Effects of substrate porosity in carbon aerogel supported copper for electrocatalytic carbon dioxide reduction

Carbon Aerogels as Electrocatalysts for Sustainable Energy Applications: Recent Developments and Prospects

Carbon aerogel (CA) based materials have multiple advantages, including high porosity, tunable molecular structures, and environmental compatibility. Increasing interest, which has focused on CAs as electrocatalysts for sustainable applications including oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and CO₂ reduction reaction (CO₂RR) has recently been raised. However, a systematic review covering the most recent progress to boost CA-based electrocatalysts for ORR/OER/HER/CO₂RR is now absent. To eliminate the gap, this critical review provides a timely and comprehensive summarization of the applications, synthesis methods, and principles. Furthermore, prospects for emerging synthesis, screening, and construction methods are outlined.

Processing of aerogels and their applications toward CO2 adsorption and electrochemical reduction: a review

Aerogels are a unique class of nanoporous ultralight materials exhibiting wide range of textural characteristic properties and tunable porosities. Due to their remarkable features such as low density, high surface area, low refractive index, small thermal conductivity, low dielectric constant and low sound velocity, they exhibit a wide range of applications in different areas such as electronics, thermal and acoustic insulation, chemistry, biomedicine and optics. The special advantages of these materials are that they can be produced in different forms such as monoliths/granular, bead/microspheres, thin films or sheets and as blankets. Aerogels are found to be potential materials for the removal of CO₂ through adsorption or electrochemical reduction. There is a plethora of research on different kinds of aerogels used for CO₂ adsorption process. Research has been going on toward the development of aerogel-based electrocatalyst, which can be used for valorization of CO₂ through electrochemical reduction methods. Although most of the review papers have covered applications of aerogels in CO₂ capture, very few discuss the processing of aerogels, more so on their applications in CO₂ valorization. In this review, we have collated literature of different forms of aerogels currently available and the steps involved in their fabrication process. In addition, we have covered applications of aerogels in CO₂ capture. Furthermore, we focussed on the basic principles involved in the development of an aerogel electrocatalyst as well as recent developments of aerogels in electrochemical CO₂ reduction.

Molecular Catalysts for the Reductive Homocoupling of CO2 towards C2+ Compounds

The conversion of CO2 into multicarbon (C2+ ) compounds by reductive homocoupling offers the possibility to transform renewable energy into chemical energy carriers and thereby create "carbon-neutral" fuels or other valuable products. Most available studies have employed heterogeneous metallic catalysts, but the use of molecular catalysts is still underexplored. However, several studies have already demonstrated the great potential of the molecular approach, namely, the possibility to gain a deep mechanistic understanding and a more precise control of the product selectivity. This Minireview summarizes recent progress in both the thermo- and electrochemical reductive homocoupling of CO2 toward C2+ products mediated by molecular catalysts. In addition, reductive CO homocoupling is discussed as a model for the further conversion of intermediates obtained from CO2 reduction, which may serve as a source of inspiration for developing novel molecular catalysts in the future.

Guiding CO2RR Selectivity by Compositional Tuning in the Electrochemical Double Layer

The electrochemical conversion of carbon dioxide to value-added chemicals provides an environmentally benign alternative to current industrial practices. However, current electrocatalytic systems for the CO₂ reduction reaction (CO₂RR) are not practical for industrialization, owing to poor specific product selectivity and/or limited activity. Interfacial engineering presents a versatile and effective method to direct CO₂RR selectivity by fine-tuning the local chemical dynamics. This Account describes interfacial design strategies developed in our laboratory that use electrolyte engineering and porous carbon materials to modify the local composition at the electrode-electrolyte interface.Our first strategy for influencing surface reactivity is to perturb the electrochemical double layer by tuning the electrolyte composition. We approached this investigation by considering how charged molecular additives can organize at the electrode surface and impact CO₂ activation. Using a combination of advanced electrochemical techniques and in situ vibrational spectroscopy, we show that the surfactant properties (the identity of the headgroup, alkyl chain length, and concentration) as well as the electrolyte cation identity can affect how surfactant molecules assemble at a biased electrode. The interplay between the electrolyte cations and the surfactant additives can be regulated to favor specific carbon products, such as HCOO^-, and suppress the parasitic hydrogen evolution reaction (HER). Together, our findings highlight how molecular assemblies can be used to design selective electrocatalytic systems.In addition to the electrolyte design, the local spatial confinement of reaction intermediates presents another strategy to direct CO₂RR selectivity. We were interested in uncovering the role of porous carbon-supported catalysts toward selective carbon product formation. In our initial study, we show that carbon porosity can be optimized to enhance C₂H₄ and CO selectivity in a series of Cu catalysts embedded in a tunable carbon aerogel matrix. These results suggested that local confinement of the active surface plays a role in CO₂ activation and motivated an investigation into probing how this phenomenon can be translated to a planar Cu electrode. Our findings show that carbon modifiers facilitated surface reconstruction and regulated CO₂ diffusion to suppress HER and improve the C_2-3 product selectivity. Given the ubiquity of carbon materials in catalysis, this work demonstrates that carbon plays an active role in regulating selectivity by restricting the diffusion of substrate and reaction intermediates. Our work in tuning the composition of the electrochemical double layer for increased CO₂RR selectivity demonstrates the potential versatility in boosting catalytic performance across an array of catalytic systems.

Engineering of Microcage Carbon Nanotube Architectures with Decoupled Multimodal Porosity and Amplified Catalytic Performance

New approaches for the engineering of the 3D microstructure, pore modality, and chemical functionality of hierarchically porous nanocarbon assemblies are key to develop the next generation of functional aerogel and membrane materials. Here, interfacially driven assembly of carbon nanotubes (CNT) is exploited to fabricate structurally directed aerogels with highly controlled internal architectures, composed of pseudo-monolayer, CNT microcages. CNT Pickering emulsions enable engineering at fundamentally different length scales, whereby the microporosity, mesoporosity, and macroporosity are decoupled and individually controlled through CNT type, CNT number density, and process energy, respectively. In addition, metal nanocatalysts (Cu, Pd, and Ru) are embedded within the architectures through an elegant sublimation and shock-decomposition approach; introducing the first approach that enables through-volume functionalization of intricate, pre-designed aerogels without microstructural degradation. Catalytic structure-function relationships are explored in a pharma-important amidation reaction; providing insights on how the engineered frameworks enhance catalyst activity. A sophisticated array of advanced tomographic, spectroscopic, and microscopic techniques reveal an intricate 3D assembly of CNT building-blocks and their influence on the functional properties of the enhanced nanocatalysts. These advances set a basis to modulate structure and chemistry of functional aerogel materials independently in a controlled fashion for a variety of applications, including energy conversion and storage, smart electronics, and (electro)catalysis.

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.

Reorganization of Interfacial Water by an Amphiphilic Cationic Surfactant Promotes CO2 Reduction

The presence of cetyltrimethylammonium bromide (CTAB) near the surface of a Cu electrode promotes the electrochemical reduction of CO₂ to fuels. CTAB increases the CO₂ reduction rate by as much as 10× and decreased the HER rate by 4×, leading to ∼75% selectivity toward the reduction of CO₂. Surface enhanced infrared absorption spectroscopy (SEIRAS) was used to probe the effects of CTAB adsorption on the structure of interfacial water and CO₂ reduction intermediates. HER suppression was found to arise from the displacement of interfacial water molecules from CTAB adsorption within the double layer. The enhanced CO₂ reduction rate can be correlated to an increased population of atop-bound CO and the emergence of a low frequency atop-CO band. These results unravel the role of additives in improving CO₂-to-fuels electrocatalysis and establishing this as a powerful methodology for directing product selectivity.

In situ templating synthesis of mesoporous Ni–Fe electrocatalyst for oxygen evolution reaction

Low-cost and efficient electrocatalysts with high dispersion of active sites and high conductivity are of high importance for oxygen evolution reaction (OER). Herein, we use amorphous mesoporous fumed silica (MFS) as a skeleton material to disperse Ni²⁺ and Fe³⁺ through a simple impregnation strategy. The MFS is in situ etched away during the OER process in 1 M KOH to prepare a stable mesoporous Ni–Fe electrocatalyst. The high specific surface area and abundant surface silanol groups in the mesoporous fumed silica afford rich anchor sites for fixing metal atoms via strong chemical metal–oxygen interactions. Raman and XPS investigations reveal that Ni²⁺ formed covalent bonds with surface Si–OH groups, and Fe³⁺ inserted into the framework of fumed silica forming Fe–O–Si bonds. The mesoporous Ni–Fe catalysts offer high charge transfer abilities in the OER process. When loaded on nickel foam, the optimal 2Ni1Fe-MFS catalyst exhibits an overpotential of 270 mV at 10 mA cm⁻² and a Tafel slope of 41 mV dec⁻¹. Notably, 2Ni1Fe-MFS shows a turnover frequency value of 0.155 s⁻¹ at an overpotential of 300 mV, which is 80 and 190 times higher than that of the state-of-the-art IrO₂ and RuO₂ catalysts. Furthermore, 2Ni1Fe-MFS exhibits 100% faradaic efficiency, large electrochemically active surface area, and good long-term durability, confirming its outstanding OER performance. Such high OER efficiency can be ascribed to the synergistic effect of high surface area, dense metal active sites and interfacial conductive path. This work provides a promising strategy to develop simple, cost-effective, and highly efficient porous Ni–Fe based catalysts for OER.

A stable mesoporous Ni–Fe–O electrocatalyst with high OER efficiency is constructed using mesoporous fumed silica as a template.