context: On the other hand, the HER and OER are the two half-reactions involved in electrocatalytic water splitting. These hydrogen-based and oxygen-based alkaline electrochemical reactions are often hindered by sluggish kinetics, which leads to a large overpotential and low round-trip efficiency. Enormous effort has been devoted to the development of highly efficient electrocatalysts for the HER and OER based on the structure–activity relationship, to increase the reaction rate and efficiency of the total system. Accordingly, Liu et al. reported AuPtPdRhRu alloy nanoparticles for HER application by an ultrasonication-assisted wet chemical method (Fig. 5a and b). The as-synthesized quinary AuPtPdRhRu nanoparticles on carbon (AuPtPdRhRu/C), whose average diameter is 2.6 ± 0.3 nm, exhibited excellent HER activity and durability in a 1.0 M KOH electrolyte. The required overpotential of AuPtPdRhRu/C to drive a current density of −30 mA cm−2 was 190 mV, which is smaller than those of quaternary AuPtPdRh/C (260 mV) and ternary AuPtRh/C (600 mV) catalysts. AuPtPdRhRu/C exhibited a lower Tafel slope of 62 mV dec−1 compared to AuPtPdRh/C (91 mV dec−1), AuPtPd/C (177 mV dec−1), and benchmark Pt/C (77 mV dec−1) catalysts, indicating its superior activity toward the alkaline HER. Moreover, AuPtPdRhRu/C exhibited stable electrocatalytic performance over 8 h, verified by chronopotentiometry at a current density of 100 mA cm−2. Likewise, Qiu et al. reported noble-metal free nanoporous HEA catalysts for the OER under alkaline conditions. Quinary nanoporous NiCoFeAlX (np-NiCoFeAlX, X = V, Cr, Mn, Cu, Zr, Nb, Mo) and senary np-MoCuNiCoFeAl catalysts were fabricated by a mechanical alloying and chemical dealloying strategy. Among the catalysts, np-MoNiCoFeAl exhibited the lowest OER overpotential (240 mV) to drive a current density of 10 mA cm−2 in a 1.0 M KOH electrolyte. Moreover, the np-MoNiCoFeAl catalyst also showed a smaller Tafel slope of 46 mV dec−1, compared to ternary np-NiFeAl (62 mV dec−1) and quaternary np-NiCoFeAl (61 mV dec−1). Moreover, the quinary np-MoNiCoFeAl exhibited high durability compared to ternary and quaternary catalysts, and retained more than 95% of its initial activity after 2000 potential cycles. Due to the oxidative potential under OER conditions, the surface of np-MoNiCoFeAl might undergo oxidation, forming surface oxides on the HEA surfaces. After a chronopotentiometry test at 20 mA cm−2 for 50 h, however, the five elements were still uniformly distributed in np-MoNiCoFeAl, forming a high-entropy-oxide (HEO) skin that exhibits high-entropy driven stability.
answer: 1.0 M KOH