Proton exchange membrane water electrolysis (PEM-WE) has emerged as a promising technology for hydrogen production and shows substantial advantages over conventional alkaline water electrolysis. To enable efficient PEM-WE in acidic media, iridium (Ir)- or ruthenium (Ru)-based catalysts are indispensable to drive the thermodynamically and kinetically demanding oxygen evolution reaction (OER). However, developing Ir/Ru catalysts with high efficiency and long-term durability still remains a formidable challenge. Herein, we report one-pot hydrothermal synthesis of ultrafine IrRu intermetallic nanoclusters loaded on conductive, acid-stable, amorphous tellurium nanoparticle support (IrRu@Te). Benefiting from the large exposed electrocatalytically active surface of ultrafine IrRu clusters and the strong electronic coupling between IrRu and Te support, the as-obtained IrRu@Te catalysts show good catalytic performance for the OER in strong acidic electrolyte (i.e., 0.5 M H2SO4), requiring overpotentials of only 220 and 303 mV to deliver 10 and 100 mA cm–2 and able to sustain continuous OER electrolysis up to 20 h at 10 mA cm–2 with minimal degradation. Moreover, IrRu@Te exhibits high specific activity, illustrating intrinsically better performance compared with that of unsupported IrRu and other commercial Ir- and Ru-based catalysts. It also demonstrates unprecedentedly high mass activity of 590 A gIrRu –1 at an overpotential of 270 mV, outperforming most Ir- and Ru-based OER catalysts reported in the literature. Furthermore, IrRu@Te catalysts reveal good OER performance in neutral electrolyte as well, holding great potential to be used for PEM-WE in environmentally friendly conditions. Density functional theory (DFT) calculations based on oxidized IrRu confirm that the catalyst/support coupling results in a lower energy barrier for the oxygen–oxygen bonding formation, offering a rational explanation to the experimentally observed OER performance.
In this work, the effect of pH on a nitrogen-doped ordered mesoporous carbon catalyst for the oxygen reduction reaction (ORR) is extensively investigated. Electrochemical methods, including cyclic voltammetry (CV), rotating ring-disk electrode (RRDE), and cathodic stripping voltammetry, are applied to investigate the electrochemical behavior in electrolyte solutions of different pHs (0–2, 7, 12–14). The CV result reveals that nitrogen-doped carbon has a variety of enriched reversible redox couples on the surface, and the pH has a significant effect. Whether these redox couples are electrochemically active or inactive to the ORR depends on the electrolyte used. In acid media, an oxygen molecule directly interacts with the redox couple, and its reduction proceeds by the surface-confined redox-mediation mechanism, yielding water as the product. Similarly, the first electron transfer in alkaline media is achieved by the surface-confined redox-mediation mechanism at the higher potentials. With decreasing potential, another parallel charge transfer process by the outer-sphere electron transfer mechanism gets pronounced, followed by parallel 2-e and 4-e reduction of oxygen. The proposed mechanisms are well supported by the following electrochemical results. At high potentials, the Tafel slope remains unchanged (60–70 mV dec–1) at all investigated pHs, and the reaction order of proton and hydroxyl ions is found to be 1 and −0.5, respectively, in acid and alkaline media. The electron transfer number is ∼4 at high potentials in both acid and alkaline media; however, at higher pHs, it shows a considerable decrease as the potential decreases, indicating the change in the reaction pathway. Finally, the nitrogen-doped carbon catalyst shows performance in alkaline media superior to that in acid media. Such a gap in performance is rationalized by considering the chemical change in the surface at different pH values.
Achieving an efficient and stable oxygen evolution reaction (OER) in an acidic or neutral medium is of paramount importance for hydrogen production via proton exchange membrane water electrolysis (PEM-WE). Supported iridium-based nanoparticles (NPs) are the state-of-the-art OER catalysts for PEM-WE, but the nonhomogeneous dispersion of these NPs on the support together with their nonuniform sizes usually leads to catalyst migration and agglomeration under strongly corrosive and oxidative OER conditions, eventually causing the loss of active surface area and/or catalytic species and thereby the degradation of OER performance. Here, we design a catalyst comprising surface atomic-step enriched ruthenium–iridium (RuIr) nanocrystals homogeneously dispersed on a metal organic framework (MOF) derived carbon support (RuIr@CoNC), which shows outstanding catalytic performance for OER with high mass activities of 2041, 970 and 205 A gRuIr –1 at an overpotential of 300 mV and can sustain continuous OER electrolysis up to 40, 45, and 90 h at 10 mA cm–2 with minimal degradation in 0.5 M H2SO4 (pH = 0.3), 0.05 M H2SO4 (pH = 1), and PBS (pH = 7.2) electrolytes, respectively. Comprehensive experimental studies and density functional theory (DFT) calculations reveal that the good performance of RuIr@CoNC can be attributed, on one hand, to the presence of abundant atomic steps that maximize the exposure of catalytically active sites and lower the limiting potential of the rate-determining step of OER and, on the other hand, to the strong interaction between RuIr nanocrystals and the CoNC support that endows homogeneous dispersion and firm immobilization of RuIr catalysts on CoNC. The RuIr@CoNC catalysts also show outstanding performance in a single-cell PEM electrolyzer, and their large-quantity synthesis is demonstrated.
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