A Review of In-Situ Techniques for Probing Active Sites and Mechanisms of Electrocatalytic Oxygen Reduction Reactions

Abstract

Electrocatalytic oxygen reduction reaction (ORR) is one of the most important reactions in electrochemical energy technologies such as fuel cells and metal–O2/air batteries, etc. However, the essential catalysts to overcome its slow reaction kinetic always undergo a complex dynamic evolution in the actual catalytic process, and the concomitant intermediates and catalytic products also occur continuous conversion and reconstruction. This makes them difficult to be accurately captured, making the identification of ORR active sites and the elucidation of ORR mechanisms difficult. Thus, it is necessary to use extensive in-situ characterization techniques to proceed the real-time monitoring of the catalyst structure and the evolution state of intermediates and products during ORR. This work reviews the major advances in the use of various in-situ techniques to characterize the catalytic processes of various catalysts. Specifically, the catalyst structure evolutions revealed directly by in-situ techniques are systematically summarized, such as phase, valence, electronic transfer, coordination, and spin states varies. In-situ revelation of intermediate adsorption/desorption behavior, and the real-time monitoring of the product nucleation, growth, and reconstruction evolution are equally emphasized in the discussion. Other interference factors, as well as in-situ signal assignment with the aid of theoretical calculations, are also covered. Finally, some major challenges and prospects of in-situ techniques for future catalysts research in the ORR process are proposed.

Achieving superior methanol oxidation electrocatalytic performance by surface reconstruction of PtNi nanoalloys during acid etching process

“…It is obvious that the catalysts anchored on the nitrogen-doped graphene substrates exhibit a relatively larger I D / I G value than PtNi 3 -rGO (1.10), indicating that the introduction of nitrogen tuned the degree of defects in the carbon skeleton and the formation of a defective carbon framework owing to the destruction of the carbon atomic lattice by nitrogen doping. 44,45 In addition, the I D / I G value of the Pt 1.5 Ni-NGA catalyst is higher than that of PtNi 3 -NGA, demonstrating that the acid etching treatment further increased the disorder of carbon skeleton and formed more defects, which is beneficial for the nucleation and anchoring of metal nanoparticles on the Pt 1.5 Ni-NGA surface due to the strengthened metal–support interaction. 46–48 This positive anchoring effect resulted in the formation of small-sized PtNi alloy nanoparticles on the carrier.…”

Identification and Understanding of Active Sites of Non‐Noble Iron‐Nitrogen‐Carbon Catalysts for Oxygen Reduction Electrocatalysis

“…The potent in-situ/operando techniques of microscopic/spectroscopic/electrochemical can further shed light on active sites, ORR intermediates, structural evolvement and degradation mechanisms. [121][122][123] Therefore, advanced in-situ/operando characterization techniques are highly desirable to provide more comprehensive understandings on the active sites to help design the expected Fe-N-C catalysts.…”

Intermetallic Nanocrystals for Fuel-Cells-Based Electrocatalysis

“…Credible characterization technologies for precisely probing intermetallic structures in order to either confirm the formation of a new phase with ordered atomic arrangement or further explicitly determine its ordering degree, is of vital importance when investigating intermetallic nanocrystals. ,, Armed with adequate characterization tools, researchers manage to explore the ordering nature and thereby unconceived properties of various intermetallic compounds so as to catch a glimpse of the enigmatic chemistry law behind numerous catalysis phenomena. , During past decades, increasingly structure-sensitive methodologies have been excavated and applied in this area, endowing people with more comprehensive understanding of the physicochemical essence of intermetallics and deeper perception of their structure–performance relation as well as deactivation mechanism in electrocatalysis. These instruments, either directly or indirectly, in situ or ex situ , macro- or microscopically, offering structural and compositional information on atomic architecture from either bulk or surface of nanocrystals, exhibit different detecting ability in different spatiotemporal scale levels. − Hence, proper selection and convincing interpretation are expected while combination of multiple characterization means are always necessary when verifying breathtaking intermetallic structure and coming to novel mechanism-related conclusions. In this section, we give a comprehensive introduction to several typical characterization techniques, including XRD, AC-TEM, XAFS, VSM, Monte Carlo simulation, and electrochemical methods, for the structure detection and ordering degree evaluation exclusively aiming for intermetallic materials (Scheme ).…”