There is now scientific consensus that in addition to decarbonization, the active removal of around 5 Gt of atmospheric CO₂ per year by 2100 will be necessary to prevent an average global warming of more than 2°C (Calvin et al., 2023). Among the carbon dioxide removal (CDR) technologies being pursued, marine carbon dioxide removal (mCDR) has the potential to reach climate-relevant scales of durable CO₂ removal (NASEM, 2022). mCDR leverages the natural role of the oceans in the carbon cycle to remove CO₂ from the air. An important group of mCDR techniques are those that remove CO₂ using aqueous acid and base that have been generated electrochemically from the seawater itself, with the two most actively pursued approaches being electrochemical ocean alkalinity enhancement (OAE) (Oschlies et al., 2023; Ringham et al., 2024) and what I will refer to here as electrochemical ocean alkalinity cycling (OAC) (de Lannoy et al., 2018). In the case of OAE, CO₂ is removed from the atmosphere and durably stored as dissolved inorganic carbon (DIC) in the ocean, whereas OAC captures CO₂ from the atmosphere in its gaseous form (Eisaman et al., 2023). In past publications electrochemical OAC been variously referred to as: CO₂ extraction from seawater (Eisaman et al., 2012), indirect ocean capture (IOC): acid process (de Lannoy et al., 2018; Eisaman et al., 2018; Eisaman, 2020), direct ocean capture (DOC) (Bui et al., 2023; Kim et al., 2023; Lucas et al., 2023), direct ocean removal (DOR) (NOAA, 2023), CO₂ removal from oceanwater (Kim et al., 2023), and electrochemical direct ocean capture (eDOC) (Aleta et al., 2023). This Perspective will use the OAC nomenclature, as it succinctly and clearly emphasizes its relation to electrochemical OAE, the approach to which it is most closely related. The purpose of this perspective is to provide an understanding of the fundamental operating principles and relative merits of electrochemical OAE and OAC.

It should be mentioned that there is what appears to be a third category of approaches, which have variously been termed indirect ocean capture (IOC): base process (Eisaman et al., 2017; de Lannoy et al., 2018; Eisaman et al., 2018) and electrolytic seawater mineralization (La Plante et al., 2023). In these methods, the base is used to increase the alkalinity and pH of seawater to induce the removal of DIC from seawater by precipitating solid carbonates and/or hydroxides. The precipitation step removes some, but not all, of the added alkalinity. Therefore, the alkalinity of the seawater remaining after precipitation is still elevated relative to the starting point. When equilibrated with the air, this alkalinity enhanced seawater will result in the removal of CO₂ from the air and storage as DIC in the ocean. These approaches are just a form of OAE where some portion of the added alkalinity goes toward precipitating DIC as solids. While this precipitation may appear inefficient because it removes some of the alkalinity that was just added, these approaches may be motivated by the reuse value or storage stability of solid carbonates or the more gravimetrically and volumetrically dense form of alkalinity in solid hydroxides.

Many methods of alkalinity generation for OAE exist other than the electrochemical generation of aqueous base, such as the addition of terrestrial alkaline minerals to the ocean (Caserini et al., 2022; Eisaman et al., 2023). However, this Perspective will focus specifically on OAE and OAC using electrochemically generated acid and base, and so will generally drop the “electrochemical” qualifier from this point forward.

From the description above, the “sponge” and “pump” labels for OAE and OAC, respectively, become clear: OAE durably absorbs additional CO₂ from the air into the ocean as added DIC, whereas OAC uses the ocean as a pass-through to pull CO₂ from air.

One important difference between OAE and OAC is the volume of seawater that must be processed per ton of CO₂ removed. For OAC, the amount of CO₂ that can be removed from the air using a given volume of seawater is the amount that exactly replaces the DIC that was extracted in the acidification and extraction steps (Steps 1 and 2 of OAC in Figures 1B, 2). A typical value for DIC in seawater is around 0.0025 M (Butler, 2019), meaning that OAC can remove 0.0025 moles of CO₂ from the air per liter of seawater, corresponding to approximately 9,089 cubic meters of seawater that must pass through the OAC system per ton of CO₂ removed from the air. The amount of seawater that must be processed by OAE for a given amount of CO₂ removal is much less than for OAC. Because OAE generates alkalinity from the salt in seawater, the amount of seawater needed for OAE is governed by its NaCl concentration (approximately 0.5 M) rather than the DIC concentration (0.0025 M). Practically, OAE can generate a maximum of approximately 0.25 moles of alkalinity per liter of seawater, with the factor of two less than the salt concentration due to the need for remaining conductivity in the outgoing salt solution. For OAE, the molar ratio of CO₂ removed to added alkalinity (ΔCO₂/ΔTA) depends on ocean conditions, but a typical range is 0.7–0.8 (He and Tyka, 2023). These values lead to a range of 113 (ΔCO₂/ΔTA = 0.8) -129 (ΔCO₂/ΔTA = 0.7) cubic meters of seawater that must pass through the OAE system per ton of CO₂ removed from the air. Therefore, OAC requires 9,089/129–9,089/113 = 69–80 times more seawater per ton of CO₂ removed than OAE. It should be noted that both OAE and OAC require the same amount of seawater to generate the acid and base used in their respective processes, but OAC must pump significant additional volumes of seawater into the system as part of the acidification and CO₂ extraction steps of the process. The need for additional seawater pumping will increase both equipment and energy costs (Eisaman et al., 2018; Eisaman, 2020).

Another difference between electrochemical OAE and OAC lies in their byproducts: aqueous acid (HCl) for this specific form of OAE and CO₂ gas for OAC. These products need to be used or stored in a manner that prevents a reversal of the CO₂ removal process, meaning that the CO₂ gas produced by OAC must be durably stored in a way that prevents its leakage to the atmosphere, and the H⁺ ions in the HCl produced by electrochemical OAE must be prevented from leaking back into the ocean. For HCl, this is most easily accomplished by using the acid in processes that result in its neutralization and the storage, disposal, or beneficial use of the resulting salts. In terms of the rate at which these products are generated, OAC produces one molecule of byproduct CO₂ for each molecule removed from the air, so removal of 1GtCO₂/y using OAC would require the durable storage of 1Gt/y of byproduct CO₂ gas. For the case of OAE, one molecule of HCl is produced for each molecule of NaOH that is generated, meaning that a typical range for (ΔCO₂/ΔTA) of 0.7–0.8 (He and Tyka, 2023) corresponds to (1/0.8–1/0.7) = 1.25–1.43 mol(HCl) generated per mol(CO₂) removed from the air, or 1.04–1.18 tons(HCl) generated per ton(CO₂) on a dry basis. Therefore, the removal of 1GtCO₂/y using electrochemical OAE would require the neutralization of 1.04–1.18 Gt/y of byproduct HCl, on a dry basis. Electrochemical OAE generates aqueous acid with typical concentrations of around 0.5 M.

For the CO₂ generated by OAC, the sequestration of CO_2, either underground or in long-lived materials, will be required (NASEM, 2019). For underground storage, the mineralization of CO₂ into solid carbonates via reaction with silicate-containing rocks has been shown to be a promising avenue that reduces concerns about leakage and durability (Matter et al., 2016; Snæbjörnsdóttir et al., 2020; White et al., 2020). To date, the maximum demonstrated rates of mineralization have been on the order of tens of thousands of tons of CO₂ per year. In scaling to injection rates of billions of tons of CO₂ per year, there is concern that slow reactions will limit the rates of mineralization (Tutolo et al., 2021). In a case where OAC and OAE may complement each other, research is ongoing as to whether this reduced CO₂ carbonation efficiency at the gigaton scale can be increased by pretreating, or co-treating (Awolayo et al., 2022), the injection formation with the acid generated by OAE. This approach aims to leverage the orders-of-magnitude increase in mineral dissolution rates and solubilities at low pH values to enhance the concentrations of carbonate forming cations. By enabling CO₂ mineralization and acid neutralization at gigaton-per-year scale, this may provide a pathway to enable the deployment of electrochemical OAE, and CDR approaches such as OAC, that require the durable storage of CO₂, at climate-relevant scales. That said, pre-existing zones of high permeability may localize injected acid, leading to increased mineral dissolution. This feedback is often termed “fingering” or “wormholing” (Szymczak and Ladd, 2014). Such effects must be minimized for acid pretreatment to realize its potential for CO₂ mineralization.

Additionally, through its ability to optimize feedstock properties, the acid generated in electrochemical OAE also potentially enables hybrid approaches with other CDR methods, including enhanced rock weathering (ERW), containerized enhanced weathering, and Biomass Carbon Removal and Storage (BiCRS). Similarly, the CO₂ generated in OAC can also be used in hybrid approaches that sequester CO₂ in non-geological locations such as concrete.

Despite the fundamental differences between OAE and OAC, some seeming similarities remain. First, with the addition of the equipment and energy needed for controllable contact between alkalinity and air, both OAE and OAC are equally capable of operating as closed or hybrid systems from the perspective of MRV. Second, as seen in Figure 1, the MRV required for OAE and OAC appear to be the same, but there is one important difference: OAE traces the CO₂ removal into a plume of TA-enhanced seawater with a typical level of DIC, while OAC traces the CO₂ removal into a plume of seawater with drastically reduced DIC and a typical level of TA. Care must be taken in the case of OAC to drive the process along the intended path on the TA-DIC-pH plot, given the almost complete loss of its buffer system after the CO₂ extraction step. For OAC, after Step 1 (acidification) and Step 2 (CO₂ extraction from seawater), the CO₂ has been removed from the seawater and this CO₂ must be durably sequestered. It is important to note, however, that this is not the CO₂ relevant to MRV. Rather, as shown in Figure 1B, the MRV required for OAC is performed on Step 4 (CO₂ extraction from air into the ocean) that occurs after Step 3 (alkalinity restoration). Just like the MRV for OAE, OAC must quantify the amount of CO₂ that flows from the air to the ocean to restore equilibrium. As an example, even if one ton of CO₂ is removed during Step 2 of OAC, if only half a ton of CO₂ is removed from the air in Step 4, perhaps because of alkalinity downwelling from the surface ocean prior to equilibration with the air, then only half a ton of CO₂ removal can be claimed. For both OAE and OAC, since the timescale for air-sea gas exchange is longer than the characteristic timescale for dilution, most of the CO₂ removal will occur far from the point of dispersion. As a result, MRV will rely on direct measurements of the seawater carbonate chemistry in the vicinity of the outfall (Cyronak et al., 2023; Schulz et al., 2023) combined with ocean modeling to estimate CO₂ removal beyond the range of direct detection (Fennel et al., 2023).

In addition to developing MRV for the measurement and verification of CO₂ removal, MRV methodologies must also be designed to quantify the environmental and ecological impact of CDR interventions, a subset of MRV sometimes labeled “eMRV.” For example, both OAC and OAE release seawater with elevated pH back to the ocean, resulting in a mixing zone near the point of dispersal where the pH is at its maximum before decreasing as it diffuses away from the mixing zone and mixes with untreated ocean water. Experiments are underway to determine the effect of these changes in carbonate chemistry within the mixing zone on marine ecosystems, with some experiments starting to report results (Gately et al., 2023). The results of these experiments will inform eMRV measurement strategies and establish safe bounds of operation for OAC and OAE.

Scaling CDR to gigatons of CO₂ removal per year is a daunting challenge that will almost certainly require the deployment of multiple complimentary approaches. A major thrust of research and development in the next few years should focus on the potential for hybrid approaches, such as electrochemical OAE coupled to CO₂ mineralization, ERW, or BiCRS, or the coupling of OAC to CO₂ utilization in materials such as concrete. Determining the optimal times and places for deploying various CDR methods will require a clear understanding of their fundamental operating principles and relative merits. Such an understanding and direct comparison for electrochemical OAE and OAC has been lacking to date but is especially important given their superficial similarity in that they both electrochemically generate acid and base from salt. In this Perspective, I have aimed to provide a clear framework for comparing these two promising electrochemical mCDR solutions.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

ME: Conceptualization, Funding acquisition, Writing – original draft, Writing – review & editing.