Carbon monoxide separation from industrial waste gases could contribute largely to carbon circularity. Traditional separation technologies are unable to separate CO from N₂ selectively. Instead, electroactive carriers show promise in selective separation of CO from N₂, where CO binds a complex in one oxidation state and releases in another oxidation state. We study Cu(i)/Cu(ii)-chloride complexes as potential carrier materials with high binding affinity to CO, good solubility and low energy consumption of the process. We show that the electrolyte composition of a copper chloride system affects the binding affinity and stability of the copper carrier (Cu⁺). Cyclic voltammetry measurements reveal that the CO binding constant increase from the previously reported 1600 M⁻¹ for 1 M KCl to 5500 M⁻¹ for 0.5 M CaCl₂. However, this increase in binding constant is not reflected to the same extent in the CO capture capacity, showing a smaller increase in CO capture. In general, the binding constant decreases with chloride concentration, while the Cu⁺ stability window increases. This highlights a trade-off that needs to be considered for electrolyte selection in electrochemical CO separation with copper chlorides.

We present a novel concept for coupling energy storage and water desalination using an acid–base flow battery architecture. In this device, electrical energy is stored through the reversible generation of acid and base, while salt is simultaneously removed from a central salt chamber. The device operates with non-toxic, earth-abundant electrolytes - NaOH and HCl - and utilizes hydrogen as an efficient redox mediator, avoiding crossover of redox active species and enabling high reversibility. We demonstrate that the degree of desalination directly impacts the desalination flow battery's open-circuit voltage and internal resistance, with high efficiency achieved at partial desalination. At 7 mA cm⁻², the device desalinates 0.5 M NaCl by 31% with 90–97% ion removal efficiency and 50% water recovery. Modelling of specific energy consumption indicates values as low as 14–18 kJ mol(NaCl)⁻¹ are achievable using state-of-the-art membranes and compartment designs. This places the device performance in line with leading desalination flow batteries while unlocking additional value through energy storage using abundant chemicals. We propose its use in decentralized coastal grids powered by intermittent renewables, where it can balance energy supply for downstream processes while at the same time desalinating seawater. This work outlines a scalable sustainable approach to address the water-energy nexus using benign and abundant chemicals.

The long-term operation of CO₂ electrolyzers using membrane electrode assemblies (MEAs) is limited by challenges related to water management. However, the water balance in CO₂ electrolyzer cells still has not been fully understood, and conflicting observations have been reported in the literature. In this study, a one-dimensional non-isothermal multiphysics model of an exchange MEA CO₂ electrolyzer with a Tokuyama A201 anion exchange membrane is developed to investigate the role of different physical and chemical phenomena on the water balance. The relative contributions of these processes vary with current density and membrane transport properties, which shift the dominant water transport mechanism in the cell. Our results highlight the significant contribution of homogeneous reactions, particularly OH⁻, to the water balance across the membrane. At low currents (i < 130 mA cm⁻²), homogeneous buffer reactions dominate the water balance and result in net water production near the catalyst layer. At higher currents (i > 130 mA cm⁻²), the flux is governed by electro-osmotic drag and a temperature gradient over the cathode gas diffusion electrode (GDE) with their relative contributions depending on membrane properties. Homogeneous buffering can re-emerge as the dominant mechanism at high currents if the hydroxide ion concentration in the membrane increases, for example under CO₂-limited cathode conditions, allowing hydroxide ions to react with depleted bicarbonate near the anode.

Bipolar membranes (BPMs) emerge as a valuable component in novel energy conversion devices utilizing a water-splitting reaction within BPMs. However, the opposite process, proton and hydroxide recombination (forward bias), remains challenging to control due to its strong dependence on the electrolyte composition. Even minor contamination of acid and base solutions by salt can significantly compromise the BPM performance. This study examines the impact of salt contamination on the BPM performance under forward bias. The results reveal that, during neutralization, salt ions accumulate near the BPM junction, hindering H⁺and OH^–transport toward the catalytic interface. Notably, the anion-exchange layer exhibits a high sensitivity to salt contamination in the base solution, with active site swapping between OH^–and anions emerging as the rate-determining step. The extent of this transport limitation depends on the acid/base-to-salt ratio. To address this issue, mitigation strategies are explored, including asymmetric BPMs. Reducing the thickness of the anion-exchange layer significantly enhances OH^–mobility, thereby increasing the limiting current density of neutralization in salt-contaminated electrolytes. These insights offer a deeper understanding of mass-transport limitations in BPMs and highlight pathways to optimize performance in energy conversion applications.

Low-temperature carbon dioxide electrolysis (CO₂E) provides a one-step means of converting CO₂ into carbon-based fuels using electrical inputs at temperatures below 100 °C. Over the past decade, an abundance of work has been carried out at ambient temperature, and high CO₂E rates and product selectivities have been achieved. With scaling of CO₂E technologies underway, greater discourse surrounding heat management and the viable operating temperatures of larger systems is important. In this Perspective we argue that, owing to the energy inefficiency of electrolysers, heat generation in CO₂E stacks will favour operating temperatures of between 40 and 70 °C, far from the ambient temperatures used so far. Such elevated temperatures put further pressure on catalyst and membrane stability and on the stack design. On the other hand, elevated temperatures could alleviate challenges in salt precipitation, water management and high cell voltages, aiding the technology. We reflect on these aspects and discuss the opportunities for waste heat valorization to increase the economic feasibility of the process.

Electrochemical reactors, such as water electrolyzers, CO₂ electrolyzers, fuel cells, and flow batteries, will be essential in electrifying industry as part of the global transition towards a defossilized and sustainable economy. These technologies require further optimization to enhance efficiency and reduce costs for widespread adoption. Hydrodynamics and mass transfer at electrode–electrolyte interfaces significantly affect electrochemical conversion reactions by influencing the reactant availability and pH in the local reaction environment. 3D electrodes, such as flow-through foams and suspension electrodes, hold a great advantage over 2D electrodes as they moderate pH changes and reactant depletion by spreading the current over a larger electrode area and electrolyte volume. We study the diffusion boundary layer in operando around a single mm-sized particle, representing an element of a 3D electrode. We visualize the local and transient pH with Fluorescence Lifetime Imaging Microscopy (FLIM) during H₂O reduction at various current densities and electrolyte flow velocities at a resolution down to 9 μm and 2 Hz. In addition, we apply an intermittent current to investigate how long the capacitive electric double layer of a suspension electrode particle can maintain an electrochemical reaction during their time of non-contact with a current collector, mimicking applications with Faradaic charge transfer (i.e. flow batteries, microbial fuel cells, capacitance-based electrolyzers). We demonstrate that the diffusion boundary layer is not symmetrical, but depend on the direction of the electric field, the current density and the flow conditions. The substantial pH gradients and boundary layer formation at the scale of hundreds of micrometers underline the importance of controlling flow in or around electrodes, making 3D electrodes an important asset for creating suitable reaction conditions in mass transport-limited electrochemical conversions.

Electrochemical conversion of CO₂ to hydrocarbons is limited by the low solubility and slow transport of CO₂ in aqueous systems. We demonstrate that we can reach partial current densities for CO₂-to-CO of 40 mA/cm² in fully aqueous systems, without the use of gas diffusion electrodes. We alleviate the mass transfer limitation by combining a suspension of catalytically active silver nanoparticles (Ag NPs) with a flow-through current collector. This extends the reactive area into the electrolyzer channel and improves the accessibility of dissolved CO₂ in a larger volume of electrolyte. The flow-through electrode system also outperforms a fully suspended electrode (based on carbon black particles), due to enhanced electric conductivity and smaller carbon area to minimize parasitic side-reactions. Additionally, we show that the distribution of the Ag NPs is pivotal for high CO₂ conversion rates, demonstrated by the highest CO current density obtained when a suspension of Ag NPs and SDS as surfactant is flowing through the 3D electrodes as pre-treatment. A stable CO current density can be sustained for more than 4 h. Although the conversion rate is still moderate compared to gas-fed CO₂ electrolzyers, the partial current density for flow-through electrodes is more than an order of magnitude larger than for planar flow systems. This work shows that CO₂ conversion in aqueous systems can be enhanced considerably by exploiting larger electrolyte volumes via smart electrode designs, such as a flow-through principle.

The decoupled power and energy output of a redox flow battery (RFB) offers a key advantage in long-duration energy storage, crucial for a successful energy transition. Iodide/iodine and hydrogen/water, owing to their fast reaction kinetics, benign nature, and high solubility, provide promising battery chemistry. However, H₂-I₂ RFBs suffer from low open circuit potentials, iodine crossover, and their multiphase nature. We demonstrate a H₂-I₂ operation with a combined neutral-pH catholyte (I₃^-/I^-) and an alkaline anolyte (KOH), producing an open circuit cell voltage of 1.28 V. Additionally, we incorporate a pressure-balanced gas diffusion electrode (GDE) to mitigate mass transport limitations at the anode. These improvements result in a maximum power density of 230 W/m² when allowing a mild breakthrough of H₂ through the GDE. While minimal crossover occurs, side reactions of permeating active species were found reversible, enabling long-term operation. Future work should address the stability of the GDE and optimization of the electrolyte thickness and concentration to fully leverage the potential unlocked by balancing the pressure and pH in the H₂-I₂ RFB.

Dense polymer membranes enable a diverse range of separations and clean energy technologies, including gas separation, water treatment, and renewable fuel production or conversion. The transport of small molecular and ionic solutes in the majority of these membranes is described by the same solution-diffusion mechanism, yet a comparison of membrane separation performance across applications is rare. A better understanding of how structure-property relationships and driving forces compare among applications would drive innovation in membrane development by identifying opportunities for cross-disciplinary knowledge transfer. Here, we aim to inspire such cross-pollination by evaluating the selectivity and electrochemical driving forces for 29 separations across nine different applications using a common framework grounded in the physicochemical characteristics of the permeating and rejected solutes. Our analysis shows that highly selective membranes usually exhibit high solute rejection, rather than fast solute permeation, and often exploit contrasts in the size and charge of solutes rather than a nonelectrostatic chemical property, polarizability. We also highlight the power of selective driving forces (e.g., the fact that applied electric potential acts on charged solutes but not on neutral ones) to enable effective separation processes, even when the membrane itself has poor selectivity. We conclude by proposing several research opportunities that are likely to impact multiple areas of membrane science. The high-level perspective of membrane separation across fields presented herein aims to promote cross-pollination and innovation by enabling comparisons of solute transport and driving forces among membrane separation applications.

Membrane technology has attracted great industrial interest in carbon capture and separation owing to the merits of energy-efficiency, environmental friendliness and low capital investment. Conventional polymeric membranes for CO₂ separation suffer from the trade-off between permeability and selectivity. Introducing porous fillers in polymers is one approach to enhance membrane separation performance. Metal-organic frameworks (MOFs), with ordered porous structure and diverse chemical functionalities, are promising fillers to prepare mixed matrix membranes (MMMs) for CO₂ separation. However, the main issue of MOF based MMMs in industry is their stability and processability. This review analyses recent work on stable and scalable MOF based MMMs for CO₂ separation. The typical stable MOFs, MOF-based MMMs and the scalable MOF synthesis are summarized. A large number of MOF-based MMM suffer from instability upon exposure to contaminants. For that reason, we also discuss the use of covalent organic frameworks (COFs) as an alternative to prepare MMMs for CO₂ separation, considering their excellent stability and good compatibility with polymers. Finally, a brief conclusion and current challenges on obtaining scalable and stable MMMs are outlined. This review may provide some guidance for designing high performance MMMs for industrial CO₂ capture and separation to help achieving carbon neutrality.

To make green hydrogen more economically attractive, the energy losses in alkaline electrolysis need to be minimized while operating at high current densities (1 A cm⁻²). At these current densities the ohmic resistance and gas bubbles effects contribute largely to the energy losses. To mitigate the gas bubbles losses, we demonstrate, for the first time, a pressure swing to remove gas bubbles in a zero-gap alkaline water electrolyzer. The pressure swing leverages the ideal gas law to increase the volume of gas in the system periodically, for a short duration (<2 s). This temporal volume increase effectively removes bubbles from the electrolyzer. We show that pressure swing can be used to measure the effect of bubbles on the ohmic resistance (R_Bubbles). Our results reveal that foam electrodes have a significantly larger R_Bubbles than perforated plate electrodes (1.8 Ω cm² vs 0.3 Ω cm²). The time-averaged cell voltage reduces by 170 mV when applying pressure swings to an electrolyzer operating at 200 mA cm⁻² in 1 M KOH with foam electrodes. The bubble resistance further depends on the electrolyte conductivity (inversely proportional) and is only moderately affected by operating pressure (25 % lower when increasing pressure amplitude from 1–2 to 1–5 bar). By implementing these findings in a model, we estimate that the pressure swing could reduce the cell voltage by ∼0.1 V for an electrolyzer operating at industrial conditions (6 M KOH, 80 °C, 1 A cm⁻²) for foam electrodes. For perforated plate electrodes, however, the reduced cell voltage is lower and does not outweigh the additional compression energy.

Nanofluidic membranes (NFMs) are gaining prominence as alternative ion-exchange membranes, because of their distinct selectivity mechanism, which does not rely on functional groups on a polymeric backbone but rather on charged nanopores that allow straight ion-conductive pathways for efficient ion transport. We measured the conductivity of commercial anodized aluminum oxide membranes with different pore sizes under different current densities and electrolyte concentrations. We also simulated a nanopore channel with charged walls between two electrolyte reservoirs. Our findings indicate that electrolyte concentration is the main parameter that determines NFM conductivity, with a linear dependence at least up to 1 M. Our study shows that the optimal pore length is between 0.5 and 5 μm considering the trade-off between selectivity and conductance. On the other hand, the conductance is not sensitive to the pore diameter. Conical nanopores are a way to increase conductance, but according to our results, this increase comes at the expense of selectivity. Our findings suggest that NFMs can outperform polymeric ion-exchange membranes in certain electrochemical applications, such as reverse electrodialysis, but not in applications that use low electrolyte concentrations on both sides of the membrane.

Electrochemical oxygen reduction is a promising and sustainable alternative to the current industrial production method for hydrogen peroxide (H₂O₂), which is a green oxidant in many (emerging) applications in the chemical industry, water treatment, and fuel cells. Low solubility of O₂ in water causes severe mass transfer limitations and loss of H₂O₂ selectivity at industrially relevant current densities, complicating the development of practical-scale electrochemical H₂O₂ synthesis systems. We tested a flow-by and flow-through configuration and suspension electrodes in an electrochemical flow cell to investigate the influence of electrode configuration and flow conditions on mass transfer and H₂O₂ production. We monitored the H₂O₂ production using Cu-tmpa (tmpa = tris(2-pyridylmethyl)amine) as a homogeneous copper-based catalyst in a pH-neutral phosphate buffer during 1 h of catalysis and estimated the limiting current density from CV scans. We achieve the highest H₂O₂ production and a 15-20 times higher geometrical limiting current density in the flow-through configuration compared to the flow-by configuration due to the increased surface area and foam structure that improved mass transfer. The activated carbon (AC) material in suspension electrodes, which have an even larger surface area, decomposes all produced H₂O₂ and proves unsuitable for H₂O₂ synthesis. Although the mass transfer limitations seem to be alleviated on the microscale in the flow-through system, the high O₂ consumption and H₂O₂ production cause challenges in maintaining the initially reached current density and Faradaic efficiency (FE). The decreasing ratio between the concentrations of the O₂ and H₂O₂ in the bulk electrolyte will likely pose a challenge when proceeding to larger systems with longer electrodes. Tuning the reactor design and operating conditions will be essential in maximizing the FE and current density.