Thickness- and Particle-Size-Dependent Electrochemical Reduction of Carbon Dioxide on Thin-Layer Porous Silver Electrodes

Improved electrochemical reduction of CO2 to syngas with a highly exfoliated Ti3C2Tx MXene-gold composite

Transforming carbon dioxide (CO2) into valuable chemicals via electroreduction presents a sustainable and viable approach to mitigating excess CO2 in the atmosphere. This report provides fresh insights into the design of a new titanium-based MXene composite as a catalyst for the efficient conversion of CO2 in a safe aqueous medium. Despite its excellent electrocatalytic activity towards CO2 reduction and high selectivity for CO production, the high cost of Au and the decline in catalytic activity on a larger scale hinder its large-scale CO2 conversion applications. In this research, we have successfully prepared an Au/Ti3C2Tx composite and tested its catalytic activity in the electrochemical CO2 reduction reaction (ECRR). The as-prepared composite features strong interactions between gold atoms and the MXene support, achieved through the formation of metal-oxygen/carbon bonds. The Au/Ti3C2Tx electrode demonstrated a significant current density of 17.3 mA cm-2 at a potential of -0.42 V vs. RHE, in a CO2 saturated atmosphere (faradaic efficiency: CO = 48.3% and H2 = 25.6%). Nyquist plots further indicated a reduction in the charge-transfer resistance of the Au/Ti3C2Tx layer, signifying rapid charge transfer between the Au and Ti3C2Tx. Furthermore, it is known that liquid crossover through the Gas Diffusion Electrode (GDE) significantly improves CO2 diffusion to catalyst active sites, thereby enhancing CO2 conversion efficiency. The goal of this work is to design an interface between metal and MXene so that CO2 can be electroreduced to fuels and other useful chemical compounds with great selectivity.

Rate-Determining Step for Electrochemical Reduction of Carbon Dioxide into Carbon Monoxide at Silver Electrodes

Silver is one of the most studied electrode materials for the electrochemical reduction of carbon dioxide into carbon monoxide, a product with many industrial applications. There is a growing number of reports in which silver is implemented in gas diffusion electrodes as part of a large-scale device to develop commercially relevant technology. Electrochemical models are expected to guide the design and operation toward cost-efficient devices. Despite decades of investigations, there are still uncertainties in the way this reaction should be modeled due to the absence of scientific consensus regarding the reaction mechanism and the nature of the rate-determining step. We review previously reported studies to draw converging conclusions on the value of the Tafel slope and existing species at the electrode surface. We also list conflicting experimental observations and provide leads to tackling these remaining questions.

Effect of pore diameter and length on electrochemical CO2 reduction reaction at nanoporous gold catalysts

In this work, we employ differential electrochemical mass spectrometry (DEMS) to track the real-time evolution of CO at nanoporous gold (NpAu) catalysts with varying pore parameters (diameter and length) during the electrochemical CO₂ reduction reaction (CO₂RR). We show that due to the increase in the local pH with increasing catalyst roughness, NpAu catalysts suppress the bicarbonate-mediated hydrogen evolution reaction (HER) compared to a flat Au electrode. Additionally, the geometric current density for CO₂RR increases with the roughness of NpAu catalysts, which we attribute to the increased availability of active sites at NpAu catalysts. Together, the enhancement of CO₂RR and the suppression of competing HER results in a drastic increase in the faradaic selectivity for CO₂RR with increasing pore length and decreasing pore diameter, reaching near 100% faradaic efficiency for CO in the most extreme case. Interestingly, unlike the geometric current density, the specific current density for CO₂RR has a more complicated relation with the roughness of the NpAu catalysts. We show that this is due to the presence of ohmic drop effects along the length of the porous channels. These ohmic drop effects render the pores partially electrocatalytically inactive and hence, they play an important role in tuning the CO₂RR activity on nanoporous catalysts.

In this work, we employ differential electrochemical mass spectrometry (DEMS) to track the real-time evolution of CO at nanoporous gold (NpAu) catalysts with varying pore parameters (diameter and length) during the electrochemical CO₂ reduction reaction (CO₂RR).

Nanoporous bismuth for the electrocatalytic reduction of CO2 to formate

Bi is an attractive catalyst towards the electrochemical reduction of CO₂ to formate. In this work, nanoporous bismuth was prepared by dealloying Mg₃Bi₂ with tartaric acid (TA) solution, and the size of the primary Bi nanoparticles was adjusted according to the concentration of TA. When the concentration of TA increased from 2 wt% to 20 wt%, the particle size of Bi increased from about 70 nm to 400 nm. The synthesized nanoporous Bi samples were investigated as electrocatalysts for the reduction of CO₂ in KHCO₃ electrolyte, and it was found that the smaller the particle size, the higher the catalytic activity. However, nanoporous Bi comprising 70 nm particles suffered from mass transfer difficulty and sintering during the reaction, whereas the 100 nm nanoporous Bi delivered both a high formate formation current and faradaic efficiency (FE) (16 mA cm^-2, FE > 90% at -0.88 V vs. RHE) and showed excellent durability.

Electrocatalysis for CO2 conversion: from fundamentals to value-added products

This timely and comprehensive review mainly summarizes advances in heterogeneous electroreduction of CO₂: from fundamentals to value-added products.

Solid Nanoporosity Governs Catalytic CO2 and N2 Reduction

Global demand for green and clean energy is increasing day by day owing to ongoing developments by the human race that are changing the face of the earth at a rate faster than ever. Exploring alternative sources of energy to replace fossil fuel consumption has become even more vital to control the growing concentration of CO₂, and reduction of CO₂ into CO or other useful hydrocarbons (e.g., C₁ and C_≥2 products), as well as reduction of N₂ into ammonia, can greatly help in this regard. Various materials have been developed for the reduction of CO₂ and N₂. The introduction of pores in these materials by porosity engineering has been demonstrated to be highly effective in increasing the efficiency of the involved redox reactions, over 40% increment for CO₂ reduction to date, by providing an increased number of exposed facets, kinks, edges, and catalytically active sites of catalysts. By shaping the surface porous structure, the selectivity of the redox reaction can also be enhanced. In order to better understand this area benefiting rational design for future solutions, this review systematically summarizes and constructively discusses the porosity engineering in catalytic materials, including various synthesis methods, characterization of porous materials, and the effects of porosity on performance of CO₂ reduction and N₂ reduction.

Photoelectrochemical Reduction of CO2 to Syngas by Reduced Ag Catalysts on Si Photocathodes

The photoelectrochemical reduction of CO2 to syngas that is used for many practical applications has been emerging as a promising technique to relieve the increase of CO2 in the atmosphere. Si has been considered to be one of the most promising materials for photoelectrodes, but the integration of electrocatalysts is essential for the photoelectrochemical reduction of CO2 using Si. We report an enhancement of catalytic activity for CO2 reduction reaction by Ag catalysts of tuned morphology, active sites, and electronic structure through reducing anodic treatment. Our proposed photocathode structure, a SiO2 patterned p-Si photocathode with these reduced Ag catalysts, that was fabricated using electron-beam deposition and electrodeposition methods, provides a low onset-potential of −0.16 V vs. the reversible hydrogen electrode (RHE), a large saturated photocurrent density of −9 mA/cm2 at −1.23 V vs. RHE, and faradaic efficiency for CO of 47% at −0.6 V vs. RHE. This photocathode can produce syngas in the ratio from 1:1 to 1:3, which is an appropriate proportion for practical application. This work presents a new approach for designing photocathodes with a balanced catalytic activity and light absorption to improve the photoelectrochemical application for not only CO2 reduction reaction, but also water splitting or N2 reduction reaction.

Rational Design of Ag-Based Catalysts for the Electrochemical CO2 Reduction to CO: A Review

The selective electrochemical CO₂ reduction (ECR) to CO in aqueous electrolytes has gained significant interest in recent years due to its capability to mitigate the environmental issues associated with CO₂ emission and to convert renewable energy such as wind and solar power into chemical energy as well as its potential to realize the commercial use of CO₂ . In view of the thermodynamic stability and kinetic inertness of CO₂ molecules, the exploitation of active, selective, and stable catalysts for the ECR to CO is crucial to promote the reaction efficiency. Indeed, plenty of electrocatalysts for the selective ECR to CO have been explored, of which Ag is known as the most promising electrocatalyst for large-scale ECR to CO due to several competitive advantages including high catalytic performance, low price, and rich reserves compared with other metal counterparts. To provide useful guidelines for the further development of efficient catalysts for the ECR to CO, a comprehensive summary of the recent progress of Ag-based electrocatalysts is presented in this Review. Different modification strategies of Ag-based electrocatalysts are highlighted, including exposure of crystal facets, tuning of morphology and size, introduction of support materials, alloying with other metals, and surface modification with functional groups. The reaction mechanisms involved in these different modification strategies of Ag-based electrocatalysts are also discussed. Finally, the prospects for the development of next-generation Ag-based electrocatalysts are proposed in an effort to facilitate the industrialization of ECR to CO.

Tailoring Copper Foam with Silver Dendrite Catalysts for Highly Selective Carbon Dioxide Conversion into Carbon Monoxide

The present study outlines the important steps to bring electrochemical conversion of carbon dioxide (CO₂) closer to commercial viability by using a large-scale metallic foam electrode as a highly conductive catalyst scaffold. Because of its versatility, it was possible to specifically tailor three-dimensional copper foam through coating with silver dendrite catalysts by electrodeposition. The requirements of high-yield CO₂ conversion to carbon monoxide (CO) were met by tuning the deposition parameters toward a homogeneous coverage of the copper foam with nanosized dendrites, which additionally featured crystallographic surface orientations favoring CO production. The presented results evidence that Ag dendrites, owing a high density of planes with stepped (220) surface sites, paired with the superior active surface area of the copper foam can significantly foster the CO productivity. In a continuous flow-cell reactor setup, CO Faradaic efficiencies reaching from 85 to 96% for a wide range of low applied cathode potentials (<1.0 V_RHE) along with high CO current densities up to 27 mA/cm² were achieved, far outperforming other tested scaffold materials. Overall, this research provides new strategic guidelines for the fabrication of efficient and versatile cathodes for CO₂ conversion compatible with large-scale integrated prototype devices.

Progress and Perspective of Electrocatalytic CO2 Reduction for Renewable Carbonaceous Fuels and Chemicals

The worldwide unrestrained emission of carbon dioxide (CO2) has caused serious environmental pollution and climate change issues. For the sustainable development of human civilization, it is very desirable to convert CO2 to renewable fuels through clean and economical chemical processes. Recently, electrocatalytic CO2 conversion is regarded as a prospective pathway for the recycling of carbon resource and the generation of sustainable fuels. In this review, recent research advances in electrocatalytic CO2 reduction are summarized from both experimental and theoretical aspects. The referred electrocatalysts are divided into different classes, including metal-organic complexes, metals, metal alloys, inorganic metal compounds and carbon-based metal-free nanomaterials. Moreover, the selective formation processes of different reductive products, such as formic acid/formate (HCOOH/HCOO-), monoxide carbon (CO), formaldehyde (HCHO), methane (CH4), ethylene (C2H4), methanol (CH3OH), ethanol (CH3CH2OH), etc. are introduced in detail, respectively. Owing to the limited energy efficiency, unmanageable selectivity, low stability, and indeterminate mechanisms of electrocatalytic CO2 reduction, there are still many tough challenges need to be addressed. In view of this, the current research trends to overcome these obstacles in CO2 electroreduction field are summarized. We expect that this review will provide new insights into the further technique development and practical applications of CO2 electroreduction.

Imidazolium Ions with an Alcohol Substituent for Enhanced Electrocatalytic Reduction of CO2

Electrochemical reduction of CO₂ to CO is an attractive process for carbon capture and energy regeneration. Imidazolium ionic liquids (IMILs) are promising electrolyte catalytically active additives for this important reaction. Herein, we report functionalizing the imidazolium ion with a propanol substituent at the N site can significantly enhance the catalytic activity of IMILs, causing a positive shift of the onset potential for the CO₂ reduction by about 90 mV in an acetonitrile electrolyte. Theoretical calculations indicated that the propanol hydroxyl could bridge a local hydrogen-bonding chain as shortcut for proton transfer, leading to a dramatic decrease of the activation barrier for the catalytic reduction of CO₂ in IMIL.