Enhanced water oxidation performances of birnessite and magnetic birnessite nanocomposites by transition metal ion doping†
Birnessite-type manganese oxides and their magnetic composites with iron oxides are known due to their effective properties in water oxidation catalysis (WOC). The present work demonstrates how transition metal ion doping influences the WOC performances of magnetic and non-magnetic MnOx catalysts in a comparative manner. Transition metal cation (TM = Cr3+, Co2+, Ni2+, Cu2+, Zn2+) doped layered birnessite and birnessite–manganese ferrite composite nanocatalysts were synthesized and characterized. Core–shell type, magnetic composite catalyst particles of ∼150 nm diameter were obtained by in situ deposition of birnessite shells onto 50–60 nm sized manganese ferrite spheres by reducing MnO4− ions with propionic acid in the presence of the TM ions. The resulting TM-doped catalyst materials were analyzed by XRD, SEM, TEM, IR, TGA and AAS to verify their compositions, structures and morphologies. WOC activities and stabilities were comparatively examined in both chemical (using Ce4+ as sacrificial oxidant at pH ∼ 1–2) and electrochemical (pH 7, screen-printed particles on FTO) test setups. In both types of experiments, the highest catalytic activity was observed for catalysts doped with Co2+ ions, followed by Ni2+-doped samples. In comparison to the K-birnessite reference material, the Co2+- and Ni2+-doped analogs were additionally found to be more stable and more active in the chemical and long-term electrocatalysis measurements, demonstrating the potential of TM-doping to improve the performance of magnetic-MnOx-based WOC materials. In addition, the formation of a magnetic core–shell structure converts the micron-sized amorphous MnO2 particles to spherical at the nanoscale and thus facilitates the mass-energy transfer throughout the entire catalyst surface.