Recent advances in single atom catalysts for the electrochemical carbon dioxide reduction reaction

The electrochemical carbon dioxide reduction reaction (CO₂RR) offers a promising solution to mitigate carbon emission and at the same time generate valuable carbonaceous chemicals/fuels. Single atom catalysts (SACs) are encouraging to catalyze the electrochemical CO₂RR due to the tunable electronic structure of the central metal atoms, which can regulate the adsorption energy of reactants and reaction intermediates. Moreover, SACs form a bridge between homogeneous and heterogeneous catalysts, providing an ideal platform to explore the reaction mechanism of electrochemical reactions. In this review, we first discuss the strategies for promoting the CO₂RR performance, including suppression of the hydrogen evolution reaction (HER), generation of C₁ products and formation of C₂₊ products. Then, we summarize the recent developments in regulating the structure of SACs toward the CO₂RR based on the above aspects. Finally, several issues regarding the development of SACs for the CO₂RR are raised and possible solutions are provided.

The electrochemical carbon dioxide reduction reaction (CO₂RR) offers a promising solution to mitigate carbon emission and at the same time generate valuable carbonaceous chemicals/fuels.     

Insights into electrochemical nitrogen reduction reaction mechanisms:Combined effect of single transition-metal and boron atom

Developing single-atom catalysts (SACs) for electrochemical devices is a frontier in energy conversion.The comparison of stability,activity and selectivity betw...     

Recent advances of MXene as promising catalysts for electrochemical nitrogen reduction reaction

Electrochemical reduction of N_2,as an eco-friendly alternative,not only allows the use of protons in water as a source of hydrogen under mild conditions but also can be driven by renewable electric energy.The major challenge is to identify high-efficiency electrocatalysts.MXene is a new class of 2D transition metal carbides,nitrides,and carbonitrides that have received significant attention in electrocatalysis.The investigations on MXene in electrocatalytic nitrogen fixation are rapidly proceeding,and some breakthroughs have emerged ve ry recently due to MXenes' satisfacto ry catalytic activity.Here,the recent progress concerning the MXene-based catalysts for electrochemical N_2 reduction reaction(NRR) is highlighted.In regards to giving guidelines for exploring more efficient MXene-based catalysts for the NRR,the fabrication and surface modification of MXene are discussed.Besides,the shortcomings and challenges of current research are summarized and the future research directions are prospected.     

Amino-metalloporphyrin polymers derived Fe single atom catalysts for highly efficient oxygen reduction reaction

Recently, nitrogen-doped porous carbon supported single atom catalysts(SACs) have become one of the most promising alternatives to precious metal catalysts in oxygen reduction reaction(ORR) due to their outstanding performance, especially those derived from porphyrin-based materials. However, most of them involve other metal residuals, which would cause the tedious pre-and/or post-treatment, even mislead the mechanistic investigations and active-site identification. Herein, we report a precursor-dilution strategy to synthesize Fe SACs through the Schiff-based reaction via co-polycondensation of amino-metalloporphyrin, followed by pyrolysis at high temperature. Systematic characterization results provide the compelling evidence of the dominant presence of atomically dispersed Fe-Nxspecies. Our catalyst shows superior ORR performance with positive half-wave potential(E_(1/2)=0.85 V vs. RHE) in alkaline condition and moderate activity(E_(1/2)=0.68 V vs. RHE) under the acidic condition, excellent methanol tolerance and good long-term stability. All the results indicate Fe SACs would be a promising candidate for replacing the precious Pt in metal-air batteries and fuel cells.     

Defect and interface engineering for electrochemical nitrogen reduction reaction under ambient conditions

Electrochemical nitrogen reduction reaction(e-NRR)under ambient conditions is an emerging strategy to tackle the hydrogen-and energy-intensive operations for traditional Haber-Bosch process in industrial ammonia(NH₃)synthesis.However,the e-NRR performance is currently impeded by the inherent inertness of N₂ molecules,the extremely slow kinetics and the overwhelming competition from the hydrogen evolution reaction(HER),all of which cause unsatisfied yield and ammonia selectivity(Faradaic efficiency,FE).Defect and interface engineering are capable of achieving novel physical and chemical properties as well as superior synergistic effects for various electrocatalysts.In this review,we first provide a general introduction to the NRR mechanism.We then focus on the recent progress in defect and interface engineering and summarize how defect and interface can be rationally designed and functioned in NRR catalysts.Particularly,the origin of superior NRR catalytic activity by applying these approaches was discussed from both theoretical and experimental perspectives.Finally,the remaining challenges and future perspectives in this emerging area are highlighted.It is expected that this review will shed some light on designing NRR electrocatalysts with excellent activity,selectivity and stability.     

Valence state of transition metal center as an activity descriptor for CO2 reduction on single atom catalysts

Carbon dioxide(CO₂)catalytic reduction has been passionately pursued for a long period of time due to its special importance in alleviating the greenhouse effect as well as generating valuable fuels and chemicals[1].Among the various approaches for achieving CO₂conversion,considerable efforts have been devoted to electrochemical reduction of CO₂since this technology operates at ambient and mild environments and can potentially produce various useful products[2].     

The atomic-level regulation of single-atom site catalysts for the electrochemical CO2 reduction reaction

The electrochemical CO₂ reduction reaction (CO₂RR) is viewed as a promising way to remove the greenhouse gas CO₂ from the atmosphere and convert it into useful industrial products such as methane, methanol, formate, ethanol, and so forth. Single-atom site catalysts (SACs) featuring maximum theoretical atom utilization and a unique electronic structure and coordination environment have emerged as promising candidates for use in the CO₂RR. The electronic properties and atomic structures of the central metal sites in SACs will be changed significantly once the types or coordination environments of the central metal sites are altered, which appears to provide new routes for engineering SACs for CO₂ electrocatalysis. Therefore, it is of great importance to discuss the structural regulation of SACs at the atomic level and their influence on CO₂RR activity and selectivity. Despite substantial efforts being made to fabricate various SACs, the principles of regulating the intrinsic electrocatalytic performances of the single-atom sites still needs to be sufficiently emphasized. In this perspective article, we present the latest progress relating to the synthesis and catalytic performance of SACs for the electrochemical CO₂RR. We summarize the atomic-level regulation of SACs for the electrochemical CO₂RR from five aspects: the regulation of the central metal atoms, the coordination environments, the interface of single metal complex sites, multi-atom active sites, and other ingenious strategies to improve the performance of SACs. We highlight synthesis strategies and structural design approaches for SACs with unique geometric structures and discuss how the structure affects the catalytic properties.

Electrochemical CO₂ reduction reaction (CO₂RR) is a promising way to remove CO₂ and convert it into useful industrial products. Single-atom site catalysts provide opportunities to regulate the active sites of CO₂RR catalysts at the atomic level.     

Nickel single atom overcoordinated active sites to accelerate the electrochemical reaction kinetics for Li-S cathode

Lithium-sulfur(Li-S) batteries with high theoretical energy density are promising advanced energy storage devices.However,shuttling of dissolute lithium polysulfide(LiPSs) and sluggish conversion kinetics impede their applications.Herein,single nickel(Ni) atoms on two-dimensional(2D) nitrogen(N)-doped carbon with Ni-N₄-O overcoordinated structure(SANi-N₄-O/NC) are prepared and firstly used as a sulfur host of Li-S batteries.Due to the efficient polysulfides traps and highly...     

Particle-size effect of nanoscale platinum catalysts in oxygen reduction reaction: an electrochemical and 195Pt EC-NMR study

Oxygen reduction reaction (ORR) measurements and (195)Pt electrochemical nuclear magnetic resonance (EC-NMR) spectroscopy were combined to study a series of carbon-supported platinum nanoparticle electrocatalysts (Pt/CB) with average diameters in the range of roughly 1-5 nm. ORR rate constants and H(2)O(2) yields evaluated from hydrodynamic voltammograms did not show any particle size dependency. The apparent activation energy of 37 kJ mol(-1), obtained for the ORR rate constant, was identical to that obtained for bulk platinum electrodes. Pt/CB catalysts on Nafion produced only 0.7-1% of H(2)O(2), confirming that the direct four-electron reduction of O(2) to H(2)O is the predominant reaction. NMR spectral features showed characteristic size dependence, and the line shapes were reproduced by using the layer-deconvolution model. Namely, the variations in the NMR spectra with particle size can be explained as due to the combined effect of the layer-by-layer variation of the s-type and d-type local density of states. However, the surface peak position of (195)Pt NMR spectra and the spin-lattice relaxation time of surface platinum atoms showed practically no change with the particle size variation. We conclude that there is a negligible difference in the surface electronic properties of these Pt/CB catalysts due to size variations and therefore, the ORR activities are not affected by the differences in the particle size.     

Why heterogeneous single-atom catalysts preferentially produce CO in the electrochemical CO2 reduction reaction

Formate and CO are competing products in the two-electron CO₂ reduction reaction (2e CO₂RR), and they are produced via *OCHO and *COOH intermediates, respectively. However, the factors governing CO/formate selectivity remain elusive, especially for metal–carbon–nitrogen (M–N–C) single-atom catalysts (SACs), most of which produce CO as their main product. Herein, we show computationally that the selectivity of M–N–C SACs is intrinsically associated with the CO₂ adsorption mode by using bismuth (Bi) nanosheets and the Bi–N–C SAC as model catalysts. According to our results, the Bi–N–C SAC exhibits a strong thermodynamic preference toward *OCHO, but under working potentials, CO₂ is preferentially chemisorbed first due to a charge accumulation effect, and subsequent protonation of chemisorbed CO₂ to *COOH is kinetically much more favorable than formation of *OCHO. Consequently, the Bi–N–C SAC preferentially produces CO rather than formate. In contrast, the physisorption preference of CO₂ on Bi nanosheets contributes to high formate selectivity. Remarkably, this CO₂ adsorption-based mechanism also applies to other typical M–N–C SACs. This work not only resolves a long-standing puzzle in M–N–C SACs, but also presents simple, solid criteria (i.e., CO₂ adsorption modes) for indicating CO/formate selectivity, which help strategic development of high-performance CO₂RR catalysts.

This report discloses a nontrivial role of the CO₂ adsorption mode in governing the CO/formate selectivity of single-atom catalysts towards two-electron CO₂ reduction.     

Noble metal-free catalysts for oxygen reduction reaction

Developing noble metal-free catalysts with low cost, high performance and stability for oxygen reduction reaction(ORR) in fuel cells is of great interest to promote sustainable energy devices. In this review, we summarized noble metal-free catalysts for ORR,including non-noble metal-based and heteroatom-doped carbon nanomaterials. Mesoporous structure, homogeneous distribution of nanocrystals and synergistic effect of carbon base and nanocrystals/doped heteroatoms have great effect on the ORR property.The noble metal-free nanomaterials showed comparable catalytic property, better stability and methanol tolerance than commercial platinum(Pt)-based catalysts, showing great potential as substitutes for noble metal-based catalysts. In addition, the challenges and chances of developing noble metal-free ORR catalysts are also discussed.     

Oxygen reduction reaction catalysts prepared from acetonitrile pyrolysis over alumina-supported metal particles

Noble-metal-free active catalysts for the oxygen reduction reaction (ORR) in an acidic environment were prepared from the pyrolysis of acetonitrile at 900 degrees C over alumina and metal-doped alumina. This work includes analyses of the nitrogen-doped carbon preparation process, characterization of the carbon materials formed, and activity testing for the ORR. The nitrogen-containing carbon nanostructures that formed during the pyrolysis of acetonitrile could be purified by washing the product with hydrofluoric acid. A wide range of techniques were used to characterize the solid carbon products of the acetonitrile decomposition. While the samples have many similar physical properties, X-ray photoelectron spectroscopy and transmission electron microscopy showed evidence that differences in the nanostructure and surface functional groups of the samples are likely to account for observed differences in oxygen reduction activity. The most active catalysts were prepared over alumina impregnated with up to 2 wt % Fe, although the catalysts that were prepared by acetonitrile pyrolysis over alumina with no metal doping still had significant activity. In comparison to a 20 wt % platinum on Vulcan carbon catalyst, the most active samples only have an additional 100 mV overpotential. The selectivity of the catalysts for complete oxygen reduction to water followed a trend similar to activity. The best selectivity to water versus peroxide obtained was 99%, or equivalently, an n of 3.98 (i.e., 3.98 electrons transferred out of a maximum of 4 electrons per mole of oxygen that is reduced), as determined by rotating ring-disk electrode testing.     

Design and synthesis of thermally stable single atom catalysts for thermochemical CO2 reduction

The continuous and excessive emission of CO₂ into the atmosphere presents a pressing challenge for global sustainable development.In response,researchers have been devoting significant efforts to develop methods for converting CO₂ into valuable chemicals and fuels.These conversions have the potential to establish a closed artificial carbon cycle and provide an alternative resource to depleting fossil fuels.Among the various conversion routes,thermochemical CO₂ re...     

Nanostructured heterogeneous catalysts for electrochemical reduction of CO2

Electrochemical reduction of CO₂ provides a sustainable solution to address the intermittent renewable electricity storage while recycling CO₂ to produce fuels and chemicals. Highly efficient catalytic materials and reaction systems are required to drive this process economically. This Review highlights the new trends in advancing the electrochemical reduction of CO₂ by developing and designing nanostructured heterogeneous catalysts. The activity, selectivity and reaction mechanism are significantly affected by the nano effects in nanostructured heterogeneous catalysts. In the future, energy efficiency and current density in electrochemical reduction of CO₂ need to be further improved to meet the requirements for practical applications.     

Dual metal atom catalysts: Advantages in electrocatalytic reactions

The dual-metal-atom catalysts(DACs) have aroused much attention as they possess the advantages of single-atom and metal alloy catalysts. And the DACs have exhibited enhanced performance in various electrocatalytic reactions, such as hydrogen/oxygen evolution and oxygen/carbon dioxide/nitrogen reduction. In this review, we mainly overview the latest understanding of the advantages of DACs for these reactions. This review will start with the familiar characterization methods for DACs, then the pri...     

利用探针分子红外光谱识别和表征铂族金属基单原子催化剂

单原子催化是提高贵金属利用率的有效手段,而表征单原子催化剂是理解单原子催化的基础.探针分子红外光谱可用于识别和定量催化剂样品中孤立的Pt族金属物种的浓度,从而得到负载的孤立的Pt族金属物种的局部几何形状、稳定性、活性及其分散性.本文讨论了该技术用于识别和表征含负载型孤立的Pt族金属原子催化剂的效能、应用、以及未来的发展方向.     

Highly durable Pd metal catalysts for the oxygen reduction reaction in fuel cells; coverage of Pd metal with silica

Pd cathode catalysts for polymer electrolyte fuel cells have been covered with silica layers a few nanometres thick. The silica-coated Pd catalysts showed high activity and excellent durability for the oxygen reduction under the severe cathode conditions of PEFCs, while Pd catalysts without silica-coating were seriously deactivated under the same conditions. The coverage of Pd metal with silica prevents the diffusion of Pd species out of the silica layers.     

Biomass-derived synthesis of nitrogen and phosphorus Co-doped mesoporous carbon spheres as catalysts for oxygen reduction reaction

A low-cost, high-performance, and durable catalyst for oxygen reduction reaction (ORR) is prerequisite for the commercialization of fuel cells. Continuous efforts are made to explore nonprecious, efficient catalysts to replace the expensive and scare platinum-based catalysts. Here, we demonstrate a kind of novel nitrogen and phosphorus co-doped mesoporous carbon spheres (NPMCS) synthesized by hydrothermal assisting pyrolysis of food yeast. The as-prepared mesoporous carbon hollow spheres exhibit a high specific surface area of 1223 m² g^?1, deliver an excellent electrocatalytic performance for ORR in alkaline media, superior durability and high resistance to methanol cross-over effect.     

碳辅助金属配合物热解法制备高含量Fe-N4单原子催化剂用于氧还原反应

单原子催化剂凭借其超高的原子利用率及在某些反应中表现出的出色催化效果,被认为是最有前途的电催化剂之一,引起了研究人员的极大热情和兴趣.制备高金属含量的单原子催化剂是基础研究和实际应用的前提和关键.然而,由于原子表面自由能随着尺寸的减小而急剧增加,在制备和催化过程中,单原子催化剂的金属原子很容易聚集成团簇甚至颗粒,因此如...     

Theoretical study on the application of the largest aluminum-pyrazole ring in electrochemical nitrogen reduction reaction

Electrochemical nitrogen reduction reaction (NRR) is a mild and sustainable method for ammonia synthesis. Therefore, developing high activity, selectivity, and economic efficiency catalysts with considering the synergistic effects between catalysts and carriers to design novel structural models is very important. Considering the non-noble metal NRR catalyst, Mo₃, we tried to find a suitable carrier which is stable and economical. Herein, we used the largest atomically precise aluminum-pyrazole ring (AlOC-69) to date (diameter up to 2.3 nm). The larger ring cavities and the presence of abundant hydroxy groups make AlOC-69 an ideal molecular carrier model and provide a basis for studying its structure-activity relationship. The formation energy (−0.76 eV) and stable Mo-O bonds indicate that Mo₃ can be stabilized on the Al₁₀O₁₀ surface. Additionally, N₂ has fully activated due to the strong interaction between the p-orbital of N and the d-orbital of Mo. The low limiting potential (−0.28 V) emerges that Mo₃@Al₁₀O₁₀ has ideal catalytic activity and selectivity. This research provides a promising catalyst model and an understanding of its catalytic process at the atomic level, providing a new approach for the co-design of catalyst and carrier in NRR.