The electrocatalytic conversion of CO₂ to produce fuels and chemicals holds great promise, not only to provide an alternative to fossil feedstocks, but also to use renewable electricity to convert and recycle the greenhouse gas CO₂ to mitigate climate problems. However, the selectivity and reaction rates for the conversion of CO₂ into desirable carbon-based products, especially multicarbon products with high added value, are still insufficient for commercial applications, which is attributed to insufficiently favourable microenvironmental conditions in the vicinity of the catalyst. The construction of catalysts/electrodes with confined structures can effectively improve the reaction microenvironment in the vicinity of the electrodes and thus effectively direct the reaction towards the desired pathway. In this review, we firstly introduce the effects of the microenvironment at the electrode–electrolyte interface including local pH, local intermediate concentration, and local cation concentration on CO₂ reduction reaction (CO₂RR) as well as the mechanism of action, and then shed light on the microenvironmental modulation within the confined space, and finally and most importantly, introduce the design strategy of CO₂RR catalyst/electrode based on the confinement effect.

In the quest to enhance the efficiency of sodium-ion batteries, the dynamics of solid electrolyte interphase (SEI) formation are of paramount importance. The SEI layer’s integrity is integral to the charge–discharge efficiency and the overall longevity of the battery. Herein, a novel two-dimensional Ti₃C₂ fragments enmeshed on iron-nitrogen-carbon (Fe-N-C) nanosheets (Ti₃C₂/Fe-N-C) has been synthesized. This electrode features a matrix which has been shown to expedite SEI layer formation through the facilitation of selective anion adsorption, thus augmenting battery performance. Density functional theory calculation reveals that the SEI evolution energy of NaPF₆ at the Ti₃C₂/Fe-N-C interface is 0.81 eV, significantly lower than the Ti₃C₂ (1.23 eV). This process is driven by the electron transportation from Ti₃C₂ to Fe-N-C substrate, facilitated by their work-function difference, leading to the formation of ferromagnetic Fe species, which possesses Fe 3d d_xzd_yz ${\mathrm{d}}_{z^2}$ orbitals and undergoes hybridization with the π and σ orbitals of NaF, creating a key intermediate during charging. This process diminishes the antibonding energy and attenuates the orbital interaction with NaF, thus reducing the activation energy and improving the SEI formation reaction kinetics. Consequently, it leads to the creation of multi-interface SEI characterized by high-throughput ion transport and an efficient reaction network.

Rechargeable sodium-ion batteries (SIBs) are considered as the next-generation secondary batteries. The performance of SIB is determined by the behavior of its electrode surface and the electrode–electrolyte interface during charging and discharging. Thus, the characteristics of these surfaces and interfaces should be analyzed to realize large-scale energy storage systems with high energy density and long-cycle stability. Although various studies have investigated the properties of electrode materials, few studies have focused on the construction of stable and efficient SIB interfaces, and even fewer have explored the mechanisms of interfacial effects; however, the strategies of regulating interfacial effects are yet to be completely developed. Moreover, the results obtained thus far are insufficient to draw systematic conclusions. The present study reviews the literature on the mechanism of interfacial effects in Na⁺ storage devices. The interfaces in a sodium-ion storage device include a heterogeneous interface between electrode materials, a solid electrolyte interphase, and a cathode electrolyte interphase. The interfacial effects during the intercalation, transformation, and alloy reactions and the resulting overall battery performance were theoretically analyzed. In this review, we aim to provide a theoretical basis for optimizing the structures of electrode surface and electrode–electrolyte interface to optimize the performance of SIBs. In addition, the challenges of investigating interfacial effects and several possible helpful methods and opportunities for studying the mechanisms of interfacial effects in SIBs will be presented.

The processes of photocatalytic CO₂ reduction (pCO₂R) and electrochemical CO₂ reduction (ECO₂R) have attracted considerable interest owing to their high potential to address many environmental and energy-related issues. In this aspect, a single Cu atom decorated on a carbon nitride (CN) surface (Cu–CN) has gained increasing popularity because of its unique advantages, such as excellent atom utilization and ultrahigh catalytic activity. CN—particularly graphitic CN (g-C₃N₄)—is a photo- and electrocatalyst and used as an important support material for single Cu atom-based catalysts. These key functions of Cu–CN-based catalysts can improve the catalytic performance and stability in the pCO₂R and ECO₂R during the application process. In this review, we focus on Cu as a single metal atom decorated on CN for efficient photoelectrochemical CO₂ reduction (pECO₂R), where ECO₂R increases the electrocatalytic active area and promotes electron transfer, while pCO₂R enhances the surface redox reaction by efficiently using photogenerated charges and offering integral activity as well as an active interface between Cu and CN. Interactions of single Cu atom-based photo-, electro-, and photoelectrochemical catalysts with g-C₃N₄ are discussed. Moreover, for a deeper understanding of the history of the development of pCO₂R and ECO₂R, the basics of CO₂ reduction, including pCO₂R and ECO₂R over g-C₃N₄, as well as the structural composition, characterization, unique design, and mechanism of a single atom site are reviewed in detail. Finally, some future prospects and key challenges are discussed.

In order to reduce the considerable usage of expensive but scarce platinum at the cathode in proton exchange membrane fuel cells (PEMFCs), it is necessary to pursue alternatives to platinum. The most promising platinum group metal (PGM)-free catalysts for oxygen reduction reaction (ORR) are atomically dispersed, and nitrogen-coordinated metal site catalysts denoted as M-N-C (M = Fe, Co, or Mn, etc.). Over the last few decades, there have been great advances in these catalysts with high ORR activity and promising initial fuel cell performance approaching traditional Pt/C catalysts. However, the stability of these highly active Fe-N-C catalysts under practical fuel cell conditions is still far from satisfactory. This review highlights recent advances in synthesizing efficient PGM-free catalysts for the ORR in PEMFCs, emphasizing our efforts on confinement strategies and spin state regulation methods. We also summarize several effective methods of improving mass and intrinsic activities. Furthermore, significant research efforts toward understanding the degradation mechanisms are made and the results are summarized, such as metal leaching, carbon corrosion, protonation, and micropore flooding. We also document several mitigation strategies to improve the lifetime of PGM-free catalysts, including controlling S1/S2 in Fe-N-C catalysts, using non-iron-based catalysts, enhancing metal-nitrogen bonds, improving the corrosion resistance of carbon carriers, and using buffered protonated liquids. Finally, the remaining challenges and possible solutions to the current atomic dispersion M-N-C catalyst are proposed in detail.

Developing stable but high active metal-nitrogen-carbon (M-N-C)-based hard carbon anode is a promising way to be the alternatives to graphene and blank hard carbon for sodium-ion batteries (SIBs), requiring the precise tailoring of the electronic structure for optimizing the Na⁺ intercalation behavior, yet is greatly challenging. Herein, Fe-N-C graphitic layer-encapsulating Fe₃C species within hard carbon nanosheets (Fe-N-C/Fe₃C@HCNs) are rationally engineered by pyrolysis of self-assembled polymer. Impressively, the Fe-N-C/Fe₃C@HCNs exhibit outstanding rate capacity (242 mAh·g⁻¹ at 2,000 mA·g⁻¹), which is 2.1 and 4.2 times higher than that of Fe-N-C and N-doped carbon (N-C), respectively, and prolonged cycling stability (176 mAh·g⁻¹ at 2,000 mA·g⁻¹ after 2,000 cycles). Theoretical calculations unveil that the Fe₃C species enhance the electronic transfer from Na to Fe-N-C, resulting in the charge redistribution between the interfaces of Fe₃C and Fe-N-C. Thus, the optimized adsorption behavior towards Na⁺ reduces the thermodynamic energy barriers. The synergistic effect of Fe₃C and Fe-N-C species maintains the structural integrity of electrode materials during the sodiation/desodiation process. The in-depth insight into the advanced Na⁺ storage mechanisms of Fe₃C@Fe-N-C offers precise guidance for the rational establishment of confinement heterostructures in SIBs.