Towards the electrochemical conversion of carbon dioxide into methanol

Dominguez-Ramos A., Singh B., Zhang X., Hertwich E.G., Irabien A.

Carbon dioxide Capture and Storage (CCS) is an important technological option for climate change mitigation. Utilization (U) of this captured CO2 as raw material for Electrochemical Reduction (ER) has been suggested as a valorization option to produce organics such as formate-based products. Previous work has focused on the influence of operating conditions or the selected cathodic material on the faradaic efficiency and distribution of products. The environmental sustainability of formate production through the ER of CO2 has been assumed rather than investigated. In this study, we perform a life cycle assessment, focusing on resources and greenhouse gas (GHG) emissions. Even though the processes reported in the literature result in a wide range of GHG emissions for the ER of CO2 to formate-based products from 32 to 519 kg CO2-eq center dot(kg HCOO-)(-1), this is higher than the current commercial production processes (3.1 kg CO2-eq center dot(kg HCOO-)(-1)). The consumption of chemicals by the electrolysis and of steam in the purification of the final formate products are found to be the dominant causes of environmental burdens of the integrated process. A future scenario under very optimistic conditions suggests 0.33 kg CO2-eq center dot(kg HCOO-)(-1) thus presenting a potential pathway to an environmentally sustainable CO2 utilization option. (C) 2013 Elsevier Ltd. All rights reserved.

Fiorani G., Guo W., Kleij A.W.

The conversion of carbon dioxide (CO2), an abundant renewable carbon reagent, into chemicals of academic and industrial interest is of imminent importance to create a higher degree of sustainability in chemical processing and production. Recent progress in this field is characterised by a plethora of organic molecules able to mediate the conversion of suitable substrates in the presence of CO2 into a variety of value-added commodities with advantageous features combining cost-effectiveness, metal-free transformations and general substrate activation profiles. In this review, the latest developments in the field of CO2 catalysis are discussed with a focus on organo-mediated conversions and their increasing importance in serving as practicable alternatives for metal-based processes. Also a critical assessment of the state-of-the-art methods is presented with attention to those features that need further development to increase the usefulness of organocatalysis in the production of organic molecules of potential commercial interest.

Xiang D., Magana D., Dyer R.B.

The catalytic reduction of CO2 is of great current interest because of its role in climate change and the energy cycle. We report a pterin electrocatalyst, 6,7-dimethyl-4-hydroxy-2-mercaptopteridine (PTE), that catalyzes the reduction of CO2 and formic acid on a glassy carbon electrode. Pterins are natural cofactors for a wide range of enzymes, functioning as redox mediators and C1 carriers, but they have not been exploited as electrocatalysts. Bulk electrolysis of a saturated CO2 solution in the presence of the PTE catalyst produces methanol, as confirmed by gas chromatography and 13C NMR spectroscopy, with a Faradaic efficiency of 10–23%. FTIR spectroelectrochemistry detected a progression of two-electron reduction products during bulk electrolysis, including formate, aqueous formaldehyde, and methanol. A transient intermediate was also detected by FTIR and tentatively assigned as a PTE carbamate. The results demonstrate that PTE catalyzes the reduction of CO2 at low overpotential and without the involvement of any metal.

Xie J., Huang Y., Li W., Song X., Xiong L., Yu H.

The existing Cu-based electrocatalysts for electrochemical reduction of CO 2 to chemical commodities suffer instability and large reaction overpotentials. Here we report a novel Cu nanoflower (NF) catalyst with a unique 3D chrysanthemum-like structure. Derived from CuO NFs, the Cu NFs were found to efficiently catalyze the CO 2 reduction with 400 mV lower overpotential than polycrystalline Cu. Besides, H 2 production was suppressed to be below 25% in terms of Faradaic efficiency in a wide potential window, implying an excellent catalytic selectivity. Prolonged electrolysis showed that the Cu NFs kept a high catalytic activity towards CO 2 reduction for over 9 h. In addition, for the first time, the deposition of amorphous carbon on the electrodes after catalysis was directly observed in this study. The reaction pathways of electrocatalytic CO 2 reduction and the involvement of the surface carbon in this process were elucidated.

Setterfield-Price B.M., Dryfe R.A.

The influence of supporting electrolyte cations on the voltammetric behaviour and product distribution in N-methylpyrrolidone-based carbon dioxide electroreduction systems is investigated. The reduction potentials associated with TBABF 4 (0.1 M) and corresponding alkali metal (M + ) electrolytes; LiBF 4 , NaBF 4 and RbBF 4 (focussing mainly on the reduction of the widely employed Li + species) were established in both the presence and absence of CO 2 at polycrystalline noble metal working electrodes. In situ and ex situ Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and qualitative element identification via flame testing were used to aid the assignment of reduction processes. It was established that CO 2 reduction products in the metal cationic systems were formed at a much less negative potential than those found with the non-metal cation (−1.5 V vs. Ferrocene, c.f. −2.2 V), however the resultant alteration of the surface environment was found to deactivate the electrode to further CO 2 reduction. The presence of CO 2 in solution was found to affect the potential required for the bulk deposition of metal from the electrolyte through the same process. Where TBA + and M + were employed simultaneously in the system, the resultant voltammetry shared the majority of features with the pure M + system with CO 2 reduction suppressed at more negative potentials therefore supporting the conclusion that any ‘catalytic effect’ associated with TBA + is in fact a lack of deactivation given by the M + system, rather than any enhancement offered by the former.

Lim R.J., Xie M., Sk M.A., Lee J., Fisher A., Wang X., Lim K.H.

In this review article, we report the development and utilisation of fuel cells, metal electrodes in aqueous electrolyte and molecular catalysts in the electrochemical reduction of CO2. Fuel cells are able to function in both electrolyser and fuel cell mode and could potentially reduce CO2 and produce energy at the same time. However, it requires considerably high temperatures for efficient operation. Direct reduction using metal electrodes and molecular catalysts are possible at room temperatures but require an additional applied potential and generally have low current densities. Density functional theory (DFT) studies have been used and have begun to unveil possible mechanisms involved which could lead to improvements and development of more efficient catalysts.

Lan Y., Gai C., Kenis P.J., Lu J.

We investigate the electrochemical behavior of different loadings of a Cu(core)/CuO(shell) catalyst in a standard three-electrode cell, which reveals transformations between Cu, CuI, and CuO species. The electroreduction of CO2 on the catalyst is performed in an aqueous solution within a flow reactor. CO and HCOOH are the main products, with H2 as a byproduct. When the catalyst loading is 1.0 mg cm−2, the faradaic efficiencies of CO and HCOOH are higher than those with other catalyst loadings, with a short reaction time. This paper also focuses on the experimental and kinetic details of the electroreduction process of CO2 to CO and HCOOH when using the Cu/CuO catalyst. Reactions are modeled by using an in-series, first-order reaction, and the models are verified to reasonably describe the two products formed during the electroreduction of CO2. According to the kinetic analysis, the rate constant for HCOOH production is higher than that of CO.

Del Castillo A., Alvarez-Guerra M., Irabien A.

Electrochemical valorization may be a strategy for mitigating climate change, as the process allows for CO2 to be converted into industrially useful chemicals. The aim of this work is to study the influence of key variables on the performance of an experimental system for continuous electroreduction of CO2 to formate with a gas diffusion electrode (GDE) loaded with Sn. A 23 factorial design of experiments at different levels of current density (j), electrolyte flow rate/electrode area ratio (Q/A ratio) and GDE Sn load was followed. Higher rates and concentrations (i.e., 1.4·10−3 mol m−2 s−1 and 1348 mg L−1 with efficiencies of approximately 70%) were obtained with GDEs than with plate electrodes. The statistical design of experiments demonstrated that the Sn load had the most significant effect on rate and efficiency. However, despite these promising results, further research is required to optimize the process. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3557–3564, 2014

Garcia-Herrero I., Alvarez-Guerra M., Irabien A.

Karamad M., Tripkovic V., Rossmeisl J.

One of the main challenges associated with the electrochemical CO or CO2 reduction is poor selectivity toward energetically rich products. In order to promote selectivity toward hydrocarbons and alcohols, most notably, the hydrogen evolution reaction (HER) should be suppressed. To achieve this goal, we studied intermetallic compounds consisting of transition metal (TM) elements that can reduce CO (Ru, Co, Rh, Ir, Ni, Pd, Pt, and Cu) separated by TM and post transition metal elements (Ag, Au, Cd, Zn, Hg, In, Sn, Pb, Sb, and Bi) that are very poor HER catalysts. In total, 34 different stable binary bulk alloys forming from these elements have been investigated using density functional theory calculations. The electronic and geometric properties of the catalyst surface can be tuned by varying the size of the active centers and the elements forming them. We have identified six different potentially selective intermetallic surfaces on which CO can be reduced to methanol at potentials comparable to or even slig...

Alvarez-Guerra M., Del Castillo A., Irabien A.

Electrochemical reduction has been pointed out as a promising method for CO 2 valorisation into useful chemicals. This paper studies the influence of key variables on the performance of an experimental system for continuous electro-reduction of CO 2 to formate, when a tin plate is used as working electrode. Particular emphasis is placed on comparing the performance of Sn and Pb as cathodes. As was previously found with Pb, the influence of current density (“ j ”) using Sn was particularly noteworthy, and when j was raised up to a limit value of 8.5 mA cm −2 , important increases of the rate of formate production were observed at the expense of lowering the Faradaic efficiency. However, unlike what was found with Pb, the performance using Sn improved when the electrolyte flow rate/electrode area ratio was increased within the range studied (0.57–2.3 mL min −1  cm −2 ). In this way, the use of Sn as cathode allowed achieving rates of formate production that were 25% higher than the maximum rates obtained with Pb, together with Faradaic efficiencies close to 70%, which were 15 points higher than those with Pb. These results reinforce the interest in Sn as electrode material in the electro-reduction of CO 2 to formate.

Jia F., Yu X., Zhang L.

Electrochemical reduction of CO2 in an aqueous 0.5 M KHCO3 solution is studied by use of novel nanostructured Cu–Au alloys, which are prepared through electrochemical deposition with a nanoporous Cu film (NCF) as template. Linear voltammetry results show that the as-synthesized Cu–Au alloys exhibit obvious catalysis towards electrochemical reduction of CO2. Further analysis of products reveals that faradic efficiencies of alcohols (methanol and ethanol) are greatly dependent on the nanostructures and compositions of Cu–Au alloys. It is expected that this work could provide new insight into the development of powerful electrocatalysts for reduction of CO2 to alcohols.

Li P., Hu H., Xu J., Jing H., Peng H., Lu J., Wu C., Ai S.

In this paper, the highly-ordered TiO 2 nanotube arrays (TiO 2 NTs) were obtained by improved anodic oxidation method and the MoS 2 -rods were assembled to the TiO 2 NTs by a facile hydrothermal method, obtained the MoS 2 -rods/TiO 2 NTs heterojunction. According to the UV–vis DRS and XPS analysis, the obtained MoS 2 -rods/TiO 2 NTs exhibited excellent absorption in the visible area (400–600 nm) and its energy band gap was 1.55 eV, but its conduction band (−0.15 eV) was more positive than the CO 2 reduction potential, it indicated that MoS 2 -rods/TiO 2 NTs had no ability for photocatalytic reduction of CO 2 . Electrocatalysis could reduce CO 2 , but the products yield was very low and the faraday efficiency gradually reduced with reaction going on. Interestingly, when illumination was introduced into the electrocatalytic process, the light greatly enhanced the CO 2 electrocatalytic reduction ability of the MoS 2 -rods/TiO 2 NTs. Furthermore, the faraday efficiency increased to 2.65 times from 42.20% (electrocatalysis) to 111.58% (photo-enhanced electrocatalysis), and the methanol yield increased to 2.29 times from 6.32 mmol L −1 to 14.49 mmol L −1 . The new insights for how light enhanced electrocatalytic reduction of CO 2 was elaborated systematically from three aspects, that is, reduction overpotential, enhanced electron transmission ability, and generated p–n heterojunction. Furthermore, the generation mechanism of methanol with photo-enhanced electrocatalysis was also deduced. Especially, we deduced that the protons involving into the CO 2 reduction came from two aspects, one was from the electrocatalytic oxidation water on the anode, and the other was from the in situ photocatalytic oxidation water on the cathode.

Barbato L., Centi G., Iaquaniello G., Mangiapane A., Perathoner S.

The utilization of carbon dioxide (CO2) to produce methanol (to be used as energy vector and raw material for chemical production) in remote areas, where cheap renewable H2 could be produced from renewable sources, is a technology with a potential impact estimated to be more than 7 Gt CO2 equivalents. By using a techno-economic analysis, it is possible to evidence that methanol can be produced at competitive costs with respect to deriving energy from fossil fuels. This result, together with an analysis of the potential unexploited sources of renewable energy that are too far from users and grids, shows that the impact on the mitigation of climate change by this route is large, up to potentially 7 Gt CO2 equivalents, and at least comparable with that of carbon capture and storage (CCS). There are also advantages in terms of i) lower costs, ii) reduced impact on the environment, and iii) enhanced energy security. Further benefits are in terms of effective integration with the actual energy and chemical production value chains. The technology may be also used to store energy to solve the issue of generation intermittency present in most of the renewable energy sources. These aspects make this CO2 conversion path using renewable energy a potentially valuable approach to mitigate climate change and increase the use of renewable energy.

Nie X., Luo W., Janik M.J., Asthagiri A.

Density functional theory (DFT) was used to determine the potential-dependent reaction free energies and activation barriers for several reaction paths of carbon dioxide (CO2) electrochemical reduction on the Cu(1 1 1) surface. The role of water solvation on CO2 reduction paths was explored by evaluating water-assisted surface hydrogenation and proton (H) shuttling with various solvation models. Electrochemical O H bond formation reactions occur through water-assisted H-shuttling, whereas C H bond formation occurs with negligible H2O involvement via direct reaction with adsorbed H* on the Cu(1 1 1) surface. The DFT-computed kinetic path shows that the experimentally observed production of methane and ethylene on Cu(1 1  1) catalysts occurs through the reduction of carbon monoxide (CO *) to a hydroxymethylidyne (COH*) intermediate. Methane is produced from the reduction of the COH* to C* and then sequential hydrogenation. Ethylene production shares the COH* path with methane production, where the methane to ethylene selectivity depends on CH 2 ∗ and H* coverages. The reported potential-dependent activation barriers provide kinetics consistent with observed experimental reduction overpotentials and selectivity to methane and ethylene over methanol for the electroreduction of CO2 on Cu catalysts.

Garcia A.E., Sanito R.C., Gao M., Chuang S.S., Nur Azizah L.A.

Min H., Kim C., Lin S., Choi J., Sim Y., Yu B., Moon J.H.

AbstractThe electrochemical conversion of methane offers a sustainable alternative to traditional thermochemical syngas pathways; however, the rational design of catalysts that ensure high productivity remains a significant challenge. In this study, a high‐entropy oxide (HEO) catalyst composed of Co, Cr, Fe, Mn, and Ni is explored, with a targeted element enriched, and identify that a Co‐rich HEO demonstrates high efficiency in room‐temperature electrochemical methane conversion. This analysis of the projected density of states (PDOS) reveals that Co sites in the HEO catalyst possess an optimally positioned p‐band center for methane activation. The Co‐rich HEO catalyst achieves an ethanol production rate of 12315 µmol/gcat/hr at 1.6 VRHE, with a Faradaic efficiency of 63.5%; a flow cell electrolyzer equipped with this catalyst achieves continuous methane‐to‐ethanol conversion at a rate of 26533 µmol/gcat/hr over 100 h. Process modeling evaluates the economic and environmental implications, demonstrating that a commercially viable process can be realized through economies of scale while significantly reducing CO₂ emissions.

Inocencio-García P.J., Cardona Alzate C.A.

Abstract Carbon dioxide (CO2) emissions have a significant impact on climate change and global warming, with concentrations exceeding the value established as a planetary limit (350 ppm CO2). In Colombia, the manufacturing industries and the final consumption in households contribute to the highest emissions of CO2 to the atmosphere. Sucre region, known for basing its economy on livestock and social services, is responsible for an annual emission of more than 3 Mton the CO2 eq. Then, the state of novelty of this study is the applicability evaluation of methanol and ethanol production technologies based on CCU systems, in terms of techno-economic indicators, to be implemented in Sucre. Technical and economic assessment of the CO2 valorization technologies towards methanol and ethanol production was carried out for a base case corresponding to a CO2 inlet flow corresponding to 10% of the net CO2 emissions in the region (i.e., 1750 kgCO2/h). The results for methanol production through CO2 hydrogenation presented a yield of 59.35% (kgmethanol/kgCO2), a CO2,out/CO2,in ratio of 0.35, a profit margin of 51.07%, and a NPV of 33.42 M.USD. Moreover, the analysis of ethanol production by a biotechnological route to convert CO2 using cyanobacteria (specifically S. elongatus sp. PCC 7942), presented a product yield of 17.61% (kgethanol/kgCO2), a CO2,out/CO2,in ratio of 1.49 × 10–4, a profit margin of 3.86%, and a NPV of − 71.28 M.USD. As conclusions, methanol production is the most viable option to be implemented in the Sucre region at all raw material scales considered. Furthermore, future studies should consider the impact of carbon credits within economic feasibility. Graphical Abstract

Lv H., Hu A., Ding L., Zhang K., Xu Y., Cheng J.

Lin J., Huang C., Wang C., Chang H., Tsai Y.

Xie K., Lin L., Wang P., Shi P., Huang W., Guo X., Zhang S., He C., Frauenheim T.

Laghari Z.A., Yahya W.Z., Mohammed S.A., Bustam M.A.

Carbon dioxide (CO2) can be electrochemically, thermally, and photochemically reduced into valuable products such as carbon monoxide (CO), formic acid (HCOOH), methane (CH4), and methanol (CH3OH), contributing to carbon footprint mitigation. Extensive research has focused on catalysts, combining experimental approaches with computational quantum mechanics to elucidate reaction mechanisms. Although computational studies face challenges due to a lack of accurate approximations, they offer valuable insights and assist in selecting suitable catalysts for specific applications. This study investigates the electrocatalytic pathways of CO2 reduction on cuprous oxide (Cu2O) catalysts, utilizing the computational hydrogen electrode (CHE) model based on density functional theory (DFT). The electrocatalytic performance of flat Cu2O (100) and hexagonal Cu2O (111) surfaces was systematically analysed, using the standard hydrogen electrode (SHE) as a reference. Key parameters, including free energy changes (ΔG), adsorption energies (Eads), reaction mechanisms, and pathways for various intermediates were estimated. The results showed that CO2 was reduced to CO(g) on both Cu2O surfaces at low energies. However, methanol (CH3OH) production was observed preferentially on Cu2O (111) at ΔG = −1.61 eV, whereas formic acid (HCOOH) and formaldehyde (HCOH) formation were thermodynamically unfavourable at interfacial sites. The CO2-to-methanol conversion on Cu2O (100) exhibited a total ΔG of −3.38 eV, indicating lower feasibility compared to Cu2O (111) with ΔG = −5.51 eV. These findings, which are entirely based on a computational approach, highlight the superior catalytic efficiency of Cu2O (111) for methanol synthesis. This approach also holds the potential for assessing the catalytic performance of other transition metal oxides (e.g., nickel oxide, cobalt oxide, zinc oxide, and molybdenum oxide) and their modified forms through doping or alloying with various elements.

Agyekum E.B., Okonkwo P.C., Rashid F.L.

AbstractThe use of methanol as a chemical precursor and fuel additive has increased recently on a global scale. Hence, this study combined bibliometric and traditional review methods to assess the recent trends and evolution of methanol production, as well as its use. According to the study, producing methanol on a large scale from renewable sources is still hampered by the immature technologies used in its production. For instance, methanol production via the process of biochemical conversion still remains at the laboratory level even though it has proven to be a promising production option. Cu-based catalysts, especially Cu-Zn-based catalysts, were found to be the most frequently used catalysts for the hydrogenation of CO2 to methanol due to their superior activity. The bibliometric study shows an annual growth rate of 3.63% in research within the last decade, with 867 authors involved. China leads globally in methanol production and consumption research. The highest collaboration occurred between China and the United States of America with a frequency of six. The study proposed future research directions, including the evaluation of the environmental impact of CO2 conversion to methanol, focusing on the entire life cycle, comparing approaches, and streamlining procedures. It is also recommended to conduct research on flow chemistry and novel reactor designs that enhance mass and heat transfer in catalytic reactors. Graphical Abstract

Yan J., Wang J., Zhang Q., Ni Z., Wang X.

Khan M.J., Safdar M., Jafari M., Arellano-García H.

Abdulkareem-Alsultan G., Lee H.V., Asikin-Mijan N., Samidin S., Adzahar N.A., Kurniawan T.A., Taufiq-Yap Y.H.

Shafiee P., Arellano-Garcia H.

Alipour F., Rahimpour M.R.

Kumar A., Kodamana H.

Mahmood A., Aljohani K., Aljohani B.S., Bukhari A., Abedin Z.U.

The global energy crisis and the urgent need to mitigate carbon emissions have spurred intensive research into sustainable energy sources and efficient catalytic systems. This review integrates recent advancements in two key areas: electrocatalytic methanol oxidation and CO2 reduction to methanol, leveraging metal-organic frameworks (MOFs) and multi-metal nanomaterials. Despite methanol's effectiveness as an energy source, its electro-oxidation requires highly active electrocatalysts. Recent studies have highlighted the superior performance of MOF-based materials, especially when combined with multiple metals, in enhancing the electrocatalytic oxidation of methanol. Downsizing components further boosts MOF activity, while the addition of carbon-containing supports like graphene oxide (GO) and reduced graphene oxide (rGO) improves catalytic capabilities through increased surface area and enhanced dispersion of active materials. Similarly, the electrocatalytic reduction of CO2 to methanol using MOFs has gained traction due to their simplicity, large surface area, and unique structural properties. This review addresses the challenges of selective and efficient CO2 electroreduction, proposing avenues to enhance MOF-based electrocatalysts for methanol production. Strategies include the development of novel MOFs with improved conductivity, chemical durability, and catalytic efficiency. Furthermore, exploration of multi-metal nanomaterials, including tri and tetra-metals, holds promise for advancing electrodes tailored for electrochemical methanol oxidation. By synergistically leveraging MOFs and multi-metal nanomaterials, this review underscores their pivotal roles in addressing energy scarcity and climate change while advancing the field of electrocatalysis towards sustainable methanol oxidation.