Carbon-neutral Energy Cycle Research Articles

First-Principles Study of C–C Coupling Pathways for CO2 Electrochemical Reduction Catalyzed by Cu(110)

To build a carbon-neutral energy cycle, the development of electrocatalysts that can reduce CO2 into products containing at least two carbon atoms (C2+) is crucial. This process would require at le...

Efficient Electrocatalytic CO2 Reduction to C2+ Alcohols at Defect-Site-Rich Cu Surface

Summary Electrochemical CO2 reduction is a promising approach for upgrading excessive CO2 into value-added chemicals, while the exquisite control of the catalyst atomic structures to obtain high C2+ alcohol selectivity has remained challenging due to the intrinsically favored ethylene pathways at Cu surface. Herein, we demonstrate a rational strategy to achieve ∼70% faradaic efficiency toward C2+ alcohols. We utilized a CO-rich environment to construct Cu catalysts with stepped sites that enabled high surface coverages of ∗CO intermediates and the bridge-bound ∗CO adsorption, which allowed to trigger CO2 reduction pathways toward the formation alcohols. Using this defect-site-rich Cu catalyst, we achieved C2+ alcohols with partial current densities of > 100 mA·cm−2 in both a flow-cell electrolyzer and a membrane electrode assembly (MEA) electrolyzer. A stable alcohol faradaic efficiency of ∼60% was also obtained, with ∼500 mg C2+ alcohol production per cm2 catalyst during a continuous 30-h operation.

Conductive Phthalocyanine-Based Covalent Organic Framework for Highly Efficient Electroreduction of Carbon Dioxide.

The electroreduction of CO2 to value-added chemicals such as CO is a promising approach to realize carbon-neutral energy cycle, but still remains big challenge including low current density. Covalent organic frameworks (COFs) with abundant accessible active single-sites can offer a bridge between homogeneous and heterogeneous electrocatalysis, but the low electrical conductivity limits their application for CO2 electroreduction reaction (CO2 RR). Here, a 2D conductive Ni-phthalocyanine-based COF, named NiPc-COF, is synthesized by condensation of 2,3,9,10,16,17,23,24-octa-aminophthalocyaninato Ni(II) and tert-butylpyrene-tetraone for highly efficient CO2 RR. Due to its highly intrinsic conductivity and accessible active sites, the robust conductive 2D NiPc-COF nanosheets exhibit very high CO selectivity (>93%) in a wide range of the applied potentials of -0.6 to -1.1V versus the reversible hydrogen electrode (RHE) and large partial current density of 35mA cm-2 at -1.1V versus RHE in aqueous solution that surpasses all the conventional COF electrocatalysts. The robust NiPc-COF that is bridged by covalent pyrazine linkage can maintain its CO2 RR activity for 10 h. This work presents the implementation of the conductive COF nanosheets for CO2 RR and provides a strategy to enhance energy conversion efficiency in electrocatalysis.

Tunable Selectivity and High Efficiency of CO2 Electroreduction via Borate-Enhanced Molten Salt Electrolysis.

SummaryConverting CO2 into value-added chemical fuels and functional materials by CO2 reduction reaction (CO2RR) is conducive to achieving a carbon-neutral energy cycle. However, it is still challenging to efficiently navigate CO2RR toward desirable products. Herein, we report a facile strategy to extend product species in borate-containing molten electrolyte at a positively shifted cathodic potential with a high current density (e.g. 100 mA/cm2), which can selectively electro-transform CO2 into desired products (either CO or solid carbon nanofibers, respectively reaching a high selectivity of ∼90%). The borates can act as a controller of electrolyte alkalinity to buffer the concentration of sequentially generated O2− during CO2RR, positively shifting the reduction potential of the captured CO2 and concurrently extending the product species. The sustainable buffering effect is available under CO2 atmosphere. Compared with borate-free electrolyte, the CO2 conversion efficiency is over three times higher, while the electrolysis energy consumption is decreased by over 40%.

Nanostructures for Electrocatalytic CO2 Reduction.

One of the most effective ways to cope with the problems of global warming and the energy shortage crisis is to develop renewable and clean energy sources. To achieve a carbon-neutral energy cycle, advanced carbon sequestration technologies are urgently needed, but because CO2 is a thermodynamically stable molecule with the highest carbon valence state of +4, this process faces many challenges. In recent years, electrochemical CO2 reduction has become a promising approach to fix and convert CO2 into high-value-added fuels and chemical feedstock. However, the large-scale commercial use of electrochemical CO2 reduction systems is hindered by poor electrocatalyst activity, large overpotential, low energy conversion efficiency, and product selectivity in reducing CO2 . Therefore, there is an urgent need to rationally design highly efficient, stable, and scalable electrocatalysts to alleviate these problems. This minireview also aims to classify heterogeneous nanostructured electrocatalysts for the CO2 reduction reaction (CDRR).

Improving CO2 Electrochemical Reduction to CO Using Space Confinement between Gold or Silver Nanoparticles.

Developing electrocatalysts that are stable and efficient for CO2 reduction is important for constructing a carbon-neutral energy cycle. New approaches are required to drive input electricity toward the desired CO2 reduction reaction (CO2RR) rather than the competitive hydrogen evolution reaction (HER). In this study, we have used quantum mechanics to demonstrate that the space confinement formed in the gaps of adjacent gold or silver nanoparticles can be used to improve the Faradaic efficiency of CO2RR to CO. This behavior is due to the space confinement stabilizing *COOH, which is the key intermediate in the CO2RR. However, space confinement has almost no effect on *H, which is the key intermediate in the HER. Possible experimental approaches for the preparation of this type of gold or silver electrocatalyst have been proposed.

Two-Dimensional Metal-Organic Framework Nanosheets with Cobalt-Porphyrins for High-Performance CO2 Electroreduction.

The electrochemical reduction of CO2 presents a promising strategy to mitigate the greenhouse effect and reduce excess carbon dioxide emission to realize a carbon-neutral energy cycle, but it suffers from the lack of high-performance electrocatalysts. In this work, catalytic active cobalt porphyrin [TCPP(Co)=(5,10,15,20)-tetrakis(4-carboxyphenyl)porphyrin-CoII ] was precisely anchored onto water-stable 2D metal-organic framework (MOF) nanosheets (Zr-BTB) to obtain ultrathin 2D MOF nanosheets [TCPP(Co)/Zr-BTB] with accessible catalytic sites for the CO2 reduction reaction. Compared with molecular cobalt porphyrin, the TCPP(Co)/Zr-BTB exhibits an ultrahigh turnover frequency (TOF=4768 h-1 at -0.919 V vs. reversible hydrogen electrode, RHE) owing to high active-site utilization. In addition, three post-modified 2D MOF nanosheets [TCPP(Co)/Zr-BTB-PABA, TCPP(Co)/Zr-BTB-PSBA, TCPP(Co)/Zr-BTB-PSABA] were obtained, with the modifiers of p-(aminomethyl)benzoic acid (PABA), p-sulfobenzoic acid potassium (PSBA), and p-sulfamidobenzoic acid (PSABA), to change the micro-environments around TCPP(Co) through the tuning of steric effects. Among them, the TCPP(Co)/Zr-BTB-PSABA exhibited the best performance with a faradaic efficiency (FECO ) of 85.1 %, TOF of 5315 h-1 , and jtotal of 6 mA cm-2 at -0.769 V (vs. RHE). In addition, the long-term durability of the electrocatalysts is evaluated and the role of pH buffer is revealed.

Copper atom-pair catalyst anchored on alloy nanowires for selective and efficient electrochemical reduction of CO2.

The electrochemical reduction of CO2 could play an important role in addressing climate-change issues and global energy demands as part of a carbon-neutral energy cycle. Single-atom catalysts can display outstanding electrocatalytic performance; however, given their single-site nature they are usually only amenable to reactions that involve single molecules. For processes that involve multiple molecules, improved catalytic properties could be achieved through the development of atomically dispersed catalysts with higher complexities. Here we report a catalyst that features two adjacent copper atoms, which we call an 'atom-pair catalyst', that work together to carry out the critical bimolecular step in CO2 reduction. The atom-pair catalyst features stable Cu10-Cu1x+ pair structures, with Cu1x+ adsorbing H2O and the neighbouring Cu10 adsorbing CO2, which thereby promotes CO2 activation. This results in a Faradaic efficiency for CO generation above 92%, with the competing hydrogen evolution reaction almost completely suppressed. Experimental characterization and density functional theory revealed that the adsorption configuration reduces the activation energy, which generates high selectivity, activity and stability under relatively low potentials.

Liquid-phase exfoliated ultrathin Bi nanosheets: Uncovering the origins of enhanced electrocatalytic CO2 reduction on two-dimensional metal nanostructure

Electrochemical CO2 reduction has been considered as a promising route for renewable energy storage and carbon-neutral energy cycle. However, the selectivity and stability of electrocatalysts for CO2 reduction need to be improved. Two-dimensional (2D) layered electrocatalysts with high conductivity and abundant active sites have been considered as good candidates for CO2 reduction. Herein, we propose a liquid-exfoliation strategy to prepare ultrathin 2D bismuth (Bi) nanosheets towards efficient electrocatalytic CO2 conversion. Compared with bulk Bi, the increased edge sites on ultrathin Bi nanosheets played a vital role in CO2 adsorption and reaction kinetics, significantly facilitating CO2-to-formate (HCOOH/HCOO-) conversion. Through density functional theory (DFT) calculation, we found that the *OCOH formation step tended to occur on edge sites rather than on facet sites, as confirmed by the lower Gibbs free energies. Benefited from the high conductivity and rich edge sites, Bi nanosheets exhibited a Faradaic efficiency of 86.0% for formate production and a high current density of 16.5 mA cm−2 at − 1.1 V (vs. RHE), much superior to bulk Bi. Moreover, the Bi nanosheets could maintain well-preserved catalytic activity after long-term testing for over consecutive 10 h. We hope this study may provide new insights for the fabrication of novel 2D nanostructured metals for highly-efficient and long-life electrocatalytic CO2 conversion.

Perovskite SrFeO3−δ decorated with Ni nanoparticles for high temperature carbon dioxide electrolysis

High temperature carbon dioxide electrolyzers are expected to play a key role on carbon-neutral energy cycle. Here we report a novel composite, namely perovskite SrFeO3−δ with impregnated Ni nanoparticles, as an advanced cathode materials for solid oxide electrolysis cells (SOECs). The XRD, SEM and TEM together indicate that the homogeneously distributed Ni nanoparticles on SrFeO3−δ substrate create sufficient and active triple-phase boundary (TPB) for electrochemical carbon dioxide splitting. Electrochemical measurements show that this tailored SrFeO3−δ cathode demonstrates better performance, and then exhibits long-term stability after 100 h operation and 8 redox cycles.

Electrochemical impedance spectroscopy as a tool to investigate the electroreduction of carbon dioxide: A short review

Abstract Carbon dioxide electroreduction (CO2ER) exploited in conjugation with renewable energy sources can open the path to a carbon-neutral energy cycle, concurrently offering the possibility to synthesize low-carbon fuels. In the last decades, a huge amount of work has been carried out by the scientific community in order to obtain high-performing and low-cost catalysts, and to design reactors able to maximize the efficiency of CO2ER. Electrochemical impedance spectroscopy (EIS) proved to be a useful tool in studying this electrochemical reaction, and has been widely employed to characterize novel materials and reactor architectures. The aim of this Review is to provide an insight on the application of EIS for the study of CO2ER. A brief introduction on the technique and on the reaction will be given, followed by a review of the most important applications in this field. Finally, a brief discussion of future research directions will be presented.

Design of Rutile Oxide Electrocatalysts for Selective Reduction of CO2 into Liquid Fuels

Efficient electrochemical CO2 reduction (CO2RR) into liquid fuels can potentially meet three goals at once – large-scale energy storage, CO2 capture and a carbon neutral energy cycle through utilization of renewable energy. Metal catalysts have high overpotential and low Faradic Efficiencies for direct conversion of CO2 to methanol and other liquid fuels. Using computational electrocatalysis simulations based on density functional theory, we show that RuO2-based electrocatalysts, which have been observed experimentally to evolve methanol at low overpotential, can catalyze the reaction through an alternative reaction mechanism. These pathways have oxygen-coordinated intermediates and the oxide catalyst can thus potentially bypass the limitations of the CO*-CHO* scaling relations on metal catalysts. Here, we establish adsorbate scaling relations for such reaction mechanisms and propose an activity volcano with OH* binding as descriptor. We also predict the ideal binding free energy for H* and OH* to facilitate selective CO2RR over H2/CO evolution. 1 Our study shows that adsorbate−adsorbate interaction effects are strong on the RuO2(110) surface and can alter the reaction thermodynamics substantially. Steric and electronic effects on reaction intermediates from varying coverage of CO* spectators can lead to different observed product compositions from experiments. 50% CO* coverage is necessary for obtaining methanol as a product from CO2RR.2 IrO2 and RuO2 binds OH* and other intermediates from CO2RR strongly and falls in the strong binding leg of the activity volcano. We observe that the miscible system IrxRu(1 - x)O2 exhibits anomalous weaker OH* binding energy in the presence of CO* spectators. We attribute this to a Ru-Ir ligand effect, based on electronic structure analysis. Ir atoms at the bridge site with Ru neighbors, have electron depletion and a t2g orbital shift. Synergistic effects from adsorbate interaction and ligand effects in compositions with intermediate Ru/Ir character for active sites places them close to the top of the activity volcano. The predicted onset potential for formic acid / methanol evolution is −0.2 V RHE. 3

Hierarchical Mesoporous SnO2 Nanosheets on Carbon Cloth: A Robust and Flexible Electrocatalyst for CO2 Reduction with High Efficiency and Selectivity.

Electrochemical reduction of CO2 into liquid fuels is a promising approach to achieve a carbon-neutral energy cycle. However, conventional electrocatalysts usually suffer from low energy efficiency and poor selectivity and stability. A 3D hierarchical structure composed of mesoporous SnO2 nanosheets on carbon cloth is proposed to efficiently and selectively electroreduce CO2 to formate in aqueous media. The electrode is fabricated by a facile combination of hydrothermal reaction and calcination. It exhibits an unprecedented partial current density of about 45 mA cm-2 at a moderate overpotential (0.88 V) with high faradaic efficiency (87±2 %), which is even larger than most gas diffusion electrodes. Additionally, the electrode also demonstrates flexibility and long-term stability. The superior performance is attributed to the robust and highly porous hierarchical structure, which provides a large surface area and facilitates charge and mass transfer.

Carbon Nanostructures for the Electrochemical Conversion of CO2 to Fuels: Defect & Electrocatalytic Activity

The advantages of the electrochemical conversion of carbon dioxide to fuels using renewable energy sources are twofold: (1) it has the potential to accomplish a carbon-neutral energy cycle and (2) it can provide an approach to tackle the environmental challenges caused by anthropogenic carbon dioxide emissions. Although thermodynamically possible, the kinetics of carbon dioxide reduction to fuels remains challenging and therefore, an efficient and robust electrocatalyst is needed to promote the reaction. The ideal catalyst for the electrochemical CO2 reduction must be capable of mediating multiple proton-coupled electron transfer reactions at low overpotentials, suppressing the concurrent hydrogen evolution reaction, converting CO2 to desired chemicals with high selectivity, and achieving long-term stability. Extensive research has been carried out on metallic electrocatalysts during the past three decades; however, further studies are need to develop materials that are simultaneously efficient and stable for practical purposes.The focus of this presentation is to report the use of N-doped carbon nanostructures as the electrocatalysts, which are highly efficient, selective and more importantly, stable catalysts to achieve CO2 conversion to CO. The catalytic activity of N-doped carbon nanotubes (NCNTs) was further benchmarked against other metallic catalysts reported in literature. Compared to noble metals Ag & Au, these NCNTs exhibited a lower overpotential to achieve similar selectivity towards CO formation. The dependence of catalytic activity, i.e., the overpotential and selectivity for CO formation on the defect structures (pyridinic, graphitic, pyrrolic-N) inside NCNTs will be discussed. The presence of both pyridinic and graphitic-N was found to significantly decrease the absolute overpotential and increase the selectivity towards CO formation. In addition, the electrocatalytic activity of two-dimensional materials will be presented. Overall, pyridinic-N defects exhibited the highest catalytic activity; thereby suggesting the directions for developing carbon nanostructures as metal-free electrocatalysts for CO2 reduction.

Dehydrogenation of Formic Acid at Room Temperature: Boosting Palladium Nanoparticle Efficiency by Coupling with Pyridinic-Nitrogen-Doped Carbon.

The use of formic acid (FA) to produce molecular H2 is a promising means of efficient energy storage in a fuel-cell-based hydrogen economy. To date, there has been a lack of heterogeneous catalyst systems that are sufficiently active, selective, and stable for clean H2 production by FA decomposition at room temperature. For the first time, we report that flexible pyridinic-N-doped carbon hybrids as support materials can significantly boost the efficiency of palladium nanoparticle for H2 generation; this is due to prominent surface electronic modulation. Under mild conditions, the optimized engineered Pd/CN0.25 catalyst exhibited high performance in both FA dehydrogenation (achieving almost full conversion, and a turnover frequency of 5530 h(-1) at 25 °C) and the reversible process of CO2 hydrogenation into FA. This system can lead to a full carbon-neutral energy cycle.

The role of metal–organic frameworks in a carbon-neutral energy cycle

Reducing society's reliance on fossil fuels presents one of the most pressing energy and environmental challenges facing our planet. Hydrogen, methane and carbon dioxide, which are some of the smallest and simplest molecules known, may lie at the centre of solving this problem through realization of a carbon-neutral energy cycle. Potentially, this could be achieved through the deployment of hydrogen as the fuel of the long term, methane as a transitional fuel, and carbon dioxide capture and sequestration as the urgent response to ongoing climate change. Here we detail strategies and technologies developed to overcome the difficulties encountered in the capture, storage, delivery and conversion of these gas molecules. In particular, we focus on metal–organic frameworks in which metal oxide ‘hubs’ are linked with organic ‘struts’ to make materials of ultrahigh porosity, which provide a basis for addressing this challenge through materials design on the molecular level. The capture, storage and conversion of gases such as hydrogen, methane and carbon dioxide may play a key role in the provision of carbon-neutral energy. This Review explores the role of metal–organic frameworks — porous networks of metal ions or clusters connected by organic linkers — for such applications.

Development of Nanoalloy Catalysts for Realization of Carbon-Neutral Energy Cycles

Increase of CO2 concentration in the atmosphere is one of reasons for the global warming. Development of energy circulation systems, which do not emit CO2 in the atmosphere, is an emergent issue for present-generation scientists [1]. As an answer, we have proposed a new type of energy circulation system, namely, carbon-neutral energy (CN) cycle. With a practical application in mind, three limitations are imposed on the CN cycle; (1) no CO2 emissions, (2) utilization of liquid fuels and (3) minimizing the use of precious metal catalysts. In anticipation of a practical use in the near future, an alkaline fuel cell will be adapted for the CN cycle where non-platinum catalysts can work. For our purpose, electric power will be generated by partial oxidation of alcohols to carboxylic acids.[2] In view of ease in handling, fuels having a high boiling point (b.p.) are favorable for the CN cycles. To this end, glycol (EG) of which b.p. is 470 K an ideal candidate as a fuel. In this case, an oxidized product of EG can be oxalic acid. Compared to the energy obtained by the complete oxidation of EG into CO2, we can derive ca. 80 % of energy even in the partial oxidation of EG to oxalic acid, implying that the EG/oxalic cycle possibly works as an energy cycle. We herein show an example of selective EG oxidation catalysts working in alkaline conditions.

Conversion of Solar Energy to Fuels by Inorganic Heterogeneous Systems

Over the last several years, the need to find clean and renewable energy sources has increased rapidly because current fossil fuels will not only eventually be depleted, but their continuous combustion leads to a dramatic increase in the carbon dioxide amount in atmosphere. Utilisation of the Sun's radiation can provide a solution to both problems. Hydrogen fuel can be generated by using solar energy to split water, and liquid fuels can be produced via direct CO 2 photoreduction. This would create an essentially free carbon or at least carbon neutral energy cycle. In this tutorial review, the current progress in fuels' generation directly driven by solar energy is summarised. Fundamental mechanisms are discussed with suggestions for future research.