Membrane-Based Electrochemical Separation

Capture Mechanism

Membrane

Furthest Progress*

TRL 3

Highest Risks

Energy
Cost
Environment

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Method Overview

Definition:

Ion-exchange membrane separation in which the CO₂ is separated from other gases in an electrochemical cell by means of a membrane and electron transfer.*

Example:

Electrochemically generated ions bind with CO2 in air and selectively permeate through a membrane to electrochemically release CO2 for storage.*

Advantages:

While current estimates indicate that this method is energy intensive, the process is electrified and thus may be capable of greater energy efficiency, relative to thermal methods. Electrochemical DAC methods can be more energy efficient, as they apply electrical energy to CO2 capture and release directly. By contrast, DAC methods that utilize thermal energy for regeneration commonly waste energy by heating entire reactors, rather than just the sorbent materials. RepAir reports that its energy consumption (2.2 GJ/tCO2) is very low.
Electrochemical DAC can easily integrate with renewable energy sources.
This DAC approach is simple and modular. The equipment consists of stacks of thin electrochemical cells, avoiding more complex equipment such as moving adsobent beds, calciners, slackers, etc. that are involved in other DAC methods.

Disadvantages:

This approach is currently limited by slow CO2 capture kinetics. This is partly due to the corresponding slow rate of electron and proton transfers through the inorganic electrode materials and membrane separators.
Coupled CO2 capture and release steps, as well as significantly varied CO2 concentrations between these steps, can hinder the continuous operation of these systems. Continuous operation is possible only if the polarity of the electrodes and gas flows are reversed periodically.

* Reproduced from The Applied Innovation Roadmap for CDR (2023) by RMI.

Company Overview

Plot of estimated funding vs. deployment status of companies utilizing this approach. Select data points to view company details. Only companies for which funding information is publicly available are included. Companies without funding information are tabulated with related details where relevant.

Summary of Deployments

View DAC deployments within this approach that have achieved or surpassed prototype scale. Planned deployments are included. Sort DAC deployments by company, scale, start of operations, and more. Because DAC is a rapidly evolving industry, this list may not be exhaustive.*

* Due to uncertain funding, plans for most DOE-funded DAC Hubs are not included in this analysis.

Risk Assessment

RMI's Applied Innovation Roadmap for CDR (2023) categorizes the risks associated with different classes of DAC technologies. Their assessment of this method is reproduced here and contextualized with current self-reported company data, where available.

Energy Requirements

Very high risk: Membrane separation is extremely energy intensive.

Energy needs meet our DAC criteria for “very high risk” (>10 GJ/tCO2). The contacting and the electrochemical cell alone requires approximately 41-62 GJ/tCO₂. DAC+S will require additional energy (e.g., compression, transport, and storage), which is excluded rom the publicly available energy value.

Self-reported energy requirements of DAC companies (GJ/tCO2): RepAir: 2.2.

Material Requirements

Medium risk: High-demand materials are required.

Apart from basic construction materials (e.g., steel, cement), this solution relies on shortened membranes. If fuel cell electric vehicles scale significantly, expanding the supply chain would be considerably easier, but high demand materials for electrodes are required.

Cost Requirements

Very high risk: Cost data is incomplete.

We base our rating on cost estimates for FOAK plants and the quality of these estimates. We apply different cut-offs for modular / non-modular technologies, as modular approaches are comparatively less expensive and are easier to scale. This technology meets our criteria for “very high risk” (FOAK <10 ktCO2; >$600 with incomplete data).

Land Requirements

Low risk: Significant areas are suitable for DAC siting.

No information is publicly available, but it is expected that plant footprint will be very competitive with other DAC approaches. The footprint is larger if land use for energy production is considered.

Water Requirements

Medium risk: Humidity/water may be required.

Depending on the specific configuration, humidity/water may be required. However, there is a lack of publicly available data.

Environmental Impact

High risk: No/incomplete LCA data is available.

No LCA is currently available. Apart from impacts associated with energy and material requirements (including disposal and recycling), minimal to no other impacts are anticipated.

Co-Location of DAC and Storage

Low risk: Co-location of DAC and storage is relatively easy.

Can be located all around the globe. Humid environments are preferable for the RepAir configuration.

Secure Storage for 100+ Years

Low risk: Storage is durable.

Geological sequestration or in-situ mineralization (primary storage options) are considered permanent storage approaches.

Path to MRV at Scale

Low risk: Measurement is straightforward.

DAC+S measurement and monitoring to inform MRV is straightforward. Capture occurs in a controlled environment and geological injection has established monitoring standards.

Key Innovations

Membrane-based electrochemical DAC is a newer method that is gaining attention due to the low energy consumption reported by its leading company, RepAir, relative to more developed electrochemical DAC approaches. While RepAir has made notable progress, including deploying a prototype unit, the technical readiness of the field in general is still low and will benefit from further research and validation of Materials, Process, and Engineering design.

To the best of our knowledge, RepAir is the sole DAC technology producer in this field. Selected characteristics of the company are outlined below alongside related research efforts that seek to devel0p this DAC approach.

Material Design:

Membranes - Studies comparing the performance of different membrane and ionomers with regards to CO2/carbonate/bicarbonate transport kinetics and back-diffusion, as well as long-term cycle stability, would support the development of this technology. An assessment of manufacturing requirements is also needed.
Electrodes - Electrode materials should be optimized to improve the CO2 diffusion and increase the rate of electron transfer to enable more efficient CO2 capture. Additionally, alternatives to Pt/C catalysts should be developed to limit costs.

Process Design:

Operating parameters - a better understanding of operating parameters such as backpressure, temperature, relative humidity for each process configuration would help validate this technology's compatibility with various climate conditions.

Equipment:

Contactor and flow field - Flat plate contactors and spiral wound membranes have both been studied. RepAir appears to use a flat plate contactor. The performance of these designs and others should be validated and compared to identify promising configurations that minimize pressure drop. In addition to the contactor, the flow field of inlet gas streams should also be optimized to decrease mass transport resistance and thereby increase CO2 capture kinetics.