A novel N,Fe-Decorated carbon nanotube/carbon nanosheet architecture for efficient CO2 reduction

The integration of bio-catalysis and electrocatalysis to produce fuels and chemicals from carbon dioxide

The dependence on fossil fuels has caused excessive emissions of greenhouse gases (GHGs), leading to climate changes and global warming. Even though the expansion of electricity generation will enable a wider use of electric vehicles, biotechnology represents an attractive route for producing high-density liquid transportation fuels that can reduce GHG emissions from jets, long-haul trucks and ships. Furthermore, to achieve immediate alleviation of the current environmental situation, besides reducing carbon footprint it is urgent to develop technologies that transform atmospheric CO₂ into fossil fuel replacements. The integration of bio-catalysis and electrocatalysis (bio-electrocatalysis) provides such a promising avenue to convert CO₂ into fuels and chemicals with high-chain lengths. Following an overview of different mechanisms that can be used for CO₂ fixation, we will discuss crucial factors for electrocatalysis with a special highlight on the improvement of electron-transfer kinetics, multi-dimensional electrocatalysts and their hybrids, electrolyser configurations, and the integration of electrocatalysis and bio-catalysis. Finally, we prospect key advantages and challenges of bio-electrocatalysis, and end with a discussion of future research directions.

Facile Synthesis of Fe@C Loaded on g-C3N4 for CO2 Electrochemical Reduction to CO with Low Overpotential

Electrochemical CO₂ reduction has been acknowledged as a hopeful tactic to alleviate environmental and global energy crises. Herein, we designed an Fe@C/g-C₃N₄ heterogeneous nanocomposite material by a simple one-pot method, which we applied to the electrocatalytic CO₂ reduction reaction (ECR). Our optimized 20 mg-Fe@C/g-C₃N₄-1100 catalyst displays excellent performance for the ECR and a maximum Faradaic efficiency (FE) of 88% with a low overpotential of -0.38 V vs. RHE. The Tafel slope reveals that the first electron transfer, which involves a surface-adsorbed *COOH intermediate, is the rate-determining step for 20 mg-Fe@C/C₃N₄-1100 during the ECR. More precisely, the coordinating capability of the g-C₃N₄ framework and Fe@C species as a highly active site promote the intermediate product transmission. These results indicate that the combination of temperature adjustment and precursor optimization is key to facilitating the ECR of an iron-based catalyst.

NiFe-CN catalysts derived from Solid-phase Exfoliation of NiFe-Layered Double Hydroxide for CO2 Electroreduction

The development of efficient carbon dioxide reducing reaction (CO2RR) catalysts is one of the practical solutions to environmental problems. Usually metal-doped catalysts were used for CO2RR, but the metal elements...

Controlled assembly of nitrogen-doped iron carbide nanoparticles on reduced graphene oxide for electrochemical reduction of carbon dioxide to syngas

The electrocatalytic CO₂ reduction reaction (CO₂RR) decreases the amount of greenhouse gas in the atmosphere while enabling a closed carbon cycle. Herein, iron oleate was used as a precursor to produce oleic acid-coated triiron tetraoxide nanoparticles (Fe₃O₄@OA NPs) by pyrolysis, which was then assembled with reduced graphene oxide (rGO) and doped with dicyandiamide as a nitrogen source to obtain nitrogen-doped iron carbide nanoparticles assembled on rGO (N-Fe₃C/rGO NPs). The catalyst prepared by nitrogen doping at 800 °C with an Fe₃O₄@OA NPs to rGO weight ratio of 20:1 showed good activity and stability for the CO₂RR. At -0.3 to -0.4 V, the H₂/CO ratio of the product from the catalyzed CO₂RR was close to 2; thus, the product can be used for Fischer-Tropsch synthesis. The results of a series of experiments and X-ray photoelectron spectroscopy analysis showed that the synergy between the CN and FeN groups in the catalyst can promote the reduction of CO₂ to CO. This work demonstrates a facile method for improving the catalytic reduction of CO₂.

Non-Innocent Role of Porous Carbon Toward Enhancing C2-3 Products in the Electroreduction of Carbon Dioxide

Selectivity for C-C coupled products remains a major challenge for electrochemical CO₂ reduction. Herein, we report a facile method by modifying a Cu foil surface with a layer of porous carbon. The structure of carbon has a major influence on C₁ and C_2,3 product selectivity. A carbon aerogel modifier leads to higher C_2,3 product formation than that of a carbon black modifier, demonstrating the non-innocent role of carbon materials. In both cases, major surface reconstruction on the Cu foil-such as pitting and particle formation-is observed during electrocatalysis. In addition, the restructured Cu surface shows distinctly lower activity toward CO₂ reduction when the carbon modifier is removed. This is likely due to the fact that the carbon modifiers influence the product distribution by (i) modulating the local pH and CO₂ concentration by serving as a highly porous and hydrophobic barrier, and (ii) restructuring the metal surface that generates more active sites. Our findings illustrate that the carbon in carbon-based catalysts can have an disproportionate role in directing product formation in electrocatalytic carbon dioxide reduction.

Atomically Dispersed Iron-Nitrogen Sites on Hierarchically Mesoporous Carbon Nanotube and Graphene Nanoribbon Networks for CO2 Reduction

Atomically dispersed metal and nitrogen co-doped carbon (M-N/C) catalysts hold great promise for electrochemical CO₂ conversion. However, there is a lack of cost-effective synthesis approaches to meet the goal of economic mass production of single-atom M-N/C with desirable carbon support architecture for efficient CO₂ reduction. Herein, we report facile transformation of commercial carbon nanotube (CNT) into isolated Fe-N₄ sites anchored on carbon nanotube and graphene nanoribbon (GNR) networks (Fe-N/CNT@GNR). The oxidization-induced partial unzipping of CNT results in the generation of GNR nanolayers attached to the remaining fibrous CNT frameworks, which reticulates a hierarchically mesoporous complex and thus enables a high electrochemical active surface area and smooth mass transport. The Fe residues originating from CNT growth seeds serve as Fe sources to form isolated Fe-N₄ moieties located at the CNT and GNR basal plane and edges with high intrinsic capability of activating CO₂ and suppressing hydrogen evolution. The Fe-N/CNT@GNR delivers a stable CO Faradaic efficiency of 96% with a partial current density of 22.6 mA cm^-2 at a low overpotential of 650 mV, making it one of the most active M-N/C catalysts reported. This work presents an effective strategy to fabricate advanced atomistic catalysts and highlights the key roles of support architecture in single-atom electrocatalysis.

Nonprecious Catalyst for Three-Phase Contact in a Proton Exchange Membrane CO2 Conversion Full Cell for Efficient Electrochemical Reduction of Carbon Dioxide

Development of a cost-effective and highly efficient electrocatalyst is essential but challenging in order to convert carbon dioxide to value-added chemicals at ambient conditions. In the current work, the activity of a full electrochemical cell has been demonstrated, utilizing a proton exchange membrane CO₂ conversion cell that can selectively convert carbon dioxide to a value-added chemical (formic acid) at room temperature and pressure. A cost-effective, nonprecious-metal-based electrocatalyst, nitrogen-doped carbon nanotubes encapsulating Fe₃C nanoparticles (Fe₃C@NCNTs), has been reported to exhibit superior catalytic activity toward the electrochemical CO₂ reduction reaction (CO₂RR). A facile one-step synthesis procedure has been undertaken to synthesize Fe₃C@NCNTs. CO₂ adsorption takes place via sharing of charge between the nucleophilic anchoring site (Fe₃C) and the electrophilic C site of CO₂, as shown by the DFT studies. The porous architecture, unique tubular structure, high graphitization degree, and appropriate doping of the Fe₃C-encapsulating NCNTs allow better three-phase contact of CO₂ (gas), H₂O (liquid), and catalyst (solid), which can enhance the electrocatalytic activity of the cell, as demonstrated by the experimental findings. The cell was tested under a continuous flow of CO₂ gas and has been demonstrated to produce a good amount of formic acid (HCOOH). The production of formic acid was examined by utilizing UV-vis spectroscopy and high-performance liquid chromatography (HPLC). A series of designed experiments disclosed that the maximum yield of formic acid was as high as 90% with Fe₃C@NCNTs as both anode and cathode catalysts. Technology to scale up the reduction procedure has also been proposed and shown in this particular work. These unique observations open a route for the development of cost-effective and highly active platinum-free electrocatalysts for the CO₂RR.