Employing Electrocatalytic CO2 Conversion in Modular Ocean-Based Carbon Capture Platforms

1. The Ocean as a Carbon Reservoir: Context and Opportunity

The oceans represent Earth's largest active carbon sink, absorbing approximately 25% of anthropogenic CO₂ emissions annually. Surface waters currently contain about 1,000 billion metric tons of dissolved inorganic carbon, with the oceanic carbon reservoir estimated at 38,000 billion metric tons - nearly 50 times more than the atmospheric pool.

1.1 The Case for Ocean-Based Carbon Capture

Traditional carbon capture and storage (CCS) systems face several limitations when applied to oceanic environments:

• High energy requirements for direct air capture (DAC) systems (estimated 1,500-2,500 kWh/ton CO₂)
• Limited scalability of terrestrial mineralization approaches
• Geographical constraints on pipeline infrastructure for storage sites

Modular ocean platforms address these challenges by:

• Leveraging the ocean's natural CO₂ absorption capacity
• Utilizing renewable energy sources (offshore wind, wave, and solar)
• Providing distributed deployment options across major oceanic gyres

2. Electrocatalytic CO₂ Conversion Fundamentals

Electrocatalytic reduction of CO₂ (eCO₂R) employs electrochemical cells to convert dissolved carbon species into value-added products through controlled potential application.

2.1 Reaction Pathways and Products

The primary reduction pathways in seawater environments produce different hydrocarbon products depending on catalyst selection and operating conditions:

Catalyst Material	Primary Product	Faradaic Efficiency Range	Potential (vs RHE)
Cu-based alloys	C₂H₄ (ethylene)	40-60%	-0.7 to -1.0 V
Sn/SnOₓ	HCOOH (formate)	70-90%	-0.6 to -0.8 V
Zn/ZnO	CO	80-95%	-0.5 to -0.7 V

2.2 Seawater-Specific Considerations

The presence of ionic species in seawater (Na⁺, Cl⁻, Mg²⁺, etc.) introduces unique challenges and opportunities:

• Chloride ions may participate in competing reactions (chlorine evolution) at potentials >1.36 V vs SHE
• Multivalent cations can stabilize reaction intermediates through electrostatic interactions
• Natural pH buffering from bicarbonate system maintains optimal conditions (pH ~8.1)

3. Modular Platform Design Architecture

The floating reactor system integrates multiple subsystems to enable continuous operation in marine environments.

3.1 Core System Components

The platform's technical architecture comprises:

1. Seawater Intake System: Depth-adjustable pumps with pre-filtration (200μm mesh) to remove particulates
2. Electrochemical Modules: Stacked flow cells with bipolar membrane arrangement (typical active area 2m² per module)
3. Renewable Power System: Hybrid wind-solar array with lithium-ion buffer storage (nominal 500 kW capacity)
4. Product Separation: Gas-liquid membrane contactors for hydrocarbon recovery
5. Data Telemetry: Satellite-linked monitoring of performance metrics and environmental conditions

3.2 Deployment Configurations

Platforms can be deployed in three primary configurations based on operational requirements:

• Autonomous Floating Units: 20-50m diameter platforms for distributed deployment (100-500 tons CO₂/yr capacity)
• Array Clusters: Interconnected modules forming larger installations (5,000+ tons CO₂/yr capacity)
• Ship-Mounted Systems: Retrofit solutions for existing vessels to offset operational emissions

4. Performance Metrics and Scaling Considerations

The technology's viability depends on achieving key performance benchmarks across multiple parameters.

4.1 Energy Efficiency Analysis

The total system energy requirement comprises three main components:

1. Electrolysis Energy: Typically 50-70% of total demand (20-30 kWh/kg hydrocarbons)
2. Pumping/Processing: 15-25% of demand (5-8 kWh/kg)
3. Ancillary Systems: 10-20% of demand (monitoring, stabilization, etc.)

The net carbon balance must account for:

• Direct CO₂ conversion efficiency (typically 60-80% of dissolved inorganic carbon)
• Indirect effects from enhanced air-sea CO₂ flux (estimated 10-15% additional uptake)
• Embodied energy in platform construction and maintenance

4.2 Scaling Laws and Cost Projections

Preliminary techno-economic analysis suggests the following scaling relationships:

Platform Size (tons CO₂/yr)	Capital Cost ($/ton capacity)	Levelized Cost ($/ton CO₂)
100	$1,200-1,500	$180-220
1,000	$800-1,000	$120-150
10,000	$500-700	$80-100

5. Environmental Impact and Monitoring Protocols

The deployment of ocean-based carbon capture systems requires rigorous environmental assessment frameworks.

5.1 Potential Ecosystem Interactions

The platforms may influence local marine environments through:

• pH Modulation: Alkaline plumes from cathode reactions (typically ΔpH <0.2 within 50m)
• Electromagnetic Fields: Subsurface cabling generates <5 μT fields at 10m distance
• Surface Structure Effects: Artificial substrate for biofouling communities (~15% increase in local biomass)

5.2 Monitoring Requirements

A comprehensive monitoring program should include:

1. Water Chemistry: Continuous pH, pCO₂, and O₂ sensors with monthly ICP-MS for trace metals
2. Biological Surveys: Quarterly eDNA sampling within 500m radius
3. System Performance: Real-time tracking of conversion efficiency and product yields

6. Future Development Pathways

The technology's evolution will depend on advances across multiple research fronts.

6.1 Catalyst Development Priorities

Next-generation catalysts must address:

• Chloride Tolerance: Developing selective catalysts that minimize Cl⁻ oxidation at relevant potentials
• Tandem Systems: Integrating oxidation and reduction catalysts for complete carbon utilization
• Self-Healing Materials: Coatings that regenerate active sites during operation

6.2 System Integration Challenges

The path to gigaton-scale deployment requires solving key engineering challenges:

1. Materials Durability: Achieving >10-year service life in marine environments
2. Dynamic Load Management: Adapting to intermittent renewable power inputs
3. Automated Maintenance: Robotics for biofouling removal and component replacement