Carbon dioxide removal (CDR) is a term used to describe anthropogenic activities that directly or indirectly remove carbon dioxide (CO2) from the atmosphere and durably store it in geological, terrestrial, or ocean reservoirs, or in products. Marine carbon dioxide removal (mCDR) is a subset of CDR approaches that leverage the ocean to remove CO2 and/or store captured CO2 in ocean reservoirs.  

Electrochemical carbon dioxide removal technologies utilize techniques that capture and remove dissolved inorganic carbon from seawater (either as CO₂ gas or as calcium carbonate), and/or produce a CO₂-reactive chemical base, e.g. sodium hydroxide (NaOH), that can be distributed in the surface ocean to ultimately consume atmospheric CO₂ and convert it to long-lived, dissolved, alkaline bicarbonate. These techniques are sometimes called “direct ocean capture” to draw comparisons with direct air capture and encompass both electrodialytic and electrolytic processes.

In electrodialytic approaches, electricity provides the energy to rearrange the most common components of seawater, H₂O and NaCl, into acidic (hydrochloric acid; HCl) and basic (sodium hydroxide; NaOH) solutions (House et al., 2007).  The acidic solution can then be used to strip dissolved inorganic carbon from seawater in the form of gaseous CO₂, or the basic solution can be used to strip the dissolved inorganic carbon by precipitating solid calcium carbonate (Note: Production of calcium carbonate via electrodialysis results in the basic component becoming acidified. This acidic stream may need to be neutralized through addition of a base before being recombined). In addition, the basic component can be selectively added to seawater to draw additional CO₂ into the ocean to be stably stored as bicarbonate ions.

Electrolytic approaches split water and salt into hydrogen and oxygen and/or chlorine gases, and produce alkaline metal hydroxide (e.g., sodium hydroxide) and acid as byproducts (Rau et al., 2018).  When added to surface seawater, the hydroxide reacts with CO₂ in seawater to form dissolved alkaline bicarbonate. The resulting CO₂ undersaturation in surface seawater then forces atmospheric CO₂ to enter the ocean and be stored largely as long-lived seawater bicarbonate.

Electrodialytic approaches are already commercial technologies used in applications such as whey demineralization, organic acids recovery, and desalination, among others (Bazinet & Geoffroy, 2020).

The electrolytic production of H₂, O₂/Cl₂, hydroxide, and acid is a very mature technology that globally produces some 75 Mt of NaOH per year that is an essential reagent in a variety of important industrial processes (e.g., in food processing, soap production, pulp and paper, pharmaceuticals, etc.) (Lakshmanan & Murugesan, 2014).

An electrolytic approach to precipitate calcium carbonate has recently been proposed (Callagon La Plante et al., 2021). This approach electrochemically generates alkalinity in order to precipitate calcium carbonate, generating a potentially useful end product from the captured carbon. But, as described above, precipitation of calcium carbonate from seawater acidifies the seawater, reducing the ability of seawater to absorb CO₂ from air. This proposed pathway is at a technology readiness level of ~5 and is currently undergoing small-scale field testing.

• Scalability – Given the immense stocks of dissolved inorganic carbon in seawater (~38,000 Gt (Ciais et al., 2013)), the theoretical scale of carbon capture from electrochemical methods is limitless for all practical intents and purposes. However, engineering, economic, political, and social considerations are likely to significantly reduce this upper bound. These include, but are not limited to:
• • Access to low or zero-carbon emissions energy and infrastructure (e.g., desalination plants, offshore wind) with the ability to pump large quantities of seawater into large-scale electrochemical reaction cells, perform electrodialysis or electrolysis, and capture outputs and byproducts. The exact energy and infrastructure needs are pathway-dependent. For instance, electrolysis requires more energy than does electrodialysis, but the relative value of beneficial products (e.g., H₂, O₂ in electrolysis, HCl in electrodialysis) produced by each process must also be considered (Campione et al., 2018).
    • In the cases where electrodialysis is used to produce HCl and strip CO₂ gas out of seawater or used to produce NaOH that precipitates calcium carbonate, the cost is high (>$350/ton CO₂) because of the cost of pumping seawater (Eisaman et al., 2018), and in the case of CO₂ stripping, the additional cost of safely storing or utilizing the captured CO₂ (Eisaman, 2020)
    • Costs can be lowered to ~$100/ton CO₂ if captured CO₂ is stored in the ocean as bicarbonate due to the elimination of seawater pumping costs^[5]Rau, G.H. (2021) ‘Testimony to The National Academies of Sciences, Engineering, and Medicine: A Research Strategy for Ocean Carbon Dioxide Removal and Sequestration’ 27-January-2021.
    • Electrolytic pathways may offer CDR for $150-100/ton CO₂, depending upon whether revenues generated from co-products are used to offset gross costs of CO₂ capture (Callagon La Plante et al., 2021).
  • The collective market size for a suite of co-products and services offered, including concentrated CO₂, H₂, O₂, HCl, control over critical seawater chemistry parameters like pH and total alkalinity, and CDR.

Ranges from recent synthesis reports vary with the 2021 NASEM report suggesting a range of 0.1-1.0 GtCO₂/yr with medium confidence, while the 2023 synthesis report from NOAA suggests 1-10 GtCO₂/yr. 

• Sequestration Permanence – Electrochemical methods that produce reactive alkaline minerals (e.g., NaOH) will result in additional CO₂ from the atmosphere being sequestered in the ocean as bicarbonate ions, which cannot exchange with the atmosphere. This is the same process that results in CO₂ sequestration for rock-based forms of ocean alkalinity enhancement. The residence time of bicarbonate ions in the ocean is ~10,000 years, suggesting that electrochemical alkalinity production will generate CO₂ sequestration with permanence of ~10,000 years.

Electrochemical methods that produce carbonate mineral products (e.g., Callagon La Plante et al., 2021) with sequestered carbon are stable on timescales of hundreds to thousands of years. 

Electrochemical methods that capture and remove CO₂ gas from seawater have variable permanence depending upon the storage location for the captured CO₂. If the captured CO₂ is sequestered in geologic storage, permanence can be long (thousands to millions of years)

• Electrochemical CDR would likely provide localized reductions in ocean acidification, with expected benefit(s) to marine ecosystems.
• Carbon-negative byproducts from electrochemical CDR can be substituted for more C-intensive sources (Rau, 2008):
  • Electrolysis: hydrogen gas, chlorine gas, and oxygen gas, as well as hydrochloric acid
  • Electrodialysis: hydrochloric acid
• In comparison to rock-based forms of alkalinity enhancement, electrochemical methods may offer:
  • Reduced risk of toxicity from metals present in rock-based forms of alkalinity addition (Hartmann et al., 2013).
  • However, the purity of outputs from electrochemical CDR is determined partially by the purity of the input materials to the electrochemical processes, and purer input materials are more expensive.
  • Unlike ocean liming and coastal enhanced weathering, electrochemical alkalinity additions will not increase the silica concentration in the ocean and, therefore, may offer a reduced risk of disturbing phytoplankton community dynamics between calcifiers and silicifiers (Bach et al., 2019).

1. Risks Shared by Electrolysis and Electrodialysis
   • The potential for changes in water column particle concentrations, turbidity, and optical properties if precipitation (inorganic mineral formation) of carbonates occurs due to local increases in alkalinity and pH.  
   • Mortality of marine life through seawater intake pumps and filters
   • Shifts in phytoplankton, invertebrate and vertebrate physiology, competition and/or mortality due to decreases in acidity as CO₂ is removed and/or alkalinity is added (Renforth & Henderson, 2017).  For example, do the preceding seawater chemistry changes provide an increased competitive advantage for calcifiers over and above the restoration of calcifiers/calcification from ongoing ocean acidification?
   • Risks about permanence and impacts of elevated concentrations of bicarbonate in the oceans.
2. Risks Specific to Electrolysis
   • Chlorine gas production and handling
3. Risks Specific to Electrodialysis
   • Production of large quantities of acid via electrodialysis that must be safely consumed or neutralized (e.g., via reaction with alkaline minerals).
   • For pathways that strip CO₂ gas from seawater and sequester the CO₂, risks of permanence of storage and potential for leakage
   • For pathways that remove dissolved inorganic carbon from seawater as calcium carbonate (Callagon La Plante et al., 2021), the impacts of large quantities of new calcium carbonate on the diversity, abundance, and ecosystem function of marine environments

There have been very few studies on the benefits of electrochemical ocean-based CDR to societies and how they will be distributed both in the local communities where they will be established as well as globally. Among the studies that are in progress are those from the OceanNETs project. As this field progresses and demonstration projects continue, it will be critically important that work to assess social impacts progresses in turn.

Some of the potential benefits of electrochemical ocean-based CDR approaches could include

1. Localized reductions in ocean acidification, with expected benefit(s) to industries that rely on these localized improvements such as aquaculture and tourism
2. Creation of employment in industries producing carbon-negative byproducts from electrochemical CDR (Rau, 2008):
   • Electrolysis: hydrogen gas, chlorine gas, and oxygen gas, as well as hydrochloric acid
   • Electrodialysis: hydrochloric acid

It is difficult to assess the range of potential impacts to society from electrochemical techniques due in large part to the remaining list of unknowns around the technical and scientific aspects of these techniques. As this field progresses and demonstration projects continue, it will be critically important that work to assess social impacts progresses in turn (Nawaz et al., 2023)