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.

Charge density wave (CDW) is a phenomenon that occurs in materials, accompanied by changes in their intrinsic electronic properties. The study of CDW and its modulation in materials holds tremendous significance in materials research, as it provides a unique approach to controlling the electronic properties of materials. TiSe₂ is a typical layered material with a CDW phase at low temperatures. Through V substitution for Ti in TiSe₂, we tuned the carrier concentration in V_xTi_1−xSe₂ to study how its electronic structures evolve. Angle-resolved photoemission spectroscopy (ARPES) shows that the band-folding effect is sustained with the doping level up to 10%, indicating the persistence of the CDW phase, even though the band structure is strikingly different from that of the parent compound TiSe₂. Though CDW can induce the band fold effect with a driving force from the perspective of electronic systems, our studies suggest that this behavior could be maintained by lattice distortion of the CDW phase, even if band structures deviate from the electron-driven CDW scenario. Our work provides a constraint for understanding the CDW mechanism in TiSe₂, and highlights the role of lattice distortion in the band-folding effect.

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.

The rational design of materials at atomic scale as efficient and stable electrocatalysts for hydrogen evolution reaction (HER) is critical for energy conversion. Herein, we report a novel hybrid nanostructure with iridium (Ir) and cobalt (Co) atomic pair configuration anchored in porous nitrogen-doped carbon (pNC) nanosheets (denoted as IrCo-pNC) for electrocatalytic HER. Experimental investigations and theoretical calculations reveal that the interaction between Ir and Co species in pNC promotes electron accumulation and depletion around isolated Ir and Co atoms, respectively, resulting in a local asymmetry electron density distribution. Density functional theory calculations also suggest that the electrons transfer from Co to adjacent Ir atom causing the down shift of the d-band center of Ir 5d in IrCo-pNC catalyst, thus optimizing the adsorption of hydrogen on Ir sites. The as-prepared IrCo-pNC exhibits significant HER performance with an overpotential of 21 mV to achieve a current density of 10 mA·cm⁻² in 0.5 M H₂SO₄. This work provides insight into the role of asymmetry electron density distribution in nanomaterials in regulating HER electrocatalysis.

To achieve a complete industrial chain of hydrogen energy, the development of efficient electrocatalysts for hydrogen evolution reaction (HER) is of great concerns. Herein, a nickel nitride supported platinum (Pt) catalyst with highly exposed Pt(110) facets (Pt₍₁₁₀₎-Ni₃N) is obtained for catalyzing HER. Combined X-ray spectra and density functional theory studies demonstrate that the interfacial electronic interaction between Pt and Ni₃N support can promote the hydrogen evolution on Pt(110) facets by weakening hydrogen adsorption. As a result, the Pt₍₁₁₀₎-Ni₃N catalyst delivers an obviously higher specific activity than commercial 20 wt.% Pt/C in acidic media. This work suggests that the suitable interface modulation may play a vital role in rationally designing advanced electrocatalysts.

Taking advantage of the unique layered structure of TiSe₂, the intrinsic electronic properties of two-dimensional materials can easily be tuned via heteroatomic engineering. Herein, we show that the charge density wave (CDW) phase in 1T-TiSe₂ single-crystals can be gradually suppressed through Sn atoms intercalation. Using angle-resolved photoemission spectroscopy (ARPES) and temperature-dependent resistivity measurements, this work reveals that Sn atoms can induce charge doping and modulate the intrinsic electronic properties in the host 1T-TiSe_2. Notably, our temperature-dependent ARPES results highlight the role exciton-phonon interaction and the Jahn-Teller mechanism through the formation of backfolded bands and exhibition of a downward Se shift of 4p valence band in the formation of CDW in this material.

Nontrivial topological behaviors in superconducting materials provide resourceful ground for the emergence and study of unconventional quantum states. Charge doping by the controlled intercalation of donor atoms is an efficient route for enhancing/inducement of superconducting and topological behaviors in layered topological insulators and semimetals. Herein, we enhanced the superconducting temperature of TaSe₂ by 20-folds (~ 3 k) through Sn atoms intercalation. Using first-principles calculations, we demonstrated the existence of nontrivial topological features. Sn_0.5TaSe₂ displays topological nodal lines around the K high symmetry point in the Brillouin zone, with drumhead-like shaped surface states protected by inversion symmetry. Altogether, the coexistence of these properties makes Sn_0.5TaSe₂ a potential candidate for topological superconductivity.

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.

Atomic intercalation in two-dimensional (2D) layered materials can be used to engineer the electronic structure at the atomic scale and generate tuneable physical and chemical properties which are quite distinct in comparison with the pristine material. Among them, electron-doped engineering induced by intercalation is an efficient route to modulate electronic states in 2D layers. Herein, we demonstrate a semiconducting to metallic phase transition in zirconium diselenide (ZrSe₂) single crystals via controllable incorporation of copper (Cu) atoms. Our angle resolved photoemission spectroscopy (ARPES) measurements and first-principles density functional theory (DFT) calculations clearly revealed the emergence of conduction band dispersion at the M/L point of the Brillouin zone due to Cu-induced electron doping in ZrSe₂ interlayers. Moreover, electrical measurements in ZrSe₂ revealed semiconducting behavior, while the Cu-intercalated ZrSe₂ exhibited a linear current–voltage curve with metallic character. The atomic intercalation approach may have high potential for realizing transparent electron-doping systems for many specific 2D-based nanoelectronic applications.

Atomically dispersed catalysts have attracted attention in energy conversion applications because their efficiency and chemoselectivity for special catalysis are superior to those of traditional catalysts. However, they have limitations owing to the extremely low metal-loading content on supports, difficulty in the precise control of the metal location and amount as well as low stability at high temperatures. We prepared a highly doped single metal atom hybrid via a single-step thermal pyrolysis of glucose, dicyandiamide, and inorganic metal salts. High-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and X-ray absorption fine structure spectroscopy (XAFS) revealed that nitrogen atoms doped into the graphene matrix were pivotal for metal atom stabilization by generating a metal-Nx coordination structure. Due to the strong anchoring effect of the graphene matrix, the metal loading content was over 4 wt.% in the isolated atomic hybrid (the Pt content was as high as 9.26 wt.% in the Pt-doped hybrid). Furthermore, the single iron-doped hybrid (Fe@N-doped graphene) showed a remarkable electrocatalytic performance for the oxygen reduction reaction. The peak power density was ~199 mW·cm^-2 at a current density of 310 mA·cm^-2 and superior to that of a commercial Pt/C catalyst when it was used as a cathode catalyst in assembled zinc-air batteries. This work offered a feasible approach to design and fabricate highly doped single metal atoms (SMAs) catalysts for potential energy applications.