Noble-Metal-Free Doped Carbon Nanomaterial Electrocatalysts

Electrocatalytic processes, such as oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER) and carbon dioxide reduction reaction (CO₂RR), play key roles in various sustainable energy storage and production devices and their optimization in an ecological manner is of paramount importance for mankind. In this inclusive Review, we aspire to set the scene on doped carbon-based nanomaterials and their hybrids as precious-metal alternative electrocatalysts for these critical reactions in order for the research community not only to stay up-to-date, but also to get inspired and keep pushing forward towards their practical application in energy conversion.     

A Water-Soluble Sodium Pectate Complex with Copper as an Electrochemical Catalyst for Carbon Dioxide Reduction

A selective noble-metal-free molecular catalyst has emerged as a fruitful approach in the quest for designing efficient and stable catalytic materials for CO₂ reduction. In this work, we report that a sodium pectate complex of copper (PG-NaCu) proved to be highly active in the electrocatalytic conversion of CO₂ to CH₄ in water. Stability and selectivity of conversion of CO₂ to CH₄ as a product at a glassy carbon electrode were discovered. The copper complex PG-NaCu was synthesized and characterized by physicochemical methods. The electrochemical CO₂ reduction reaction (CO₂RR) proceeds at −1.5 V vs. Ag/AgCl at ~10 mA/cm² current densities in the presence of the catalyst. The current density decreases by less than 20% within 12 h of electrolysis (the main decrease occurs in the first 3 h of electrolysis in the presence of CO₂). This copper pectate complex (PG-NaCu) combines the advantages of heterogeneous and homogeneous catalysts, the stability of heterogeneous solid materials and the performance (high activity and selectivity) of molecular catalysts.     

Selective and Efficient Reduction of Carbon Dioxide to Carbon Monoxide on Oxide‐Derived Nanostructured Silver Electrocatalysts

In this work, the selective electrocatalytic reduction of carbon dioxide to carbon monoxide on oxide‐derived silver electrocatalysts is presented. By a simple synthesis technique, the overall high faradaic efficiency for CO production on the oxide‐derived Ag was shifted by more than 400 mV towards a lower overpotential compared to that of untreated Ag. Notably, the Ag resulting from Ag oxide is capable of electrochemically reducing CO₂ to CO with approximately 80 % catalytic selectivity at a moderate overpotential of 0.49 V, which is much higher than that (ca. 4 %) of untreated Ag under identical conditions. Electrokinetic studies show that the improved catalytic activity is ascribed to the enhanced stabilization of COOH^. intermediate. Furthermore, highly nanostructured Ag is likely able to create a high local pH near the catalyst surface, which may also facilitate the catalytic activity for the reduction of CO₂ with suppressed H₂ evolution.     

过渡金属单原子电催化剂还原CO2制CO

电催化二氧化碳还原(ECR)技术是实现“碳中和”目标的一种理想途径,而过渡金属单原子催化剂具有电子结构可调、原子利用率高和活性位点均一等特点,在ECR研究中具有显著优势。本文首先介绍了单原子电催化剂在还原CO₂尤其是在选择性生成CO研究中的优势,然后综述了近年来Fe、Co、Ni及其他单原子电催化剂的反应位点调控策略与电催化选择性的调控机制,重点对质子耦合CO₂还原生成CO的中间过程调控进行了归纳总结,并简要展望了发展方向,以期为推动单原子催化剂在ECR中规模化应用提供指导和参考。     

氧化还原刻蚀铜表面对二氧化碳电催化还原性能的研究

大规模化石燃料的使用排放了大量的二氧化碳（CO₂）,导致环境中二氧化碳的含量急剧增加. 为了降低大气中二氧化碳的含量,以电催化的方法将二氧化碳转化为有用的化工原料和燃料是解决能源和环境问题的重要途径. 本文主要利用氧化还原刻蚀法,在铜表面形成复合纳米结构,用于二氧化碳的电催化还原反应研究. 首先,作者通过一定浓度的三氯化铁（FeCl₃）溶液与铜片的氧化还原反应,在刻蚀铜表面时形成具有立方体结构的氯化亚铜纳米材料,用于二氧化碳的电催化还原反应. 为了研究反应时间对催化性能的影响,作者通过改变反应时间（1、2、3和4 h）合成了不同结构的铜基催化剂. 研究发现,在反应3 h后,Cu-3h催化剂对二氧化碳的电催化还原具有较小的起始电压（-0.3 V vs. RHE）和较大的电流密度值,表现出了较强的还原能力. 经检测,所得到主要还原产物为一氧化碳（CO）和甲烷（CH₄）. 在-0.6 V时,二氧化碳催化还原的法拉第效率可达到60%,表明以氧化还原法刻蚀铜表面具有较好的改善二氧化碳电催化还原的能力.     

三相界面电催化二氧化碳还原研究进展

电催化二氧化碳还原是能源化学及催化科学的研究重点与难点.气-固-液三相界面模型作为物理化学中的基本概念,近年来被越来越多地应用于电催化二氧化碳还原反应的研究,其相比于传统固-液两相体系表现出了诸多优点.本综述阐述了三相界面电催化二氧化碳还原研究进展,对三相界面电催化体系进行分类及原理探究.再具体到二氧化碳还原反应,讨论...     

The Functionality of Surface Hydroxy Groups on the Selectivity and Activity of Carbon Dioxide Reduction over Cuprous Oxide in Aqueous Solutions

Carbon dioxide (CO₂) reduction in aqueous solutions is an attractive strategy for carbon capture and utilization. Cuprous oxide (Cu₂O) is a promising catalyst for CO₂ reduction as it can convert CO₂ into valuable hydrocarbons and suppress the side hydrogen evolution reaction (HER). However, the nature of the active sites in Cu₂O remains under debate because of the complex surface structure of Cu₂O under reducing conditions, leading to limited guidance in designing improved Cu₂O catalysts. This paper describes the functionality of surface‐bonded hydroxy groups on partially reduced Cu₂O(111) for the CO₂ reduction reaction (CO₂RR) by combined density functional theory (DFT) calculations and experimental studies. We find that the surface hydroxy groups play a crucial role in the CO₂RR and HER, and a moderate coverage of hydroxy groups is optimal for promotion of the CO₂RR and suppression of the HER simultaneously. Electronic structure analysis indicates that the charge transfer from hydroxy groups to coordination‐unsaturated Cu (Cu_CUS) sites stabilizes surface‐adsorbed COOH*, which is a key intermediate during the CO₂RR. Moreover, the CO₂RR was evaluated over Cu₂O octahedral catalysts with {111} facets and different surface coverages of hydroxy groups, which demonstrates that Cu₂O octahedra with moderate coverage of hydroxy groups can indeed enhance the CO₂RR and suppress the HER.     

Probing the Local Reaction Environment During High Turnover Carbon Dioxide Reduction with Ag-Based Gas Diffusion Electrodes

Discerning the influence of electrochemical reactions on the electrode microenvironment is an unavoidable topic for electrochemical reactions that involve the production of OH⁻ and the consumption of water. That is particularly true for the carbon dioxide reduction reaction (CO₂RR), which together with the competing hydrogen evolution reaction (HER) exert changes in the local OH⁻ and H₂O activity that in turn can possibly affect activity, stability, and selectivity of the CO₂RR. We determine the local OH⁻ and H₂O activity in close proximity to a CO₂-converting Ag-based gas diffusion electrode (GDE) with product analysis using gas chromatography. A Pt nanosensor is positioned in the vicinity of the working GDE using shear-force-based scanning electrochemical microscopy (SECM) approach curves, which allows monitoring changes invoked by reactions proceeding within an otherwise inaccessible porous GDE by potentiodynamic measurements at the Pt-tip nanosensor. We show that high turnover HER/CO₂RR at a GDE lead to modulations of the alkalinity of the local electrolyte, that resemble a 16 m KOH solution, variations that are in turn linked to the reaction selectivity.     

二氧化碳还原电催化剂的结构设计及性能研究进展

随着人类社会工业化进程的推进, 化石能源被过度消耗, 人类向大气中排放过量的二氧化碳, 造成能源危机和环境问题. 通过电催化二氧化碳还原反应来制备高附加值精细化学品是积极探索建立人工碳循环的方向之一, 引起了基础研究与工业应用领域研究者的广泛关注. 设计与制备具有高活性、高选择性和高稳定性的电催化剂对于实现二氧化碳高效还原具有重要意义. 近年来, 关于催化剂与电极材料的结构设计和应用案例有许多报道, 取得了显著的进步. 分别从尺寸效应、表面特性、缺陷工程和多级结构四个方面对催化剂结构与电极结构的调控策略进行了综述, 并对电催化二氧化碳还原领域的发展进行了展望.     

Oxygenates from the Electrochemical Reduction of Carbon Dioxide

Electrochemical reduction of carbon dioxide (CO₂) driven by renewable electricity to give chemicals and fuels is considered an ideal approach that can alleviate both carbon emission and energy tension stress. High‐value chemicals such as oxygenates can be effectively produced from the electroreduction of CO₂, and this is highly attractive to promote the economy and applicability of CO₂ utilization. This review focuses on recent advancements in the electrochemical reduction of CO₂ to formic acid, methanol, ethanol, acetic acid, and other oxygenates. The principles of the process, influencing factors, and typical catalysts are summarized. On the basis of the aforementioned discussions, we present future prospects for further development of the electroreduction of CO₂ to oxygenates.     

Electrochemical Reduction of Carbon Dioxide to 1-Butanol on Oxide-Derived Copper

The electroreduction of carbon dioxide using renewable electricity is an appealing strategy for the sustainable synthesis of chemicals and fuels. Extensive research has focused on the production of ethylene, ethanol and n-propanol, but more complex C₄ molecules have been scarcely reported. Herein, we report the first direct electroreduction of CO₂ to 1-butanol in alkaline electrolyte on Cu gas diffusion electrodes (Faradaic efficiency=0.056 %, j_1-Butanol=−0.080 mA cm⁻² at −0.48 V vs. RHE) and elucidate its formation mechanism. Electrolysis of possible molecular intermediates, coupled with density functional theory, led us to propose that CO₂ first electroreduces to acetaldehyde-a key C₂ intermediate to 1-butanol. Acetaldehyde then undergoes a base-catalyzed aldol condensation to give crotonaldehyde via electrochemical promotion by the catalyst surface. Crotonaldehyde is subsequently electroreduced to butanal, and then to 1-butanol. In a broad context, our results point to the relevance of coupling chemical and electrochemical processes for the synthesis of higher molecular weight products from CO₂.     

锡气体扩散电极上电化学还原二氧化碳制甲酸性能的稳定性

研究了Sn气体扩散电极(SGDE)上电化学还原CO₂制甲酸(ERCF)性能的稳定性。采用X射线衍射(XRD)、扫描电子显微镜(SEM)、X射线能量色散谱(EDX)和活性表面积测试等技术手段 分别表征SGDE在电化学还原CO₂制甲酸过程前后的物相结构、表面形貌、元素组成和活性表面积。 采用生成甲酸的法拉第效率(f_HCOOH)评价SGDE上电化学还原CO₂制甲酸的性能。 结果显示,f_HCOOH随电解时间的延长急剧地降低,电解时间12 h的f_HCOOH((36.6±1.6)%)比电解时间0.5 h时的f_HCOOH((78.5±0.1)%)降低了53%。 SGDE在12 h电还原反应后,表面沉积了微量Fe,而且Sn含量(质量分数)减少了66%,活性表面积降低了41%。 进一步的研究发现,沉积的微量Fe对电化学还原CO₂制甲酸过程基本没有影响,Sn含量和活性表面积的降低可能是SGDE上电化学还原CO₂制甲酸性能降低的主要原因。     

On the Role of Metals in Nitrogen‐Doped Carbon Electrocatalysts for Oxygen Reduction

The notion of metal‐free catalysts is used to refer to carbon materials modified with nonmetallic elements. However, some claimed metal‐free catalysts are prepared using metal‐containing precursors. It is highly contested that metal residues in nitrogen‐doped carbon (NC) catalysts play a crucial role in the oxygen reduction reaction (ORR). In an attempt to reconcile divergent views, a definition for truly metal‐free catalysts is proposed and the differences between NC and M‐N_x/C catalysts are discussed. Metal impurities at levels usually undetectable by techniques such as XPS, XRD, and EDX significantly promote the ORR. Poisoning tests to mask the metal ions reveal the involvement of metal residues as active sites or as modifiers of the electronic structure of the active sites in NC. The unique merits of both M‐N_x/C and NC catalysts are discussed to inspire the development of more advanced nonprecious‐metal catalysts for the ORR.     

Electrochemical Reduction of Carbon Dioxide to Value‐Added Products: The Electrocatalyst and Microbial Electrosynthesis

The electrochemical reduction of carbon dioxide (CO₂) to value‐added products obtains great attention and investigation worldwide in recent years. The commercialization of this green process relies on the progress of relating high‐performance electrocatalysts and their feasibility with proper reactor design. The microbial electrosynthesis (MES) is an alternative route to reduce CO₂ with electroactive bio‐film electrode as catalyst. This review presents the research status and development of cathode catalysts, particularly focusing on the active sites and development tendency, for highly efficient electrochemical reduction CO₂ from personal viewpoint. Some of our results are also presented to exhibit contributions. MES shows a similar process to the typical electrochemical reduction of CO₂. Their combination is an important trend, and the future research in this field is full of challenges and opportunities.     

H型电解池中CO2电化学还原的阳极电解液问题

采用自制的H型电解池开展了KHCO₃溶液中电化学还原CO₂制甲酸的研究. 研究发现,在电解池中长时间电解时阴阳两极间的电压（槽电压）会持续升高,导致电解过程不可持续. 经过恒电位电解、恒电流电解、pH测试以及电解前后阳极室KHCO₃浓度分析等实验研究,作者发现,这是由以下过程引起的：阳极上的析氧反应产生的H⁺与电解液中的HCO₃^-反应生成水和CO₂,导致阳极室的HCO₃^-的消耗,之后阳极室的K⁺被迫扩散进入阴极室而导致阳极室电解质浓度下降. 因此,阳极室电解液导电性下降,进而引起阳极电位的升高. 研究发现,阳极电解液具有碱性时,都可能发生此种现象,因此,为了保证电解过程可持续且保持高的能量转换效率,阳极液的电解质不能是任何具有碱性的物质.     

Facile Synthesis of Sub-Nanometric Copper Clusters by Double Confinement Enables Selective Reduction of Carbon Dioxide to Methane

Previous density-functional theory (DFT) calculations show that sub-nanometric Cu clusters (i.e., 13 atoms) favorably generate CH₄ from the CO₂ reduction reaction (CO₂RR), but experimental evidence is lacking. Herein, a facile impregnation-calcination route towards Cu clusters, having a diameter of about 1.0 nm with about 10 atoms, was developed by double confinement of carbon defects and micropores. These Cu clusters enable high selectivity for the CO₂RR with a maximum Faraday efficiency of 81.7 % for CH₄. Calculations and experimental results show that the Cu clusters enhance the adsorption of *H and *CO intermediates, thus promoting generation of CH₄ rather than H₂ and CO. The strong interactions between the Cu clusters and defective carbon optimize the electronic structure of the Cu clusters for selectivity and stability towards generation of CH₄. Provided here is the first experimental evidence that sub-nanometric Cu clusters facilitate the production of CH₄ from the CO₂RR.     

Formate Dehydrogenase Mimics as Catalysts for Carbon Dioxide Reduction

Formate dehydrogenases (FDH) reversibly catalyze the interconversion of CO₂ to formate. They belong to the family of molybdenum and tungsten-dependent oxidoreductases. For several decades, scientists have been synthesizing structural and functional model complexes inspired by these enzymes. These studies not only allow for finding certain efficient catalysts but also in some cases to better understand the functioning of the enzymes. However, FDH models for catalytic CO₂ reduction are less studied compared to the oxygen atom transfer (OAT) reaction. Herein, we present recent results of structural and functional models of FDH.     

Densely Packed,Ultra Small SnO Nanoparticles for Enhanced Activity and Selectivity in Electrochemical CO2 Reduction

Controlling the selectivity in electrochemical CO₂ reduction is an unsolved challenge. While tin (Sn) has emerged as a promising non‐precious catalyst for CO₂ electroreduction, most Sn‐based catalysts produce formate as the major product, which is less desirable than CO in terms of separation and further use. Tin monoxide (SnO) nanoparticles supported on carbon black were synthesized and assembled and their application in CO₂ reduction was studied. Remarkably high selectivity and partial current densities for CO formation were obtained using these SnO nanoparticles compared to other Sn catalysts. The high activity is attributed to the ultra‐small size of the nanoparticles (2.6 nm), while the high selectivity is attributed to a local pH effect arising from the dense packing of nanoparticles in the conductive carbon black matrix.     

DFT and Empirical Considerations on Electrocatalytic Water/Carbon Dioxide Reduction by CoTMPyP in Neutral Aqueous Solutions**

A combined experimental and density functional theory (DFT) investigation was employed in order to examine the mechanism of electrochemical CO₂ reduction and H₂ formation from water reduction in neutral aqueous solutions. A water soluble cobalt porphyrin, cobalt [5,10,15,20-(tetra-N-methyl-4-pyridyl)porphyrin], (CoTMPyP), was used as catalyst. The possible attachment of different axial ligands as well as their effect on the electrocatalytic cycles were examined. A cobalt porphyrin hydride is a key intermediate which is generated after the initial reduction of the catalyst. The hydride is involved in the formation of H₂ and formate and acts as an indirect proton source for the formation of CO in these H⁺-starving conditions. The experimental results are in agreement with the computations and give new insights into electrocatalytic mechanisms involving water soluble metalloporphyrins. We conclude that in addition to the porphyrin's structure and metal ion center, the electrolyte surroundings play a key role in dictating the products of CO₂/H₂O reduction.     

Molybdenum–Bismuth Bimetallic Chalcogenide Nanosheets for Highly Efficient Electrocatalytic Reduction of Carbon Dioxide to Methanol

Methanol is a very useful platform molecule and liquid fuel. Electrocatalytic reduction of CO₂ to methanol is a promising route, which currently suffers from low efficiency and poor selectivity. Herein we report the first work to use a Mo‐Bi bimetallic chalcogenide (BMC) as an electrocatalyst for CO₂ reduction. By using the Mo‐Bi BMC on carbon paper as the electrode and 1‐butyl‐3‐methylimidazolium tetrafluoroborate in MeCN as the electrolyte, the Faradaic efficiency of methanol could reach 71.2 % with a current density of 12.1 mA cm^?2, which is much higher than the best result reported to date. The superior performance of the electrode resulted from the excellent synergistic effect of Mo and Bi for producing methanol. The reaction mechanism was proposed and the reason for the synergistic effect of Mo and Bi was discussed on the basis of some control experiments. This work opens a way to produce methanol efficiently by electrochemical reduction of CO₂.