The platinum-group metals (PGMs) are six neighboring elements in the periodic table of the elements. Each PGM can efficiently promote unique reactions, and therefore, alloying PGMs would create ideal catalysts for complex or multistep reactions that involve several reactants and intermediates. Thus, high-entropy-alloy (HEA) nanoparticles (NPs) of all six PGMs (denoted as PGM-HEA) having a great variety of adsorption sites on their surfaces could be ideal candidates to catalyze complex reactions. Here, we report for the first time PGM-HEA and demonstrate that PGM-HEA efficiently promotes the ethanol oxidation reaction (EOR) with complex 12-electron/12-proton transfer processes. PGM-HEA shows 2.5 (3.2), 6.1 (9.7), and 12.8 (3.4) times higher activity than the commercial Pd/C, Pd black and Pt/C catalysts in terms of intrinsic (mass) activity, respectively. Remarkably, it records more than 1.5 times higher mass activity than the most active catalyst to date. Our findings pave the way for promoting complex or multistep reactions that are seldom realized by mono- or bimetallic catalysts.

RuRhPdIrPt high-entropy-alloy nanoparticles with a broad and featureless valence band spectrum show high hydrogen evolution reaction activity.

Promoting the oxygen evolution reaction (OER) with saline water is highly desired to realize seawater splitting. This requires OER catalysts to resist serious corrosion and undesirable chloride oxidation. We introduce a 5d transition metal, Ir, to develop a monolayer NiIr-layered double hydroxide (NiIr-LDH) as the catalyst with enhanced OER performance for seawater splitting. The NiIr-LDH catalyst delivers 500 mA/cm 2 at only 361 mV overpotential with ∼99% O 2 Faradaic efficiency in alkaline seawater, which is more active than commercial IrO 2 (763 mV, 23%) and the best known OER catalyst NiFe-LDH (530 mV, 92%). Moreover, it shows negligible activity loss at up to 650 h chronopotentiometry measurements at an industrial level (500 mA/cm 2 ), while commercial IrO 2 and NiFe-LDH rapidly deactivated within 0.2 and 10 h, respectively. The incorporation of Ir into the Ni(OH) 2 layer greatly altered the electron density of Ir and Ni sites, which was revealed by X-ray absorption fine structure and density functional theory (DFT) calculations. Coupling the electrochemical measurements and in situ Raman spectrum with DFT calculations, we further confirm that the generation of rate-limiting intermediate *O and *OOH species was accelerated on Ni and Ir sites, respectively, which is responsible for the high seawater splitting performance. Our results also provide an opportunity to fabricate LDH materials containing 5d metals for applications beyond seawater splitting.

The compositional space of high-entropy-alloy nanoparticles (HEA NPs) significantly expands the diversity of the materials library. Every atom in HEA NPs has a different elemental coordination environment, which requires knowledge of the local electronic structure at an atomic level. However, such structure has not been disclosed experimentally or theoretically. We synthesized HEA NPs composed of all eight noble-metal-group elements (NM-HEA) for the first time. Their electronic structure was revealed by hard X-ray photoelectron spectroscopy and density function theory calculations with NP models. The NM-HEA NPs have a lower degeneracy in energy level compared with the monometallic NPs, which is a common feature of HEA NPs. The local density of states (LDOS) of every surface atom was first revealed. Some atoms of the same constituent element in HEA NPs have different LDOS profiles, whereas atoms of other elements have similar LDOS profiles. In other words, one atom in HEA loses its elemental identity and it may be possible to create an ideal LDOS by adjusting the neighboring atoms. The tendency of the electronic structure change was shown by supervised learning. The NM-HEA NPs showed 10.8-times higher intrinsic activity for hydrogen evolution reaction than commercial Pt/C, which is one of the best catalysts.

Developing efficient bifunctional electrocatalysts for overall water splitting in acidic conditions is the essential step for proton exchange membrane water electrolyzers (PEMWEs). We first report the synthesis of core−shell structure nanoparticles (NPs) with an Au core and an AuIr 2 alloy shell (Au@AuIr 2 ). Au@AuIr 2 displayed 4.6 (5.6) times higher intrinsic (mass) activity toward the oxygen evolution reaction (OER) than a commercial Ir catalyst. Furthermore, it showed hydrogen evolution reaction (HER) catalytic properties comparable to those of commercial Pt/C. Significantly, when Au@AuIr 2 was used as both the anode and cathode catalyst, the overall water splitting cell achieved 10 mA/cm 2 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) and is among the most efficient bifunctional catalysts reported. Theoretical calculations coupled with Xray-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.