0428同步年報-2021-全

Energy Science 059 T he oxygen-evolution reaction (OER) is an important electrochemical anodic reaction that provides protons and electrons for fuel-generating cathodic reactions such as the hydrogen-evolution reaction and CO 2 reduction reactions. Tremendous efforts have been made to develop efficient and scalable OER electrocatalysts. For instance, mixed metal oxides containing Co, Fe or Ni have been found to be the most active heterogeneous OER catalysts in an alkaline medium, but the heterogeneous nature of these metal oxides makes it difficult to study and to understand the fundamental properties and mechanisms of these catalysts. Fortunately, atomically dispersed catalysts including single- atom catalysts and discrete sub- nano clusters are an emerging class of heterogeneous electrocatalysts with high atomic efficiency. These catalysts possess uniform and well defined active sites, providing a unique opportunity for a mechanistic understanding. Xile Hu (Ecole Polytechnique Fédérale de Lausanne, Switzerland) and Hao Ming Chen (National Taiwan University) recently developed a general synthesis of Co-, Fe- and Ni-containing double-atom catalysts from their single-atom precursors via electrochemical transformation in situ . Employing operando X-ray absorption spectra (XAS) at beamlines TPS 44A , TLS 01C1 , SP 12B1 and SP 12U1 , 1 the results demonstrated how atomic-configuration and chemical state could affect their resulting catalytic nature. For example, operando extended X-ray absorption fine structure (EXAFS) spectra of the Ni K-edge and the Fe K-edge were recorded under varied conditions, as plotted in Fig. 1 . The results indicated that the Ni ion in Ni–N–C is most probably coordinated with 3N, 1C and 1O. The N and C donors come from the N-doped carbon support; the O donor is assumed to be an adsorbed hydroxyl group or water. When a single-atom Ni catalyst (Ni–N–C) was immersed in 1-M KOH, two N donors were replaced by three new O donors (OH − /H 2 O), as shown in Fig. 1(a) . After five cyclic voltammetry scans How Does the Synergism Work? A stable and non-precious double-atom catalyst was demonstrated for the next generation of superior oxygen-evolution catalysts. Fig. 1 : Operando XAS study of Ni–Fe–N–C. (a) Operando EXAFS spectra of Ni K-edge for Ni–N–C (as prepared) and Ni–Fe–N–C under various conditions. (b) Edge-jump energies of Ni K-edge spectra for Ni–N–C (as prepared) and Ni–Fe–N–C under various conditions. Inset: enlarged graph for the oxidation states. (c) Operando EXAFS spectra of Fe K-edge for Ni–Fe–N–C under various reaction durations. (d) Edge-jump energies of Fe K-edge spectra for Ni– Fe–N–C under various reaction durations. Inset: enlarged graph for the oxidation state. (e) A scheme showing the formation of double-atom Ni–Fe–N–C from the single-atom Ni–N–C precatalyst. [Reproduced from Ref. 1] (a) (b) (c) (d) (e)