Electrochemical Reduction of CO2 to Oxalic Acid: Experiments, Process Modeling, and Economics

Non-Aqueous Electrochemical CO2 Reduction to Multivariate C2-Products Over Single Atom Catalyst at Current Density up to 100 mA cm-2

Electrochemical CO2 reduction reaction (CO2-RR) in non-aqueous electrolytes offers significant advantages over aqueous systems, as it boosts CO2 solubility and limits the formation of HCO3 - and CO3 2- anions. Metal-organic frameworks (MOFs) in non-aqueous CO2-RR makes an attractive system for CO2 capture and conversion. However, the predominantly organic composition of MOFs limits their electrical conductivity and stability in electrocatalysis, where they suffer from electrolytic decomposition. In this work, electrically conductive and stable Zirconium (Zr)-based porphyrin MOF, specifically PCN-222, metalated with a single-atom Cu has been explored, which serves as an efficient single-atom catalyst (SAC) for CO2-RR. PCN- 222(Cu) demonstrates a substantial enhancement in redox activity due to the synergistic effect of the Zr matrix and the single-atom Cu site, facilitating complete reduction of C2 species under non-aqueous electrolytic conditions. The current densities achieved (≈100 mA cm- 2) are 4-5 times higher than previously reported values for MOFs, with a faradaic efficiency of up to 40% for acetate production, along with other multivariate C2 products, which have never been achieved previously in non-aqueous systems. Characterization using X-ray and various spectroscopic techniques, reveals critical insights into the role of the Zr matrix and Cu sites in CO2 reduction, benchmarking PCN-222(Cu) for MOF-based SAC electrocatalysis.

Reverse-engineered Electro-Fenton for the selective synthesis of oxalic or oxamic acid through the degradation of acetaminophen: A novel green electrocatalytic refinery approach

The Electro-Fenton process (EF) has been conventionally applied to efficiently degrade refractory and/or toxic pollutants. However, in this work, EF was used as a reverse engineering tool to selectively synthesize highly value-added products (oxalic or oxamic acid) through the degradation of the model pollutant acetaminophen, a widely used analgesic and antipyretic drug. It was found that the production of either oxalic or oxamic acid is dictated by the applied current density. Hence, oxalic acid is favored at low current densities trough a mechanism dominated by homogeneous •OH radical oxidation, while oxamic acid is the majoritarian product at high current densities where electron transfer at the anode surface becomes an important mechanism in combination with •OH oxidation. Under optimal reaction conditions (0.71 mA cm-2 and 100 mg l-1 of initial total organic carbon (TOC) concentration), up to 227.1 ± 26.3 mg l-1 of oxalic acid were produced, with high yield and selectivity of 54.9 ± 5.1 % and 94.7 ± 9.9 %, respectively (the TOC removal was 42.0 ± 2.4 %). In the case of oxamic acid, the highest concentration of 33.8 ± 2.1 mg l-1 was produced at 2.13 mA cm-2 and an initial TOC concentration of 50 mg l-1, which represented a yield of 18.7 ± 0.3 % and 60.9 ± 9.3 % selectivity (71.1 ± 4.4 % of TOC removal). It is worth noting that at low current density when oxalic acid is favored, the selectivity for both products was 100 %, meaning that those were the only products remaining in the solution, with oxalic acid as the major product (94.7 ± 9.9 % with initial TOC of 100 mg l-1, and 98.7 ± 0.9 % with initial TOC of 50 mg l-1). This is a pioneer work on EF applications to the field of wastewater valorization/refining through the recovery of value-added products within a circular economy.

Parameter Dependency of Electrochemical Reduction of CO2 in Acetonitrile - A Data Driven Approach

The electrochemical CO2 reduction reaction (CO2RR) is a promising technology for the utilization of captured CO2. Though systems using aqueous electrolytes is the state-of-the-art, CO2RR in aprotic solvents are a promising alternative that can avoid the parallel hydrogen evolution reaction (HER). While system parameters, such as electrolyte composition, electrode material, and applied potential are known to influence the reaction mechanism, there is a lack of intuitive understanding as to how. We show that by using multivariate data analysis on a large dataset collected from the literature, namely random forest modelling, the most important system parameters can be isolated for each possible product. We find that water content, current density, and applied potential are powerful determinants in the reaction pathway, and therefore in the Faradaic efficiency of CO2RR products.

Proposing Oxalic Acid as Chemical Storage of Carbon Dioxide to Achieve Carbon Neutrality

Increasing emissions of carbon dioxide into the atmosphere due to the use of fossil fuels and ongoing deforestation are affecting the global climate. To reach the Paris climate agreement, in the coming decades low emission technologies must be developed, which allow for carbon removal on a Gt per year-scale. In this regard, we propose the electrochemical conversion of carbon dioxide to oxalic acid as a potentially viable pathway for large scale CO2 utilization and storage. Combined with water oxidation, in principle this transformation does not need stoichiometric amounts of co-reagents and minimize the necessary electrons for the reduction of carbon dioxide.

The Effect of Salts on the CO2 Reduction Product Distribution in an Aprotic Electrolyte

Electrochemical CO2 reduction in non-aqueous solvents is promising due to the increased CO2 solubility of organic-based electrolytes compared to aqueous electrolytes. Here the effect of nine different salts in propylene carbonate (PC) on the CO2 reduction product distribution of polycrystalline Cu is investigated. Three different cations (tetraethylammonium (TEA), tetrabutylammonium (TBA), and tetrahexylammonium (THA)) and three different anions (chloride (Cl), tetrafluoroborate (BF4), and hexafluorophosphate (PF6)) were used. Chronoamperometry and in-situ FTIR measurements show that the size of the cation has a crucial role in the selectivity. A more hydrophobic surface is obtained when employing a larger cation with a weaker hydration shell. This stabilizes the CO2 - radical and promotes the formation of ethylene. CO2 reduction in 0.7 M THACl/PC shows the highest hydrocarbon formation. Lastly, we hypothesize that the hydrocarbon formation pathway is not through C-C coupling, as the CO solubility in PC is very high, but through the dimerization of the COH intermediate.

Electrochemical CO2 Reduction Reaction: Comprehensive Strategic Approaches to Catalyst Design for Selective Liquid Products Formation

The escalating concern regarding the release of CO2 into the atmosphere poses a significant threat to the contemporary efforts in mitigating climate change. Amidst a multitude of strategies for curtailing CO2 emissions, the electrochemical CO2 reduction presents a promising avenue for transforming CO2 molecules into a diverse array of valuable gaseous and liquid products, such as CO, CH3OH, CH4, HCO2H, C2H4, C2H5OH, CH3CO2H, 1-C3H7OH and others. The mechanistic investigations of gaseous products (e. g. CO, CH4, C2H4, C2H6 and others) broadly covered in the literature. There is a noticeable gap in the literature when it comes to a comprehensive summary exclusively dedicated to coherent roadmap for the designing principles for a selective catalyst all possible liquid products (such as CH3OH, C2H5OH, 1-C3H7OH, 2-C3H7OH, 1-C4H9OH, as well as other C3-C4 products like methylglyoxal and 2,3-furandiol, in addition to HCO2H, AcOH, oxalic acid and others), selectively converted by CO2 reduction. This entails a meticulous analysis to justify these approaches and a thorough exploration of the correlation between materials and their electrocatalytic properties. Furthermore, these insightful discussions illuminate the future prospects for practical applications, a facet not exhaustively examined in prior reviews.

The Effect of the Tetraalkylammonium Cation in the Electrochemical CO2 Reduction Reaction on Copper Electrode

Aprotic organic solvents such as acetonitrile offer a potential solution to promote electrochemical CO2 reduction over the competing hydrogen evolution reaction. Tetraalkylammonium cations (TAA+) are widely used as supporting electrolytes in organic media due to their high solubility and conductivity. The alkyl chain length of TAA+ cations is known to influence electron transfer processes in electrochemical systems by the adsorption of TAA+, causing modifications of the double layer. In this work, we elucidate the influence of the cation chain length on the mechanism and selectivity of the CO2RR reaction under controlled dry and wet acetonitrile conditions on copper cathodes. We find that the hydrophobic hydration character of the cation, which can be tuned by the chain length, has an effect on product distribution, altering the reaction pathway. Under dry conditions, smaller cations (TEA+) preferentially promote oxalate production via dimerization of the CO2 ·- intermediate, whereas formate is favored in the presence of water via protonation reaction. Larger cations (TBA+ > TPA+ > TEA+) favor the generation of CO regardless of water content. In situ FTIR analysis showed that TBA+ cations are able to stabilize adsorbed CO more effectively than TEA+, explaining why larger cations generate a higher proportion of CO. Our findings also suggest that higher cation concentrations suppress hydrogen evolution, particularly with larger cations, highlighting the role of cation chain length size and hydrophobic hydration shell.

From Ethylene Glycol to Glycolic Acid: Electrocatalytic Conversion on Pt-Group Metal Surfaces

Ethylene glycol (EG) is one of the most attractive platform molecules derived from biomass and waste plastics. Thus, the selective electrooxidation of ethylene glycol (EGOR) into value-added chemicals (especially glycolic acid (GA)) can promote its recycling and upgrading. However, the understanding of the EG-to-GA process on Pt-group metal (PGM) electrodes is far limited now. It has been shown that the Pt and Pd electrodes could show considerable EGOR activity but not Rh and Ir electrodes. Meanwhile, EGOR mainly produces the glycolate, oxalate, and formate on Pt and Pd electrodes, whereas it can obtain minute amounts of glycolate and oxalate on Rh and Ir electrodes. Impressively, the selectivity of glycolate on Pt and Pd electrodes can be over 85% (apparent Faradaic efficiency) in alkaline media, although the stability should be further improved through interfacial tuning and/or engineering. This work might deepen the fundamental understanding of the EGOR process on the nature of PGM electrodes.

A comprehensive review of recent advances in the applications and biosynthesis of oxalic acid from bio-derived substrates

Organic acids are important compounds with numerous applications in different industries. This work presents a comprehensive review of the biological synthesis of oxalic acid, an important organic acid with many industrial applications. Due to its important applications in pharmaceuticals, textiles, metal recovery, and chemical and metallurgical industries, the global demand for oxalic acid has increased. As a result, there is an increasing need to develop more environmentally friendly and economically attractive alternatives to chemical synthesis methods, which has led to an increased focus on microbial fermentation processes. This review discusses the specific strategies for microbial production of oxalic acid, focusing on the benefits of using bio-derived substrates to improve the economics of the process and promote a circular economy in comparison with chemical synthesis. This review provides a comprehensive analysis of the various fermentation methods, fermenting microorganisms, and the biochemistry of oxalic acid production. It also highlights key sustainability challenges and considerations related to oxalic acid biosynthesis, providing important direction for further research. By providing and critically analyzing the most recent information in the literature, this review serves as a comprehensive resource for understanding the biosynthesis of oxalic acid, addressing critical research gaps, and future advances in the field.

Evaluating the native oxide of titanium as an electrocatalyst for oxalic acid reduction

Herein, we show that unmodified titanium electrodes bearing the naturally-forming native TixOy coating display superior activity for the electroreduction of oxalic acid to glyoxylic acid and glycolic acid compared to Ti-based electrodes that have been deliberately modified for this purpose, in terms of both oxalic acid conversion and overall yields of reduced products.

Catalyst-Free Carbon Dioxide Conversion in Water Facilitated by Pulse Discharges

By inducing CO2-pulsed discharges within microchannel bubbles and regulating thus-forming plasma microbubbles, we observe high-performance, catalyst-free coformation of hydrogen peroxide (H2O2) and oxalate directly from CO2 and water. With isotope-labeled C18O2 as the feedstock, peaks of H218O16O and H216O2 observed by ex situ surface-enhanced Raman spectra indicate that single-atom oxygen (O) from CO2 dissociations and H2O-derived OH radicals both contribute to H2O2 formation. The global plasma chemistry modeling suggests that high-density, energy-intense electron supply enables high-density CO2- (aq) and HCO2- (aq) formation and their subsequent coupling to produce oxalate. The enhanced solvation of CO2, facilitated by the efficient transport of CxOy ionic species and CO, is demonstrated as a crucial benefit of spark discharges interacting with water at the bubble interface. We expect this plasma microbubble approach to provide a novel power-to-chemical avenue to convert CO2 into valuable H2O2 and oxalic acid platform chemicals, thus leveraging renewable energy resources.

A Quantitative Analysis of Electrochemical CO2 Reduction on Copper in Organic Amide and Nitrile-Based Electrolytes

Aqueous electrolytes used in CO₂ electroreduction typically have a CO₂ solubility of around 34 mM under ambient conditions, contributing to mass transfer limitations in the system. Non-aqueous electrolytes exhibit higher CO₂ solubility (by 5-8-fold) and also provide possibilities to suppress the undesired hydrogen evolution reaction (HER). On the other hand, a proton donor is needed to produce many of the products commonly obtained with aqueous electrolytes. This work investigates the electrochemical CO₂ reduction performance of copper in non-aqueous electrolytes based on dimethylformamide (DMF), n-methyl-2-pyrrolidone (NMP), and acetonitrile (ACN). The main objective is to analyze whether non-aqueous electrolytes are a viable alternative to aqueous electrolytes for hydrocarbon production. Additionally, the effects of aqueous/non-aqueous anolytes, membrane, and the selection of a potential window on the electrochemical CO₂ reduction performance are addressed in this study. Experiments with pure DMF and NMP mainly produced oxalate with a faradaic efficiency (FE) reaching >80%; however, pure ACN mainly produced hydrogen and formate due to the presence of more residual water in the system. Addition of 5% (v/v) water to the non-aqueous electrolytes resulted in increased HER and formate production with negligible hydrocarbon production. Hence, we conclude that aqueous electrolytes remain a better choice for the production of hydrocarbons and alcohols on a copper electrode, while organic electrolytes based on DMF and NMP can be used to obtain a high selectivity toward oxalate and formate.

Titanium dioxide-based anode materials for lithium-ion batteries: structure and synthesis

Lithium-ion batteries (LIBs) have high energy density, long life, good safety, and environmental friendliness, and have been widely used in large-scale energy storage and mobile electronic devices. As a cheap and non-toxic anode material for LIBs, titanium dioxide (TiO2) has a good application prospect. However, its poor electrical conductivity leads to unsatisfactory electrochemical performance, which limits its large-scale application. In this review, the structure of three TiO2 polymorphs which are widely investigated are briefly described, then the preparation and electrochemical performance of TiO2 with different morphologies, such as nanoparticles, nanowires, nanotubes, and nanospheres, and the related research on the TiO2 composite materials with carbon, silicon, and metal materials are discussed. Finally, the development trend of TiO2-based anode materials for LIBs has been briefly prospected.