Surface-assembled Fe-Oxide colloidal nanoparticles for high performance electrocatalytic water oxidation

Nanoscale electrocatalytic materials having enhanced electroactive sites has been considered trendier and can drive kinetically uphill OER at much lower energy cost with high efficiency. However, very complex synthetic strategies, extensive functionalization processes, and less stability have stimulated quest for economically viable, straightforward and facile preparative methods for designing stable, robust and active nanoscale electrocatalysts engaging geologically abundant materials to ensure their industrial implications. Here we present surface-assembled Fe(OH)_x/FeO_x type colloidal catalytic thin-films, with or without post annealing, derived from Fe-colloidal NPs in simple carbonate system for efficient water oxidation. Comprehensive electrochemical studies including cyclic voltammetry, chronoamperometry, chronopotentiometry, impedance spectroscopy, Tafel slope analysis, mass activity, electrochemically active surface area measurements are conducted to comparatively evaluate the performance of simple (FeO_x/HCO₃⁻@FTO and annealed (FeO_x/HCO₃⁻@FTO₂₅₀, FeO_x/HCO₃⁻@FTO₅₀₀) catalysts for oxygen evolution reaction (OER) under employed conditions. The FeO_x/HCO₃⁻@FTO₂₅₀ annealed at 250 °C initiates water oxidation at much lower overpotential of 1.52 V vs. RHE with remarkable stability during long-term electrochemical experimentations. In addition to enhanced OER activity as evidence by better onset potential (<1.55 V vs. RHE), lower Tafel slope value (36 mV dec¹⁻) and negligible charge transfer resistance, the Fe(OH)_x/HCO₃⁻@FTO type catalyst presented excellent electroactive nature during long term controlled potential electrolysis experiments where more and more electroactive sites were getting exposed during continuous hours of electrolysis. The catalysts behave as a potential enduring, inexpensive and competent candidate for catalyzing water oxidation reaction when tested under begin conditions.