With the development of proton exchange membrane water electrolyzers (PEMWEs), hydrogen production by electrolysis of water under acidic conditions is considered to be the most promising way to efficiently convert sustainable hydrogen energy.
Electrocatalytic water splitting contains the oxygen evolution reaction (OER) at the anode and the hydrogen evolution reaction (HER) at the cathode. Compared with the outstanding HER performance realized by Pt-based catalysts at low overpotentials, the sluggish OER kinetics and the rapid deactivation of OER catalysts in acidic electrolytes limit the wide commercialization of PEMWEs.
In a study published in J. Am. Chem. Soc., the research group led by Prof. CAO Rong and Prof. CAO Minna from Fujian Institute of Research on the Structure of Matter (FJIRSM) of the Chinese Academy of Sciences, reported an Au@AuIr2 core-shell alloy nanocatalyst with partial oxidation surface, which exhibited excellent overall water splitting performance in acidic media.
Ir-based nanomaterials have been widely studied owing to effective OER performance under acidic electrolytes. To make the scarcely stored precious metals cost-effective, the researchers have to improve atomic utilization rate without sacrificing performance in order to meet commercial demand.
The researchers used a one-pot reaction to synthesize AuIr core-shell nanoparticles with HAuCl43H2O and IrCl3 xH2O being the precursors, and oleylamine being both the solvent and the reducing agent.
At low temperature, Au, with a higher redox potential, is reduced prior to Ir and then forms as a core. As the temperature increased, Ir atoms were deposited on the surface of Au to form a Au-Ir alloy surface by atomic diffusion. When the reaction time was prolonged to 3 h, all the nanoparticles (NPs) evolved into uniform core-shell structure NPs with Au core and AuIr2 alloy shell (Au@AuIr2).
Through powder X-ray diffraction (PXRD), the researchers confirmed two components with lattice constants of a = 4.078(5) Å and 3.889(4) Å in Au@AuIr2, which can be assigned as Au and AuIr alloy. By means of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), high-resolution transmission electron microscopy (HRTEM), energy dispersive X-ray spectroscopy (EDS) and energy dispersive X-ray (EDX) line scan, they confirmed the core-shell alloy structure.
There is a gradient of declining Au distribution from core to shell, with Au core and AuIr2 alloy shell. The local atomic and electronic structures of Au@AuIr2 were characterized by X-ray photoelectron spectroscopy (XPS) and (X-ray absorption fine structure spectroscopy) AFS. The results suggest that amorphous IrOx in the surface of Au@AuIr2 NPs, and the partially oxidized surface was mainly from the interaction between Au and Ir.
Au@AuIr2 showed excellent catalytic activity under acidic conditions, and displayed 4.6 (5.6) times higher intrinsic (mass) activity toward OER than a commercial Ir catalyst. It presented HER catalytic properties comparable to those of commercial Pt/C. Significantly, when Au@AuIr2 was used as both the anode and cathode catalyst, the overall water splitting cell achieved 10 mA/cm2 with a low cell voltage of 1.55 V and maintained this activity for more than 40 h, which greatly outperformed the commercial couples (Ir/C||Pt/C, 1.63 V, activity decreased within minutes).
Density Functional theory (DFT) calculations demonstrated that the partially oxidized Au@AuIr2 core-shell alloy nanoparticles achieve a better balance for intermediates binding and thus exhibit a better OER performance. Theoretical calculations coupled with X-ray-based structural analyses suggest that partially oxidized surfaces originating from the electronic interaction between Au and Ir provide a balance for different intermediates binding and realize significantly enhanced OER performance.
This study realizes the regulation of the nanostructure and electronic structure of core-shell alloy at the atomic scale, which is helpful to understand the structure-activity relationship between the structure and properties of catalysts, and provides an idea for material design. The rational design of the surface oxidation and material composition can enable a suitable balance for intermediates binding, which not only improves the activity and stability of the catalyst to a greater extent, but also greatly improves the utilization efficiency of precious metal catalyst.
Core-shell alloy nano-catalyst composed of Au core and AuIr2 alloy shell (Au@AuIr2) with partially oxidized surfaces enhanced water splitting performance in acidic media (Image by Prof. CAO’s group)
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