Hydrogel Polymer–PBA Nanocomposite Thin Film-Based Bifunctional Catalytic Electrode for Water Splitting: The Unique Role of the Polymer Matrix in Enhancing the Electrocatalytic Efficiency

A novel approach to efficient bifunctional catalytic electrodes for water splitting is developed, based on a counterintuitive choice of an insulating hydrogel polymer (chitosan, CS)–Prussian blue analogue (PBA, KCoFe) nanocomposite thin film on nickel foam. The polymer matrix in KCoFe-CS enables the formation of framelike structures of the non-noble metal-based catalyst nanocrystals, in addition to improving their stability. An optimized cycling protocol leads to a substantial enhancement of the electrocatalytic efficiency for oxygen evolution reaction (OER) as well as hydrogen evolution reaction (HER), achieving relatively low overpotentials of 272 and 320 mV (@ 10 and 20 mA cm^–2) and 146 mV (@ 10 mA cm^–2), respectively, reduced Tafel slopes, and increased Faradaic efficiencies of 98 and 96%; the overpotentials estimated based on the electrochemically active surface area show similar trends. The polymer encapsulation and the cycling protocol are key to the realization of the desirable combination of enhanced efficiency and stability demonstrated up to 50 h for both OER and HER. Detailed characterizations of the postcycling catalytic electrode show that favorable morphological changes of the polymer matrix with concomitant reduction in the PBA nanocrystal size lead to the enhanced activity. The bifunctional activity of the catalytic electrode is demonstrated by the stable water splitting achieved with a 20 mA cm^–2 current density at 1.55 V. The present study unravels the utility of hydrogel polymer matrices (without the use of binders like Nafion) in realizing sustainable water splitting electrocatalysts with high stability and efficiency, through the combined effect of confining the electrolyte within and favorably modifying the catalyst nanoparticles and the nanocomposite morphology.