Synthesis, Experimental, and Theoretical Study of Co 3- x Ni x O 4 Mixed Oxides: Potential Candidates for Hydrogen Production via Solar Redox Cycles

Recently, green hydrogen production via solar thermochemical water splitting (STWS) as a clean and sustainable method is becoming a subject of interest to many researchers. Great efforts are being made to develop materials for STWS with suitable operating conditions, low cost, and good cycling stability. In this context, the study of mixed cobalt and nickel oxides with the general formula Co Ni O (0 ≤ ≤ 1) was carried out, where four mixed metal oxides Co Ni O , Co Ni O , Co Ni O , and Co NiO have been successfully synthesized through the sol-gel method modified Pechini route. The structural investigation demonstrated that pure spinel structures were obtained for 0 ≤ ≤ 0.75. A deep study was carried out with the main goal of finding the best phase that provides low redox temperature. Interesting reduction temperatures for all the compositions have been found, and the lowest values of 675 and 710 °C have been reported for Co Ni O and Co Ni O , respectively. The thermal cycling results of this latest material using TGA measurement have proven attractive cycling stability of which the complete reoxidation of the samples was achieved. In addition, thermodynamic analysis of a reduction step was performed and good agreement of the theoretical reduction temperature of Co Ni O with the experimental one has been found.