Low-Cost Green Hydrogen—Scalable Hybrid Electrode System for Controlled Nucleation of OER Catalysts

Green hydrogen plays a key role in the sustainability transition: it can be used as a renewable fuel, chemical building block, or an energy storage vector. Water electrolysis, powered by renewable energy, is currently the most promising method for green hydrogen production on an industrially relevant scale(1). Among different types of electrolysis, alkaline reactors hold the promise of enabling the use of low-cost anion exchange membranes (AEM) and abundant catalysts such as Nickel (Ni), yet the energy efficiency and stability of those catalysts are too low to make green hydrogen economically competitive(2). To address this problem, we focused on one of the major sources of energy loss in hydrogen electrolyzers – sluggish oxygen evolution reaction (OER). Here, we report an abundant, cheap, energy-efficient, and stable Ni/NiFe catalyst for alkaline water splitting synthesized in a “Hybrid Electrode” electrochemical system. We deployed electroplating as a synthetic procedure, since it has already been adopted in the industry to synthesize diverse catalysts. To increase the electroplating efficiency for the challenging OER reaction, we hypothesized that by carefully controlling the nucleation mechanism(3), we could achieve a more uniform deposition of the target co-catalyst (Ni and Fe and Ni foam) resulting in a higher current density at lower overpotentials. Our work shows how to achieve this control by rationally designing the counter electrode used in the electroplating step. Through chronoamperometry analysis, we identify and optimize the regimes in which crystal nuclei formation and growth occur. As a result, we achieved better controllability of the electrodeposition process and a significant increase in the current density of the OER catalyst. Using 1M potassium hydroxide solution (KOH), a milder concentration than what is currently used in the industry (5-10 M KOH), we achieved 167 mA/cm2 at 1.6V applied against an Ag/AgCl electrode. Previous literature reports on Ni-based catalysts reported max. 100 mA/cm2 with the same energy efficiency(4,5), confirming that our nucleation control strategy can significantly enhance the electrocatalytic performance. While using conditions closer to these deployed in the industry (7M KOH), we recorded up to 500 mA/cm2 at 0.7V (against Ag/AgCl), at only 25°C - a temperature significantly lower than what is used in the industrial electrolyzers. High current densities obtained for a wide concentration of the electrolytes and low temperatures enable a broad application of the “Hybrid Electrode” synthesis systems. This synthetic procedure could be used for the development of more efficient and cost-effective anode materials not only for water splitting but also systems of Carbon Dioxide (CO2) electro-reduction to valuable products such as syngas and ethanol as well as for anodic electro-oxidation of glycerol and other liquid waste products(6).<br/>(1)Shiva Kumar, S.; Lim, H. An Overview of Water Electrolysis Technologies for Green Hydrogen Production. Energy Reports 2022, 8, 13793-13813<br/>(2)Du et al. Anion-Exchange Membrane Water Electrolyzers.Chemical Reviews 2022, 122 (13), 11830 11895<br/>3 Tan et al Nucleation and Growth Mechanisms of an Electrodeposited Ni Se Cu Coating on Nickel Foam. Journal of Colloid and Interface Science 2021, 600, 492–502<br/>(4)Lu and Zhao. Electrodeposition of hierarchically structured three-dimensional nickel–iron electrodes for efficient oxygen evolution at high current densities. Nat Commun 6, 6616 (2015)<br/>(5)Zhang and Zou. Advanced Transition Metal Based OER Electrocatalysts: Current Status, Opportunities, and Challenges. Small 2021, 17 (37), 2100129<br/>(6)Xi et al. Electrochemical CO2 Reduction Coupled with Alternative Oxidation Reactions: Electrocatalysts, Electrolytes, and Electrolyzers. Applied Catalysis B: Environmental 2024, 341, 123291