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The catalytic conversion of CO2 into high value‐added chemical products driven by electricity was an effective strategy to reduce atmospheric CO2 concentration. The development of high efficiency catalysts had always been the core for electrocatalytic reduction of CO2. The single transition metal sites anchored on nitrogen‐doped carbon showed great potential in CO2 reduction reaction (CO2RR). In this paper, the atomically dispersed Ni−N sites are constructed on the two‐dimensional mesoporous carbon nanosheets by the secondary pyrolysis method. The rich mesoporous structure of the two‐dimensional nanosheets is beneficial to CO2 diffusion and adsorption during CO2RR, and the high nitrogen contents could increase the Ni single atom loadings. The nitrogen content and type could be controlled by regulating the ratio of nitrogen source to carbon source, and the catalyst activity is improved as the proportion of nitrogen sources increases. When the ratio of nitrogen source to carbon source is four, the oxidation state of Ni single atoms Ni@NMCN4 is between 0 and +2, and Ni single atoms exists as Ni−N4 structure with pyridinic N of 32.78 %, which exhibits the best electrochemical CO2RR activity. The active site of Ni@NMCN4 is confirmed as the highly stable Ni single atoms. Moreover, the pyridinic N species are proved to play an important role in enhancing the CO2 electroreduction performance. This work provides a new perspective to adjust the nitrogen content and type in transition metal monatomic‐N‐doped carbon electrocatalysts for CO2RR. [ABSTRACT FROM AUTHOR]

[Display omitted] • Synergy between Ni 12 P 5 nanocrystals and NSG reinforces the HER/IRR activity. • Ni 12 P 5 /NSG yields a low overpotential of 133 mV in alkaline HER. • Ni 12 P 5 /NSG gives an impressive photovoltaic performance. • The catalytic mechanism for HER/IRR was revealed by DFT calculation. Designing high-efficient bifunction catalysts with excellent catalytic activity and enhanced charge-transfer capability in both alkaline hydrogen evolution reaction (HER) and triiodide reduction reaction (IRR) is of utmost significance to advance the development of green hydrogen production and photovoltaics, respectively, yet remains a crucial challenge. Here, highly dispersed and small-sized Ni 12 P 5 nanocrystals with narrow size distribution was well attached on the surface of N, S co-doped graphene (Ni 12 P 5 /NSG) by the facile hot-injection method. As expected, the optimized Ni 12 P 5 /NSG requires a relatively low overpotential of 132.94 ± 0.08 mV to deliver a current density of 10 mA cm−2 in alkaline condition, accompanied with remarkable long-time durability with negligible attenuation over 50 h. Density functional theory (DFT) calculations revealed that the positively synergic effect between Ni 12 P 5 and NSG are in favor of modulating the rate determining step of the dissociation of H 2 O to *(OH-H), thereby upgrading its HER activity. When used as the counter electrode catalyst for IRR in DSSCs, the resultant Ni 12 P 5 /NSG exhibits extraordinary Pt-like catalytic activity and well electrochemical stability in iodide-based electrolyte, delivering a high photoelectric conversion performance (PCE) comparable to Pt. The improved PCE can be attributed to the accelerated interfacial charge-transfer capability around active sites for facilitating the reaction kinetics of IRR, as demonstrated by DFT calculations. This work provides an effective strategy for synthesizing cost-effective composites with multi-active sites and offering valuable insight into the structure-performance relationship, which is conducive to guide the synthesis of promising catalysts in the energy conversion field. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fe(III)/Ni@NC catalyst was synthesized through competitive complexation between Ni2+ and functionalized auxiliary agents (Fe(acca) 3) with chitosan and achieved uniform dispersion of Ni atoms. • The formation of CO 2 •− intermediates is a kinetic rate limiting step of CO 2 RR and Ni single atom sites provide reaction kinetics and faster charge transfer. • FTIR confirms that the excellent product CO selectivity in CO 2 RR is due to the accumulation of *COOH intermediates at the active sites. • Fe(III)/Ni@NC shows excellent performance in the flow cell with FE CO over 96 % from −40 to −140 mA cm−2. Converting CO 2 into useful chemicals is an effective measure to address the energy crisis and the greenhouse effect. CO 2 RR has become a research hotspot due to its mild conditions. Ni-N-C catalysts are known as promising electrocatalysts, while achieving atomic level dispersion of Ni on the surface of the carrier is still facing challenges. In this work, various N-doped carbon supported Ni-based catalysts were synthesized by selecting Fe2+/Fe3+ as functionalized auxiliary agents in precursors. Different coordination situations of Fe(III)/Ni@NC and Fe(II)/Ni@NC are formed which derived from the valence state of Fe. During the synthesis process of Fe(III)/Ni@NC, Ni2+ replaced Fe3+ through competitive complexation, thereby achieving atomic level dispersion of Ni atoms on the catalyst surface. XAS confirms that Fe(III)/Ni@NC is a Ni-single-atom catalyst with Ni-N 4 coordination. In contrast, Fe(II)/Ni@NC contains Ni 3 C/Ni nanoparticles coated with carbon layer, which shows a characteristic peak approaching to 2.12 Å related to the metallic Ni-Ni bonds. In the H-type cell, the FE CO of Fe(III)/Ni@NC achieves 97.8 % at −0.83 V and maintains over 95 % in a wide potential window. The in-situ Fourier transform infrared (FTIR) spectroscopy confirms that the excellent product CO selectivity in CO 2 RR is due to the accumulation of *COOH intermediates at the active sites. Significantly, Fe(III)/Ni@NC shows excellent performance in the flow cell with FE CO over 96 % from −40 to −140 mA cm−2. This work reveals the impact of Fe valence state of functionalized auxiliary agents on the electrocatalytic performance of materials, which provides a new approach for designing high-performance atomically dispersed electrocatalysts. [ABSTRACT FROM AUTHOR]

[Display omitted] • The coordination structure of Ni single atoms can be easily controlled by the regulation of pyrolysis temperature. • The electrocatalytic CO 2 RR performance was promoted by low-coordination nickel-nitrogen sites. • Ni SA @N 3 -C displayed a maximum FE CO of 96.0% with a j CO of −18.87 mA cm−2 at −0.83 V vs. RHE. Single-atom catalysis has been considered as a powerful approach for CO 2 reduction reaction (CO 2 RR) to achieve efficient resource conversion and carbon neutrality. The electrocatalytic activity of single-atom catalysts (SACs) is closely related to the local coordination environment. Herein, Ni SACs with well-defined low-coordination nickel-nitrogen sites (denoted as Ni SA @N 3 -C) have been successfully developed via a facile sacrificial template method. XAS results reveal that the coordination environment of the atomically dispersed Ni active sites can be controlled by the pyrolysis temperature. Significantly, Ni SA @N 3 -C displays remarkably excellent activity toward electrocatalytic CO 2 RR with CO Faradaic efficiency (FE CO) of 96.0% at −0.83 V vs. RHE and remains high FE CO exceeding 90% over a broad potential range from −0.63 to −0.93 V vs. RHE, outperforming those of Ni SA @N 4 -C and Ni NP @NC. More importantly, Ni SA @N 3 -C exhibits an excellent CO selectivity of 99.2% with a considerable current density of −160 mA cm−2 in the flow cell reactor. Density functional theory (DFT) calculations further suggest that the Ni single atoms coordinated by three N atoms possesses a suitable free energy barrier for *COOH formation and *CO desorption, thereby exhibiting the most excellent CO 2 RR performance. This study sheds a new light on the design of SACs with controllable coordination structures for CO 2 RR. [ABSTRACT FROM AUTHOR]