Metal–nitrogen–carbon (M–N–C) molecular catalysts with NiN₄ active structure have been extensively studied as selective and active catalysts toward electrochemical reduction of CO₂ to CO. The key challenge for a practical M–N–C catalyst is to increase the density of atomic metal active sites that achieves the partial current density of CO (j_CO) relevant to the industrial level at lower overpotentials. Here, we revealed the effect of physical and chemical properties of carbon substrates and synthetic processes on the tuning of the density of atomic metal active sites as well as the role of reaction chemistry in enhancing the j_CO and reducing the overpotential. The achievable loading of NiN₄ active site in the Ni–N–C is determined by the combined content of pyridinic and pyrrolic N functionalities and Ni–N coordination efficiency derived from the pyrolytic step rather than the uptake capability of Ni²⁺ in the adsorption step in the case of carbon black with high specific surface area (>1000 m²/g). The N dopant content can be improved by modifying oxygen functional groups on the surface of carbon black, optimizing the pyrolytic temperature, and iterating the doping step. Through a combination of all optimum factors, the resultant Ni–N–C catalyst has a maximum loading of ∼4.4 wt % for atomic Ni. This Ni–N–C catalyst exhibited Faradaic efficiency (FE) of CO of 97% and j_CO of −152 mA cm^–2 at −0.93 V vs RHE in a flow cell using 0.5 M KHCO₃ electrolyte while showing 93% FE of CO and j_CO of −67 mA cm^–2 at −0.61 V vs RHE at 1 M KOH. Adding KI to the base electrolyte significantly magnified the j_CO to larger than −200 mA cm^–2 at a potential of −0.51 V vs RHE while maintaining the almost unity FE of CO. The Ni–N–C is compatible with the membrane-electrode-assembly-based electrolyzer in which the j_CO also achieved >200 mA cm^–2 at a cell voltage of around 2.7 V.

中文翻译：

---

高速率CO ₂电还原为CO的镍-氮-碳分子催化剂：关于碳底物的作用和反应化学

具有NiN ₄活性结构的金属-氮-碳（M-N-C）分子催化剂已被广泛研究用作将CO ₂电化学还原为CO的选择性和活性催化剂。实用M-N-C催化剂的主要挑战是以增加原子金属活性位点的密度，从而在较低的超电势下实现与工业水平相关的CO（j _CO）的部分电流密度。在这里，我们揭示了碳底物的物理和化学性质以及合成工艺对原子金属活性位点密度的调整的影响，以及反应化学在增强j _CO和减少过电位方面的作用。NiN ₄的可达到负载Ni–N–C中的活性位点取决于吡啶和吡咯N官能团的总含量以及热解步骤产生的Ni–N配位效率，而不是碳吸附情况下的Ni ²⁺吸收能力黑色，比表面积高（> 1000 m ² / g）。可以通过修饰炭黑表面上的氧官能团，优化热解温度并重复掺杂步骤来提高N掺杂剂含量。通过所有最佳因素的综合，最终的Ni–N–C催化剂对原子Ni的最大负载量约为4.4 wt％。这种Ni–N–C催化剂的CO的法拉第效率（FE）为97％，j _CO的−152 mA cm ^–2在使用0.5 M KHCO ₃电解液的流通池中，RHE为-0.93 V时的RHE，而在-0.61 V时相对于1 M KOH时的RHE，则显示出93％的FE和j _CO为-67 mA cm ^-2。在基础电解液中添加KI可以显着将j _CO放大至−200 mA cm ^–2，相对于RHE的电势为-0.51 V，同时保持CO的FE几乎统一。Ni–N–C与膜兼容。基于电极组件的电解槽，其中j _CO在2.7 V左右的电池电压下也达到了> 200 mA cm ^–2。