To assist reaching net-zero emissions, the dissolved carbon in the ocean can be extracted to enable an indirect air capture. An electrochemical bipolar membrane electrodialysis (BPMED) is a sustainable method for such capture. The BPMED enables a pH-swing that manipulates the oceanic carbonate-equilibrium using electricity. However, at alkaline-pH, an in-situ process suffers from inorganic fouling within the stack, increasing the cost of capture. In the current work, we investigate fouling management strategies including fouling control (i.e., membrane- configuration and current-flow rate optimization) and fouling removal methods. Fouling removal methods including air and CO₂(g) sparging, dissolved CO₂ (aq) cleaning, back-pressure, flow rate increase, and acid-wash are investigated under accelerated fouling conditions. The stack configuration containing the BPM-AEM pairs shows 4 × lower fouling than the BPM-CEM stack, while the carbonate-extraction and faradaic efficiency are similar for both configurations. From the scaling removal methods, only the acid wash combined with the back-pressure removed all the inorganic fouling, recovering both the cell voltage and pressure drop to their initial values. Upon the air sparging, the total cell voltage and pressure drop increased even more due to the trapped gas inside the netted spacers. Cleaning via dissolved and gaseous CO₂ decreases the cell pH, dissolving hydroxide/carbonate-based fouling, but decreases the carbonate-removal significantly which is not preferred. Applying the back-pressure and higher flow rates decelerated the scaling buildup but was not enough to remove the fouling. Using BPM-AEM stacks in combination with periodic acid cleaning has potential as resilient oceanic carbon removal via BPMED.

Bipolar membrane electrodialysis (BPMED) can provide a sustainable route to capture the oceanic-dissolved inorganic carbon (DIC) using an electrochemical pH-swing concept. Previous works demonstrated how gaseous CO₂ (through acidification) can be obtained from ocean water, and how carbonate minerals can be provided via ex situ alkalinization. In this work, we present, for the first time, the in situ mineralization via the alkalinization route using both real and synthetic seawater. An in situ pH-swing, inside of the BPMED cell, allows reducing the energy consumption of the oceanic-DIC capture. We demonstrate that, by accurately controlling the applied current density and cell residence time, the energy required for the process can be indeed lowered through facilitating an optimized pH in the cell (i.e., base-pH 9.6–10). Within this alkaline pH-window, we capture between 60% (for real seawater) up to 85% (for synthetic seawater) of the DIC from the feed, together with minor Mg(OH)₂ precipitates. The CaCO₃(s) production increases linearly with the applied current density, with a theoretical maximum extraction of 97 %. The energy consumption is dominated by the ohmic losses and BPM-overpotential. Through tuning the current density and flow rate, we optimised the energy consumption by applying a mild in situ pH-swing of ca. pH 3.2 – 9.75 (for real seawater). As a result, aragonite was extracted by using of 318 ± 29 kJ mol⁻¹ CaCO₃(s) (i.e., ca. 0.88 kWh kg⁻¹ CaCO₃(s)) from real seawater in a cell containing ten bipolar – cation exchange membrane cell pairs, which is less than half of the previously lowest energy consumption for carbonate mineralization from (synthetic) seawater.

The practical energy required for water dissociation reaction in bipolar membrane (BPM) is still substantially higher compared to the thermodynamic equivalent. This required energy is determined by the bipolar membrane voltage, consisting of (1) thermodynamic potential and (2) undesired voltage losses. Since the pH gradient over the BPM affects both voltage components, in this work, pH gradient is leveraged to decrease the BPM-voltage. We investigate the effect of four flow orientations: 1) co-flow, 2) counter-flow, 3) co-recirculation, and 4) counter-recirculation, on the pH gradient and BPM-voltage, using an analytical model and chronopotentiometry experiments. The analytical model predicts the experimentally obtained pH accurately and confirms the importance of the flow orientation in determining the longitudinal pH gradient profile over the BPM in the bulk solution. However, in contrast to the simulated results, our observations show the effect of flow orientations on the BPM-voltage to be insignificant under practical operating conditions. When the water dissociation reaction in the BPM is dominant, the internal local pH inside of the membrane determines its final voltage, shadowing the effect of the external pH-gradient in the bulk solution. Therefore, although changing the flow orientation affects the bulk pH, it does not influence the local pH at the BPM junction layer and hence the BPM-voltage. Instead, opportunities for reducing the membrane voltage are in the realm of improved catalysts and ion exchange layers of the BPM.

A bipolar membrane (BPM), consisting of a cation and an anion exchange layer (CEL and AEL), can be used in an electrochemical cell in two orientations: reverse bias and forward bias. A reverse bias is traditionally used to facilitate water dissociation and control the pH at either side. A forward bias has been proposed for several applications, but insight into the ion transport mechanism is lacking. At the same time, when implementing a BPM in a membrane electrode assembly (MEA) for CO2 reduction, the BPM orientation determines the environment of the CO2 reduction catalyst, the anolyte interaction and the direction of the electric field at the interface layer. In order to understand the transport mechanisms of ions and carbonic species within a bipolar membrane electrode assembly (BPMEA), these two orientations were compared by performing CO2 reduction. Here, we present a novel BPMEA using a Ag catalyst layer directly deposited on the membrane layer at the vapour-liquid interface. In the case of reverse bias, the main ion transport mechanism is water dissociation. CO2 can easily crossover through the CEL as neutral carbonic acid due to the low pH in the reverse bias. Once it enters the AEL, it will be transported to the anolyte as (bi)carbonate because of the presence of hydroxide ions. When the BPM is in the forward bias mode, with the AEL facing the cathode, no net water dissociation occurs. This not only leads to a 3 V lower cathodic potential but also reduces the flux of carbonic species through the BPM. As the pH in the AEL is higher, (bi)carbonate is transported towards the CEL, which then blocks the majority of those species. However, this forward bias mode showed a lower selectivity towards CO production and a higher salt concentration was observed at the cathode surface. The high overpotential and CO2 crossover in reverse bias can be mitigated via engineering BPMs, providing higher potential for future application than that of a BPM in forward bias owing to the intrinsic disadvantages of salt recombination and poor faradaic efficiency for CO2 reduction. This journal is