Heterogeneous electrochemical CO2 reduction using nonmetallic carbon-based catalysts: current status and future challenges

Nanoscale Metal Particle Modified Single-Atom Catalyst: Synthesis, Characterization, and Application

Single-atom catalysts (SACs) have attracted considerable attention in heterogeneous catalysis because of their well-defined active sites, maximum atomic utilization efficiency, and unique unsaturated coordinated structures. However, their effectiveness is limited to reactions requiring active sites containing multiple metal atoms. Furthermore, the loading amounts of single-atom sites must be restricted to prevent aggregation, which can adversely affect the catalytic performance despite the high activity of the individual atoms. The introduction of nanoscale metal particles (NMPs) into SACs (NMP-SACs) has proven to be an efficient approach for improving their catalytic performance. A comprehensive review is urgently needed to systematically introduce the synthesis, characterization, and application of NMP-SACs and the mechanisms behind their superior catalytic performance. This review first presents and classifies the different mechanisms through which NMPs enhance the performance of SACs. It then summarizes the currently reported synthetic strategies and state-of-the-art characterization techniques of NMP-SACs. Moreover, their application in electro/thermo/photocatalysis, and the reasons for their superior performance are discussed. Finally, the challenges and perspectives of NMP-SACs for the future design of advanced catalysts are addressed.

Controlling the C1/C2+ product selectivity of electrochemical CO2 reduction upon tuning bimetallic CuIn electrocatalyst composition and operating conditions

Electrochemical carbon dioxide (CO2) reduction (eCO2R) over Cu-based bimetallic catalysts is a promising technique for converting CO2 into value-added multi-carbon products, such as fuels, chemicals, and materials. For improving the process efficiency, electrocatalyst development for the eCO2R must be integrated with tuning of operating conditions. For example, CuIn-based materials typically lead to preferential C1 product selectivity, which delivers the desired C2+ products upon varying the In/Cu ratio and operating conditions (i.e., in 0.1 M KHCO3 electrolytes using an H-type cell with a cation exchange membrane vs. in 1 M KOH electrolytes using a flow cell with an anion exchange membrane). At lower Cu-loading (i.e., InCu5Ox material), the maximum faradaic efficiency of HCOOH (FEHCOOH) of 70% was achieved at -1 V versus the reversible hydrogen electrode (vs. RHE) in an H-type cell. However, upon increasing the Cu loading, the preferential product selectivity could be altered: the InCu73Ox material led to a high CO selectivity (maximum FE of 51%) in the H-type cell at -0.8 V vs. RHE and delivered a current density of 100 mA cm-2 with a FEC2+ of up to 37% at -0.8 V vs. RHE in the flow cell configuration. Various characterization tools were also employed to probe the catalytic materials to rationalize the electrocatalytic performance.

Fabrication and characterization of ZnGa1.01Te2.13/g-C3N4 heterojunction with enhanced photocatalytic activity

The extensive consumption of fossil fuels increases CO2 concentration in the atmosphere, resulting in serious global warming problems. Meanwhile, the problem of water contamination by organic substances is another significant global challenge. We have successfully synthesized ZnGa1.01Te2.13/g-C3N4 (ZGT/GCN) composites for the first time as effective photocatalysts for both pollutant degradation and CO2 reduction. ZGT/GCN composites were synthesized by a simple hydrothermal method. The prepared photocatalysts were characterized by XRD, SEM, TEM-EDS, DRS, BET, PL, and XPS. The ZGT/GCN heterojunction exhibited considerably enhanced photocatalytic activity in the degradation of crystal violet (CV) as well as in the photoreduction of CO2 when compared to pure ZGT and GCN semiconductors. The optimal rate constant for CV degradation was obtained with the ZGT-80%GCN composite (0.0442 h-1), which is higher than the constants obtained with individual ZGT and GCN by 7.75 and 1.63 times, respectively. Moreover, the CO2 reduction yields into CH4 by ZGT-80%GCN was 1.013 μmol/g in 72 h, which is 1.21 and 1.08 times larger than the yields obtained with ZGT and GCN. Scavenger and ESR tests were used to propose the photocatalytic mechanism of the ZGT/GCN composite as well as the active species in the CV degradation.

Unravelling the Effect of Activators used in The Synthesis of Biomass-Derived Carbon Electrocatalysts on the Electrocatalytic Performance for CO2 Reduction

N-doped carbon materials can be efficient and cost-effective catalysts for the electrochemical CO2 reduction reaction (CO2 RR). Activators are often used in the synthesis process to increase the specific surface area and porosity of these carbon materials. However, owing to the diversity of activators and the differences in physicochemical properties that these activators induce, the influence of activators used for the synthesis of N-doped carbon catalysts on their electrochemical performance is unclear. In this study, a series of bagasse-derived N-doped carbon catalysts is prepared with the assistance of different activators to understand the correlation between activators, physicochemical properties, and electrocatalytic performance for the CO2 RR. The properties of N-doped carbon catalysts, such as N-doping content, microstructure, and degree of graphitization, are found to be highly dependent on the type of activator applied in the synthesis procedure. Moreover, the overall CO2 RR performance of the synthesized electrocatalysts is not determined only by the N-doping level and the configuration of the N-dopant, but rather by the overall surface chemistry, where the porosity and the degree of graphitization are jointly responsible for significant differences in CO2 RR performance.

Heusler alloy catalysts for electrochemical CO2 reduction

Developing efficient catalysts for electrochemical CO2 reduction reaction (ECO2RR) to hydrocarbons is becoming increasingly important but still challenging due to their high overpotential and poor selectivity. Here, the famous Heusler alloys are investigated as ECO2RR catalysts for the first time by means of density functional theory calculations. The linear scaling relationship between the adsorption energies of CHO (and COOH) and CO intermediates is broken and, thus, the overpotential can be tuned regularly by chemically permuting different 3 d, 4 d, or 5 d transition metals (TMs) in Heusler alloy Cu2TMAl. Cu2ZnAl shows the best activity among all the 30 Heusler alloys considered in the present study, with 41% improvement in energy efficiency compared to pure Cu electrode. Cu2PdAl, Cu2AgAl, Cu2PtAl, and Cu2AuAl are also good candidates. The calculations on the competition between hydrogen evolution reaction and CO2RR indicate that Cu2ZnAl is also the one having the best selectivity toward hydrocarbons. This work identifies the possibility of applying the Heusler alloy as an efficient ECO2RR catalyst. Since thousands of Heusler alloys have been found in experiments, the present study also encourages the search for more promising candidates in this broad research area.

Recent advances in low-temperature electrochemical conversion of carbon dioxide

Since the onset of the industrial revolution, fossil fuels have been the primary source of energy generation, and the continued exploitation of fossil fuels has led to an increase in the amount of atmospheric carbon dioxide. A lot of research currently focuses much on decreasing dependence on fossil fuels by replacing them with green energy. However, this technique poses a number of challenges, such as the need for improved infrastructure and technology and the high market penetration of renewable energy technologies. Capturing and converting carbon dioxide using electrochemical approaches can help to stabilize atmospheric greenhouse gas levels and create a positive future for the transformation of carbon dioxide into a number of value-added products. The conversion of carbon dioxide via electrochemical approach is a major challenge, and consideration must be given to the development and production of low-cost, stable, and highly efficient electrocatalysts. Hence, this review presents an overview of the current developments in the electrochemical conversion of carbon dioxide. In addition, this study discusses the current progress of electrocatalysts, in particular, the homogeneous and heterogeneous catalyst, which has a high level of activity and selectivity of low overpotential preferred products. The overview of the mechanisms and kinetics of the carbon dioxide reduction using the computational method are also addressed.

Proton Capture Strategy for Enhancing Electrochemical CO2 Reduction on Atomically Dispersed Metal-Nitrogen Active Sites*

Electrocatalysts play a key role in accelerating the sluggish electrochemical CO₂ reduction (ECR) involving multi-electron and proton transfer. We now develop a proton capture strategy by accelerating the water dissociation reaction catalyzed by transition-metal nanoparticles (NPs) adjacent to atomically dispersed and nitrogen-coordinated single nickel (Ni-N_x ) active sites to accelerate proton transfer to the latter for boosting the intermediate protonation step, and thus the whole ECR process. Aberration-corrected scanning transmission electron microscopy, X-ray absorption spectroscopy, and calculations reveal that the Ni NPs accelerate the adsorbed H (H_ad ) generation and transfer to the adjacent Ni-N_x sites for boosting the intermediate protonation and the overall ECR processes. This proton capture strategy is universal to design and prepare for various high-performance catalysts for diverse electrochemical reactions even beyond ECR.

Graphene-supported single-atom catalysts and applications in electrocatalysis

Supported metal nanostructures are the most extensively studied heterogeneous catalysts, benefiting from easy separation, regeneration and affordable cost. The size of the supported metal species is one of the decisive factors in determining the activity of heterogeneous catalysts. Particularly, the unsaturated coordination environment of metal atoms preferably act as the active centers, minimizing these metal species can significantly boost the specific activity of every single metal atom. Single-atom catalysts/catalysis (SACs), containing isolated metals atomically dispersed on or coordinated with the surface of a support material, represent the ultimate utilization of supported metals and maximize metal usage efficiency. Graphene, a two-dimensional star material, exhibiting extraordinary physical and chemical properties, has been approved as an excellent platform for constructing SACs. When atomically dispersed metal atoms are strongly anchored on the graphene surface, featuring ultra-high surface area and excellent electronic properties, SACs offer a great potential to significantly innovate the conventional heterogeneous catalysis, especially in the field of electrocatalysis. In this review, a detailed discussion of graphene-supported SACs, including preparation approaches, characterization techniques and applications on typical electrocatalytic reactions is provided. The advantages and unique features of graphene-supported SACs as efficient electrocatalysts and the upcoming challenges for improving their performance and further practical applications are also highlighted.

Recent Advances in the Electroreduction of CO2 over Heteroatom-Doped Carbon Materials

Ever-growing anthropogenic activity has increased global energy demands, resulting in growing concentrations of greenhouse gases such as CO2 in the atmosphere. The electroreduction of CO2 has been proposed as a potential solution for reducing anthropogenic CO2 emissions. Despite the promising results obtained so far, some limitations hinder large-scale applications, especially those associated with the activity and selectivity of electrocatalysts. A good number of metal catalysts have been studied to overcome this limitation, but the high cost and low earth abundance of some of these materials are important barriers. In this sense, carbon materials doped with heteroatoms such as N, B, S, and F have been proposed as cheaper and widely available alternatives to metal catalysts. This review summarizes the latest advances in the utilization of carbon-doped materials for the electroreduction of CO2, with a particular emphasis on the synthesis procedures and the electrochemical performance of the resulting materials.

Efficient electrocatalytic CO2 reduction to CO with high selectivity using a N-doped carbonized wood membrane

A N-doped carbonized wood membrane (N-CWM) is developed and directly used as a self-supported electrode for CO₂ electroreduction to CO.

Solar driven reduction of CO2 using Pt-Cu/C as a catalyst in a photoelectrochemical cell: experiment and mechanism study

Carbon supported nano-metal catalysts are expected to improve CO₂ reduction selectivity and efficiency due to the addition of more active sites and enhancement of electron transport ability. In this study, HKUST-1 was pyrolyzed and decorated with Pt to prepare Pt-Cu/C catalysts. The catalytic effect of the catalysts with different Pt contents in the CO₂ photoeletrochemical reduction reaction (CO₂PRR) were compared. The total carbon atom conversion rate in CO₂PRR experiments using Pt-Cu/C catalysts first increased to a peak when using 1.6 wt% Pt-Cu/C catalyst and then decreased with the increase of Pt content. The 1.6 wt% Pt-Cu/C catalyst showed good hydrogen evolution reaction (HER) inhibiting ability compared with other Pt-Cu/C catalysts. Density functional theory (DFT) calculations were conducted to give an insight into the CO₂PRR mechanism on some possible active sites in Pt-Cu/C catalysts. The result demonstrated that HER was more likely to be inhibited on the Cu/Pt active surface and at the same time CO₂PRR was promoted.

Two-Dimensional Materials on the Rocks: Positive and Negative Role of Dopants and Impurities in Electrochemistry

Two-dimensional (2D) materials, such as graphene and transition-metal chalcogenides, were shown in many works as very potent catalysts for industrially important electrochemical reactions, such as oxygen reduction, hydrogen and oxygen evolution, and carbon dioxide reduction. We critically discuss here the development in the field, showing that not only dopants but also impurities can have dramatic effects on catalysis. Note here that the difference between dopant and impurity is merely semantic-dopant is an impurity deliberately added to the material. We contest the general belief that all doping has a positive effect on electrocatalysis. We show that in many cases, dopants actually inhibit the electrochemistry of 2D materials. This review provides a balanced view of the field of 2D materials electrocatalysis.

N-CND modified NH2-UiO-66 for photocatalytic CO2 conversion under visible light by a photo-induced electron transfer process

A N-CNDs/NH₂-UiO-66 composite exhibits enhanced photocatalytic activity for CO₂ conversion under visible light irradiation. This is attributed to a photo-induced electron transfer process from the light absorption antenna of the N-CNDs to the NH₂-UiO-66 photocatalyst.

Single Sb sites for efficient electrochemical CO2 reduction

We report the facile synthesis of Sb single atoms for efficient electrocatalytic CO₂ reduction to CO.

Synergistic catalysis of CuO/In2O3 composites for highly selective electrochemical CO2 reduction to CO

We demonstrate synergistic catalysis of CuO and In₂O₃ for efficient electrochemical CO₂ reduction to CO.

Carbon-supported Ni nanoparticles for efficient CO2 electroreduction

The development of highly selective, low cost, and energy-efficient electrocatalysts is crucial for CO2 electrocatalysis to mitigate energy shortages and to lower the global carbon footprint. Herein, we first report that carbon-coated Ni nanoparticles supported on N-doped carbon enable efficient electroreduction of CO2 to CO. In contrast to most previously reported Ni metal catalysts that resulted in severe hydrogen evolution during CO2 conversion, the Ni particle catalyst here presents an unprecedented CO faradaic efficiency of approximately 94% at an overpotential of 0.59 V, even comparable to that of the best single Ni sites. The catalyst also affords a high CO partial current density and a large CO turnover frequency, reaching 22.7 mA cm-2 and 697 h-1 at -1.1 V (versus the reversible hydrogen electrode), respectively. Experiments combined with density functional theory calculations showed that the carbon layer coated on Ni and N-dopants in carbon material both play important roles in improving catalytic activity for electrochemical CO2 reduction to CO by stabilizing *COOH without affecting the easy *CO desorption ability of the catalyst.

Nanocellulose: Recent advances and its prospects in environmental remediation

Among many other sustainable functional nanomaterials, nanocellulose is drawing increasing interest for use in environmental remediation technologies due to its numerous unique properties and functionalities. Nanocellulose is usually derived from the disintegration of naturally occurring polymers or produced by the action of bacteria. In this review, some invigorating perspectives on the challenges, future direction, and updates on the most relevant uses of nanocellulose in environmental remediation are discussed. The reported applications and properties of nanocellulose as an adsorbent, photocatalyst, flocculant, and membrane are reviewed in particular. However, additional effort will be required to implement and commercialize nanocellulose as a viable nanomaterial for remediation technologies. In this regard, the main challenges and limitations in working with nanocellulose-based materials are identified in an effort to improve the development and efficient use of nanocellulose in environmental remediation.

Preparation of a Cu(BTC)-rGO catalyst loaded on a Pt deposited Cu foam cathode to reduce CO2 in a photoelectrochemical cell

To increase the reaction productivity and selectivity of the CO₂ photoelectrochemical reduction reaction, a Cu (benzene 1,3,5-tricarboxylic acid [BTC])-reduced graphite oxide (rGO) catalyst was prepared by using a facile hydrothermal method and used in a CO₂ photoelectrochemical cell (PEC) as a cathode catalyst. Characterization of the catalyst proved that successfully bonding of rGO to Cu(BTC) not only facilitated faster transfer of electrons on the surface of the catalyst but also created more active sites. CO₂ photoelectrochemical reduction experimental results showed that the total carbon atom conversion rate was up to 3256 nmol h⁻¹ cm⁻² which was much higher than when pure Cu(BTC) was used as a cathode catalyst. The liquid product's selectivity to alcohols was up to 95% when −2 V voltage was applied to the system with Cu(BTC)-rGO used as the cathode catalyst.

Schematic of a photoelectrochemical cell for CO₂ reduction: the H⁺ generation process and the CO₂ process run in two separated chambers respectively.

Tuning the Pd-catalyzed electroreduction of CO2to CO with reduced overpotential

The electrochemical CO₂reduction to CO can be greatly enhanced by controlling the Pd–ceria interface and doping with tellurium atoms.

Boron-doped graphene as a promising electrocatalyst for NO electrochemical reduction: a computational study

The B-doped graphene is a quite promising metal free electrocatalyst for NO reduction to N₂O and NH₃.