A cobalt oxide–polypyrrole nanocomposite as an efficient and stable electrode material for electrocatalytic water oxidation

Abstract : Developing electrolyzers operating under neutral or near-neutral conditions with catalysts based only on earth-abundant metals is highly desirable with a view to reduce the cost of hydrogen production from water splitting reaction and avoid the environmental issues related to corrosion usually encountered with alkaline electrolyzers. Herein, we report a highly active and stable anode material for oxygen evolving reaction (OER) in mild-pH conditions based on cobalt oxide-nanoparticles embedded into a poly(pyrrole-alkylammonium) matrix (denoted PPN+-CoOx). Examples of hybrid materials combining metal oxide nanoparticles as OER catalysts within a polymer film are still rare. However, they are very promising to control the formation and the size of metal particles in view to enhance the electrochemically active surface area and thus the electrocatalytic performances. Our strategy consists in electroprecipitating Co0 nanoparticles by reduction of an anionic cobalt oxalate complex into the cationic PPN+ film, the latter being previously deposited onto an electrode surface by electropolymerization. The Co0 nanoparticles within the composite are then partially in-situ oxidized under air exposure into CoO, and then finally fully oxidized into CoOx by successive scans between 0 and 1.2 V vs Ag/AgCl in a borate buffer at pH 9.2. This nanocomposite material is highly structured with around 30 nm-large CoOx nanoparticles well dispersed into the polypyrrole film conferring a high OER electrocatalytic activity at near neutral pH of 9.2 with exceptional values of mass activity and turnover frequency of 3.01 A mg-1 and 0.46 s-1 respectively, at an overpotential of 0.61 V and with a cobalt loading of 1.34 µg cm-2. These performances place the PPN+-CoOx electrode among the most active anodes described in the literature employing cobalt oxide under mild pH conditions. In addition, when the PPN+-CoOx material is electrodeposited on carbon paper with a higher roughness than a simple carbon electrode, the physisorption of the film on the electrode is considerably enhanced resulting in a stable catalytic current over more than 43 h. Post electrolysis characterizations by SEM and EDX confirm the integrity of the PPN+-CoOx material after long hours of electrocatalysis. This demonstrates the beneficial role of the polypyrrole matrix in the achievement of very stable and highly active anodes for water oxidation.