CO2 valorisation towards alcohols by Cu-based electrocatalysts: challenges and perspectives

Advances and strategies of electrocatalytic CO2 conversion to alcohols on Cu-based catalysts is assessed with an outlook of current challenges for a practical application of this technology

Influence of sonication on co-precipitation synthesis of copper oxide catalyst for CO2 electroreduction

The need to reduce greenhouse gas emissions and increase our energy supply makes the electrochemical reduction of CO2 (CO2R) a very attractive alternative to produce non-fossil-based fuels or chemicals. Copper-based catalysts is one of the catalyst that most efficiently promote the formation of species with one or more carbon-carbon bonds from the electrochemical reduction of CO2 [1]. Because the catalyst preparation method has an influence on the physicochemical properties and on the electrocatalytic performance[2], in this work, it was decided to evaluate the effect of the ultrasound application (US) on the shape and size of the particles obtained, its electrocatalytic activity and its selectivity to products of interest. For this purpose, sonication was carried out at different percentage amplitudes of ultrasonic power (23, 30 and 37%) during the aging time of the synthesis. Physical characterization was carried out by using different techniques including X-ray diffraction, BET and filed-emission scanning electron microscopy (FESEM). Electrochemical tests for CO2 reduction were done under ambient conditions. Regarding the physical characteristics, we found that pore size distribution is narrower by increasing the US amplitude. On the other hand, there is no significant difference in morphology and dimension of particles. However, the surface area increased with the use of ultrasound, this is attributed to a better dispersion created by acoustic cavitation. Ultrasound has also an effect on Copper-based catalysts performance; in this case, the selectivity towards H2 and C1 products (CO and formate) was enhanced. In addition, an increase in productivity of CO2R products was obtained with respect to the synthesized catalysts that were not assisted by ultrasound (> 3-fold). These results motivate us to further explore in what other ways acoustic cavitation phenomenon can influence the physical characteristics of the catalysts and, in turns, their performance for the electrochemical reduction of CO2

Development of Cu-based hybrid catalysts for the electrocatalytic CO2 reduction to added value products

The simultaneous need to reduce greenhouse gas emissions and increase our energy supply makes the electrochemical reduction of CO2 a very attractive alternative [1]. In this context, science seeks effective methods to transform CO2 into chemicals of economic value. Among the possible products to obtain, we are especially interested in species with one or more carbon-carbon bonds, these types of compounds are favoured using copper as catalyst. Six catalysts were synthesized with different ratios of Cu, Zn Al and subsequently exposed to a thermal treatment to obtain the correspondent oxidized compounds. These kinds of catalyst are traditionally used in thermocatalysis for the efficient production of methanol at high temperature and pressure conditions [2]. Noting the good performance of this catalyst in thermocatalysis, it was chosen to carry out the experiment in the electrochemical reduction of CO2 at ambient conditions. Electrochemical tests were carried out in the rotating disk electrode (RDE) in order to reduce the mass transfer limitations that may exist due to the low solubility of CO2 in an aqueous medium. The chemical-physical properties of the catalyst were studied by several characterization techniques (e.g. XRD, XPS, BET, among others) to understand the role of the modification of the catalyst components during operation in the final selectivity and activity. Among the liquid products obtained are acetone, ethanol, isopropanol, formic acid and in some cases, methanol was also found. Moreover, gaseous products obtained were hydrogen, carbon monoxide and methane, being these last - gaseous products - those that present the highest faradaic efficiencies. These results were compared with the performance of the catalysts in a Gas Diffusion Electrode (GDE) cell, to obtain commercially-relevant current densities

Optimization of Cu-based catalyst for the electrocatalytic reduction of CO2 to fuels

In the last century, with the intensification of human industrial activities, carbon dioxide levels in the environment increased, making global warming and greenhouse effect pressing issues. In this sense, the electroreduction of CO2 is an interesting strategy, also if coupled with renewable energy sources to store the intermittent electric energy in form of chemical bonds [1]. Catalysts composed of a mixture of commercial copper nanoparticles (NPs) were studied. NPs with particles sizes of 25nm, 40-60nm and ZnO: 20-25 nm, named CZ 25_B and CZ_40-60_B. The molar ratio between copper and ZnO is equal to 65/35. Another commercial catalyst that was analysed because it is active for the CO2 hydrogenation is composed of CuO/ZnO/Al2O3 and traces of MgO (named CZA CC_B). These catalytic mixtures were prepared in a planetarian ball mill. Another sample was prepared by pre-oxidation of the Cu NPs and then by manual mixing with the ZnO (called CZ calc 2h). Carbon Nanotubes (CNT) was used as carbon substrate to increase the conductivity and allow a better catalytic ink dispersion. All the samples were characterized by BET analysis, FESEM microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and electrochemical analysis. The electrocatalytic activity was tested by using a rotating disk electrode (RDE) at ambient conditions. The best conditions were appreciated at a potential equal to -2 V vs Ag/AgCl, with the lowest FE for H2 and the highest current density (mA/cm2) in absolute value. In Figure 1, the FE % of gaseous and liquid products are reported for the different prepared catalysts. From all test, the best catalytic activity with the lowest FEH2 was obtained with the Cu/ZnO peroxidised material (CZ calc 2h) at -2 V vs Ag/AgCl. In conclusion, it can be said that Cu-based catalysts were confirmed to be active towards CO2RR via the electrochemical method, with the advantage of performing the reactions at ambient conditions

Catalytic vs electrocatalytic reduction of CO2 to added-value products

Currently, around 85% of the energy matrix is dependent on fossil fuels. Burning fossil fuels provokes environmental pollutants such as CO2, which is the most representative GHG and its concentration has increased in the atmosphere after industrial revolution to >410 ppm[1]. Therefore, to mitigate CO2 emissions into the atmosphere, it can be exploited as a raw material to synthesize high added-value products (i.e. methanol) [2]. The electrochemical (EC) reduction of CO2 is a sustainable and technologically interesting process to produce chemicals or fuels using renewable electricity sources[3]. The main challenge is to find a suitable electrocatalyst to establish this technology at an industrial level. In such context, our group have exploited, for this EC process, a Cu-based material typically used as catalyst in Thermochemical (TC) catalysis for the production of methanol. A commercial catalyst (Cu-Zn-Al-based) was tested for both processes for comparison. The TC CO2 reduction reaction in H2 atmosphere (25 bars and 250 °C) leads to a methanol selectivity of 50% and CO as side-product, whereas the EC process (at atmospheric conditions) yields different alcohols and other C-based products (C1 to C3) with an overall faradaic efficiency of ~70%. The EX situ X-ray diffraction pattern, Field-Emission Electron Microscopy and Transmission Electron Microcopy of the catalyst were compared before and after both experiments in order to study the role of the modification of the catalyst components during operation in the final selectivity. These results demonstrated that there is synergy between both processes that can be exploited to develop new electrocatalysts

Facile and scalable synthesis of Cu2O-SnO2 catalyst for the photoelectrochemical CO2 conversion

The conversion of the atmospheric CO2 to value-added compounds is more and more attractive to the scientific community, since natural sink cannot keep up with the constant anthropogenic emission and amplification processes. Recently, CO2 concentration in the atmosphere exceeds 410 ppm, and its growth has remained constant since the 50s[1]. Renewable and green approaches to CO2 recovery are aimed to minimize the worrying impact of its emission to the environment, and to drive the transition to a new circular economy approach in chemistry and energy production. Within the depicted scenario, electrochemical and photoelectrochemical CO2 reduction are being widely investigated as promising methods to transform CO2, under mild reaction conditions, into useful chemicals or fuels. For instance, alcohols, CO and HCOOH that can be exploited as renewable energy sources or as key intermediates for the chemical industry. Among the non-precious metal oxides, Cu2O is a cheap, abundant and intrinsically p-type semiconductor. Due to its narrow band gap (~ 2 eV) and the suitable positioning of conduction and valence bands, Cu2O is an ideal photocatalyst for CO2RR. Simultaneously, SnO2 is an n-type direct band-gap semiconductor with noticeable electron mobility together with an intrinsic stability. It openly transpires the dual role of Cu2O, as photoabsorber and forming a p-n junction with Tin Oxide. In this work, the synthesis of photoactive copper-tin-oxide-based catalyst was optimized by a co-precipitation method[2], employing Cu(NO3)2·3H2O and SnCl4∙5H2O into a stirred and heated reactor. A solution of Na2CO3 was added as a precipitant agent, while NaBH4 as the reducing one[3], in order to promote the Cu2O formation. The work-up protocol, based on copious MilliQ water washings, was implemented and finally optimized. The characterization step included Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive X-ray Analysis (EDX) X-rays Diffraction Analysis(XRD), UV-Visible Spectroscopy analysis, among others, and allowed the morphological assessment, the porosity value estimation and the crystalline phase evaluation. With the latter, it has been found a correspondence to the cubic crystalline phase (cuprite) of Cu2O and consequently confirmation that the reduction process has been successfully carried out. The so obtained catalyst was then deposited it onto a GDL (Gas Diffusion Layer) and FTO-based substrates by spray coating of an ink containing: the catalysts, Vulcan carbon (to increase the electrode conductivity), Nafion as binder and Isopropanol as carrier. The photo-electrochemical activity for the CO2 reduction reaction was tested in the dark and under sunlight simulated conditions by means of Linear Sweep Voltammetry (LSV) and Chrono-Potentiometry (CP) analyses. Relevant current density (j) values of up to 40 mA/cm2 were observed, and from the products analysis during the CP a high Faradaic Efficiency to CO was obtained. The influence of the ink composition was accurately investigated in terms of interaction among all the components and with respect to the employed substrate, taking into account sun-light activity and stability of the prepared electrodes towards their future utilization in a device for the sun-driven CO2 conversion to high-added value products. AKCNOWLEDGMENT This work has received funding from the European Union’s Horizon 2020 Research and Innovation Action programme under the Project SunCoChem (Grant Agreement No 862192). [1] X. Lan, B. D. Hall, G. Dutton, J. Mühle, and J. W. Elkins. (2020). Atmospheric composition [in State of the Climate in 2018, Chapter 2: Global Climate]. [2] Schuth F., et al., Journal of Catalysis, (2008), 258 [3] Angew. Chem. Int. Ed. 10.1002/anie.20180896

Hierarchical TiN-Supported TsFDH Nanobiocatalyst for CO2 Reduction to Formate

The electrochemical reduction of CO2 to value-added products like formate represents a promising technology for the valorization of carbon dioxide. We propose a proof-of-concept bioelectrochemical system (BES) for the reduction of CO2 to formate. For the first time, our device employs a nanostructured titanium nitride (TiN) support for the immobilization of a formate dehydrogenase (FDH) enzyme. The hierarchical TiN nanostructured support exhibits high surface area and wide pore size distribution, achieving high catalytic loading, and is characterized by higher conductivity than other oxide-based supports employed for FDHs immobilization. We select the oxygen-tolerant FDH from Thiobacillus sp. KNK65MA (TsFDH) as enzymatic catalyst, which selectively reduces CO2 to formate. We identify an optimal TiN morphology for the enzyme immobilisation through enzymatic assay, reaching a catalyst loading of 59 μg cm−2 of specifically-adsorbed TsFDH and achieving a complete saturation of the anchoring sites available on the surface. We evaluate the electrochemical CO2 reduction performance of the TiN/TsFDH system, achieving a remarkable HCOO− Faradaic efficiency up to 76 %, a maximum formate yield of 44.1 μmol mg−1FDH h−1 and high stability. Our results show the technological feasibility of BES devices employing novel, nanostructured TiN-based supports, representing an important step in the optimization of these devices

Hierarchical TiN-Supported TsFDH Nanobiocatalyst for CO2 Reduction to Formate

AbstractThe electrochemical reduction of CO2 to value‐added products like formate represents a promising technology for the valorization of carbon dioxide. We propose a proof‐of‐concept bioelectrochemical system (BES) for the reduction of CO2 to formate. For the first time, our device employs a nanostructured titanium nitride (TiN) support for the immobilization of a formate dehydrogenase (FDH) enzyme. The hierarchical TiN nanostructured support exhibits high surface area and wide pore size distribution, achieving high catalytic loading, and is characterized by higher conductivity than other oxide‐based supports employed for FDHs immobilization. We select the oxygen‐tolerant FDH from Thiobacillus sp. KNK65MA (TsFDH) as enzymatic catalyst, which selectively reduces CO2 to formate. We identify an optimal TiN morphology for the enzyme immobilisation through enzymatic assay, reaching a catalyst loading of 59 μg cm−2 of specifically‐adsorbed TsFDH and achieving a complete saturation of the anchoring sites available on the surface. We evaluate the electrochemical CO2 reduction performance of the TiN/TsFDH system, achieving a remarkable HCOO− Faradaic efficiency up to 76 %, a maximum formate yield of 44.1 μmol mg−1FDH h−1 and high stability. Our results show the technological feasibility of BES devices employing novel, nanostructured TiN‐based supports, representing an important step in the optimization of these devices

Investigation of Gas Diffusion Electrode Systems for the Electrochemical CO2 Conversion

Electrochemical CO2 reduction is a promising carbon capture and utilisation technology. Herein, a continuous flow gas diffusion electrode (GDE)-cell configuration has been studied to convert CO2 via electrochemical reduction under atmospheric conditions. To this purpose, Cu-based electrocatalysts immobilised on a porous and conductive GDE have been tested. Many system variables have been evaluated to find the most promising conditions able to lead to increased production of CO2 reduction liquid products, specifically: applied potentials, catalyst loading, Nafion content, KHCO3 electrolyte concentration, and the presence of metal oxides, like ZnO or/and Al2O3. In particular, the CO productivity increased at the lowest Nafion content of 15%, leading to syngas with an H2/CO ratio of ~1. Meanwhile, at the highest Nafion content (45%), C2+ products formation has been increased, and the CO selectivity has been decreased by 80%. The reported results revealed that the liquid crossover through the GDE highly impacts CO2 diffusion to the catalyst active sites, thus reducing the CO2 conversion efficiency. Through mathematical modelling, it has been confirmed that the increase of the local pH, coupled to the electrode-wetting, promotes the formation of bicarbonate species that deactivate the catalysts surface, hindering the mechanisms for the C2+ liquid products generation. These results want to shine the spotlight on kinetics and transport limitations, shifting the focus from catalytic activity of materials to other involved factors