The electrocatalytic production of hydrogen peroxide (H 2 O 2 ) through the two-electron oxygen reduction reaction (ORR) requires costeffective catalysts with high selectivity, activity, and stability. Herein we report the synthesis and electrocatalytic assessment of nickel−nitrogen−carbon (Ni−N−C) electrocatalysts to gain insight into ORR activity and selectivity toward the production of H 2 O 2 . The activity and selectivity of the catalysts depended on the amount of nickel added during synthesis as well as the pH of the electrolyte. The materials were found to be heterogeneous in nature, consisting of nitrogen-doped carbon structures containing Ni species, including Ni 3 S 2 and covered metallic Ni particles. The presence of Ni during synthesis was imperative for the ORR performance in acidic electrolytes but had minimal impact on the performance in alkaline electrolytes. By experimentally demonstrating that Ni 3 S 2 , metallic Ni, and N-doped carbon species were not the source of activity, we postulate that atomically dispersed Ni−N x /C sites are responsible for the ORR performance in acidic electrolytes, with an activity of −0.3 mA cm −2 and a H 2 O 2 selectivity of 43% measured for the best Ni−N−C catalyst at 0.5 V vs RHE. This work highlights the potential and generates scientific insight into Ni− N−C catalysts to guide the design of improved performance metal−nitrogen−carbon catalysts based on inexpensive precursors and simplistic syntheses.

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Soft X-ray Scanning Transmission X-ray microscopy (STXM) is a synchrotron-based technique which can provide both spectroscopic characterization (near edge X-ray absorption fine structure, NEXAFS) and chemically selective imaging with high spatial resolution (~30 nm). Recently, we have developed in situ flow electrochemical devices [1,2] which allow control of the electrochemical environment while conducting STXM measurements, thus providing a platform for in-situ studies of electrochemical oxidation and reduction processes. This presentation reports results of in situ flow electrochemical STXM studies on three different systems to demonstrate the present capabilities. First, the ability to rapidly exchange the electrolyte is demonstrated by a STXM Fe 2p and in situ electrochemical study of the ferro/ferricyanide solution redox system. Second Cu and Ag-doped Cu catalysts for CO2 electrochemical reduction (CO2R) were successfully prepared using in situ electrodeposition. Third, the electrolyte was changed from CuSO4 to NaHCO3 (as substrate for CO2R) and the cell was operated under electrochemical CO2 reduction conditions, while monitoring the changes to the Cu deposited layer at various potentials, including –0.5 V where CO2 reduction is expected [3]. The figure shows a cyclic voltammogram (CV) and color-coded Cu oxidation state maps which were derived from Cu 2p stacks measured under chronoamperometric conditions at the indicated potentials. These results demonstrate that in situ flow electrochemical STXM measurements can be performed in our device under varying electrochemical reaction conditions, enabling visualization of the morphology changes from selective energy imaging, and quantitative tracking of electrochemical transformations from spectromicroscopy. This system will be used for in situ studies of CO2 electrochemical reduction catalysis with the goal of obtaining mechanistic insights to guide the development of catalysts with improved efficiency and selectivity. In addition, the system will be used to study a variety of material science, chemistry and environmental science related questions associated with oxidation or reduction processes, such as mechanisms of extra-cellular electron transport in marine sediment microbial biofilms [4]. This research is supported by NSERC (Canada). STXM measurements were performed at the ambient STXM facility at the Canadian Light Source, which is funded by the Canadian Foundation for Innovation. [1] V. Prabu et al., Rev. Sci. Inst. 89 (2018) 063702. [2] P. Ingino, et al., in preparation [3] L. Wang, D.C. Higgins, et al., Proc. Nat. Acad. Sci. (2020) 01821683. DOI: 10.1073/pnas.1821683117 [4] M. Obst, et al., Microsc. Microanal. 24 (S-2) (2018) 502-504. Figure 1

Atomically dispersed metal–nitrogen–carbon (M–N–C) materials are a class of electrocatalysts for fuel cell and electrochemical CO2 reduction (CO2R) applications. However, it is challenging to characterize the identity and concentration of catalytically active species owing to the structural heterogeneity of M–N–C materials. We utilize scanning transmission X-ray microscopy (STXM) as a correlative spectromicroscopy approach for spatially resolved imaging, identification, and quantification of structures and chemical species in mesoscale regions of nickel–nitrogen–carbon (Ni–N–C) catalysts, thereby elucidating the relationship between Ni content/speciation and CO2R activity/selectivity. STXM results are correlated with conventional characterization approaches relying on either bulk average (X-ray absorption spectroscopy) or spatially localized (scanning transmission electron microscopy with electron energy loss spectroscopy) measurements. This comparison illustrates the advantages of soft X-ray STXM to provide spatially resolved identification and quantification of active structures in Ni–N–C catalysts. The active site structures in these catalysts are identified to be atomically dispersed NiN x /C sites distributed throughout entire catalyst particles. The NiN x /C sites were notably demonstrated by spectroscopy to possess a variety of chemical structures with a spectroscopic signature that most closely resembles nickel­(II) tetraphenylporphyrin molecules. The quantification and spatial distribution mapping of atomically dispersed Ni active sites achieved by STXM address a target that was elusive to the scientific community despite its importance in guiding advanced material designs.