Breakthroughs in energy storage and conversion devices depend heavily on the exploration of low-cost and high-performance materials. Carbon-supported electrocatalysts with dimensional varieties have recently attracted significant attention due to their strong structural flexibility and easy accessibility. Nevertheless, understanding the connection between their electronic, structural properties, and catalytic performance must remain a top priority. Synchrotron radiation (SR) X-ray absorption spectroscopy (XAS) techniques, including hard XAS and soft XAS, are recognized as efficient and comprehensive platforms for probing the surface, interface, and bulk electronic structure of elements of interest in the materials community. In the past decade, the flourishing development of materials science and advanced characterization technologies have led to a deeper understanding at different temporal, longitudinal, and spatial scales. In this review, we briefly describe the concept of XAS techniques and summarize their recent progress in addressing scientific questions on carbon-supported electrocatalysts through the development of advanced instruments and experimental methods. We then discuss the remaining challenges and potential research directions in next-generation materials frontiers, and suggest challenges and perspectives for shedding light on the structure–activity relationship.

Transition metal selenides have aroused great attention in recent years due to their high theoretical capacity. However, the huge volume fluctuation generated by conversion reaction during the charge/discharge process results in the significant electrochemical performance reduction. Herein, the carbon-regulated copper(I) selenide (Cu₂Se@C) is designed to significantly promote the interface stability and ion diffusion for selenide electrodes. The systematic X-ray spectroscopies characterizations and density functional theory (DFT) simulations reveal that the Cu–Se–C bonding forming on the surface of Cu₂Se not only improves the electronic conductivity of Cu₂Se@C but also retards the volume change during electrochemical cycling, playing a pivotal role in interface regulation. Consequently, the storage kinetics of Cu₂Se@C is mainly controlled by the capacitance process diverting from the ion diffusion-controlled process of Cu₂Se. When employed this distinctive Cu₂Se@C as anode active material in Li coin cell configuration, the ultrahigh specific capacity of 810.3 mA·h·g⁻¹ at 0.1 A·g⁻¹ and the capacity retention of 83% after 1,500 cycles at 5 A·g⁻¹ is achieved, implying the best Cu-based Li⁺-storage capacity reported so far. This strategy of heterojunction combined with chemical bonding regulation opens up a potential way for the development of advanced electrodes for battery storage systems.

"Intrinsic" strategies for manipulating the local electronic structure and coordination environment of defect-regulated materials can optimize electrochemical storage performance. Nevertheless, the structure–activity relationship between defects and charge storage is ambiguous, which may be revealed by constructing highly ordered vacancy structures. Herein, we demonstrate molybdenum carbide MXene nanosheets with customized in-plane chemical ordered vacancies (Mo_1.33CT_x), by utilizing selective etching strategies. Synchrotron-based X-ray characterizations reveal that Mo atoms in Mo_1.33CT_x show increased average valence of +4.44 compared with the control Mo₂CT_x. Benefited from the introduced atomic active sites and high valence of Mo, Mo_1.33CT_x achieves an outstanding capacity of 603 mAh·g⁻¹ at 0.2 A·g⁻¹, superior to most original MXenes. Li⁺ storage kinetics analysis and density functional theory (DFT) simulations show that this optimized performance ensues from the more charge compensation during charge–discharge process, which enhances Faraday reaction compared with pure Mo₂CT_x. This vacancy manipulation provides an efficient way to realize MXene's potential as promising electrodes.

The electrocatalysis of oxygen evolution reaction (OER) plays a key role in clean energy storage and transfer. Nonetheless, the sluggish kinetics and poor durability under acidic and neutral conditions severely hinder practical applications such as electrolyzer compatible with the powerful proton exchange membrane and biohybrid fuel production. Here, we report a boron-doped ruthenium dioxide electrocatalyst (B-RuO₂) fabricated by a facile boric acid assisted strategy which demonstrates excellent acidic and neutral OER performances. Density functional theory calculations and advanced characterizations reveal that the boron species form an anomalous B–O covalent bonding with the oxygen atoms of RuO₂ and expose the fully coordinately bridge ruthenium site (Ru-bri site), which seems like a switch that turns on the inactive Ru-bri site into OER-active, resulting in more exposed active sites, modified electronic structure, and optimized binding energy of intermediates. Thus, the B-RuO₂ exhibits an ultralow overpotential of 200 mV at 10 mA/cm² and maintains excellent stability compared to commercial RuO₂ in 0.5 M sulfuric acid. Moreover, the superior performance is as well displayed in neutral electrolyte, surpassing most previously reported catalysts.

The phase transformation of catalysts has been extensively observed in heterogeneous catalytic reactions that hinder the long cycling catalysis, and it remains a big challenge to precisely control the active phase during the complex conditions in electrochemical catalysis. Here, we theoretically predict that carbon-based support could achieve the phase engineering regulation of catalysts by suppressing specific phase transformation. Taken single-walled carbon nanotube (SWCNT) as typical support, combined with calculated E-pH (Pourbaix) diagram and advanced synchrotron-based characterizations technologies prove there are two different active phases source from cobalt selenide which demonstrate that the feasibility of using support effect regulating the potential advantageous catalysts. Moreover, it is worth noting that the phase engineering derived Co₃O₄-SWCNT exhibits a low overpotential of 201 mV for delivering the current density of 10 mA/cm² in electrocatalytic oxygen evolution reaction (OER). Also, it reaches a record current density of 529 mA/cm² at 1.63 V (vs. RHE) in the electrocatalytic urea oxidation reaction (UOR), overwhelming most previously reported catalysts.

Despite acknowledgment of structural reconstruction of materials following oxygen evolution reaction (OER) reaction, the role of support during the reconstruction process has been ignored. Given this, we directly in situ transform the residual iron present in raw single-walled carbon nanotubes (SWCNT) into Fe₂O₃ and thus build Fe₂O₃-CNT as the model system. Intriguingly, an anomalous self-optimization occurred on SWCNT and the derived components show satisfactory electrochemical performance. Soft X-ray absorption spectroscopy (sXAS) analysis and theory calculation correspondingly indicate that self-optimization yields stronger interaction between SWCNT and Fe₂O₃ nanoparticles, where the electrons migrate from Fe₂O₃ to optimized SWCNT. Such polarization will generate a positive charge center and thus boost the OER activity. This finding directly observes the self-optimization of support effect, providing a new perspective for OER and related electrochemical reactions.

The intercalation of metal is a promising method for the modulating electronic properties in transition metal dichalcogenides (TMDs). However, there still lacks enough knowledge about how the intercalated atoms directly impact the two-dimensional structural layers and modulate the band structures therein. Taking advantage of X-ray absorption fine structure and angle-resolved photoemission spectroscopy, we studied how Cu intercalation influences the host TaSe₂ layers in Cu_0.03TaSe₂ crystals. The intercalated Cu atoms form bonds with Se of the host layers, and there is charge transfer from Cu to Se. By examining the changes of band dispersions, we show that the variation of electronic structures is beyond a simple rigid band model with merely charge doping effect. This work reveals that the unusual change of band dispersions is associated with the formation of bonds between the intercalated metal elements and anion ions in the host layers, and provides a reference for the comprehensive understanding of the electronic structures in intercalated materials.

In most cases, layered transition metal dichalcogenides (LTMDs), containing metallic phases, show electrochemical behavior different from their semiconductor counterparts. Typically, two-dimensional layered metallic 1T-MoS₂ demonstrates better electrocatalytic performance for water splitting compared to its 2H counterpart. However, the characteristics of low metallic phase concentration and poor stability limit its applications in some cases. Herein, we demonstrate a simple and efficient bottom-up wet-chemistry strategy for the large-scale synthesis of nanoscopic ultrathin Mo_1-xW_xS₂ nanosheets with enlarged interlayer spacing and high metallic phase concentration. Our characterizations, including X-ray absorption fine structure spectroscopy (XAFS), high-angle annular dark-fieldscanning transmission electron microscopy (HAADF-STEM), and X-ray photoelectron spectroscopy (XPS) revealed that the metallic ultrathin ternary Mo_1-xW_xS₂ nanosheets exhibited distorted metal-metal bonds and a tunable metallic phase concentration. As a proof of concept, this optimized catalyst, with the highest metallic phase concentration (greater than 90%), achieved a low overpotential of about-155 mV at a current density of -10 mA/cm², a small Tafel slope of 67 mV/dec, and an increased turnover frequency (TOF) of 1.3 H₂ per second at an overpotential of -300 mV (vs. reversible hydrogen electrode (RHE)), highlighting the importance of the metallic phase. More importantly, this study can lead to a facile solvothermal route to prepare stable and high-metallicphase-concentration transition-metal-based two-dimensional materials for future applications.

Rational design and facet-engineering of nanocrystal is an effective strategy to optimize the catalytic performance of abundant and economic semiconductor-based photocatalysts. In this study, we demonstrate a novel ternary Cu₂MoS₄ nanotube with the {010} facet exposed, synthesized via a hydrothermal method. Compared with two-dimensional Cu₂MoS₄ nanosheet with the {001} facet exposed, this one-dimensional nanotube exhibits highly enhanced performance of photodegradation and water splitting. Both theoretical calculations and experimental results suggest that the conduction band minimum (CBM) of the {010} facet crystal shows lower potential than that of the {001} facet. In particular, the up-shifted CBM in Cu₂MoS₄ nanotube is significantly beneficial for the absorption of dye molecules and reduction of H⁺ to H₂. These results may open a new route for realizing high-efficiency photocatalysts based on Cu₂MX₄ by facet engineering.