Identifying the catalytically active site(s) in the oxygen reduction reaction (ORR), under real-time electrochemical conditions, is critical to the development of fuel cells and other technologies. We have employed in situ synchrotron-based X-ray absorption spectroscopy (XAS) to investigate the synergistic interaction of a Co–Mn oxide catalyst which exhibits impressive ORR activity in alkaline fuel cells. X-ray absorption near edge structure (XANES) was used to track the dynamic structural changes of Co and Mn under both steady state (constant applied potential) and nonsteady state (potentiodynamic cyclic voltammetry, CV). Under steady state conditions, both Mn and Co valences decreased at lower potentials, indicating the conversion from Mn­(III,IV) and Co­(III) to Mn­(II,III) and Co­(II), respectively. Rapid X-ray data acquisition, combined with a slow sweep rate in CV, enabled a 3 mV resolution in the applied potential, approaching a nonsteady (potentiodynamic) state. Changes in the Co and Mn valence states were simultaneous and exhibited periodic patterns that tracked the cyclic potential sweeps. To the best of our knowledge, this represents the first study, using in situ XAS, to resolve the synergistic catalytic mechanism of a bimetallic oxide. Strategies developed/described herein can provide a promising approach to unveil the reaction mechanism for other multimetallic electrocatalysts.

Electrocatalysis has been the cornerstone for enhancing energy efficiency, minimizing environmental impacts and carbon emissions, and enabling a more sustainable way of meeting global energy needs. Elucidating the structure and reaction mechanisms of electrocatalysts at electrode–electrolyte interfaces is fundamental for advancing renewable energy technologies, including fuel cells, water electrolyzers, CO2 reduction, and batteries, among others. One of the fundamental challenges in electrocatalysis is understanding how to activate and sustain electrocatalytic activity, under operating conditions, for extended time periods and with optimal activity and selectivity. Although traditional ex situ methods have provided a baseline understanding of heterogeneous (electro)­catalysts, they cannot provide real-time interfacial structural and compositional changes under reaction conditions, which calls for the use of in situ/operando methods. Herein, we provide a selective review of in situ and operando characterizations, in particular, the use of operando synchrotron-based X-ray techniques and in situ atomic-scale scanning transmission electron microscopy (STEM) in liquid/gas phases to advance our understanding of electrode–electrolyte interfaces at macro- and microscopic levels, which dictate the charge transfer kinetics and overall reaction mechanisms. The use of scanning electrochemical microscopy (SECM) enables direct probing of the local activity of electrocatalysts at the nanometer scale. In addition, differential electrochemical mass spectrometry (DEMS) and the electrochemical quartz crystal balance (EQCM) enable the simultaneous identification of multiple reaction intermediates and products for mechanistic studies of electrocatalyst selectivity and durability. We anticipate that continuous advances of in situ/operando techniques and probes will continue to make significant contributions to establishing structure/composition-reactivity correlations of electrocatalysts at unprecedented atomic-scale and molecular levels under realistic, real-time reaction conditions.

Ordered intermetallic nanoparticles are promising electrocatalysts with enhanced activity and durability for the oxygen-reduction reaction (ORR) in proton-exchange membrane fuel cells (PEMFCs). The ordered phase is generally identified based on the existence of superlattice ordering peaks in powder X-ray diffraction (PXRD). However, after employing a widely used postsynthesis annealing treatment, we have found that claims of “ordered” catalysts were possibly/likely mixed phases of ordered intermetallics and disordered solid solutions. Here, we employed in situ heating, synchrotron-based, X-ray diffraction to quantitatively investigate the impact of a variety of annealing conditions on the degree of ordering of large ensembles of Pt3Co nanoparticles. Monte Carlo simulations suggest that Pt3Co nanoparticles have a lower order–disorder phase transition (ODPT) temperature relative to the bulk counterpart. Furthermore, we employed microscopic-level in situ heating electron microscopy to directly visualize the morphological changes and the formation of both fully and partially ordered nanoparticles at the atomic scale. In general, a higher degree of ordering leads to more active and durable electrocatalysts. The annealed Pt3Co/C with an optimal degree of ordering exhibited significantly enhanced durability, relative to the disordered counterpart, in practical membrane electrode assembly (MEA) measurements. The results highlight the importance of understanding the annealing process to maximize the degree of ordering in intermetallics to optimize electrocatalytic activity.

A facile, seed-mediated growth method and postannealing treatment have been employed to synthesize a single-phase 40% mass loading intermetallic Pt 3 Co/Vulcan electrocatalyst, enabling the electrocatalytic layer to be thinner and thus lowering the mass transport resistance for the ORR in proton exchange membrane fuel cells (PEMFCs). These 40% Pt 3 Co/C nanocatalysts exhibited superior kinetics toward the ORR, relative to a 20% Pt 3 Co/C catalyst, and only showed a slight degradation in the half-wave potential and electrochemical surface area after a 4000-cycle stability test. The high activity and stability are attributed to a stable intermetallic Pt 3 Co core with a Pt shell two to three atomic layers thick, as well as to its good dispersion on the support. This seed-mediated growth approach and postannealing treatment provide a new strategy to ensure single-phase formation and uniform particle size distribution for other high-mass-loading binary intermetallic/alloy electrocatalyst systems and can contribute to the practical commercialization of PEMFCs, especially in high-current-density applications, such as automotive.

Developing high-performance, low-cost, and conductive nonprecious electrocatalysts for the oxygen reduction reaction (ORR) has been a key challenge for advancing fuel cell technologies. Here, we report on a novel family of cobalt nitrides (Co x N/C, x = 2, 3, 4) as ORR electrocatalysts in alkaline fuel cells. Co 4 N/C exhibited the highest ORR activity among the three types of cobalt nitrides studied, with a half-wave potential (E 1/2 ) of 0.875 V vs RHE in 1 M KOH, rivaling that of commercial Pt/C (0.89 V). Moreover, Co 4 N/C showed an 8-fold improvement in mass activity at 0.85 V, when compared to cobalt oxide, Co 3 O 4 /C, and a negligible degradation (ΔE 1/2 = 14 mV) after 10 000 potential cycles. The superior performance was ascribed to the formation of a conductive nitride core surrounded by a naturally formed thin oxide shell (about 2 nm). The conductive nitride core effectively mitigated the low conductivity of the metal oxide, and the thin oxide shell on the surface provided the active sites for the ORR. Strategies developed herein represent a promising approach for the design of other novel metal nitrides as electrocatalysts for fuel cells.