Kinetic and thermodynamic analysis of ammonia electro-oxidation over alumina supported copper oxide (CuO/Al2O3) catalysts for direct ammonia fuel cells

Abstract

In this work, ammonia electrooxidation (AEO) is studied to probe the electrochemical behavior of aqueous NH3 as direct fuel by implication of gamma alumina supported copper oxide catalysts (CuO/Al2O3). Precipitation and impregnation techniques are adapted to synthesize the substrate Al2O3, and loading of different ratios (1%, 2%, 3%, 4%) of active precursor i.e., CuO, respectively. Small average crystallite sizes (DAvg) in range of 1.46–6.76 nm and smaller particle sizes from micrographs proposed a superb activity of the catalysts. Modified GCE exhibited the excellent conductive properties towards standard redox probe K4[Fe(CN)6] and KCl, displaying a high active electrochemical surface area of up to 0.0021 cm2. Current profiles in response to increase in scan rates, concentrations of ammonia and temperature have been observed in 0.1 M KOH, thereby estimating the thermodynamic and kinetic constraints of the AEO process. Attributed to the electroactive properties towards AEO, CuO/Al2O3 is found to exhibit the desirable physiochemical properties owing to large oxidation current, large diffusion coefficient “D°” (3.6 × 10-9 cm2 s-1), large rate constant “ko” (1.2 × 10-5 cm s-1), large system entropy “ΔS” (-108 J K-1 mol-1), high change in enthalpy “ΔH” (72.3 J mol-1) and low activation energy “ΔG” (32.8 kJ mol-1). Resultingly, the oxidation of ammonia is found to be facile and robust by incorporation of CuO/Al2O3 catalysts owing to large ko, ΔH and ΔH. This study opened a gateway towards eco-benign and economical efficient energy generation and viable market entry of direct ammonia fuel cells.