Scaling CO2 electrolysis cell to relevant channel lengths introduces thermal and compositional gradients that can affect salt precipitation behavior. In alkaline systems, these gradients may shift where salt supersaturation first becomes thermodynamically feasible. Here, a spatially resolved non-isothermal model is applied to a 40 cm high CO2 flow cell operated at 200–800 mA cm⁻2. Local predicted states are evaluated against temperature-dependent ternary solubility limits for H2O–HCOOK–K2CO3 and H2O–HCOOK–KHCO3, as well as a mixed carbonate and bicarbonate solubility envelope, using a supersaturation ratio (SR). The simulated axial temperature rise increases from 8 °C at 200 mA cm⁻2 to 39 °C at 800 mA cm⁻2, while the contribution of ohmic dissipation to the cathode-side heat budget increases from 5

CO₂ electroreduction (CO₂ER) offers a sustainable pathway for producing value-added chemicals from CO₂ using renewable electricity. Among its products, formate (HCOO⁻) is particularly attractive for energy storage and industrial applications. However, scaling CO₂ER systems from laboratory to industrial dimensions presents challenges including inhomogeneous reactant distribution, mass transport limitations, and local pH gradients. These effects are exacerbated at high current densities, leading to intensified CO₂ depletion and increased hydrogen evolution (HER). In this study, a validated 2D transient-state model is developed to investigate the impact of cell height, operating pressure, and electrolyte flow rate on formate production performance. Results show that shorter cells (4 cm) better maintain Faradaic efficiency by reducing HER and reactant depletion, whereas longer cells (40 cm) exhibit pronounced concentration gradients and non-uniform current densities. Operating at elevated pressures (5.5 atm) improves CO₂ solubility, limiting efficiency loss to 11 %, compared to 16 % at 1.5 atm under current densities of 150–400 mA cm⁻². These insights provide design and operation guidelines for optimizing industrial-scale CO₂ electrolyzers for efficient formate production.

Electrochemical carbon dioxide (CO2) reduction reaction (CO2RR) is gaining attraction as it enables the generation of highly valuable chemicals while mitigating greenhouse gas emissions. Numerous efforts have been dedicated to designing efficient catalysts that selectively reduce CO2 into various chemicals such as formic acid/formate, carbon monoxide, methane, ethylene, and ethanol. The efficiency of the CO2RR is evaluated through selectivity to the desired product, overpotential, current density, and stability. This work reviews emerging strategies to improve the CO2RR performance to produce formate/formic acid by tuning catalyst and electrode structures. The catalyst-developing strategies are discussed in terms of engineering electronic and geometric structures, surface oxidation structure, and enlarging surface active area. The design of gas diffusion electrodes and reactor configuration, which are essential in enhancing the efficiency of the electrochemical system, are also mentioned. Finally, insights and perspectives are given on how to overcome the instability of catalysts and limitations of reactor designs.

Since the dawn of the industrial revolution, the Earth's atmosphere has experienced a gradual increase in carbon dioxide (CO2) concentrations, leading to the trapping of solar energy and consequent global warming. This shift in climate dynamics carries significant consequences, including heightened levels of secondary pollutants in urban areas and an increase in the frequency and severity of natural disasters. To address these pressing environmental challenges, there is a widespread global inclination toward adopting sustainable technologies and reducing CO2 and other greenhouse gas emissions [1,2]. However, the primary source of CO2 emissions remain the combustion of fossil fuels, which is expected to persist as the dominant energy source for the foreseeable future. It is driven by extensive usage across various sectors, notably in the chemical industry and transportation. Despite growing awareness of the need to curb greenhouse gas emissions, transitioning to viable alternatives in these sectors presents formidable challenges. Factors such as entrenched infrastructure, economic dependencies, and technological barriers complicate the shift away from fossil fuels, highlighting the complexity of the transition toward sustainable solutions [1,3]. The emergence of CO2 electrolyzers, which can be powered by various energy sources including renewable options like solar power, can redefine the production of chemicals, reducing our reliance on fossil fuels in this sector and thereby mitigating CO2 emissions. This shift offers a promising solution to address global warming problems. These devices facilitate the conversion of CO2 into valuable chemicals through electrochemical processes, specifically the electrochemical CO2 Reduction Reaction (CO2RR), thus aiding emission reduction and integrating renewable energy into global infrastructure. However, scaling this technology to industrial levels reveals significant hurdles. Achieving consistent reaction rates across the whole cell's surface area, optimizing current distribution, and addressing challenges related to CO2 mass transfer are critical considerations. Among these challenges, maintaining electrochemical performance over time emerges as the most important factor. Industrial-scale implementation is further complicated by the inherent complexity of operational modes, expanded surface areas, and intricate flow paths. Consequently, adopting a model-based approach becomes paramount. Leveraging computational models allows for comprehensive analysis and optimization of large-scale CO2 electrolysis systems, helping to understand system dynamics and to refine pivotal components such as gas diffusion electrodes. Advancing CO2 electrolyzer technology holds significant importance for both academic research and industrial applications in the pursuit of sustainable energy solutions [4,5]. In this study, we present a two-dimensional transient-state multiphase Gas Diffusion Electrode (GDE) model, which describes mass and heat transfer, electrochemical kinetics, aqueous-phase reactions, and species concentrations within the total flowing electrolyte volume in the system over time. Our study focuses on the conversion of CO2 to formate due to its industrial significance and commercial viability. We provide experimental validation of the model using results obtained from an industrial cell, specifically the first industrial 1526 cm² electrochemical cell. This represents a significant advancement, with a 60-fold increase from the largest three-compartment electrolyzer reported to date [6]. Subsequently, we apply the model to predict the scalability of the GDE and assess the impact of key parameters on system performance. We observe that electrolyzers with greater heights exhibit significantly stronger reactions at the same current density. Moreover, the model effectively captures the pH time dependency at various points in the GDE and aids in identifying the stability and evolution of the chemical species composing the GDE. It also accurately represents variations in carbonate and bicarbonate production, as well as the decrease of hydroxide over time in the catholyte. Furthermore, in the context of stability, we numerically investigate the impact of catalyst degradation over time on system performance. [1] Konderla, Vojtěch. "Numerical modelling of a gas diffusion-based CO2 electrolyser with flowing catholyte." (2022). [3] Kibria, Md Golam, et al. "Electrochemical CO2 reduction into chemical feedstocks: from mechanistic electrocatalysis models to system design." Advanced Materials 31.31 (2019): 1807166. [3] Smith, Wilson A., et al. "Pathways to industrial-scale fuel out of thin air from CO2 electrolysis." Joule 3.8 (2019): 1822-1834. [4] Yang, Ziming, et al. "Modeling and upscaling analysis of gas diffusion electrode-based electrochemical carbon dioxide reduction systems." ACS Sustainable Chemistry & Engineering 9.1 (2020): 351-361. [5] Gawel, A., et al., Electrochemical CO2 reduction-the macroscopic world of electrode design, reactor concepts & economic aspects. IScience, 2022. [6] Fink, A.G., et al., Scale‐up of Electrochemical Flow Cell towards Industrial CO2 Reduction to Potassium Formate. ChemCatChem: p. e202300977. Figure 1

Gas diffusion electrodes (GDEs) are promising for scaling up industrial CO2 electrochemical reduction cells. This study introduces a transient numerical model representing an industrial electrolyzer. The model incorporates electrochemical kinetics, homogeneous reaction kinetics, and transport phenomena within the cathode compartment. By integrating a global mass balance over the entire electrolyte, it analyzes time-dependent performance variations such as Faradaic efficiency (FE). This allows us to simulate formate production and understand mass transport limitations within the GDE. Our results demonstrated a 4% increase in FE when the electrolyte flow rate was increased from 120 to 360 mL/min. However, further increasing the flow rate to 830 mL/min showed diminishing returns. Additionally, increasing the KOH concentration in the catholyte from 0.5 to 1 M resulted in a 7-10% increase in FE. A slight further increase was observed when increasing from 3 to 4 M. This analysis provides valuable insights into optimizing electrochemical reduction processes at an industrial scale.

The CO2 electroreduction reaction (CO2RR) presents a pathway to decarbonize the manufacturing industry by using clean electricity and CO2 as feedstocks instead of relying on fossil fuels. Although known for over 100 years, this technology has yet only been developed at bench-top scale (1-100 cm(2)). In this manuscript, we report CO2 electroreduction to potassium formate (HCOOK) in a stack of two 1526-cm(2) three-compartments electrochemical cells with gas diffusion electrode (GDE) cathodes. In this stack, we achieved over 60 % current selectivity towards HCOOK at current densities of 200 mA cm(-2) and under cell voltages of similar to 4.0 V. We reached these performance metrics by tuning electrolyte composition and cell architecture. We also show that a minimum of +10-14 kPa of pressure difference must be applied between gaseous and catholyte compartments to enable the CO2RR to take place. We emphasize the challenges associated with scaling-up a CO2 electrochemical cell, specifically by demonstrating that optimal operation parameters are strongly correlated to cell architecture. This study demonstrates the feasibility of developing CO2RR electrochemical cells to an industrial scale.

The kinetics of water adsorption in powder sorbent layers are important to design a scaled-up atmospheric water capture device. Herein, the adsorption kinetics of three sorbents, a chromium (Cr)-based metal-organic framework (Cr-MIL-101), a carbon-based material (nanoporous sponges/NPS), and silica gel, have been tested experimentally, using powder layers ranging from ∼0 to 7.5 mm in thickness, in a custom-made calibrated environmental chamber cycling from 5 to 95% RH at 30 °C. A mass and energy transfer model was applied onto the experimental curves to better understand the contribution of key parameters (maximum water uptake, kinetics of single particles, layer open porosity, and particle size distribution). Open porosity (i.e., the void-to-particle ratio in the sorbent layer) shows the highest influence to improve the kinetics. Converting the sorbent kinetics data into a daily yield of captured water demonstrated (i) the existence of an optimal open porosity for each sorbent, (ii) that thinner layers with moderate open porosity performed respectively better than thicker layers with high open porosity, and (iii) that high maximum water uptake and fast single-particle kinetics are not necessarily predictive of high daily water yield.

The thermodynamic properties of various sorbents, namely carbon-based sorbents, silica gel and metal-organic frameworks (MOFs) were assessed and compared based on their adsorption isotherms at 25, 30 and 35 degrees C. The isotherms were measured in a custom-made and calibrated environmental chamber using a gravimetric method. Gibbs free energy demonstrated the spontaneity of the adsorption process and the hygroscopicity variation of the sorbents depending on their surface chemistry. The carbon-based sorbent, nanoporous sponges (NPS), and one of the MOFs, Cr-MIL-101, had lower sorbent-adsorbate interactions and thus had integral enthalpies converging rapidly to the heat of vaporization of pure water. As such, these samples would release less heat during an adsorption step with partial filling of the sorbent. Integral entropy showed that, for most of the environmental conditions, adsorbed water molecules had an entropy equivalent to pure water for most of the sorbent materials, except for silica gel, due to its higher energy sites and higher water-sorbent interactions. NPS and Cr-MIL-101 were shown to be entropically advantageous for the recovery/removal of water. Enthalpy and entropy can provide insight to select favorable conditions to perform adsorption-desorption cycles in a practical water capture system.

CO2 electroreduction flow cells with three compartments (CO2, catholyte, and anolyte) are promising as scalable and viable systems for industrial applications. CO2 and catholyte are separated by a porous gas diffusion layer (GDL), allowing enhanced CO2 diffusion to the catalyst. While CO2 diffusion is an expected phenomenon in the electroreduction cell, a pressure difference between both sides of the GDL can lead to unfavorable CO2-to-catholyte or catholyte-to-CO2 crossflows. These two crossflows are detrimental to the electrochemical performance by decreasing the diffusion mass transfer while damaging the cell. The particular geometry and feed conditions of a cell stack can induce pressure variations along the fluid path, leading to these undesirable crossflows. Using a commercial GDL with no microporous layer for the present numerical study, we found that the amplitude of the pressure difference between both sides must be lower than 5 kPa to prevent these unfavorable crossflows. The effects of CO2 and catholyte mass flow rates, channel size in the CO2 distribution side, and surface area of the cell are evaluated in a single-cell system and in a 50-cell stack, made possible by our mathematical resolution of the Z-manifold systems in a straight parallel channel configuration. Small cells (10 cm(2)) have a wider range of favorable conditions for enabling crossflows compared to larger cells (0.5 m(2)). CO2 and catholyte mass flow rates of, respectively, 1.5 x 10(-4) and 6.6 x 10(-2) kg/s for the 0.5 m(2) cell were found to prevent any unfavorable crossflows by limiting the pressure difference within the desirable range.

Formic acid is a liquid, safe, and energy-dense carrier for fuel cells. Above all, it can be sustainably produced from the electroreduction of CO2. The formic acid market is currently saturated, and it requires alternative applications to justify additional production capacity. Fuel cell technologies offer a chance to expand it, while creating an opportunity for sustainability in the energy sector. Formic acid-based fuel cells represent a promising energy supply system in terms of high theoretical open-circuit voltage (1.48 V). Compared to common fuel cells running on H-2 (e.g., proton-exchange membrane fuel cells), formic acid has a lower storage cost and is safer. This review focuses on the sustainable production of formic acid from CO2 and on the detailed analysis of commercial examples of formic acid-based fuel cells, in particular direct formic acid fuel cell stacks. Designs described in the literature are mostly at the laboratory scale, still, with 301 W as the maximum power output achieved. These case studies are fundamental for the scale-up; however, additional efforts are required to solve crossover and increase performance.

Water scarcity threatens more and more people in the world. Moisture adsorption from the atmosphere represents a promising avenue to provide fresh water. Nanoporous sponges ("NPSs"), new carbon-based sorbents synthesized from the pyrolysis of resorcinol-formaldehyde resin, can achieve comparable performance to metal organic framework-based systems, but at a significantly lower cost. Oxygen and nitrogen functionalities can be added to the NPS surface, through oxidation and addition of phenanthroline to the initial reagent mixture, respectively. The resulting NPS sorbents have high specific surface areas of 347 to 527 m(2).g(-1) and an average capillary-condensation-compatible pore size of 1.5 nm. When oxidized, the NPS can capture up to 0.28 g of water per gram of adsorbent at a relative pressure of 0.90 (0.14 g.g(-1) at P/P-sat = 0.40) and maintain this adsorption capacity over multiple adsorption/desorption cycles. Scaled-up synthesis of the NPS was performed and tested in an experimental water capture setup, showing good agreement between small- and larger-scale adsorption properties. Water adsorption isotherms fitted with the theoretical model proposed by Do and Do demonstrate that hydroxyl functionalities are of key importance to NPS behavior.

A versatile pilot-scale reactor has been designed in such a way that it can be readily converted from a dielectric barrier discharge "PECVD" operating mode into a photoinitiated "PICVD" one; in the latter, low-pressure mercury (Hg) lamps replace the high-voltage glow discharge plasma. Both processes operate at ambient temperature and atmospheric pressure, 100 kPa, using acetylene (C2H2) monomer. In both sets of experiments, it was found that efficient gas-to-solid conversion can occur in the form of a nanoparticulate amorphous hydrocarbon polymer-like material. It was found that in the PICVD case, great care was required to exclude even traces of O-2 contamination, because it not only reduced the growth rate of solid, but the latter then became highly oxidized ([O] similar to 50 at.%) and water-soluble. [GRAPHICS]

Photo-initiated chemical vapor deposition (PICVD) functionalizes carbon nanotube (CNT)-enhanced porous substrates with a highly polar polymeric nanometric film, rendering them super-hydrophilic. Despite its ability to generate fully wettable surfaces at low temperatures and atmospheric pressure, PICVD coatings normally undergo hydrophobic recovery. This is a process by which a percentage of oxygenated functional group diffuse/re-arrange from the top layer of the deposited film towards the bulk of the substrate, taking the induced hydrophilic property of the material with them. Thus, hydrophilicity decreases over time. To address this, a vertical chemical gradient (VCG) can be deposited onto the CNT-substrate. The VCG consists of a first, thicker highly cross-linked layer followed by a second, thinner highly functionalized layer. In this article, we show, through water contact angle and XPS measurements, that the increased cross-linking density of the first layer can reduce the mobility of polar functional groups, forcing them to remain at the topmost layer of the PICVD coating and to suppress hydrophobic recovery. We show that employing a bi-layer VCG suppresses hydrophobic recovery for five days and reduces its effect afterwards (contact angle stabilizes to 42 ± 1° instead of 125 ± 3°).

Contaminants in an electrochemical cell converting CO2 to formic acid can lead to the deactivation of cathode catalysts through several pathways, causing severe performance loss over time. Potential contaminants from flue gas emissions of principal fossil fuels include N2, O2, H2O, CO, NO2, SO2, particulate matter and hydrocarbons. Contaminant effects on the CO2 to formic acid electroreduction are scarcely covered in the literature. We describe in the present study these effects based on catalysts reported for the electroreduction of CO2 to formic acid, focusing principally on copper, tin and lead in the two most popular configurations, 2 and 3 compartment cells. Water solubility, metal affinity through chemisorption, known chemical reactions and altered electrochemical activities are the main focus of this review. We herein highlight that O2, SO2 and particulate matter have especially detrimental effects. While O2 can be efficiently removed from flue gas, additional treatment to remove SO2 and particulate matter is required. Our conclusions should raise interest in experimentally validating the effect of such contaminants.

The present electrocatalyst consists of copper and copper sulfide nanoparticles dispersed on graphene nanoflakes (GNFs), a stack of 5–20 highly crystalline graphene layers having typical side lengths of 100 by 100 nm. Plasma-based functionalization is used for adding covalently bonded nitrogen sites on the surface of the GNFs to a level of up to 2 at%. The catalyst nanoparticles are synthesized through wet chemistry, where graphene and a copper sulfate salt are mixed in a solvent and dried prior to pyrolysis under inert atmosphere at 700 °C. The amount of copper added to the carbon support is well controlled through this method, and is comprised between 20 and 50 wt%. Tests made towards the electrocatalytic reduction of carbon dioxide show these catalysts generate a faradaic efficiency of up to 42.5% toward liquid products at −0.6 V vs RHE, with a high selectivity for formate. The performance of the catalysts is mostly driven by the amount of the copper sulfide phase rather than the metallic copper content, and competes with similar catalysts found in the literature for a fraction of the metal content.

A batch process is developed to generate sulphur functionalized graphene nanoflakes (S-GNFs), corresponding to nanoparticles of stacked graphene. The growth and functionalization of the catalysts are done in a single thermal plasma reactor. The GNFs are first grown through the decomposition of methane in the thermal plasma volume followed by homogeneous nucleation of the nanoparticles in the well-controlled recombining plasma stream allowing the 2-dimensional evolution of the nanoparticle morphology. The precursor feeding conditions are then changed to liquid carbon disulphide in order to generate sulphur-based functional groups on the nanoparticles. The plasma conditions and carbon disulphide injection are varied, and samples with tuneable amount of sulphur between 4 and 28 at% are obtained. The functional groups generated include polythiophene polymer partly covering the GNFs, sulphur functionalities implemented directly on the graphitic structure, and traces of orthorhombic sulphur. The S-GNFs exhibit higher electrocatalytic activity toward the oxygen reduction reaction in alkaline medium for the samples containing the highest amounts of sulphur.

Graphene nanoflakes (GNFs) are a stack of 5-20 layers of highly crystalline graphene sheets having a planar size of approximately 100 by 100 nm and low defect concentration. They are produced here through homogeneous nucleation following the plasma decomposition of a carbon source in an inductively coupled thermal plasma reactor. Following synthesis within the plasma reactor, the GNFs are functionalized downstream in the plasma recombination zone with the addition of oxygen to the main plasma stream. Tunable oxygen functionalization is obtained and values up to 14.2 atomic percent are reached on the surface of the particles. While the non-functionalized GNFs are hydrophobic, the oxygen-functionalized GNFs (O-GNFs) show full stability of all the produced powders when directly dispersed in water or ethanol without any surfactant. The O-GNFs keep their structural integrity even after the implementation of the hydrophilic groups on the surface of the nanoparticles, and maintain their dispersion stability in water and ethanol over a long period of time. (C) 2016 Elsevier Ltd. All rights reserved.

Graphene nanoflakes (GNFs), a stack of 5-20 layers of graphene sheets, are generated here using methane decomposition in a thermal plasma followed by homogeneous nucleation of the 2-dimensional structures in the gas stream. The GNFs are functionalized with nitrogen and iron to improve their electrocatalytic activity. The iron functionalization step is carried out as a post-processing step within the same thermal plasma reactor used to grow the nanoparticles. Two different iron precursors are tested in the reactor, iron powder and iron (II) acetate solution. The iron source carried by a nitrogen flow is injected in the argon plasma, and parameters such as the plasma power, pressure, and the exposure time during functionalization are optimized for enhanced catalyst activity. Structure and composition of the resulting catalysts are characterized, and their electrocatalytic performances in terms of onset potential, half wave potential and current density show an increase compared to the non-functionalized GNFs. This study proves the ability to entirely produce a pure and highly crystalline graphene-based non-noble metal catalyst using a thermal plasma single batch process with simple precursors such as methane and nitrogen gas, and an iron powder or iron acetate solution. (C) 2016 Elsevier B.V. All rights reserved.

Graphene nanoflakes (GNF), a stack of 5 to 20 layers of graphene sheets with typically 100 nm side lengths, are the product of methane decomposition using an argon ICP thermal plasma. GNFs are good candidates to support non-noble catalytic sites for the oxygen reduction reaction. This material has a high crystallinity allowing the graphene to be acid resistant, together with a high electrical conductivity, these properties providing a good basis for a stable catalyst material in fuel cells. The GNFs are functionalized with nitrogen to support iron atoms and create catalytic sites dispersed at the atomic level on the nano-structured powders. The iron functionalization step is realized in situ as a post-processing step within the synthesis reactor through the vaporization of two different Fe precursors in the core of the plasma. The first consists of pure iron powders carried by a nitrogen flow; this method having the advantage of avoiding impurities during the functionalization step. The second precursor is an iron(II) acetate solution also carried by a nitrogen flow, with the iron already in atomic form once dissociated in the thermal plasma core. The effects of the type of precursor, the power of the plasma, and the reactor chamber pressure on the iron functionalization are studied in the present contribution. The structure, composition, and activity of the resulting catalyst are also fully characterized.