1. Technology Selection Rationale
Removing CO₂ from ocean water can be achieved through several possible methods. We evaluated chemical, biological, and electrochemical approaches with the aim of finding the cheapest viable, efficient, and environmentally safe technique that fits within the constraints of a $100 unit. After comparative analysis, an electrochemical pH-swing process was selected as the optimal method for these small-scale CO₂ extraction units.
Chemical absorption methods (such as amine scrubbers or alkaline absorbents) were considered but deemed impractical for ocean deployment at small scale. While effective in industrial settings, liquid amine or hydroxide sorbents would require handling of large reagent volumes and regeneration steps involving heat or concentrated chemicals. This would add complexity, cost, and potential ecological risk if any sorbent leaked into seawater. Likewise, biological approaches (e.g. using algae or engineered microbes to fix CO₂) would be slow and hard to control in an open marine environment, and would require mechanisms to permanently sequester the biomass carbon. These approaches were not ideal for an inexpensive, maintenance-free device.
We focused on electrochemical approaches, which directly leverage the fact that seawater itself contains dissolved inorganic carbon in the form of bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) ions in equilibrium with dissolved CO₂. By applying electrical current, one can locally alter the water's pH to release CO₂ from bicarbonate, then capture that CO₂ and return the water to a neutral or alkaline state. This approach effectively "borrows" the ocean's natural carbon storage and uses electrical energy to trigger CO₂ release on demandchemistryworld.comchemistryworld.com. The advantage is that the ocean has already done the work of absorbing CO₂ from the air, at ~100× higher concentration than in airchemistryworld.com, so an electrochemical system can skip the air-capture step and directly tap the ocean's dissolved carbonchemistryworld.com. This can be more energy-efficient and cost-effective than direct air capture, provided the system components are inexpensivechemistryworld.comchemistryworld.com.
Among electrochemical CDR techniques, the most developed is the pH swing via bipolar membrane electrodialysis (BPMED). In such systems (exemplified by projects like Captura and Equatic), seawater is split into an acid stream and a base stream using stacks of ion-exchange membranes and applied voltagespectrum.ieee.org. The acidified portion of the water releases CO₂ gas (by converting bicarbonate to CO₂), which can be collected, while the basified portion (high pH) is used to neutralize the remaining water before returning it to the oceanspectrum.ieee.org. This approach works and has been demonstrated in pilot plants (e.g. a 100-tonne CO₂/year Captura system)spectrum.ieee.orgspectrum.ieee.org. However, traditional electrodialysis setups are costly: they require multiple specialized ion-exchange membranes, pumps, and careful control of electrode reactions (often needing added chemical reagents to carry charge)chemistryworld.com. The membranes themselves are expensive and can be prone to fouling or degradation, which is problematic for a $100 device. Thus, while BPMED is effective, it would not meet our cost and simplicity targets without significant breakthroughs in low-cost membranes or system miniaturization.
Traditional Methods Challenges
- Expensive membranes
- Complex pumping systems
- Chemical reagent requirements
- Membrane fouling issues
New Approach Benefits
- Membrane-free design
- Inexpensive electrode materials
- Self-regenerating process
- Lower cost per ton CO₂
Fortunately, recent innovations allow a membrane-free electrochemical pH swing, eliminating the most expensive componentschemistryworld.com. Researchers have developed an approach using inexpensive solid electrodes (notably bismuth and silver chloride) to alternately acidify and basify seawater without a bipolar membrane stackchemistryworld.com. In this scheme, a bismuth metal electrode is used to generate acidity (H⁺ ions) by electrochemically reacting with chloride ions in seawater, forming bismuth oxychloride (BiOCl)chemistryworld.com. This lowers the local pH and converts bicarbonate (HCO₃⁻) into dissolved CO₂, which can then be extractedchemistryworld.com. In a second step, the process is reversed: the electrode reactions are run in the opposite direction to generate alkalinity (consume H⁺ or produce OH⁻) and regenerate the electrodeschemistryworld.comchemistryworld.com. The net result is that CO₂ is removed from the water, and the treated water is returned slightly alkalinized, helping to counter ocean acidificationchemistryworld.com. Crucially, this approach avoids costly membranes and only uses low-cost electrode materials (bismuth is a cheap, non-toxic metal, and silver chloride is used in small quantities and is recoverable)chemistryworld.com. A preliminary techno-economic analysis of this membrane-free pH swing showed an estimated cost of ~$50–100 per tonne of CO₂ removedchemistryworld.com, which is a dramatic improvement over earlier estimates of $600+ per tonne for direct air capture or first-generation ocean capturechemistryworld.com. Given these advantages, we selected this electrochemical pH-swing via bismuth/silver chloride electrodes as the core CO₂ extraction method for the device.
This electrochemical method is not only cost-effective but also environmentally benign in its chemistry. It does not consume additives apart from the electrodes (which are continuously regenerated each cycle), and it returns all processed water to the ocean in a balanced state (no net acid or base addition beyond a slight increase in alkalinity, which is beneficial)chemistryworld.com. The only output is concentrated CO₂ gas, which can be collected for sequestration or utilization. Thus, based on technical reviews of current CDR methods, we have chosen the cheapest viable, efficient, and safe CO₂ extraction technique – a membrane-free electrochemical pH swing – as the foundation of our unit's design. The subsequent sections detail how this technology is implemented in a small, self-sufficient device along with all necessary supporting systems.