Biotemplated nickel iron oxide electrocatalysts for efficient oxygen evolution reaction

Introduction: Earth-abundant microalgae have emerged as attractive carbon-based scaffolds for the growth of electrocatalysts. In our previous research work on algal cells Tetraselmis, the cell framework was retained after carbonization treatment. To further investigate whether this approach could be extended to other types of algal cells to preserve the cell framework, in this work, we use algae Nannochloropsis oculata (N. oculata) as a biotemplate to synthesize the promising oxygen evolution reaction (OER) nanocatalysts, nanostructural NiFe oxides (NiFeOx). Materials and methods: Two approaches have been implemented. The first approach involves directly using algal cells as a biotemplate for growing NiFeOx nanoparticles at different pH levels under a relatively mild and facile condition by hydrothermal reactions. In the second approach, algal cells are first carbonized by thermal annealing. The resulting carbon nanostructures then serve as a scaffold for NiFeOx nanoparticle growth via hydrothermal reactions under different pH levels. The resulting samples have been measured by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), etc. Their OER electrocatalytic properties have been evaluated by cyclic voltammetry (CV) and linear sweep voltammetry (LSV), in comparison with those of benchmark OER catalyst Ir/C. Results: In the first approach, pH 9.0 sample has the best OER performance, with a Tafel slope of 50 mV/dec, onset potential ~1.47 V and the potential ~1.56 V at 10 mA/cm2. In the second approach, pH 8.0 sample shows a low Tafel slope of 45 mV/dec, onset potential ~1.47 V and the potential ~1.51 V at 10 mA/cm2, better than those of benchmark OER catalyst Ir/C. The Tafel slope is further reduced to 32 mV/dec after iR correction. We have also observed that the cell framework cannot be maintained after carbonization treatment. Conclusions: We have identified a few factors including cell wall thickness and thermal annealing conditions that may contribute to the cell framework conservation of carbonized cells. The current work presents the potential of biotemplating for highly efficient NiFeOx nanoelectrocatalysts.