Metal-free carbon-based catalysts exhibit diverse electrocatalytic performances in CO₂ reduction reaction (CO₂RR), but the attributions and contributions of active sites are still confusing to date. Herein, the hierarchical carbon nanocages (hCNC) doped with different heteroatoms (B, N, P, S) are prepared to examine the impact of dopants on the competitive CO₂RR and hydrogen evolution reaction (HER). The hCNC and P-doped hCNC show little CO₂RR activity, B- and S-doped hCNC show weak CO₂RR activity, while N-doped hCNC presents high CO₂RR activity. The CO Faradaic efficiency (FE_CO) of N-containing hCNC increases almost linearly with increasing the N content, even with the co-existing B or P. S- and SN-doped hCNC more facilitate the HER. 16 doping configurations are constructed, and up to 53 sites are examined for the electrochemical activities with a constant potential modelling method. The pyridinic-N(N*) is the best active site for CO₂RR to CO, while CBO₂H₂-1(αC*), CBO₂H₂-2(γC*), NO-1(βC*), PO₂H-3(αC*) and SO₃H-3(δC*) are active for HER. The optimized FE_CO achieves 83.6% for N-doped hCNC with 9.54 at.% nitrogen, and S-doped hCNC reaches ca. 30 mA·cm⁻² current density for HER. This study unveils the structure-performance correlation of heteroatom-doped hCNC, which is conducive to the rational design of advanced metal-free carbon-based catalysts.

High-entropy alloys (HEAs) are promising candidates for the electrocatalyst of hydrogen evolution reaction (HER) due to their unique properties such as cocktail electronic effect and lattice distortion effect. Herein, the ultrasmall (sub-2 nm) nanoparticles of PtRuCoNiCu HEA with uniform element distribution are highly dispersed on hierarchical N-doped carbon nanocages (hNCNC) via low-temperature thermal reduction, denoted as us-HEA/hNCNC. The optimal us-HEA/hNCNC exhibits excellent HER performance in 0.5 M H₂SO₄ solution, achieving an ultralow overpotential of 19 mV at 10 mA·cm⁻² (without iR-compensation), high mass activity of 13.1 A·mg_{noble metals}⁻¹ at −0.10 V and superb stability with a slight overpotential increase of 3 mV after 20,000 cycles of cyclic voltammetry scans, much superior to the commercial Pt/C (20 wt.%). The combined experimental and theoretical studies reveal that the Pt&Ru serve as the main active sites for HER and the CoNiCu species modify the electron density of active sites to facilitate the H* adsorption and achieve an optimum M–H binding energy. The hierarchical pore structure and N-doping of hNCNC support also play a crucial role in the enhancement of HER activity and stability. This study demonstrates an effective strategy to greatly improve the HER performance of noble metals by developing the HEAs on the unique hNCNC support.

Semihydrogenation of trace acetylene in an ethylene gas stream is a vital step for the industrial production of polyethylene, in which Pd single-site catalysts (SSCs) have great potential. Herein, two Pd SSCs with different coordination structures are prepared on hierarchical nitrogen-doped carbon nanocages (hNCNC) by regulating the nitrogen species with or without using dicyandiamide. With using dicyandiamide, the obtained Pd₁-N_dicy/hNCNC SSC features the coordinated Pd by two pyridinic N and two pyrrolic N ( ${\text{PdN}}_{\text{2}}^{\text{py}}{\text{N}}_{\text{2}}^{\text{pr}}$). Without using dicyandiamide, the obtained Pd₁/hNCNC SSC features the coordinated Pd by pyridinic N and C ( ${\text{PdN}}_x^{\text{py}}{\text{C}}_{\text{4}-x}$, x = 1–4). The former exhibits an 18-fold increase in catalytic activity compared to the latter. Theoretical results reveal the abundant unoccupied orbital states above the Fermi level of ${\text{PdN}}_{\text{2}}^{\text{py}}{\text{N}}_{\text{2}}^{\text{pr}}$ moiety, which can facilitate the activation of substrate molecules and dynamics of acetylene hydrogenation as supported by the combined theoretical and experimental results. In addition, the ${\text{PdN}}_{\text{2}}^{\text{py}}{\text{N}}_{\text{2}}^{\text{pr}}$ moiety presents a favorable desorption of ethylene. Consequently, the Pd₁-N_dicy/hNCNC SSC exhibits high C₂H₂ conversion (99%) and C₂H₄ selectivity (87%) at 160 °C. This study demonstrates the impact of Pd single-site coordination structure on catalytic performance, which is significant for the rational design of advanced Pd SSCs on carbon-based supports.

The atomically dispersed Fe³⁺ sites of Fe-N-C single-site catalysts (SSCs) are demonstrated as the active sites for CO₂ electroreduction (CO₂RR) to CO but suffer from the reduction to Fe²⁺ at ~ −0.5 V, accompanied by the drop of CO faradaic efficiency (FE_CO) and deterioration of partial current (J_CO). Herein, we report the construction of F-doped Fe-N-C SSCs and the electron-withdrawing character of fluorine could stabilize Fe³⁺ sites, which promotes the FE_CO from the volcano-like highest value (88.2%@−0.40 V) to the high plateau (> 88.5%@−0.40–−0.60 V), with a much-increasedJ_CO (from 3.24 to 11.23 mA·cm⁻²). The enhancement is ascribed to the thermodynamically facilitated CO₂RR and suppressed competing hydrogen evolution reaction, as well as the kinetically increased electroactive surface area and improved charge transfer, due to the stabilized Fe³⁺ sites and enriched defects by fluorine doping. This finding provides an efficient strategy to enhance the CO₂RR performance of Fe-N-C SSCs by stabilizing Fe³⁺.

The investigation of earth-abundant electrocatalysts for efficient water electrolysis is of central importance in renewable energy system, which is currently impeded by the large overpotential of oxygen evolution reaction (OER). NiFe sulfides show promising OER activity but are troubled by their low intrinsic conductivities. Herein, we demonstrate the construction of the porous core-shell heterojunctions of FeNi₃@(Fe, Ni)S₂ with tunable shell thickness via the reduction of hierarchical NiFe(OH)_x nanosheets followed by a partial sulfidization. The conductive FeNi₃ core provides the highway for electron transport, and the (Fe, Ni)S₂ shell offers the exposed surface for in situ generation of S-doped NiFe-oxyhydroxides with high intrinsic OER activity, which is supported by the combined experimental and theoretical studies. In addition, the porous hierarchical morphology favors the electrolyte access and O₂ liberation. Consequently, the optimized catalyst achieves an excellent OER performance with a low overpotential of 288 mV at 100 mA·cm^-2, a small Tafel slope of 48 mV·dec^-1, and a high OER durability for at least 1, 200 h at 200 mA·cm^-2. This study provides an effective way to explore the advanced earth-abundant OER electrocatalysts by constructing the heterojunctions between metal and corresponding metal-compounds via the convenient post treatment, such as nitridation and sulfidization.

Metal-nitrogen-carbon materials are promising catalysts for CO₂ electroreduction to CO. Herein, by taking the unique hierarchical carbon nanocages as the support, an advanced nickel-nitrogen-carbon single-site catalyst is conveniently prepared by pyrolyzing the mixture of NiCl₂ and phenanthroline, which exhibits a Faradaic efficiency plateau of > 87% in a wide potential window of -0.6 - -1.0 V. Further S-doping by adding KSCN into the precursor much enhances the CO specific current density by 68%, up to 37.5 A·g^-1 at -0.8 V, along with an improved CO Faradaic efficiency plateau of > 90%. Such an enhancement can be ascribed to the facilitated CO pathway and suppressed hydrogen evolution from thermodynamic viewpoint as well as the increased electroactive surface area and improved charge transfer fromkinetic viewpoint due to the S-doping. This study demonstrates a simple and effective approach to advanced electrocatalysts by synergetic modification of the porous carbon-based support and electronic structure of the active sites.