Nanoscale Ni-Mo-Pt Alloy Catalyst with Tuneable Composition for Hydrogen Economy: Electrosynthesis and Characterisation

In the search for alternative and renewable energies that will finally allow abandoning the use of fossil fuels once and for all, hydrogen energy is among the most promising solutions able to fuel any kind of device independent of its size. The energy cycle of hydrogen needs a large infrastructure of highly efficient catalysts used in both electrolysers, which produce hydrogen gas, as well as fuel cells, where the energy stored in the hydrogen bond is converted into electrical current. The conversion of hydrogen works most effectively in acidic media, where the most effective and chemically stable material is platinum. The low abundancy and associated high cost of Pt make it impossible to provide a large-scale infrastructure using the commercial Pt/C catalyst. Alternatives must be found to substantially reduce the amount of Pt used as catalyst material without compromising its sustainability. A facile electrodeposition process from aqueous media allows the one-step synthesis of a Ni-Mo-Pt alloy for use in both hydrogen evolution reaction (HER) and energy conversion systems such as fuel cells. In a previous study, mesoporous Ni-Pt films were synthesised by electrodeposition and thoroughly characterised towards HER, finding that the reaction in 0.5 M H2SO4 was efficient, stable and reproducible. However, some leaching of Ni into the sulfuric acid was observed under open circuit conditions [1,2]. In this study, molybdenum is introduced into the previously investigated Ni-Pt alloy to increase the stability of the material in acidic media. With respect to the electrolyte used for the synthesis of the Ni-Pt alloy, all bath components were kept the same except for the addition of sodium molybdate and citric acid. The latter complexes Mo(VI), thus enabling its co-deposition. Due to a pH-dependent complexation of Mo(VI) by citric acid, the composition of the Ni-Mo-Pt alloy is strongly pH-dependent and can further be fine-tuned to the needs of the specific application by changing the electrodeposition parameters. The Mo contents obtained reach from 10 at% up to 50 at%. Alloys with the highest Mo content, however, trigger phase separation. Using potentiostatic electrodeposition, continuous thin films of Ni-Mo-Pt are obtained on a Cu-coated Si substrate. However, the growth of globular particles is favoured on a hydrophobic substrate, such as a carbon-based gas diffusion layer (GDL) typically found in a fuel cell set-up. Further, using pulse electrodeposition, nanoparticles with a mean diameter down to 10 nm are successfully obtained. For HER in 0.5 M H2SO4, Ni rich alloys with low Pt contents (between 1 at% and 5 at%) are investigated, while alloys with higher Pt contents can provide sufficient electrochemical stability for oxygen reduction reaction (ORR) in a proton exchange membrane (PEM) fuel cell. The stability of alloys with varying composition is determined by incubation in 0.5 M H2SO4. Cyclic voltammetry curves in the same media are performed on Ni-Mo-Pt nanoparticles in order to activate the surface and remove any contaminants as a preparation for tests in a PEM fuel cell, and to determine their electrochemically active surface area (ECSA). The electrochemical experiments are supplemented with microstructural analysis by SEM and XRD. Acknowledgement: This work has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 764977. Additional support from the Generalitat de Catalunya (2017-SGR-292) and the Spanish Government (MAT2017-86357-C3-1-R and associated FEDER) is also acknowledged. References: [1] K. Eiler, S. Suriñach, J. Sort, E. Pellicer, Appl. Catal. B, 2020, 265, 118597 [2] K. Eiler, J. Fornell, C. Navarro-Senent, E. Pellicer, J. Sort, Nanoscale, 2020, 12, 7749