Bare-foot Ni(OH)2-clothed platinum-tetrapods enable more efficient green hydrogen production
By Chengzhang Wan
This article was originally published by UCLA Chemistry & Biochemistry
A collaborative research team led by Professor Xiangfeng Duan’s group, Professor Anastassia N. Alexandrova’s group in the UCLA Department of Chemistry & Biochemistry, and Professor Yu Huang’s group in the UCLA Department of Materials Science and Engineering has developed a new strategy to tailor the local chemical environment on Pt for boosting alkaline hydrogen evolution reaction (HER), a reaction of critical importance for water electrolysis and green hydrogen production. The research is described in a recent paper published in Nature Materials.
Water electrolysis using intermittent renewable electricity (from solar cells and windmills) offers an attractive solution to green hydrogen production that is essential for renewable energy industry and achieving carbon neutrality. The discovery of more efficient and durable catalysts for the HER is important for sustainable and cost-effective hydrogen production from renewable electricity. Although Pt is the best catalyst for HER in acidic conditions, the HER rate on Pt in the alkaline condition is orders of magnitude lower than that in the acidic electrolyte due to the sluggish water dissociation and the poor proton supply rate. Considerable efforts have been placed on tailoring the Pt active sites to improve alkaline HER kinetics.
Inspired by the natural enzymes where the precisely tailored micro-environment works in concert with the active sites to ensure superior activity, selectivity, and durability, the research team developed a unique “bare-foot Ni(OH)2-clothed Pt-tetrapod” core/shell nanostructure [Pttet@Ni(OH)2] to create a local chemical environment that can provide efficient proton (H+) supply to the Pt active sites and greatly boost HER performance in alkaline medium (Fig. 1).
“The bare-feet Ni(OH)2-clothed Pt-tetrapod structure offers an ideal geometry for isolating most of the Pt surface sites from the bulk alkaline electrolyte while allowing the “bare-feet” to make robust electrical contacts with the conductive carbon support for efficient electron transport to the catalytic sites”, commented the first author, Dr. Chengzhang Wan, previously a graduate student in Duan lab and currently is postdoc fellow in Huang lab.
Moreover, the amorphous Ni(OH)2 shell functions as an effective water dissociation catalyst and a proton permeable sieve to ensure efficient H+supply to the interfacial Pt sites, creating a proton-enriched local environment and fundamentally altering the HER to kinetics to the acidic-like Tafel-step limited pathway. The designed Pttet@Ni(OH)2 achieves a record-breaking specific activity of 27.7 mA/cm2 Pt and mass activity of 13.4 A/mgPt at -70 mV vs. reversible hydrogen electrode in alkaline electrolyte.
Additionally, the Ni(OH)2 shell effectively rejects impurity ions and retards the Oswald ripening process, endowing a high tolerance to water impurities and long-term durability not attainable in the other naked Pt-catalysts.
“The markedly improved alkaline HER performance presents an attractive catalyst material for alkaline water electrolyzers. The exceptional tolerance to ionic impurities may also open up new opportunities for other practical applications, including direct seawater electrolysis.” Prof. Duan commented.
The mechanism of water dissociation and hydrogen transfer at the Pt/NiOxHyinterface was further investigated by Alexandrova’s group. Using the grand canonical genetic algorithm to establish an accurate atomistic model of the amorphous NiOxHy structure, researchers further found that the amorphous NiOxHy features a complex H-bond network that effectively stabilizes the transition states of both water dissociation and proton transfer, resulting in significantly enhanced water dissociation rate and barrierless proton transfer processes.
“The Ni-O framework and hydrogen bond matrix in the highly disordered NiOxHy layer is more flexible than in the regular crystalline Ni(OH)2 structure, and the hydrogen is delocalized between the proton donor and acceptors.” commented co-author Zisheng Zhang from Prof. Alexandrova’s group. “Therefore, under the applied voltage, such delocalized proton inside NiOxHy may readily transfer from the NiOxHy/electrolyte interface to the Pt/NiOxHyinterface following the Grotthuss-type mechanism.”
Tailoring local chemical environment beyond the active sites opens an exciting new frontier in electrocatalyst design. “The demonstrated capability to fundamentally modify the reaction kinetics by tailoring the local chemical environment may be expanded for the design of a new generation of electrocatalysts with a favorable reaction environment and high selectivity or durability for other fundamentally and technologically important electrochemical reactions.” Prof. Huang commented.
The research team is further adapting their approach for other important electrochemical reactions and applications that are of great importance for the renewable energy industry, such as direct seawater splitting.
About the Lead UCLA Authors
Professor Xiangfeng Duan joined the chemistry and biochemistry faculty in 2008. His group’s research interests precision synthesis of atomic-scale materials, artificial heterostructures, high-order superlattices and crystalline-molecular hybrid solids with designable modulation of chemical compositions and electronic band structures, by doing so to unlock the previously inaccessible physical limits and enable new device functions future electronics, energy and healthcare technologies.
Professor Anastassia Alexandrova is a professor in the Chemistry and Biochemistry Department since 2010. Her laboratory focuses on theory and computation for studies and design of a broad range of functional materials, from heterogeneous (electro)catalysis to enzymes, and to qubits.
Professor Yu Huang is a Professor and Chair at Department of Materials Science and Engineering. Her research focuses on mechanistic understanding of nanoscale material synthesis and assembly. Exploiting the unique attributes of nanoscale surfaces and interfaces, her group is creating novel methodologies for probing and tailoring nanoscale processes that can fundamentally impact a wide range of technologies including materials synthesis, catalysis, fuel cells, and device applications.
Dr. Chengzhang Wan received his Bachelor’s degree from Nanjing University in 2015 and his Ph.D. degree in Chemistry from UCLA in 2020. He is now a postdoctoral fellow in Prof. Xiangfeng Duan and Prof. Yu Huang’s group. His research at UCLA focuses on the surface and interface engineering of platinum nanostructures for effective electrochemical energy conversion and storage.
Zisheng Zhang was born in Wuhan, PRC. He received a B.Sc. in Chemistry from South University of Science and Technology of China in 2019 advised by Prof. Jun Li. At UCLA, he was a UCLA-CSST fellow in 2018, obtained a M.Sc. in Chemistry in 2021, and is currently a Ph.D. candidate advised by Prof. Anastassia N. Alexandrova. In 2022, he worked with Dr. Maria K. Chan as a research intern at Argonne National Lab. His research interests include realistic modeling of catalytic interfaces and inverse design of functional molecules.