This journal is © The Royal Society of Chemistry 2021 Chem. Commun., 2021, 57, 10027–10030 | 10027 Cite this: Chem. Commun., 2021, 57, 10027 Thermodynamically driven metal diffusion strategy for controlled synthesis of high-entropy alloy electrocatalysts† Huilin Li, Han Zhu, * Shuhui Sun, Jiace Hao, Zhenfeng Zhu, Fangping Xu, Shuanglong Lu, Fang Duan and Mingliang Du * We report a thermodynamically driven metal diffusion strategy for the controlled synthesis of high-entropy alloy (HEA) nanocrystals using electrospun carbon nanofibers (CNFs) as nanoreactors. This conceptual pathway is resistant to high temperatures and produces a series of medium-entropy alloy (MEA) and HEA nanocrystals supported on CNFs by adjusting the numbers and kinds of ele- ments. The FeCoNiCrMn/CNFs obtained the lowest overpotential of 345 mV at 50 mA cm À2 compared to MEA. The operando electro- chemical Raman results indicate that the enhanced electron trans- fer from low-electronegativity Fe, Ni, Cr and Mn to the orbit of the Co atom makes Co a local negative charge center, leading to the decrease in absorption energy of OH. The oxygen evolution reaction (OER) is a vital step in recharge- able metal–air batteries and water electrolysis. 1–3 Producing green hydrogen energy through electrocatalytic water splitting has been utilized intensely. 4–6 The sluggish reaction kinetics caused by complex 4e À transfers demonstrate the highly energy- intensive process of the OER. 7,8 Noble metals (Ru, Ir) have been continuously studied as catalysts for the OER; however, their scarcity, high cost and poor stability have encouraged scientists to develop earth-abundant electrocatalysts. 9–11 Metal alloying, including bimetallic and trimetallic alloys, can tune the adsorp- tion energies of the reaction intermediates on catalyst surfaces, which outclasses their monometallic counterparts in terms of activity as well as stability. 12–16 Recently, as a new class of metal alloy catalyst, high-entropy alloy (HEA) materials containing five or more elements with unique synergistic functions exhibit promising properties in degrading azo dye solutions, ammonia decomposition, CO oxidation, water splitting, and the oxygen reduction reaction. 17–20 Compared with low-entropy and medium- entropy alloys (LEA and MEA), HEA exhibits three main advan- tages: (i) the configurational entropy of the mixture increases as the composition of the elements increases, leading to the formation of a stable single-phase solution structure with superior stability. (ii) The lattice distortion effect, caused by the size differences of the constituent atoms in HEA, can significantly reduce electrical conductivity and thermal con- ductivity. (iii) Malformations increase the energy barrier of atomic diffusion and contribute to the formation of nanoscale HEA. 20–23 The preparation of HEA nanostructures usually require complicated processes or harsh conditions (e.g., extre- mely high temperature up to 1700 1C) to overcome their inherent heterogeneity. 22 The development of rational strate- gies for the controlled synthesis of HEA nanocrystals to reveal the fundamental working principles of HEA electrocatalysts is still limited. Meanwhile, still less is known about the activity assessment of MEA and HEA electrocatalysts. Here, we report a thermodynamically driven metal diffusion strategy for the controlled synthesis of HEA nanocrystals using electrospun nanofiber as nanoreactors. The metal salts were firstly distributed in the polymer nanofibers, and after the graphitization process, the metal salts were reduced to metal atoms and diffused in the CNFs. Under the confinement by CNFs, the multi-metal atoms fused together into a solid solution alloy. The MEA and HEA nanocrystals can be easily synthesized by adjusting the numbers and kinds of elements. The FeCoNiCrMn/CNFs require a lower overpotential of only 345 mV to reach the current density of 50 mA cm À2 , compared to MEA FeCoNi/CNFs (407 mV), FeCoNiMn/CNFs (376 mV) and FeCoNiCr/CNFs (369 mV), suggesting that the number of ele- ments in an alloy could influence the OER activity. The typical synthesis procedure for FeCoNiCrMn/CNFs is shown in Fig. 1a. In brief, five kinds of metal salts were dissolved in polyacrylonitrile/N,N-dimethylformamide (PAN/DMF) solution. Then, the precursor solution was electro- spun into one-dimensional nanofibers. Through the Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, P. R. China. E-mail: zhysw@jiangnan.edu.cn, du@jiangnan.edu.cn † Electronic supplementary information (ESI) available. See DOI: 10.1039/ d1cc03072h Received 10th June 2021, Accepted 31st August 2021 DOI: 10.1039/d1cc03072h rsc.li/chemcomm ChemComm COMMUNICATION Published on 31 August 2021. Downloaded by Jiangnan University on 1/14/2023 3:20:04 AM. View Article Online View Journal | View Issue