Carbon‐based metal‐free catalysts for electrochemical CO2 reduction: Activity, selectivity, and stability

Zero or negative emissions of carbon dioxide(CO2)is the need of the times,as inexorable rising and alarming levels of CO2 in the atmosphere lead to global warming and severe climate change.The electrochemical CO2 reduction(eCO2R)to value‐added fuels and chemicals by using renewable electricity provides a cleaner and more sustainable route with economic benefits,in which the key is to develop clean and economical electrocatalysts.Carbon‐based catalyst materials possess desirable properties such as high offset potential for H2 evolution and chemical stability at the negative applied potential.Although it is still challenging to achieve highly efficient carbon‐based catalysts,considerable efforts have been devoted to overcoming the low selectivity,activity,and stability.Here,we summarize and discuss the recent progress in carbon‐based metal‐free catalysts including carbon nanotubes,carbon nanofibers,carbon nanoribbons,graphene,carbon nitride,and diamonds with an emphasis on their activity,product selectivity,and stability.In addition,the key challenges and future potential approaches for efficient eCO2R to low carbon‐based fuels are highlighted.For a good understanding of the whole history of the development of eCO2R,the CO2 reduction reactions,principles,and techniques including the role of electrolytes,electrochemical cell design and evaluation,product selectivity,and structural composition are also discussed.The metal/metal oxides decorated with carbon‐based electrocatalysts are also summarized.We aim to provide insights for further development of carbon‐based metal‐free electrocatalysts for CO2 reduction from the perspective of both fundamental understanding and technological applications in the future.

Electrochemical reduction of CO_2 in solid oxide electrolysis cells

The effort on electrochemical reduction of COto useful chemicals using the renewable energy to drive the process is growing fast recently. In this review, we introduce the recent progresses on the electrochemical reduction of COin solid oxide electrolysis cells（SOECs）. At high temperature, only CO is produced with high current densities and Faradic efficiency while the reactor is complicated and a better sealing technique is urgently needed. The typical electrolytes such as zirconia-based oxides, ceria-based oxides and lanthanum gallates-based oxides, anodes and cathodes are introduced in this review, and the cathode materials, such as conventional metal–ceramics（cermets）, mixed ionic and electronic conductors（MIECs） are discussed in detail. In the future, to gain more value-added products, the electrolyte, cathode and anode materials should be developed to allow SOECs to be operated at temperature range of 573–873 K. At those temperatures, SOECs may combine the advantages of the low temperature system and the high temperature system to produce various products with high current densities.

Tuning the catalytic selectivity in electrochemical CO2 reduction on copper oxide-derived nanomaterials

Electrochemical conversion of CO2 to hydrocarbons can relieve both environmental and energy stresses. However, electrocatalysts for this reaction usually suffer from a poor product selectivity and a large overpotential. Here we report that tunable catalytic selectivity for hydrocarbon formation could be achieved on Cu nanomaterials with different morphologies. By tuning the electrochemical parameters, either Cu oxide nanowires or nanoneedles were fabricated and then electrochemically reduced to the corresponding Cu nanomaterials. The Cu nanowires preferred the formation of C2H4, while the Cu nanoneedles favored the production of more CH4, rather than C2H4. Our work provides a facile synthetic strategy for preparing Cu-based nanomaterials to achieve selective CO2 reduction.

Facile synthesis of hierarchical flower-like Ag/Cu2O and Au/Cu2O nanostructures and enhanced catalytic performance in electrochemical reduction of CO2

Novel,hierarchical,flower-like Ag/Cu2O and Au/Cu2O nanostructures were successfully fabricated and applied as efficient electrocatalysts for the electrochemical reduction of CO2.Cu2O nanospheres with a uniform size of^180 nm were initially synthesized.Thereafter,Cu2O was used as a sacrificial template to prepare a series of Ag/Cu2O composites through galvanic replacement.By varying the Ag/Cu atomic ratio,Ago.12/Cu2O,having a hierarchical,flower-like nanostructure with intersecting Ag nanoflakes encompassing an inner Cu2O sphere,was prepared.The as-prepared Ag/Cu2O samples presented higher Faradaic efficiencies(FE)for CO and relatively suppressed H2 evolution than the parent Cu2O nanospheres due to the combination of Ag with Cu2O in the former.Notably,the highest CO evolution rate was achieved with Ago.12/Cu2O due to the larger electroactive surface area furnished by the hierarchical structure.The same hier-archical flower-like structure was also obtained for the Auo./Cu2O composite,where the FEco(10%)was even higher than that of Ago.12/Cu2O.Importantly,the results reveal that Ago.12/Cu2O and Auo./Cu2O both exhibit remarkably improved stability relative to Cu2O.This study presents a facile method of developing hierarchical metal-oxide composites as fficient and stable electrocatalysts for the electrochemical reduction of CO2.

A bio-inspired O2-tolerant catalytic CO2 reduction electrode

The electrochemical reduction of CO2 to give CO in the presence of O2 would allow the direct valorization of flue gases from fossil fuel combustion and of CO2 captured from air. However, it is a challenging task because O2 reduction is thermodynamically favored over that of CO2. 5% O2 in CO2 near catalyst surface is sufficient to completely inhibit the CO2 reduction reaction. Here we report an O2-tolerant catalytic CO2 reduction electrode inspired by part of the natural photosynthesis unit. The electrode comprises of heterogenized cobalt phthalocyanine molecules serving as the cathode catalyst with >95% Faradaic efficiency(FE) for CO2 reduction to CO coated with a polymer of intrinsic microporosity that works as a CO2-selective layer with a CO2/O2 selectivity of $20. Integrated into a flow electrolytic cell, the hybrid electrode operating with a CO2 feed gas containing 5% O2 exhibits a FECOof 75.9% with a total current density of 27.3 mA/cm^2 at a cell voltage of 3.1 V. A FECO of 49.7% can be retained when the O2 fraction increases to 20%. Stable operation for 18 h is demonstrated. The electrochemical performance and O2 tolerance can be further enhanced by introducing cyano and nitro substituents to the phthalocyanine ligand.

Promoting electrochemical conversion of CO2 to formate with rich oxygen vacancies in nanoporous tin oxides

Defect engineering,especially oxygen vacancies(O-vacancies) introduction into metal oxide materials has been proved to be an effective strategy to manipulate their surface electron exchange processes.However,quantitative investigation of O-vacancies on CO2 electroreduction still remains rather ambiguous.Herein,a series of nanoporous tin oxide(SnOx) materials have been prepared by thermal treatment at various temperatures and reaction conditions.The annealing temperature dependent Ovacancies property of the SnOx was revealed and attributed to the balance tunning of the desorption of oxygen species and the continous oxidation of SnOx.The as-prepared nanoporous SnOx with 300℃treatment was found to be highest O-vacant material and showed an impressive CO2 RR activity and selectivity towards the conversion of CO2 into formic acid(up to 88.6%),and superior HCOOH incomplete current density to other samples.The ideal performance of the O-vacancies rich SnOx-300 material can be ascribed to the high delocalized electron density inducing much enhanced adsorption of CO2 with O binding and benefiting the subsequent reduction with high selectively forming of formic acid.

An efficient 3D ordered mesoporous Cu sphere array electrocatalyst for carbon dioxide electrochemical reduction

The electrochemical reduction of CO2 is a promising solution for sustainable energy research and carbon emissions.However,this solution has been challenged by the lack of active and selective catalysts.Here,we report a two-step synthesis of 3D ordered mesoporous Cu sphere arrays,which is fabricated by a dual template method using a poly methyl methacrylate(PMMA) inverse opal and the nonionic surfactant Brij 58 to template the mesostructure within the regular voids of a colloidal crystal.Therefo re,the well-ordered 3D interconnected bi-continuous mesopore s structure has advantages of abundant exposed catalytically active sites,efficient mass transport,and high electrical conductivity,which result in excellent electrocatalytic CO2 RR perfo rmance.The prepared 3D ordered mesoporous Cu sphere array(3 D-OMCuSA) exhibits a low onset potential of-0.4 V at a 1 mA cm^-2 electrode current density,a low Tafel slope of 109.6 mV per decade and a long-term durability in 0.1 M potassium bicarbonate.These distinct features of 3 D-OMCuSA render it a promising method for the further develo p ment of advanced electrocatalytic materials for CO2 reduction.

Understanding of strain effects in the electrochemical reduction of CO_2:Using Pd nanostructures as an ideal platform

With the support by the National Natural Science Foundation of China,a collaborative study by the research groups led by Prof.Zeng Jie(曾杰)and Prof.Yang Jinlong(杨金龙),both from Hefei National Laboratory for Physical Sciences at the Microscale,University of Science and Technology of China,