Improving CO2 electroreduction over ZIF-derived carbon doped with Fe-N sites by an additional ammonia treatment

Coupling Atomically Dispersed Fe-N5 Sites with Defective N-Doped Carbon Boosts CO2 Electroreduction

Atomically dispersed iron immobilized on nitrogen-doped carbon catalyst has attracted enormous attention for CO₂ electroreduction, but still suffers from low current density and poor selectivity. Herein, atomically dispersed FeN₅ active sites supported on defective N-doped carbon successfully formed by a multistep thermal treatment strategy with the aid of dicyandiamide are reported. This dual-functional strategy can not only construct intrinsic carbon defects by selectively etching pyridinic-N and pyrrolic-N, but also introduces an additional N from the neighboring carbon layer coordinating to the commonly observed FeN₄ , thus creating an FeN₅ active site supported on defective porous carbon nanofibers (FeN₅ /DPCF) with a local 3D configuration. The optimized FeN₅ /DPCF achieves a high CO Faradaic efficiency (>90%) over a wide potential range of -0.4 to -0.6 V versus RHE with a maximal FE_CO of 93.1%, a high CO partial current density of 9.4 mA cm^-2 at the low overpotential of 490 mV, and a remarkable turnover frequency of 2965 h^-1 . Density functional theory calculations reveal that the synergistic effect between the FeN₅ sites and carbon defects can enhance electronic localization, thus reducing the energy barrier for the CO₂ reduction reaction and suppressing the hydrogen evolution reaction, giving rise to the superior activity and selectivity.

Electroreduction of CO2 toward High Current Density

Carbon dioxide (CO2) electroreduction offers an attractive pathway for converting CO2 to valuable fuels and chemicals. Despite the existence of some excellent electrocatalysts with superior selectivity for specific products, these reactions are conducted at low current densities ranging from several mA cm−2 to tens of mA cm−2, which are far from commercially desirable values. To extend the applications of CO2 electroreduction technology to an industrial scale, long-term operations under high current densities (over 200 mA cm−2) are desirable. In this paper, we review recent major advances toward higher current density in CO2 reduction, including: (1) innovations in electrocatalysts (engineering the morphology, modulating the electronic structure, increasing the active sites, etc.); (2) the design of electrolyzers (membrane electrode assemblies, flow cells, microchannel reactors, high-pressure cells, etc.); and (3) the influence of electrolytes (concentration, pH, anion and cation effects). Finally, we discuss the current challenges and perspectives for future development toward high current densities.

Synthetic strategies for MOF-based single-atom catalysts for photo- and electro-catalytic CO2 reduction

The excessive CO₂ emission has resulted in climate changes, which has threatened human existence. Photocatalytic and electrocatalytic CO₂ reduction, driven by wind electricity and solar energy, are feasible ways of tackling carbon dioxide emission, as both energies are clean and renewable. Single-atom catalyst (SAC) is a candidate owing to excellent electrocatalytic and photocatalytic performance. Methods for preparing an SAC by using metal-organic frameworks (MOFs) as support or precursors are summarized. Also, applications in energy conversion are exhibited. However, the real challenge is to improve the selectivity of catalytic reactions to yield higher value products, which is to be discussed.

Ni-N-Doped Carbon-Modified Reduced Graphene Oxide Catalysts for Electrochemical CO2 Reduction Reaction

Electrochemical CO2 reduction reaction (CO2RR) is eliciting considerable attention in relation to the carbon cycle and carbon neutrality. As for the practical application of CO2RR, the electrocatalyst is a crucial factor, but, even so, designing and synthesizing an excellent catalyst remains a significant challenge. In this paper, the coordination compound of Ni ions and dimethylglyoxime (DMG) was employed as a precursor to modify reduced graphene oxide (rGO) for CO2RR. The textural properties and chemical bonds of as-obtained rGO, N–C–rGO, Ni–rGO, Ni–N–C, and Ni–N–C–rGO materials were investigated in detail, and the role of Ni, N–C, and rGO in the CO2RR were researched and confirmed. Among all the catalysts, the Ni–N–C–rGO showed the optimal catalytic activity and selectivity with a high current density of 10 mA cm−2 and FE(CO)% of 85% at −0.87 V vs. RHE. In addition, there was no obvious decrease in activity for 10 h. Therefore, the Ni–N–C–rGO is a promising catalyst for CO2RR to CO.

From CO2 to Value-Added Products: A Review about Carbon-Based Materials for Electro-Chemical CO2 Conversion

The global warming and the dangerous climate change arising from the massive emission of CO2 from the burning of fossil fuels have motivated the search for alternative clean and sustainable energy sources. However, the industrial development and population necessities make the decoupling of economic growth from fossil fuels unimaginable and, consequently, the capture and conversion of CO2 to fuels seems to be, nowadays, one of the most promising and attractive solutions in a world with high energy demand. In this respect, the electrochemical CO2 conversion using renewable electricity provides a promising solution. However, faradaic efficiency of common electro-catalysts is low, and therefore, the design of highly selective, energy-efficient, and cost-effective electrocatalysts is critical. Carbon-based materials present some advantages such as relatively low cost and renewability, excellent electrical conductivity, and tunable textural and chemical surface, which show them as competitive materials for the electro-reduction of CO2. In this review, an overview of the recent progress of carbon-based electro-catalysts in the conversion of CO2 to valuable products is presented, focusing on the role of the different carbon properties, which provides a useful understanding for the materials design progress in this field. Development opportunities and challenges in the field are also summarized.

Carbon Nanomaterials From Metal-Organic Frameworks: A New Material Horizon for CO2 Reduction

The rise of CO2 in the atmosphere, which results in severe climate change and temperature increase, is known as the major reason for the greenhouse effect. Reducing CO2 to value-added products is an attractive solution to this severe problem, along with addressing the energy crisis, to which the catalysts being employed are of vital importance. Due to their high porosity and tunable compositions, metal-organic frameworks (MOFs) show great potential in energy conversion systems. By thermal or chemical treatment methods, the MOFs are easily turned into MOF-derived carbon nanomaterials. The much higher level of conductivity enables MOF-derived carbon nanomaterials to be employed in CO2 conversion processes. The present review, discusses the state of the art of MOF-derived carbon nanomaterials in CO2 electrochemical, photocatalytic, and thermal reduction applications. The corresponding reaction mechanisms and influence of various factors on catalyst performance are elaborated. Finally, the deficiencies and recommendations are provided for future progress.

Porous Carbon Membrane-Supported Atomically Dispersed Pyrrole-Type FeN4 as Active Sites for Electrochemical Hydrazine Oxidation Reaction

The rational design of catalytically active sites in porous materials is essential in electrocatalysis. Herein, atomically dispersed Fe-N_x sites supported by hierarchically porous carbon membranes are designed to electrocatalyze the hydrazine oxidation reaction (HzOR), one of the key techniques in electrochemical nitrogen transformation. The high intrinsic catalytic activity of the Fe-N_x single-atom catalyst together with the uniquely mixed micro-/macroporous membrane support positions such an electrode among the best-known heteroatom-based carbon anodes for hydrazine fuel cells. Combined with advanced characterization techniques, electrochemical probe experiments, and density functional theory calculation, the pyrrole-type FeN₄ structure is identified as the real catalytic site in HzOR.

Organic-Inorganic Hybrid Nanomaterials for Electrocatalytic CO2 Reduction

Electrochemical CO₂ reduction (ECR) to value-added chemicals and fuels is regarded as an effective strategy to mitigate climate change caused by CO₂ from excess consumption of fossil fuels. To achieve CO₂ conversion with high faradaic efficiency, low overpotential, and excellent product selectivity, rational design and synthesis of efficient electrocatalysts is of significant importance, which dominates the development of ECR field. Individual organic molecules or inorganic catalysts have encountered a bottleneck in performance improvement owing to their intrinsic shortcomings. Very recently, organic-inorganic hybrid nanomaterials as electrocatalysts have exhibited high performance and interesting reaction processes for ECR due to the integration of the advantages of both heterogeneous and homogeneous catalytic processes, attracting widespread interest. In this work, the recent advances in designing various organic-inorganic hybrid nanomaterials at the atomic and molecular level for ECR are systematically summarized. Particularly, the reaction mechanism and structure-performance relationship of organic-inorganic hybrid nanomaterials toward ECR are discussed in detail. Finally, the challenges and opportunities toward controlled synthesis of advanced electrocatalysts are proposed for paving the development of the ECR field.

Synergistic Catalysis over Iron-Nitrogen Sites Anchored with Cobalt Phthalocyanine for Efficient CO2 Electroreduction

Simultaneously achieving high Faradaic efficiency, current density, and stability at low overpotentials is essential for industrial applications of electrochemical CO₂ reduction reaction (CO₂ RR). However, great challenges still remain in this catalytic process. Herein, a synergistic catalysis strategy is presented to improve CO₂ RR performance by anchoring Fe-N sites with cobalt phthalocyanine (denoted as CoPc©Fe-N-C). The potential window of CO Faradaic efficiency above 90% is significantly broadened from 0.18 V over Fe-N-C alone to 0.71 V over CoPc©Fe-N-C while the onset potential of CO₂ RR over both catalysts is as low as -0.13 V versus reversible hydrogen electrode. What is more, the maximum CO current density is increased ten times with significantly enhanced stability. Density functional theory calculations suggest that anchored cobalt phthalocyanine promotes the CO desorption and suppresses the competitive hydrogen evolution reaction over Fe-N sites, while the *COOH formation remains almost unchanged, thus demonstrating unprecedented synergistic effect toward CO₂ RR.