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Hull Cell Investigation of Ni-Mo and Ni-Mo-O Electrodeposits from Ammonium Citrate Baths

Among different Ni-based alloys, Ni-Mo is well-known for the excellent corrosion resistance which makes them a potential alternative for the electroplated chromium. In addition, Ni-Mo alloys have been used as substrates for various purposes and as hydrogen evolution reaction electrocatalyst owing to their good mechanical strength and electrocatalytic activities. Each application requires distinct material properties of Ni-Mo alloys (e.g., composition, morphology, and crystallinity). Therefore, tunability of the material properties of Ni-Mo alloys become important to facilitate them for applications. Synthesis of Ni-Mo alloys has been achieved by various methods (e.g., e-beam evaporation, sputtering, and arc melting); however, high cost and time consumption, especially harsh synthesis conditions, limit the range of studied parameters. Hull cell, an electrodeposition technique, becomes a promising means for the high throughput investigation of synthesis parameters. Similar to other efficient electrodeposition techniques, Hull cell not only provides the tunability of morphology and composition, but also requires a simple setup and near ambient operating conditions. Ni-Mo alloys are electrodeposited via induced co-deposition, implying the dependent reduction of Ni and Mo. Therefore, complexing agent (e.g., citrate ions) is required to control the relative reduction rates of Ni and Mo and consequently tune the material properties via the complexation and the reduction reactions. The complexation between metal precursors and complexing agents generates many different species. Only chemically active species participate in the reduction reactions at the interface of electrolyte and electrode. Thereby, it is necessary to study the distribution of species after complexation to maximize the fraction of chemically active species for Ni-Mo co-deposition. In this work, effects of various synthesis parameters on the formation of Ni-Mo alloys and Ni-Mo-O composites were investigated using Hull cell. Firstly, the fraction of complex species at equilibrium is simulated using material balance with the assistance of MATLAB to understand the complexation of metals and complexing agents as well as predict the solution compositions which maximize the chemically active species for reduction reactions. Also, the effect of various synthesis parameters (e.g., complexing agents (ammonium-to-citrate ratio), Ni/Mo precursor ratio, pH) are experimentally studied and correlated with simulated data to elucidate the deposition mechanism as well as determine the conditions for the transition from Ni-Mo alloys to Ni-Mo-O composites.