Yanmei Shi

The interfacial pH distribution throughout the electrode has a considerable influence on the electrocatalytic performance. However, a local pH mapping method that can be applied to various electrocatalytic electrodes over a full pH range is still lacking. Herein, we demonstrate in situ local pH visualization with phosphates as probes via Raman spectroscopy with wide universality. An automated procedure is developed to convert the Raman mapping spectra of phosphates to a pH distribution. The full pH range is divided into three segments: acidic, neutral and alkaline. A generalized logistic function is used and fitted in each segment. Nitrate reduction on CuO supported by Cu mesh in neutral is taken as the research model, and a cross on the mesh is chosen as the imaging area. The results show a rapid pH increase on the cross while a gentle increase in the adjacent electrolyte, as well as a nonuniform pH distribution on the electrode cross ranging from 5.6 to 10.0 at the same potential. A direct comparison between the distribution of the pH and the concentration of NO₃^- as the reactant is also demonstrated. Our work provides a method to obtain the in situ pH distribution for various electrocatalytic reactions in a wide pH, and is helpful for understanding the local microenvironment near an electrode.

Single-atom Fe-N-C is a promising candidate for the oxygen reduction reaction (ORR) at the cathode of proton-exchange membrane fuel cells (PEMFCs). However, under strongly acidic conditions, Fe-N-C suffers from severe oxidation from Fenton reactions caused by trace amounts of dissolved Fe and a 2-electron (2e) ORR product of H₂O₂. Herein, we demonstrate a facile and general strategy to nearly eliminate the 2e path of the ORR by introducing phosphates. We discover that bubbling O₂ into water introduces an inherent problem in breaking the hydrogen-bond network and thus hindering proton transfer, resulting in a decreased 4e ORR selectivity. Introducing phosphates is found to recover the hydrogen-bond network to eliminate the 2e path. This strategy works well for Fe-N-C, commercial Pt/C, and even carbon catalysts with a dominant 2e selectivity, resulting in negligible H₂O₂ production and better stability in both the rotating ring-disk electrode system and flow cell. Our work provides deep insight into the ORR mechanism and a useful strategy to lower the cost and lengthen the lifetime of PEMFCs by using nonnoble metal electrocatalysts as cathodes.

In-situ and operando techniques in heterogeneous electrocatalysis are a powerful tool used to elucidate reaction mechanisms. Ultimately, they are key in determining concrete links between a catalyst’s physical/electronic structure and its activity en route to designing next-generation systems. To this end, the exact execution and interpretation of these lines of experiments is critical as this determines the strength of conclusions that can be drawn and what uncertainties remain. Instead of focusing on how techniques were used to understand systems, as is the case with most reviews on the topic, this work instead initiates a nuanced discussion of 1) how to best carry out each technique and 2) initiate a nuanced analysis of which level of insights can be drawn from the set of in-situ or operando experiments/controls carried out. We focus on several commonly used techniques, including vibrational (IR, Raman) spectroscopy, X-ray absorption spectroscopy and electrochemical mass spectrometry. In addition to this, we include sections of reactor design and the link with theoretical modelling that are applicable across all techniques. While we focus on heterogeneous electrocatalysis, we make links when appropriate to the areas of photo- and thermo-catalytic systems. We highlight common pitfalls in the field, how to avoid them, and what sets of complementary experiments may be used to strengthen the analysis. We end with an overview of what gaps remain in in-situ and operando techniques and what innovations must be made to overcome them.

This review summarizes the recent focus on the dynamic evolution of atomic metal–nitrogen-codoped catalysts in electrocatalytic reactions.

M−N−C‐type single‐atom catalysts (SACs) are highly efficient for the electrocatalytic oxygen evolution reaction (OER). And the isolated metal atoms are usually considered real active sites. However, the oxidative structural evolution of coordinated N during the OER will probably damage the structure of M−N−C, hence resulting in a completely different reaction mechanism. Here, we reveal the aggregation of M−N−C materials during the alkaline OER. Taking Ni−N−C as an example, multiple characterizations show that the coordinated N on the surface of Ni−N−C is almost completely dissolved in the form of NO₃⁻, accompanied by the generation of abundant O functional groups on the surface of the carbon support. Accordingly, the Ni−N bonds are broken. Through a dissolution‐redeposition mechanism and further oxidation, the isolated Ni atoms are finally converted to NiOOH nanoclusters supported by carbon as the real active sites for the enhanced OER. Fe−N−C and Co−N−C also have similar aggregation mechanism. Our findings provide unique insight into the structural evolution and activity origin of M−N−C‐based catalysts under electrooxidative conditions.

M−N−C‐type single‐atom catalysts (SACs) are highly efficient for the electrocatalytic oxygen evolution reaction (OER). And the isolated metal atoms are usually considered real active sites. However, the oxidative structural evolution of coordinated N during the OER will probably damage the structure of M−N−C, hence resulting in a completely different reaction mechanism. Here, we reveal the aggregation of M−N−C materials during the alkaline OER. Taking Ni−N−C as an example, multiple characterizations show that the coordinated N on the surface of Ni−N−C is almost completely dissolved in the form of NO₃⁻, accompanied by the generation of abundant O functional groups on the surface of the carbon support. Accordingly, the Ni−N bonds are broken. Through a dissolution‐redeposition mechanism and further oxidation, the isolated Ni atoms are finally converted to NiOOH nanoclusters supported by carbon as the real active sites for the enhanced OER. Fe−N−C and Co−N−C also have similar aggregation mechanism. Our findings provide unique insight into the structural evolution and activity origin of M−N−C‐based catalysts under electrooxidative conditions.

N-doped carbon materials, with their applications as electrocatalysts for the oxygen reduction reaction (ORR), have been extensively studied. However, a negletcted fact is that the operating potential of the ORR is higher than the theoretical oxidation potential of carbon, possibly leading to the oxidation of carbon materials. Consequently, the influence of the structural oxidation evolution on ORR performance and the real active sites are not clear. In this study, we discover a two-step oxidation process of N-doped carbon during the ORR. The first oxidation process is caused by the applied potential and bubbling oxygen during the ORR, leading to the oxidative dissolution of N and the formation of abundant oxygen-containing functional groups. This oxidation process also converts the reaction path from the four-electron (4e) ORR to the two-electron (2e) ORR. Subsequently, the enhanced 2e ORR generates oxidative H₂O₂, which initiates the second stage of oxidation to some newly formed oxygen-containing functional groups, such as quinones to dicarboxyls, further diversifying the oxygen-containing functional groups and making carboxyl groups as the dominant species. We also reveal the synergistic effect of multiple oxygen-containing functional groups by providing additional opportunities to access active sites with optimized adsorption of OOH*, thus leading to high efficiency and durability in electrocatalytic H₂O₂ production.

M‐N‐C single‐atom catalysts (SACs) are promising electrode materials for many electro‐reduction reactions. However, their stability is far from practical applications, and their deactivation mechanism has been rarely investigated. Herein, we demonstrate the structural degradation of M‐N‐C (M=Co, Ni, and Fe) at industrial‐grade current density for long‐term electro‐reduction. Both M‐N and N‐C bonds are broken, resulting in the gradual hydrogenation and dissolution of N in the form of ammonia. The residual M is finally converted to M‐containing core–shell nanoparticles after sequential dissolution, redeposition, and electro‐reduction. The destruction of the M‐N‐C structure and the formation of nanoparticles greatly affect the electrocatalytic performance. Our work highlights the structural degradation and deactivation mechanism of M‐N‐C‐type SACs under strong reductive conditions and provides useful information for inspiring researchers to develop new strategies to improve the electrocatalytic stability of similar types of materials.

M‐N‐C single‐atom catalysts (SACs) are promising electrode materials for many electro‐reduction reactions. However, their stability is far from practical applications, and their deactivation mechanism has been rarely investigated. Herein, we demonstrate the structural degradation of M‐N‐C (M=Co, Ni, and Fe) at industrial‐grade current density for long‐term electro‐reduction. Both M‐N and N‐C bonds are broken, resulting in the gradual hydrogenation and dissolution of N in the form of ammonia. The residual M is finally converted to M‐containing core–shell nanoparticles after sequential dissolution, redeposition, and electro‐reduction. The destruction of the M‐N‐C structure and the formation of nanoparticles greatly affect the electrocatalytic performance. Our work highlights the structural degradation and deactivation mechanism of M‐N‐C‐type SACs under strong reductive conditions and provides useful information for inspiring researchers to develop new strategies to improve the electrocatalytic stability of similar types of materials.

The difference between solution pH and local pH near an electrode surface greatly determines the electrocatalytic performance. However, there is still a lack of a facile and universal method for the local pH detection of various electrode reactions, leaving the origin of local pH changes unclear. Herein, by using phosphate species in phosphate buffer solution (PBS) as the pH probe, we demonstrate a universal local pH detection strategy through in situ Raman spectroscopy for various electrode reactions. Oxygen evolution is chosen as the example to detect the potential-dependent local pH change. Then the strategy extends to nitrate reduction, nitrobenzene reduction, and benzylamine oxidation. By comparing the local pH changes in different reactions, we reveal that the local pH change is strongly dependent on the reaction current, the ability of the system to replenish the local H⁺/OH^-, and the number of H⁺/OH^- per electron transfer of the electrode reaction.