Electrochemical regeneration techniques have renewed interest in aqueous potassium hydroxide (KOH) solution as a potential absorbent for post-combustion CO2 capture. However, absorber performance at different scales and its sensitivity to the loading of the regenerated solvent remain underexplored, primarily due to the traditional reliance on thermal regeneration methods which are not kinetically favorable to KOH.This study evaluates the CO2 capture behavior for aqueous KOH solutions and performance in terms of achieved CO2 capture efficiencies and rich (CO2) loading of the solvent. Absorption experiments were conducted using both laboratory-scale (4.2 Nm3/h gas flow capacity) and pilot scale packed absorbers (40 Nm3/h), with three lean loading levels (0, 0.25 and 0.6  mol CO2/mol K+) to establish baseline performance. The absorbers were operated to pinpoint conditions allowing for 100% CO2 capture efficiency from simulated flue gas. Higher CO2 capture efficiency was obtained at higher ratios of solvent flow rate to gas flow rate, corresponding to higher gas residence time. Regardless of K+ molality and lean loading (0.25–0.6), the obtained rich loading (up to 0.85) is closely related to ṅCO2/ṅOH- ratio, following a logarithmic trend.The results from the pilot-scale absorber were in excellent agreement with those from the laboratory-scale, reaching comparable rich loading at full capture. The consistency across scales underscores the potential for commercial-scale application and viability of KOH as a solvent for post-combustion CO2 capture. These findings provide a comprehensive understanding of the CO2 absorption process using KOH solution, which is crucial for advancing electrochemical CO2 absorption technology and designing carbon capture plants.

A first-of-a-kind electrochemical carbon capture technology has been developed and extensively tested at a magnesite calcination plant in Greece. The process uses electricity rather than heat to capture and purify the CO2 and operates at ambient temperature. During the 5-month campaign, 45 steady state points were reached, and the process was extensively examined. The process absorbed up to 9.4 kg CO2/h during the campaign of this work. The mean power consumption of the carbon capture process was found to be 21.77 GJ/ton. Depending on the process configurations, power consumptions down to 9.17 GJ/ton were obtained. The concentration of CO2 in the gas coming into the pilot plant was shown to have a major influence on power consumption, with higher CO2 levels leading to much lower power consumption. Additionally, the resistance in the electrochemical stack had a great influence on the efficiency of the process, as it increased the overall power consumption in the stack. The campaign shows a large capacity for lowering the energy consumption by optimizing process parameters such as gas and solvent flows, as well as by optimizing the design of the electrochemical stack.

The hydrogenotrophic conversion of carbon dioxide (CO2) and hydrogen (H2) gases to methane (CH4), presents a viable alternative to fossil-based natural gas. While different bioreactors have been investigated, the key challenge in these systems is overcoming mass transfer limitations of H2 to hydrogenotrophic archaea due to its low solubility. Ideally, optimal H2 bioavailability is achieved at thermophilic temperatures, directly feeding the gas to the biomass. In this study, a membrane biofilm reactor (MBfR) with gas-permeable polydimethylsiloxane (PDMS) membranes was designed to support methanogenic biofilm formation and enhance CO2 and H2 conversion under minimal mixing conditions, in a closed-loop and without additional nutrients supplementation. Over the 437-day operation, a thick, proteins-rich biofilm developed on the membrane. High-purity methane (≈95%) was produced, and a >95% conversion efficiency to methane was achieved for CO2 and H2. The MBfR sustained high methane production 397 days without additional nutrient supply, and reach the highest methane production rate of 24.3 L·m-2·day-1 after trace element supplementation, likely replenishing molybdenum. In both biofilm and mixed liquor, the microbial community was dominated by the hydrogenotrophic methanogen Methanothermobacter together with a new Hydrothermae spp. bacterium, likely utilizing H2 and CO2 for homoacetogenesis and/or fermenting detrital biomass. The stable microbial community functions ensured a complete anaerobic digestion metabolic chain, allowing for nutrients recycling and metals mobilization. The MBfR design presented in this study offers a promising strategy for scalable, energy-efficient biological methanation optimizing H2 utilization with minimal nutrient input.

This study presents the results of an electrochemically driven CO2 capture demonstration plant developed within the ConsenCUS project, funded by the EU. The demonstration was conducted using industrial flue gas (CO2 concentration of 3.5 vol%) from OMVPET, a refinery in Romania. The system integrates flue gas pre-treatment, a potassium carbonate absorption loop, and a bipolar membrane electrodialysis (BMED) stack for solvent regeneration. The demonstration unit is designed to treat flue gas slipstreams of up to 500 Nm3/h, containing 4-18 vol% CO2. Downstream, a water wash column minimises solvent emissions. The BMED stack, comprising 126 cell pairs, was operated in the range of 150 to 800 A/m2, with a K+ transport efficiency of 0.5 to 0.8 mol K+ /mol e-, producing acidic and alkaline streams for solvent regeneration. The acidic tank functioned as a flash unit, releasing CO2 at purities exceeding 98 vol%, while the alkaline stream restored lean solvent alkalinity. Continuous operation with stable integration of capture and regeneration modules was demonstrated for over 500 h. The results confirm that electrochemical solvent regeneration can be scaled to the demonstration level, delivering high-purity CO2 while avoiding the high-temperature demands of thermal desorption. However, Specific Energy Consumption (SEC) values of 15-45 GJ/tonne CO2 have been observed due to low water dissociation efficiency. Capture efficiencies in the range of 10 to 90% have been demonstrated. These findings offer critical insights into energy demand, solvent management, and process flexibility, thereby advancing the case for electrochemically enabled low-carbon CO2 capture technologies.

Lowering energy consumption is crucial for the implementation of carbon capture technology and the achievement of CO2 reduction goals. Electrochemical carbon capture, typically demands high energy input, required for the pH swing to concentrate CO2. This work presents a novel electrochemical carbon capture design based on an anion exchange membrane electrode assembly (AEMEA) in combination with an amine solution that acted as CO2 capture and pH buffer solution to narrow the pH swing range, thereby reducing energy consumption. Additionally, a tunable H2 to CO2 gas mix can be produced, suitable for subsequent carbon conversion processes such as hydrogenotrophic bio-methanation. The energy consumption for the electrochemical solvent regeneration process with an AEMEA was 21 kJ mol(-1) using a CO2-saturated amine solution at 20 A/m2, substantially lower than a similar system with CEM based design. We achieved different and stable CO2:H2 product ratios (between 1:1 to 1:4) that can be controlled through the H2 supply and the current density. Long term process stability was demonstrated in two-month continuous experiment with different model flue gas CO2 concentration (5%, 10%, and 20%). No significant degradation in performance was detected in the amine solution, or the materials employed throughout the experiment.

Electrochemical carbon capture is a potentially energy-efficient alternative to conventional solvent-based processes. However, it requires careful innovation and adjustments of operating conditions to achieve the required energy efficiency and stability. The absorber recirculation flow is a key variable and was found to be a critical lever for system performance. In this study, a demonstration-scale electrochemical capture unit was tested on industrial flue gas at a magnesia production site in Greece. Single-stack and dual-stack cell configurations were compared in terms of overall CO2 desorption and electrochemical performance metrics. The single-stack mode initially outperformed the dual-stack setup. This was caused by insufficient absorber recirculation, which limited CO2 loading and resulted in higher energy consumption. When the absorber recycle flow was adjusted to better match the available regeneration capacity, the dual-stack performance improved and became comparable to single-stack operation. The simple change was unexpected but can be explained by the increased rich loading which enhanced the performance of the electrochemical solvent regeneration. Overall, while the energy consumption of the process remains high, the findings highlight the absorber recirculation flow as a decisive parameter for future development of scalable electrochemical CO2 capture. Future development may enable operation that combines high throughput with efficient and selective ion transport for low energy consumption CO2 capture.

Bipolar membrane electrodialysis (BMED) is an electricity-driven technology that captures and purifies CO2, but its efficiency remains poorly understood. This work quantified coulombic efficiency losses in BMED for carbon capture using a fully saturated (1 M KHCO3) and a partially saturated KOH solvent. Fully saturated solutions required 50% less energy consumption than partially saturated solutions for CO2 purification and solvent regeneration. The minimum specific energy consumption was 161.2 kJ per mol CO2 (3.65 GJ per ton CO2) at 100 A/m(2) and was limited by parasitic potassium transport across the bipolar membrane, resulting in low CO2 desorption efficiency (<60%) at low current density. Additionally, incomplete CO2 desorption represented up to 10% of efficiency losses in most of the conditions tested. BMED displayed 100% CO2 desorption efficiency (mol CO2 per mol e(-)) and a specific energy consumption of 290.61 kJ per mol CO2 (6.60 GJ per ton CO2) at industrially relevant current densities (1000 A/m(2)). This work presents a robust analytical framework to identify coulombic efficiency losses and identifies the cause for low CO2 desorption efficiency at a wide range of current densities and operational conditions, providing unique insights into the transport mechanisms in BMED and unlocking new pathways to maximize technology performance.

Carbon capture is widely acknowledged as a promising strategy for achieving negative emissions. Electrochemical carbon capture technologies are considered a viable alternative to conventional temperature swing processes. Among these, employing the hydrogen oxidation and hydrogen evolution reactions as a redox couple, along with an ion exchange membrane, offers an effective means of establishing a pH swing for desorbing CO2 and regenerating the alkaline solvent. However, the practical scalability of this approach is impeded by challenges such as high energy demands resulting from a high pH differential between anodic and cathodic environments and operation with solutions with a low conductivity, required to obtain an acceptable current yield. To address these limitations, this study introduces an innovative anion exchange membrane (AEM)-based electrochemical process for solvent regeneration. Our research demonstrates the advantageous utilization of amines as chemical buffers. Selecting an amine solution with a favorable pKa (∼7 to 10) helps in maintaining bicarbonate as the predominant carbon species within the system, thereby ensuring a high current yield (>80%) across various operational conditions (current, load ratio, and solution concentration). Furthermore, our analysis indicates that the use of amine solutions effectively reduces the overpotential of the hydrogen evolution reaction due to a lower local pH. This results in a minimum energy requirement of 63 kJ/mol at a current density of 20 A/m2 to regenerate the solution (MDEA) while maintaining high (>99%) product (CO2) purity.

This work demonstrates and characterizes the use of a bipolar membrane electrodialysis for pH-driven CO2 capture and solvent regeneration using potassium hydroxide solutions. The impact of potassium concentration, current density and load ratio on the CO2 desorption efficiency was analyzed and substantiated with an equilibrium model. The system was tested with partially saturated solutions that mimic the expected carbon content of alkaline solvents that have been in contact with flue gas (carbon loading of 0.6 and K+ concentration from 0.5 M to 2 M). Among the tested current densities, 1000 A/m2 demonstrated the highest CO2 desorption efficiency but also the highest energy consumption, whereas 250 A/m2 exhibited the lowest energy consumption (8.8 GJ/ ton CO2) but lower CO2 desorption. Efficiency losses were associated with H+ transport across the membranes at high load ratios and decrease of the bipolar membranes water dissociation efficiency at low current densities. This work establishes key performance indicators and describes fundamental characteristics of continuous bipolar membrane electrodialysis systems for regeneration of alkaline solvents used in post-combustion CO2 capture.

Energy-efficient capture technologies need to be deployed by 2050 to abate global warming caused by excessive carbon dioxide (CO2) emissions. CO2 capture using alkaline solutions and absorbent regeneration mediated through bipolar membrane electrodialysis (BMED) have been tested previously as a standalone technology. However, the continuous operation of an integrated system remains largely unclear. Here, a bench-scale study was conducted using an integrated prototype to analyze the performance of CO2 capture and electrochemical regeneration using potassium hydroxide (KOH) aqueous solution. A wide range of current densities from 150 to 1000 A/m2 was applied to demonstrate the continuous operation of the CO2 capture system emphasizing the stability in attainable high rich carbon loading and CO2 desorption. The electrochemical regeneration module achieved CO2 desorption efficiency of 70% and absorbent recovery up to 89% under industrial relevant current densities of 500-1000 A/m2. The absorbent recovery has been identified to be a result of the combined effect of load ratio and rich carbon loading. The observed inefficient CO2 separation indicates significant potential to enhance energy efficiency. These results represent a pivotal step forward in electrochemically mediated CO2 capture technology, with promising potential for rapid industrial scale-up in the near future.

Reverse electrodialysis (RED) is a promising technology to harvest salinity gradient power (SGP) that is available where fresh and sea water mix, using anion (AEM) and cation exchange membranes (CEM) in a stack. Fouling of the membranes is one of the main challenges for RED, since it leads to a reduction in attainable net power output. In this study, we combined the use of profiled ion-exchange membranes (200 mu m thick compartments) with a pretreatment by a dual media filter for both natural water streams (Lake Ijssel and Wadden Sea), with four different cleaning procedures: (i) increased flow, (ii) reverse and increased flow, (iii) reverse flow and feed switch, and (iv) air sparging. Cleaning with air sparging was the most effective technique, limiting the pumping losses and not influencing the power generation capacity. The cleaning with reverse flow and feed switch also showed to be suitable, keeping the pressure drop losses lower than 100 mbar for both water streams. Post experiment membrane autopsy showed that CEMs were more subjected to particulate fouling than AEMs, and that a lower accumulation of fouling by particulate resulted in a higher concentration of humic acids and biofouling on the membrane surface.

An electrochemical process based on pH-swing has been proposed recently to regenerate spent alkaline absorbent from direct air capture (DAC). In this work, we experimentally investigated and theoretically simulated two optimization strategies to further reduce the energy consumption of such novel electrochemical process. First, partial vacuum was applied to the gas phase during CO2 desorption to increase the gas production rate. The energy consumption of the electrochemical cell decreased by 12 to 15% when the CO2 partial pressure in the gas phase was reduced from 0.9 to 0.3 atm. Second, phosphate and sulphate were tested as background electrolyte to the alkaline absorbent, reducing the energy consumption by minimizing the ohmic losses in the electrochemical cell. The optimal concentration for phosphate was 0.1 M, while the CO2 production rate was limited by either the total carbon feeding rate or the high acidifying solution pH at higher concentrations of phosphate. Moreover, due to the low pKa and high molar conductivity of sulphate compared to phosphate, sulphate addition showed an even lower energy consumption than phosphate addition. Finally, the lowest experimental energy consumption was 247 kJ mol-1 CO2 achieved with CO2 partial pressure of 0.3 atm and 0.1 M of sulphate addition at current density of 150 A m- 2 while our mathematical model predicted a theoretical minimum energy consumption of 138 kJ mol-1 under the same condition. Overall, the investigated optimization strategies advanced the development of an energy-efficient electricity-driven process for direct air capture.

A bipolar electrodialysis (BP-ED) pilot plant including 3.15 m(2) of cation exchange membrane and bipolar membrane each was operated for ammonia recovery. The pilot treated source-separated diluted urine (1 gNH(4)(+)/ L). Previously found set operation parameters for lab-scale such as current density and nitrogen load did not directly influence the stack performance. However, the effluent pH was directly related to the removal efficiency of the system. 80% nitrogen removal was achieved at a set effluent pH of 4. Operating under an effluent pH control strategy was more effective to control NH4+ removal than controlling current density or nitrogen loading, as it accounts for fluctuation in wastewater availability and composition. The pilot plant removed up to 88% of the NH4 from urine and recovered around 700 g/day (from 1 m(3) of urine). This was a significant improvement compared to the pilot plant previous performance on digestate. The energy consumption was around 13 Wh/g(N). The overall current efficiency was similar to 40% with most losses caused by parasitic ionic shortcut currents occurring at the hydraulic manifolds of the BP-ED stack. Therefore, the energy demand can be further decreased by preventing these ionic shortcuts in the new cell designs.

Nutrient recovery systems can be impacted due to the presence of organic micropollutants (OMPs). TAN (total ammonium nitrogen, sum of ammonium and ammonia) recovery from wastewater can be achieved by combining an electrochemical system with membrane stripping. Essential components of these processes are cation-exchange membranes and a hydrophobic gas-permeable membrane. These membranes are barriers between the OMPs source (wastewater) and recovered products. Despite reports about OMPs – ion exchange membrane interactions, there is limited knowledge about the transport of OMPs in ammonium recovery systems. This work gives a first detailed description of the transport mechanism of a broad group of OMPs with varying properties during electrochemical ammonium recovery supplied with a complex matrix (digested blackwater). Even after continuous exposure to OMPs and consequent system equilibration, OMP concentrations in the effluent were often lower than in the inflow stream. The highest removal and transport towards the concentrate were found for positively charged OMPs. The presence of organic matter contributed to the adsorption and transport of OMPs. Although OMPs were transported over the CEMs, the gas permeable hydrophobic membrane for ammonia recovery retained all OMPs.

Ammonia recovery from wastewater is crucial, yet technology of low carbon emission and high ammonia perm-selectivity against complex stream compositions is urgently needed. Herein, a membrane-based hybrid process of the Donnan dialysis–electrodialysis process (DD–ED) was proposed for sustainable and efficient ammonia recovery. In principle, DD removes the majority of ammonia in wastewater by exploring the concentration gradient of NH4+ and driven cation (Na+) across the cation exchange membrane, given industrial sodium salt as a driving chemical. An additional ED stage driven by solar energy realizes a further removal of ammonia, recovery of driven cation, and replenishment of OH− toward ammonia stripping. Our results demonstrated that the hybrid DD–ED process achieved ammonia removal efficiency >95%, driving cation (Na+) recovery efficiency >87.1% for synthetic streams, and reduced the OH− loss by up to 78% compared to a standalone DD case. Ammonia fluxes of 98.2 gN m−2 d−1 with the real anaerobic digestion effluent were observed using only solar energy input at 3.8 kWh kgN−1. With verified mass transfer modeling, reasonably controlled operation, and beneficial recovery performance, the hybrid process can be a promising candidate for future nutrient recovery from wastewater in a rural, remote area.

Bio-electrochemical systems (BESs) can recover ammonium at low specific energy inputs and can produce hydrogen gas. A reason hampering development of large-scale applications for ammonium recovery is the general instability of the bio-generated current and the thereby variable TAN (ammonia and ammonium) effluent concentrations. Electrochemical systems for ammonium recovery, such as the hydrogen recycling electro-chemical system (HRES), can fine-tune the applied current, but require higher specific energy inputs than BES, and may require an additional external hydrogen supply in case of HRES. This research presents for the first time an integrated BES and HRES system for ammonium recovery to combine the advantages of both types of systems to achieve high TAN removal efficiency at low specific energy input. The HRES was able to partially compensate for BES instability, which resulted in an overall high TAN removal efficiency (89-95 %) at an increased energy demand of 5.2-10.2 MJ/kgN. When the BES was performing stably and efficiently, the HRES removed almost all of remaining TAN at up to 99.8 % overall TAN removal efficiency, requiring 9.2 MJ/kgN. The combined system can remove TAN down to much lower effluent concentrations at little to no additional energy input. These results indicate that combining BES and HRES in one system can result in TAN recovery that is more efficient than in each system separately, which could facilitate new application possibilities for (bio-)electrochemical ammonium recovery.

Nitrogen (N) is an essential nutrient for plants and plays an important role in agriculture. However, the cycle between nitrogen input into agriculture and the nitrogen content in wastewater has been broken. Recently, nitrogen recovery from wastewater was demonstrated using an electrochemical system (ES) combined with a gas permeable membrane. Once concentrated, the ammonia is recovered into an acid, producing either ammonium sulfate (AS) or ammonium nitrate (AN). The ES was operated at different conditions while guaranteeing a certain nutrient removal and producing a concentrated fertilizer. An ammonium nitrate with 25.5 g(N)/L and an ammonium sulfate with 21.5 g(N)/L were recovered. To evaluate the performance of these nitrogen fertilizers, 5 treatments were applied to two crops, spinach and radish. The treatments were (1) AS recovered with the ES, 2) commercial AS, 3) AN recovered with the ES, 4) commercial AN, and 5) no addition (control). Between fertilized treatments, a significant difference was observed for fresh biomass of spinach leaf (consumable part of the crop) between recovered and commercial fertilizers. Effective fertilization was confirmed by a significantly higher fresh biomass in fertilized crops, both radish and spinach, compared to control. After harvest, soil pH was above 5.0 for both commercial and recovered fertilizers, despite the low pH of the recovered fertilizers (pH 1.8 - 2.4). This work demonstrates that fertilizers recovered by electrochemical systems can be used to improve crop growth and are a feasible alternative to commercial fertilizers.

Ammonia recovery from centrate of an anaerobic digester was investigated using an onsite bipolar-electrodialysis (BP-ED) pilot scale plant coupled to two liquid/liquid membrane contactor (LLMC) modules. To investigate the process performance and robustness, the pilot plant was operated at varying current densities, load ratio (current to nitrogen loading), and in continuous and intermittent current (Donnan) mode. A higher load ratio led to higher total ammonium nitrogen (TAN, sum of ammonia and ammonium) removal efficiency, whereas the increase in the applied current did not have a significant impact the TAN removal efficiency. Continuous current application resulted in the higher TAN removal compared with the Donnan dialysis mode. The lowest specific energy consumption of 6.3 kWh kgN(-1) was recorded in the Donnan mode, with the load ratio of 1.4, at 200 L h(-1) flowrate and current density of 75 A m(-2). Lower energy demand observed in the Donnan mode was likely due to the lower scaling and fouling of the ion exchange membranes. Nevertheless, scaling and fouling limited the operation of the BP-ED stack in all operational modes, which had to be interrupted by the daily cleaning procedures. The LLMC module enabled a highly selective recovery of ammonia as ammonium sulfate ((NH4)(2)SO4), with the concentration of ammonia ranging from 19 to 33 gN L-1. However, the analysis of per-and polyfluoroalkyl substances (PFASs) in the obtained (NH4)(2)SO4 product revealed the presence of 212-253 ng L-1 of 6:2 fluorotelomer sulfonate (FTS), a common substitute of legacy PFAS. Given the very low concentrations of 6:2 FTS (i.e., < 2 ng L-1) encountered in the concentrated stream, 6:2 FTS was likely released from the Teflon-based components in the sulfuric acid dosage line. Thus, careful selection of the pilot plant tubing, pumps and other components is required to avoid any risks associated with the PFAS presence and ensure safe use of the final product as fertilizer.

During electrochemical nutrient recovery, current and ion exchange membranes (IEM) are used to extract an ionic species of interest (e.g., ion) from a mixture of multiple ions. The species of interest (ion 1) has an opposing charge to the IEM. When ion 1 is extracted from the solution, the species fractions at the membrane and the adjunct boundary layers are affected. Hence, the species transport through the electrochemical system (ES) can no longer be described as electrodialysis-like. A dynamic state is observed in the compartments, where the ionic species are recovered. When the boundary layer-membrane interface is depleted, the IEM is at maximum current. If the ES is operated at a current higher than the maximum current, the fluxes of both ion 1 and other competing ions, with the same charge (ion 2), occur. This means, for example, ion 1 will be recovered, and the concentration of ion 2 will build up in time. Therefore, a steady state is never reached. Ideally, to prevent the effect of limiting current at the boundary layer-membrane interface, ES for nutrient recovery should be operated at low currents.