The bismuth vanadate thin layers modified by cobalt hexacyanocobaltate as visible-light active photoanodes for photoelectrochemical water oxidation

How to Build Prussian Blue Based Water Oxidation Catalytic Assemblies: Common Trends and Strategies

Prussian blue (PB) and its analogues (PBAs) have at least a three-century-long history in coordination chemistry. Recently, cobalt-based PBAs have been acknowledged as efficient and robust water oxidation catalysts. Given the flexibility in their synthesis, the structure and morphology of cobalt-based PBAs have been modified for enhanced catalytic activity under electrochemical (EC), photocatalytic (PC), and photoelectrochemical (PEC) conditions. Here, in this review, the work on cobalt-based PBAs is presented in four sections: i) electrocatalytic water oxidation with bare PBAs, ii) photocatalytic processes in the presence of a photosensitizer (PS), iii) photoelectrochemical water oxidation by coupling PBAs to proper semiconductors (SCs), and iv) the utilization of PBA-PS assemblies coated on SCs for the dye-sensitized photoelectrochemical water oxidation. This review will guide readers through the structure and catalytic activity relationship in cobalt-based PBAs by describing the role of each structural component. Furthermore, this review aims to provide insight into common strategies to enhance the catalytic activity of PBAs.

Co3O4/BiVO4 Heterostructures for Photochemical Water Oxidation: The Role of Synthesis Parameters and Preparation Route for the Physico-Chemical Properties and the Catalytic Activity

The synthesis conditions of the semiconductor BiVO4 have marked influence on its physicochemical and photocatalytic properties. In this work, a pH-controlled co-precipitation route was systematically investigated. Special attention was paid to the pH value during co-precipitation and co-precipitate post-treatment. In a sacrificial photocatalytic water oxidation test, the highest activity was observed for a sample precipitated at pH = 1 and 70 °C, which was post treated at 600 °C in air for 3 h after washing and drying. The activity was further improved by another 90% by the addition of the Co3O4-based co-catalyst. Good synergy between semiconductor and co-catalyst was obtained for the deposition of pre-formed nanoparticles at a loading of 0.1 w% (physical impregnation). The effects of different synthesis conditions of the semiconductor and impregnation method for the co-catalyst on the structure, morphology and optical properties of the catalysts were investigated by PXRD, SEM, UV-vis spectroscopy, and TEM, while the water oxidation activity was compared in the dark and with the aid of visible light using cerium (IV) or silver (I) as sacrificial agents, respectively, with the aim of establishing structure-activity correlations. The roles of semiconductor particle anisotropy and co-catalyst particle distribution for optimal photo-activity in the oxygen evolution reaction are discussed.

Enhanced Photoelectrocatalytical Performance of Inorganic-Inorganic Hybrid Consisting BiVO4, V2O5, and Cobalt Hexacyanocobaltate as a Perspective Photoanode for Water Splitting

Thin layers of BiVO4/V2O5 were prepared on FTO substrates using pulsed laser deposition technique. The method of cobalt hexacyanocobaltate (Cohcc) synthesis on the BiVO4/V2O5 photoanodes consists of cobalt deposition followed by electrochemical oxidation of metallic Co in K3[Co(CN)6] aqueous electrolyte. The modified electrodes were tested as photoanodes for water oxidation under simulated sunlight irradiation. Deposited films were characterized using UV-Vis spectroscopy, Raman spectroscopy, and scanning electron microscopy. Since the V2O5 is characterized by a narrower energy bandgap than BiVO4, the presence of V2O5 shifts absorption edge (ΔE = ~0.25 eV) of modified films towards lower energies enabling the conversion of a wider range of solar radiation. The formation of heterojunction increases photocurrent of water oxidation measured at 1.2 V vs Ag/AgCl (3 M KCl) to over 1 mA cm-2, while bare BiVO4 and V2O5 exhibit 0.37 and 0.08 mA cm-2, respectively. On the other hand, the modification of obtained layers with Cohcc shifts onset potential of photocurrent generation into a cathodic direction. As a result, the photocurrent enhancement at a wide range of applied potential was achieved.