While the carbon-based metal-free electrocatalysts for oxygen reduction reaction (ORR) have experienced great progress in recent years, the fundamental issue on the origin of ORR activity is yet far from being clarified. To date, the ORR activities of these electrocatalysts are usually attributed to different dopants, while the contribution of intrinsic carbon defects has been little touched. Herein, we report the high ORR activity of the defective carbon nanocages, which is better than that of the B-doped carbon nanotubes and comparable to that of the N-doped carbon nanostructures. Density functional theory (DFT) calculations indicate that pentagon and zigzag edge defects are responsible for the high ORR activity. The mutually corroborated experimental and theoretical results reveal the significant contribution of the intrinsic carbon defects to ORR activity, which is crucial for understanding the ORR origin and exploring the advanced carbon-based metal-free electrocatalysts.
The stabilization of transition metals as isolated centres on suitably tailored carriers with high density is crucial to exploit the technical potential of single-atom heterogeneous catalysts, enabling their maximized productivity in industrial reactors. Wet-chemical methods are best suited for practical applications due to their amenability to scale up. However, achieving single-atom dispersions at metal contents above 2 wt.% remains challenging. We introduce a versatile approach combining impregnation and two-step annealing to synthesize ultra-high-density single-atom catalysts (UHD-SACs) with unprecedented metal contents up to 23 wt.% for 15 metals on chemically-distinct carriers. Translation to an automated protocol demonstrates its robustness and provides a path to explore virtually unlimited libraries of mono or multimetallic catalysts. At the molecular level, characterization of the synthesis mechanism through experiments and simulations shows that controlling the bonding of metal precursors with the carrier via stepwise ligand removal prevents their thermally-induced aggregation into nanoparticles, ensuring atomic dispersion in the resulting UHD-SACs. The catalytic bene ts of UHD-SACs are demonstrated for the electrochemical reduction of CO 2 to CO over NiN 4 motifs on carbon.
High volumetric energy density combined with high power density is highly desired for electrical double-layer capacitors. Usually the volumetric performance is improved by compressing carbon material to increase density but at the much expense of power density due to the deviation of the compressed porous structure from the ideal one. Herein the authors report an efficient approach to increase the density and optimize the porous structure by collapsing the carbon nanocages via capillarity. Three samples with decreasing sizes of meso- and macropores provide us an ideal model system to demonstrate the correlation of volumetric performance with porous structure. The results indicate that reducing the surplus macropores and, more importantly, the surplus mesopores is an efficient strategy to enhance the volumetric energy density while keeping the high power density. The optimized sample achieves a record-high stack volumetric energy density of 73 Wh L in ionic liquid with superb power density and cycling stability.
For mass production of high‐purity hydrogen fuel by electrochemical water splitting, seawater electrolysis is an attractive alternative to the traditional freshwater electrolysis due to the abundance and low cost of seawater in nature. However, the undesirable chlorine ion oxidation reactions occurring simultaneously with seawater electrolysis greatly hinder the overall performance of seawater electrolysis. To tackle this problem, electrocatalysts of high activity and selectivity with purposely modulated coordination and an alkaline environment are urgently required. Herein, it is demonstrated that atomically dispersed Ni with triple nitrogen coordination (Ni‐N3) can achieve efficient hydrogen evolution reaction (HER) performance in alkaline media. The atomically dispersed Ni electrocatalysts exhibit overpotentials as low as 102 and 139 mV at 10 mA cm–2 in alkaline freshwater and seawater electrolytes, respectively, which compare favorably with those previously reported. They also deliver large current densities beyond 200 mA cm–2 at lower overpotentials than Pt/C, as well as show negligible current attenuation over 14 h. The X‐ray absorption fine structure (XAFS) experimental analysis and density functional theory (DFT) calculations verify that the Ni‐N3 coordination, which exhibits a lower coordination number than Ni‐N4, facilitates water dissociation and hydrogen adsorption, and hence enhances the HER activity.
Single‐atom electrocatalysts (SAECs) have recently attracted tremendous research interest due to their often remarkable catalytic responses, unmatched by conventional catalysts. The electrocatalytic performance of SAECs is closely related to the specific metal species and their local atomic environments, including their coordination number, the determined structure of the coordination sites, and the chemical identity of nearest and second nearest neighboring atoms. The wide range of distinct chemical bonding configurations of a single‐metal atom with its surrounding host atoms creates virtually limitless opportunities for the rational design and synthesis of SAECs with tunable local atomic environment for high‐performance electrocatalysis. In this review, the authors first identify fundamental hurdles in electrochemical conversions and highlight the relevance of SAECs. They then critically examine the role of the local atomic structures, encompassing the first and second coordination spheres of the isolated metal atoms, on the design of high‐performance SAECs. The relevance of single‐atom dopants for host activation is also discussed. Insights into the correlation between local structures of SAECs and their catalytic response are analyzed and discussed. Finally, the authors summarize major challenges to be addressed in the field of SAECs and provide some perspectives in the rational construction of superior SAECs for a wide range of electrochemical conversions.
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