Operando X-Ray Spectroscopic Techniques: A Focus on Hydrogen and Oxygen Evolution Reactions

Abstract

The study of structural as well as chemical properties of an electrocatalyst in its reaction environment is a challenge in electrocatalysis. This is very important for the better understanding of the dynamic changes in the reactivity with respect to the structure of catalysts to give insight into the reaction mechanism. The in situ/operando investigation of electrode/electrolyte interface has been increasingly explored in recent days due to the significant developments in technology. The review focus on operando X-ray spectroscopic techniques to understand the behavior of electrocatalysts in hydrogen evolution and oxygen evolution reactions (HER and OER). Some recent studies on the application of operando X-ray spectroscopic methods to study the dynamic nature as well as the evaluation of structural and chemical changes of the electrocatalysts for HER and OER in different reaction environment are discussed.

Tuning Reconstruction Level of Precatalysts to Design Advanced Oxygen Evolution Electrocatalysts

“…The structural properties of in situ-formed species on a catalyst surface mainly determine the final catalytic behavior related to the local electronic structure of the species, such as hydrophilicity, co-ordination, and metal-oxygen covalency [ 55 , 80 ]. Recently, a series of sophisticated in situ/operando characterizations have led to the monitoring of the dynamic structural reconstructions of various types of electrocatalysts [ 61 , 63 , 81 , 82 ]. These efforts found that typical in situ-formed species on catalyst surfaces are metal oxyhydroxides [ 53 , 62 , 78 , 79 , 82 , 83 , 84 ].…”

“…Generally, it is difficult to capture intermediates with a short lifetime, or to observe dynamic changes by ex situ characterization approaches. To track the surface change during an electrochemical reaction, in situ/operando techniques, specifically in situ/operando XRD/XPS/TEM/XAS/Raman methods, are the most powerful tools [ 61 , 63 , 82 , 94 , 138 , 139 ]. For example, in situ XAS can provide informative data regarding dynamic changes of the valence state, coordination number, and bond length [ 72 , 81 , 94 , 140 ].…”

“…In addition, electro-derived surface reconstruction also plays a significant role in determining electrocatalytic activities [ 51 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 ]. Advanced in situ/operando techniques have demonstrated that the surfaces of precatalysts tend to be converted partly or completely to oxyhydroxides during the electrochemical oxidation process, acting as actual active species for OER [ 46 , 51 , 61 , 62 , 63 , 64 , 65 ]. Although typical in situ-formed species are metal oxyhydroxides for after a reconstruction process, the generated active species can show diverse activities even for the same metal oxyhydroxides, as confirmed by different observed electrocatalytic activities from identical oxyhydroxides on different precatalysts surface [ 62 , 66 ].…”

“…The same group later reported an amorphous surface (Co(Fe)OOH phase) of BSCF during the OER process, which explained the initially enhanced activity as determined by cyclic voltammetry and potentiostat measurements [ 70 , 71 ]. These works promoted a further understanding of catalyst surface changes under electrochemical potentials, especially by what are known as in situ/operando techniques [ 50 , 51 , 61 , 72 , 73 , 74 , 75 , 76 ]. It is also important to note that further data from electrochemical experiments revealed that BSCF would lose its superior activity after long-term testing, which is more likely due to the collapse of the crystalline matrix on the surface [ 77 ].…”

Surface-Attached Molecular Catalysts on Visible-Light-Absorbing Semiconductors: Opportunities and Challenges for a Stable Hybrid Water-Splitting Photoanode

“…An emerging approach is to develop and use in situ techniques to elucidate decomposition pathways, including spectroelectrochemistry 59 and X-ray techniques. 60,61 Analysis of the products is required for determination of the faradaic efficiency and identification of any side reactions because a photocurrent can be generated from oxidative decomposition of the catalyst or electrode surface rather than representing true catalytic turnovers. GC-MS and UV−vis methods can be used to characterize solution species that indicate possible surface detachment.…”