2. Detailed Technical Design
Overview: Each CO₂ extraction unit is a self-contained module roughly the size of a small buoy or a portable appliance (on the order of 30–50 cm in diameter). It consists of a mechanical housing that holds the electrochemical reactor, a power subsystem, sensors, and a control/communication circuit. The design is optimized for corrosion resistance, continuous operation in marine conditions, and ease of attachment to existing structures (boat hulls, pier pilings, etc.). The following subsections describe the mechanical design, the electrochemical CO₂ extraction process in detail, and the electrical/control system design.
2.1 Mechanical Design and Housing
The mechanical design of the unit emphasizes simplicity, durability, and minimal maintenance. The core of the device is a cylindrical or box-shaped housing made of marine-grade plastic (e.g. UV-stabilized polyethylene or polycarbonate) or composite material. This housing is watertight for the electronics compartment, while allowing seawater to flow through the reaction chamber portion. Key mechanical features include:
Mounting system:
The unit can be strapped or bolted to structures. For boat mounting, a clamp or bracket system is provided to attach the device to the hull or rail, with the reaction chamber submerged. For piers or pilings, adjustable band clamps or bolt-on mounts secure the unit at an appropriate depth. The entire device is lightweight (target <5 kg) so that a single person can handle it, and multiple units can be installed without significant structural load. The compact size and weight also reduce shipping costs for global distribution.
Buoyancy and orientation:
If deployed as a free-floating buoy (an alternative deployment mode), the housing is designed to naturally float with the reaction chamber submerged and the solar panel (if used) facing upward. Foam or air-filled compartments can provide buoyancy. For hull/pier-mounted units, buoyancy is less critical, but the design still avoids negative buoyancy to prevent sinking if detached. The unit's center of mass is kept low so it remains upright in water.
Reaction chamber:
This is the section of the device where seawater contacts the electrodes and CO₂ is extracted. The design uses either a single chamber that is alternately operated in acidification and basification modes, or two small chambers that function in tandem (one acidifies while the other basifies). In our design, we propose a two-chamber system for continuous flow: an acidification chamber where CO₂ is released, and a neutralization chamber where the water is re-alkalized. These chambers are connected by small valves or a simple pump that moves water sequentially through them. Each chamber contains the respective electrodes (described in 2.2) and is shaped to ensure good contact between water and electrodes. The chambers have inlets/outlets covered with fine mesh screens to prevent marine organisms or debris from entering and to reduce fouling.
CO₂ collection mechanism:
When CO₂ is pulled out of solution in the acidification chamber, it forms bubbles of CO₂ gas. The chamber is oriented such that gas bubbles rise into a small headspace or dome at the top of the chamber. We include a gas capture dome with a one-way valve or hydrophobic gas-permeable membrane. The liberated CO₂ gas accumulates there and can be routed via tubing to a central collection vessel if the unit is part of a larger system (for example, on a ship, the CO₂ from multiple units could be piped into a storage tank). In a simple standalone scenario, the CO₂ may be periodically vented to the atmosphere if immediate storage is not feasible (recognizing this shifts the CO₂ but allows the ocean to absorb equivalent CO₂ from the air, achieving net removal from the atmosphere-ocean systemchemistryworld.com). The design optionally allows attaching a small CO₂ sorbent cartridge in the dome (e.g. a cartridge of solid calcium hydroxide that would convert CO₂ to calcium carbonate) for on-site sequestration, but this would require periodic replacement. For the base design, we assume venting or external collection of CO₂.
Materials and anti-fouling:
All materials in contact with seawater are chosen for corrosion resistance. Plastics or fiberglass composites are inherently corrosion-proof and can be used for the housing and chamber structure. The electrodes themselves will be housed or coated in a way that minimizes direct exposure of any sacrificial material to seawater beyond what is intended in the reaction (for example, the silver electrode will form AgCl but we prevent any free silver from leaching out). Surfaces may be treated with non-toxic anti-fouling coatings (e.g. silicone-based paints or textured surfaces that discourage organism growth) to prolong intervals between cleaning. The device design also avoids narrow crevices where biofouling could seize moving parts—most parts are static, and any moving components like valves or pump impellers are designed for biofouling tolerance or are enclosed.
Cooling and ventilation:
The electrochemical reactions generate minimal heat (since the process operates near ambient temperature), but the electronics and battery may need ventilation. The housing includes a small breathable membrane or pressure-equalizing vent for the electronics compartment that keeps water out but allows gases to exchange, preventing pressure buildup and moisture accumulation. The solar panel doubles as a top cover for the electronics compartment.
In summary, the mechanical design ensures the unit can robustly function in harsh marine environments (saltwater, waves, biofouling) for extended periods. It provides the structural framework for the reactor, protects sensitive components, and interfaces with the outside world (mounting, water flow, gas capture) in a reliable way. All of this is achieved with inexpensive manufacturing (injection-molded plastics, standard fittings) to meet the low unit cost.
2.2 Electrochemical CO₂ Extraction Process
At the heart of each unit is an electrochemical reactor that performs the CO₂ extraction via a pH swing. The design leverages the bismuth–silver chloride electrode system introduced in recent researchchemistryworld.com. The process can be understood in two phases, which we illustrate in the figure below.
(Figure: Schematic of the two-step electrochemical pH swing process for CO₂ extraction from seawater. In the acidification cell (left), a Bi metal electrode (anode) produces H⁺ (acid) and an Ag/AgCl electrode (cathode) releases Cl⁻, converting bicarbonate to CO₂ which outgasses. In the second phase or cell (right), the reactions are reversed (BiOCl is reduced and Ag is oxidized back to AgCl), consuming H⁺ (producing OH⁻) and thus re-alkalizing the water before returning it to the oceanchemistryworld.comchemistryworld.com.)
Phase 1: Acidification and CO₂ Release
When the reactor is fed with fresh seawater (which typically has pH ≈ 8.1 and contains dissolved inorganic carbon mainly as HCO₃⁻), we apply a voltage across the two electrodes to acidify the water. The bismuth electrode (Bi) serves as the anode in this phase. Under a modest anodic potential, the solid Bi metal oxidizes in the presence of chloride ions (which are abundant in seawater at ~0.5 M Cl⁻). The anodic reaction can be represented (simplified) as:
Bi (s) + 2 Cl⁻ + H₂O → BiOCl (s) + 2 H⁺ + 3 e⁻pubs.rsc.org.
This produces bismuth oxychloride (a solid that temporarily deposits on the electrode) and, critically, releases protons (H⁺) into the water, lowering the local pH. On the cathode side, we use a silver chloride (Ag/AgCl) electrode. Silver chloride is a common reference electrode material and in our case acts as a reversible chloride ion source/sink. Under the applied voltage, the cathodic reaction is:
AgCl (s) + e⁻ → Ag (s) + Cl⁻pubs.rsc.org.
This means the AgCl at the cathode is reduced to metallic silver, releasing a chloride ion into solution for each electron transferred. The net effect in the solution is that chloride ions are not being consumed overall (the Bi anode consumes Cl⁻ to form BiOCl at the same rate the AgCl cathode releases Cl⁻), but H⁺ is being produced. The water in the cell therefore becomes acidic: its pH drops significantly from the initial ~8 toward the range where dissolved inorganic carbon (DIC) is mostly CO₂. As the pH falls below around 6 or lower, the bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) in the seawater are converted to molecular CO₂ (carbonic acid equilibrium shifts to CO₂ + H₂O)pubs.rsc.orgpubs.rsc.org. This dissolved CO₂ then comes out of solution as gas (since the water becomes supersaturated with CO₂). In the reactor, we see this as bubbles forming and collecting in the headspace. Essentially, the ocean's stored CO₂ is liberated: we have "stripped" CO₂ from the seawater by acidification.
During this phase, we continue until a target fraction of the DIC is converted to CO₂ and removed. We do not necessarily need to acidify all the way to pH 4 or lower; even a moderate pH swing can release a substantial portion of CO₂. For efficiency, the system might aim for pH ~4–5 in the acid chamber, which releases the majority of inorganic carbon as CO₂ gaschemistryworld.com. The reactions at the electrodes are designed so that they proceed with high Faradaic efficiency (most of the electrons go into making H⁺ and Cl⁻ as desired). Experiments have shown this approach can achieve a high conversion efficiency with an energy input around 122 kJ per mole of CO₂ capturedpubs.rsc.org, which is orders of magnitude lower than the energy to capture CO₂ from air, thanks to the favorable chemistry and higher CO₂ concentration in seawater.
Phase 2: Re-alkalization and Discharge
Once the CO₂ has been released, the acidic water in the reaction chamber now needs to be neutralized (and ideally made slightly alkaline) before we put it back into the ocean, to avoid acidifying local seawater. In our design, after the CO₂ gas is extracted from the first chamber, that acidified water is pumped or allowed to flow into the second chamber (the neutralization chamber). In this chamber, we perform the reverse electrochemical reactions to consume the excess H⁺ and regenerate our electrodes for the next cycle. We now make the bismuth electrode the cathode (receiving electrons) and the silver electrode the anode. The reactions reverse: Bismuth oxychloride (BiOCl) that was formed is reduced back to Bi metal, consuming H⁺ in the process (essentially producing OH⁻ or removing acidity), and silver metal is oxidized to AgCl, consuming a chloride ion. In chemical terms, the cathodic reaction at BiOCl would be the reverse of the earlier equation: BiOCl (s) + 2 H⁺ + 3 e⁻ → Bi (s) + 2 Cl⁻ + H₂O (thus removing H⁺, raising pH), and the anodic reaction at silver would be Ag (s) + Cl⁻ → AgCl (s) + e⁻ (taking Cl⁻ out of solution). By the end of this phase, the water in the neutralization chamber is returned to roughly its original pH or slightly above (since we can intentionally add a bit of extra base). This treated water, now CO₂-depleted and slightly alkalinized, is released back into the ocean. Because it has a higher capacity to take up CO₂ (being alkaline), it will eventually draw down an equivalent amount of CO₂ from the atmosphere as it equilibrateschemistryworld.com, completing the removal from the atmospheric pool.
The overall electrochemical cell thus operates in a cyclic fashion, swinging the pH down and then up. We can implement this with two physical chambers as described (one always acidifying incoming water, the other always alkalizing water to discharge), or with one chamber that does batch processing (acidify, release CO₂, then switch polarity in the same chamber to neutralize). A two-chamber continuous flow design is chosen for efficiency: seawater flows through the acidification chamber (CO₂ is stripped), then into the neutralization chamber, and then is discharged. The electrodes in each chamber alternately switch roles so that each electrode is regenerated in situ.
Electrical Efficiency
The electrochemical cell operates at a low voltage (on the order of a few hundred millivolts up to perhaps 2–3 volts at most) because the reactions are near spontaneous. In fact, the theoretical cell potential for the combined Bi/AgCl reaction can be very low (the MIT researchers reported an overall reaction cell potential of ~0.07 V indicating it is thermodynamically favorable)pubs.rsc.org. In practice, some overpotential is needed to drive reasonable current, but this means the process is quite energy-efficient.
Production Rate
The current drawn will determine the CO₂ production rate. For example, every 3 electrons passed in the cycle corresponds to one Bi reaction that releases 2 H⁺ (which can liberate one CO₂ molecule from bicarbonate). Faradaic efficiency was reported to be high (close to 100%)pubs.rsc.org, so we can assume roughly one CO₂ molecule per 2–3 electrons. A device running at 1 A of current continuously would in theory produce on the order of 0.18–0.27 moles of CO₂ per hour (≈8–12 g CO₂/hour).
Auxiliary Components & Safety
To facilitate the above process, a small water pump or valve system is included to control the flow of seawater through the chambers. In a simplified design, we might rely on diffusion and natural convection: when CO₂ is removed and the water in chamber A becomes denser (more acidic), it could flow down and out, drawing in fresh water. However, to have more control, a tiny low-power pump (like those used in aquarium devices) can be used to push water through at a set rate.
Finally, safety and fail-safes in the electrochemical system are considered. If the pH swing overshoots (too much acid or base), sensors (pH sensors) can detect that and the controller will stop current to prevent any extreme pH water from being released. The electrodes are kept at relatively low voltages, so hydrogen or chlorine gas evolution (from water splitting or chloride oxidation to Cl₂) is minimized.
2.3 Electrical and Control System Design
Each CO₂ extraction unit includes on-board electronics for power management, control of the electrochemical process, sensing, and communication. The electrical system is designed to be low-cost, low-power, and robust, using largely off-the-shelf components common in IoT (Internet of Things) devices.
Power Electronics
The device's power subsystem (detailed in Section 4) produces a certain supply voltage (for example, a battery at 12 V nominal or similar). The electrochemical cell needs a controllable DC power source, typically in the range of 0–5 V and capable of supplying a few amperes. We use a small DC-DC converter or H-bridge driver that can both source and sink current, allowing us to reverse the polarity to switch between acidification and basification modes. Essentially, this is a programmable DC power supply for the cell. Given the low voltage, the design can use a high-efficiency buck converter to step down from the battery voltage to the cell voltage. A microcontroller or dedicated analog circuit controls the voltage or current delivered to the electrodes.
Microcontroller and Firmware
At the brain of the unit is a low-power microcontroller (for example, an ARM Cortex-M series or a microcontroller integrated in a system-on-chip with communication capabilities). This microcontroller handles multiple tasks:
- Running the control loop for CO₂ extraction: It periodically activates the electrochemical cell, monitors sensor readings (pH, perhaps CO₂ sensor, etc.), and adjusts the current and timing of phases.
- Power management: The microcontroller monitors battery level and input power. If power is low (e.g., at night when solar is off and battery is depleted), it can reduce or pause CO₂ extraction temporarily.
- Sensor interface: The unit is equipped with sensors for local water conditions. The primary sensor is a pH sensor that measures the pH of water in the chambers or just outside the device.
- Data logging: The device logs key performance metrics, like how much CO₂ has been extracted (which can be estimated from the total charge passed).
- Wireless communication: Each unit is equipped with a wireless module to communicate with other units and/or a central gateway.
Network & Coordination
If one of the units or a dedicated gateway has an internet uplink (via WiFi, cellular, or satellite), data from the network can be sent to cloud servers for remote monitoring. In coastal deployments, a pier-mounted base station could have WiFi or Ethernet and gather data from all nearby units via LoRa, then upload to an online dashboard.
The firmware in each unit can implement cooperative algorithms. For example, a cluster of devices can use a distributed algorithm to ensure they don't all extract at once from the same water parcel. They might take turns or modulate duty cycles if sensors detect a drop in CO₂ or pH beyond a setpoint.