Electrodeposition of Non-Noble High-Entropy Alloys for Effective Hydrogen Evolution Electrocatalysts

As society works towards reducing its carbon footprint, various sectors such as energy, steelmaking and fertilizer production have begun to look increasingly towards decarbonization. Hydrogen production through electrolysis, when linked to renewables (solar, wind), provides a promising low-emission alternative as an energy carrier and chemical feedstock. With the decreasing cost of said renewables, greater focus is now on the capital cost of the electrolyser, and particularly the membrane/electrode structures. Conventional Proton Exchange Membrane (PEM) electrolysers rely on the use of electrocatalysts made with expensive noble metals such as platinum, ruthenium, and palladium [1]. Anion Exchange Membrane (AEM) electrolysers present an effective means of alkaline water splitting with low carbon emissions and can effectively utilize non-noble metal electrocatalysts for the Hydrogen Evolution Reaction (HER). A novel class of materials, High-Entropy Alloys (HEAs), made of non-noble metals or with reduced noble metal content have shown great promise as active electrocatalytic materials [2]. HEAs are typically defined as metal alloys made of five or more constituent elements each comprising at least 5% of the total material [3]. Having such a large number of principal alloying elements can lead to enhanced phase stability due to an increased configurational entropy, have unique chemical clustering, and can help prevent degradation of the material in harsh environments [2]. Additionally, the large lattice parameter mismatch between constituent elements can result in a high degree of strain which can help shift electronic states in a way that is favourable to hydrogen electrocatalysis [4]. These properties along with the so called “cocktail effect,” whereby combining multiple elements can result in unexpected synergistic interactions, makes HEAs uniquely suited to electrocatalytic applications [5]. In this work, we investigate the electrocatalytic performance of electrodeposited FeNiCoMoW HEAs. The goal is to combine three hyper-d transition metals with two hypo-d transition metals to alter the electronic structure and improve electrocatalytic performance. Using chronoamperometric measurements, the Tafel slopes for FeNiCoMoW HEAs electrodeposited at pH 5, 6 and 7 were determined, with the pH 5 HEA demonstrating the smallest average Tafel slope of approximately 83 mV/dec. Compared to some electrodeposited binary alloys reported on in the literature, this marks an improvement and may be the result of unexpected interactions within the compositionally complex alloy. In the FeNiCoMoW HEA, the magnitude of the Tafel slope increased in tandem with the electrochemically active surface area (ECSA) as determined by Cyclic Voltammetry (CV) which suggests that the composition may be more important for improving performance. As the pH of the electrodeposition solution increased, the amount of cobalt in the as-deposited HEA decreased while the amounts of molybdenum and tungsten remained constant. The composition of nickel and iron appeared to vary inversely, reaching a maximum and minimum respectively at pH 6. From these results it seems that maximizing cobalt content and minimizing nickel content could potentially help to improve HER performance in the FeNiCoMoW system. References: [1] J. Song et al. , “Implementation of Proton Exchange Membrane Water Electrolyzer with Ultralow Pt Loading Cathode through Pt Particle Size Control,” ACS Sustain. Chem. Eng. , vol. 11, no. 45, pp. 16258–16266, Nov. 2023, doi: 10.1021/acssuschemeng.3c04679. [2] G. M. Tomboc, T. Kwon, J. Joo, and K. Lee, “High entropy alloy electrocatalysts: a critical assessment of fabrication and performance,” J. Mater. Chem. A , vol. 8, no. 30, pp. 14844–14862, Aug. 2020, doi: 10.1039/D0TA05176D. [3] J.-W. Yeh et al. , “Nanostructured High-Entropy Alloys with Multiple Principal Elements: Novel Alloy Design Concepts and Outcomes,” Adv. Eng. Mater. , vol. 6, no. 5, pp. 299–303, 2004, doi: 10.1002/adem.200300567. [4] T. Löffler et al. , “Toward a Paradigm Shift in Electrocatalysis Using Complex Solid Solution Nanoparticles,” ACS Energy Lett. , vol. 4, no. 5, pp. 1206–1214, May 2019, doi: 10.1021/acsenergylett.9b00531. [5] Y. Zhang, D. Wang, and S. Wang, “High-Entropy Alloys for Electrocatalysis: Design, Characterization, and Applications,” Small , vol. 18, no. 7, p. 2104339, 2022, doi: 10.1002/smll.202104339. Figure 1