Investigation of electrocatalytic performance of prussian blue analogues for water splitting

Abstract

Research on H2 production has recently been directed to the development of cost-efficient and robust heterogeneous catalysts for splitting of water. While several Prussian blue analogues (PBAs) have been investigated as water oxidation catalysts, the field lacks a comprehensive study that focuses on the design of the ideal PBA for efficient water oxidation. Herein this thesis, a series of PBAs with different cyanide precursors were investigated to study the effect of hexacyanometal group to their electrocatalytic catalytic water oxidation activities. Cyclic voltammetry, chronoamperometry, and chronopotentiometry measurements reveal the close relation between the electron density of electroactive cobalt sites and electrocatalytic activity, which is also confirmed by Infrared and XPS studies. pH dependent cyclic voltammetry studies were also performed to gain insight about the catalytic mechanism and electronic structure of cyanide-based systems, to identify possible intermediates, and assign the rate-determining step of the process. In addition, a N-donor ligand, 1-heptyl 4-(4 pyridyl) pyridinium bromide, was used to prepare a new pentacyanoferrate complex. This complex was used to make a new series of PB analogues to investigate the effect of the change in the morphology to electrocatalytic water oxidation performance. Synthesis, characterization, and electrochemical experiments were performed to investigate electrocatalytic properties of PB analogues. With this synthesis route, the electroactive cobalt sites are increased approximately two-fold. It is observed that amorphous nature has positive impact on the catalytic activity when compared to cobalt hexacyanoferrates. A current density of 1 mA.cm-2 was achieved at an overpotential of 421 mV, which is much lower than those obtained with metal hexacyanometalates. Given the promising catalytic activities of several cobalt-based systems and the robustness of Prussian blue analogues in harsh catalytic processes including water oxidation, a Co-Co Prussian blue analogue was investigated as a Hydrogen evolution catalyst for the first time. Co-Co Prussian Blue modified FTO electrode demonstrated a significant Hydrogen evolution activity with an onset overpotential of 257 mV, a Tafel slope of 80.2 mV.dec-1, and a turnover frequency of 0.090 s-1 at an overpotential of 250 mV. Comparative XPS, Infrared, and XRD studies performed on pristine and post-catalytic electrodes confirm the stability of the catalyst.