTO glass for further physico-chemical characterization. Moreover, the photoelectrochemical (PEC) performance under illumination and in the dark is investigated for the prepared electrodes. Finally, the ink coated electrodes are compared to BiVO4 photoanodes, which are prepared via solution-based methods, as reported in literature.[1-4]

For the physico-chemical analysis, the ink coated photoanodes are characterized by various microscopic and spectroscopic techniques. SEM images reveal that homogeneous catalyst layers are prepared. Additionally, LSM measurements show that ink coating leads to a catalyst layer with a high surface roughness. XRD and Raman measurements reveal crystalline and pure BiVO4 structures. Furthermore, for the investigation of the PEC performance, an H-cell which is equipped with an LED light source is utilized, allowing for rapid and flexible data acquisition at constant temperatures. In the photoelectrochemical measurements, a significant reduction of the OER onset potential as well as a significant increase in photocurrent density in comparison to dark scans is achieved for neat BiVO4 photoanodes. For now, the ink coated BiVO4 could lower the onset potential by up to 0.6 V under illumination and increase the current density by up to 10 mA/cm² in comparison to the dark scans. In addition, modification of the ink coated BiVO4 by doping and co-catalyst deposition results in an even higher enhancement of the PEC performance for the OER. At this, the ink coated catalyst layers show an excellent stability even after several hours of PEC measurements. Moreover, it is shown that the ink coated BiVO4 photoanodes perform well in comparison to the solution coated electrodes. Overall, the ink coating procedure was successfully established for BiVO4 photoanodes, opening the door for the adaptation to the electrode preparation for different Power-to-X technologies.

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