Review

Carbon capture, storage, and utilization

Nickel-nitrogen-carbon (Ni-N-C) electrocatalysts toward CO₂ electroreduction: Advances, optimizations, challenges and prospects

Qingqing Pang, Xizheng Fan, Kaihang Sun, Kun Xiang, Baojun Li, Shufang Zhao, Young Dok Kim, Qiaoyun Liu,* Zhongyi Liu,* and Zhikun Peng*

((Optional Dedication))

Q. Pang, X. Fan, K. Sun, B. Li, Dr. Q. Liu, Prof. Z. Liu, Dr. Z. Peng

School of Chemical Engineering, College of Chemistry, Henan Institute of Advance Technology, Zhengzhou University, Zhengzhou 450001, China E-mail: liuqiaoyun@zzu.edu.cn; liuzhongyi@zzu.edu.cn; pengzhikun@zzu.edu.cn

K. Xiang School of Chemistry and Environmental Engineering

Wuhan Institute of Technology, Wuhan 430205, China

S. Zhao, Y. D. Kim Department of Chemistry

Sungkyunkwan University, Suwon 16419, Republic of Korea

Keywords: Ni-N-C electrocatalysts, Active sites, Optimization strategies, Electrocatalysis, CO₂ reduction

Abstract: Electrocatalytic reduction of CO₂ into high energy-density fuels and value-added chemicals under mild conditions can promote the sustainable cycle of carbon and decrease current energy and environmental problems. Constructing electrocatalyst with high activity, selectivity, stability and low cost is really matter to realize industrial application of electrocatalytic CO₂ reduction (ECR). Metal-nitrogen-carbon (M-N-C) electrocatalysts, especially Ni-N-C, display excellent performance, such as nearly 100% CO selectivity, high current density, outstanding tolerance, etc., which is considered to possess broad application prospects. Based on the current research status, starting from the mechanism of ECR and the existence form of Ni active species, the latest research progress of Ni-N-C electrocatalysts in CO₂electroreduction is systematically summarized. An overview is emphatically interpreted on the regulatory strategies for activity optimization over Ni-N-C, including N coordination modulation, vacancy defects construction, morphology design, surface modification, heteroatom activation, bimetallic cooperation. Finally, some urgent problems and future prospects on designing Ni-N-C catalyst for ECR are discussed. This review aims to provide the guidance for the design and development of Ni-N-C catalyst with practical application.

1. Introduction

Excess emission of greenhouse gas CO₂ from massive exploitation of fossil fuels, breaks the carbon cycle system in nature, causing the global warming and biodiversity reduction.^[1-3]How to reduce CO₂ emissions and turn them into reusable fuels or chemicals has become one of the most urgent tasks. A great deal of effort has been devoted to finding effective solutions to achieve the important task of converting CO₂.^[4-6] Compared with the thermal and photocatalytic conversion of CO₂, the electrocatalytic CO₂ reduction (ECR) technology with mild conditions, controllability, environmental friendliness and high efficiency is getting more and more attention.^[7-9] Overcoming the obstacles such as desirable catalysts, steerable reaction conditions and devices, is extremely crucial for realizing the maximum application value of ECR technology. At present, although the ECR reaction achieved a variety of gaseous and liquid products including formic acid (HCOOH), carbon monoxide (CO), hydrocarbons (CH₄ and C₂H₄) and alcohols (CH₃OH and C₂H₅OH),^[10-14]the development of electrocatalysts with high selectivity and stability to overcome slow kinetics and high overpotential is still the core content for ECR.

In recent years, diverse electrocatalysts have been tried and employed in ECR reaction, including precious metals,^[15-17]transition metals^[18-22] and metal-free catalysts^[23-25]. Among them, transition metal-nitrogen-carbon (M-N-C) materials are considered to have great potential as ECR electrocatalysts. On the one hand, nitrogen-doped carbon as the substrate can anchor metal atoms or particles to enrich the active sites and accelerate the electron transport rate; On the other hand, the charge distribution and electronic structure of active species can be regulated by the morphology design, defect construction and heteroatom doping of the nitrogen-doped carbon matrix, achieving the regulation of product type and selectivity.^[26,27]Notably, Ni-coupled N-doped carbon (Ni-N-C) catalyst has demonstrated to possess more advantages than other transition metals including current density, CO selectivity and stability, which is the hot topic in ECR field.^[28-31]For instance, Yang et al. reported an extended multi-step pyrolysis process for the preparation of monatomic nitrogen-doped carbon nanotubes catalysts with high metal loads. The activity evaluation of nitrocarbon catalyst for multifarious metals (MSA-N-CNTs, where M = Ni, Co, NiCo, CoFe and NiPt) showed that NSA-N-CNTs exported the highest ECR activity, and the CO selectivity was up to 91.3% at −0.7 V.^[32] Furthermore, Hao et al. studied the intrinsic catalytic activity, CO turnover and CO Faraday efficiency of M-N-C (M = Mn, Fe, CO, Ni, Cu) electrocatalysts containing the active species M-N_x . It was confirmed that Ni-N_x single site exhibited unique reactivity and Faraday efficiency in the reduction process from CO₂ to CO, which produced the most CO at high potential. The above findings illustrate the great potential of Ni-N-C materials in ECR, including excellent electrical conductivity and stability, high current density, and near 100% CO selectivity, meeting and even exceeding the mass activity of advanced precious metal catalysts.^[33,34] Based on the extraordinary potential of Ni-N-C catalysts in the ECR field, the relatively systematic summary is essential.

Combining the experimental and theoretical investigations of Ni-N-C, we systematically summarized the recent progress of Ni-N-C electrocatalyst in the field of CO₂ electroreduction technology. On account of the mechanism of CO₂ electrocatalytic reduction, the construction strategy, systematic classification, activity regulation and product selectivity of advanced Ni-N-C electrocatalysts were systematically reviewed. The structure-activity relationship of Ni-N-C electrocatalyst was emphasized by surface modification, defect engineering, change of coordination environment over active sites and reactor optimization. At the end of the paper, the challenges and future prospects are presented. Through this review, we anticipate providing new insights that can benefit for enhancing ECR. Our work may provide the basis and strategies for program upgrading in terms of improving the efficiency of electrocatalytic reduction over CO₂ and achieving industrialization.