Direct air capture of carbon dioxide is one of the technologies whose development is constrained not by the process of binding CO₂ itself, but by the energy required to subsequently release it and prepare it for further use or storage. Two recent studies published in Nature journals show two different ways of addressing this problem. One simplifies the design of the electrochemical capture system by removing the conventional membrane, while the other shows that a cell based on nickel hydroxide can operate for 5,000 hours and be scaled up to a much larger stack.
Direct air capture, or the direct capture of carbon dioxide from the air, has to deal with an exceptionally low concentration of the gas. The atmosphere contains around 0.04 percent CO₂, so a facility has to process very large volumes of air to obtain a significant amount of carbon dioxide. Binding the molecule itself does not solve the problem, however. The sorbent must subsequently be regenerated, and the resulting CO₂ must be separated in a form suitable for transport, utilisation or permanent storage. The regeneration stage is one of the main sources of energy demand in the entire process.
Electrochemical capture methods attempt to change this relationship. Instead of supplying large amounts of heat needed to regenerate a conventional sorbent, they use electricity to control its ability to bind and release CO₂. In systems using redox active sorbents, this means changing their electrochemical state and then reversing that change. If the process can be carried out with low energy consumption, the technology could be much easier to integrate with renewable electricity.
Ion exchange membranes used in electrochemical CO₂ capture devices have nevertheless remained one of the design obstacles. They perform an important function, but increase system complexity, can limit performance and make scaling more difficult. A recent paper published in Nature Chemical Engineering addresses precisely this problem. The authors proposed a membrane free architecture using solid counter electrodes instead of a conventional membrane based configuration.
Electrochemical capture without a conventional membrane
The new system uses a molecular redox active sorbent that reversibly binds carbon dioxide depending on its electrochemical state. The key change lies in how the process is conducted. Instead of separating the individual parts of the system with an ion exchange membrane, the researchers used a solid counter electrode.
This solution removes one of the components that can complicate the construction of larger devices. It does not mean, however, that the problem has been completely solved. A membrane free system introduces the phenomenon of self discharge. Intermediate forms of the sorbent can transfer electrons uncontrollably to the counter electrode, causing losses and reducing process efficiency.
The research showed that the rate of this self discharge is strongly linked to the kinetics of ion transport within the solid material. This is an important design finding because it means that solving the problem does not necessarily require looking only for materials with more favourable thermodynamic properties. Designing the electrode material in a way that limits unwanted ion transport becomes equally important.
The authors used sodium iron phosphate as the counter electrode material, working with an azopyridine based sorbent. The system maintained stable operation for 75 CO₂ capture and release cycles, corresponding to around 350 hours of operation. It was also tested under conditions closer to practical applications, including high current density, a diluted gas stream and the presence of oxygen.
In their technoeconomic analysis, the authors estimated that the membrane free architecture could reduce the cost of capture by 28.9 percent compared with the membrane based system analysed in their study. This is a technoeconomic analysis result, however, rather than a cost achieved by a commercial installation. The actual value of the technology will depend, among other things, on material durability, electrode manufacturing costs and the ability to build devices with a much larger active surface area.
The second cell passes the long duration test
A different problem is examined by a team whose work was published in Nature Energy. In this case, the focus is an electrochemical direct air capture system using nickel hydroxide, Ni(OH)₂. The design is not membrane free. It uses an anion exchange membrane, so its results should not be treated as another demonstration that membranes can be completely eliminated. Its significance lies elsewhere, in the durability of the device and its scale up.
The researchers tested a cell with an area of 25 square centimetres for 5,000 hours. This duration is significant for DAC technology because many electrochemical concepts remain at the stage of small devices tested for short periods. In this case, the experiment lasted more than six months, making it possible to examine the system’s behaviour over a much longer period.
The authors did not stop at the small cell. As part of the test, they scaled the design up to a stack measuring 9 by 300 square centimetres. During 48 hours of operation, the stack achieved an energy consumption of 132 kilojoules per mole of CO₂, corresponding to around 0.83 megawatt hours per tonne of captured CO₂. At the same time, the resulting CO₂ flux was 0.19 moles of CO₂ per square metre per hour.
An additional important parameter concerns airflow. In direct air capture, enormous amounts of air have to pass through the device, so flow resistance can become a significant energy consumer. In the study, the larger stack met the assumed pressure drop requirement of 300 pascals, which the authors adopted as a requirement for DAC applications.
0.83 MWh per tonne is an important figure, but not the whole calculation
The energy result achieved by the new cell is interesting primarily because it concerns a specific device rather than merely the properties of a material. It cannot, however, be equated with the total energy consumption of a future DAC facility. An industrial system must also provide for the movement of large quantities of air, CO₂ preparation and compression, water circulation and other auxiliary processes.
This distinction also matters when comparing different technologies. Electrochemical capture can reduce the demand for heat and replace it with electricity, but the source of that electricity and the efficiency of the entire chain then become crucial. A low figure for the cell itself does not automatically mean a low cost of removing a tonne of CO₂ from the atmosphere.
The study’s authors nevertheless presented a pathway towards a cost below 100 dollars per tonne of CO₂, based on assumptions concerning subsequent generations of the technology and the learning effect. This is a development forecast, not the current cost of commercial capture. Its significance lies in showing what parameters would have to be maintained or improved for the technology to become economically competitive.
Two different problems, one goal
Both publications show that the development of electrochemical CO₂ capture is entering a stage in which the focus is increasingly less on demonstrating the chemical reaction itself. In the study of the membrane free design, the problem is simplifying the device architecture without sacrificing efficiency. In the Ni(OH)₂ study, the key issues are durability, active surface area, airflow and the possibility of building a larger stack.
These are precisely the parameters that determine the transition from a laboratory material to an industrial technology. A DAC device must operate for thousands of hours, maintain the properties of its sorbent and electrodes, work with a highly diluted CO₂ stream and process enormous volumes of air. Each of these elements can increase energy consumption or capital costs.
For the Ni(OH)₂ system, the important signal is therefore not only the energy consumption figure, but the combination of several results: 5,000 hours of operation of the small cell, demonstration of a larger stack, a defined CO₂ flux and compliance with the assumed pressure drop requirement. In the membrane free architecture, it is equally important that removing the membrane does not necessarily mean giving up control over the electrochemical process, although the current demonstration covers a much shorter operating period than the Ni(OH)₂ test.
The real test will be the cost of the entire device
The biggest question surrounding both approaches is therefore industrial. Laboratory results show that energy consumption can be reduced, operating time extended and device surface area increased. They do not yet determine how much a tonne of CO₂ captured in a real installation operating for years will cost.
The scale is particularly demanding in DAC because the low concentration of CO₂ requires enormous volumes of air to be processed. If electrochemical systems are to play a larger role, they must simultaneously reduce energy consumption, maintain high productivity per unit of active surface area, provide long material lifetimes and allow large modules to be built without a sharp increase in flow resistance. The results published in Nature Chemical Engineering and Nature Energy address precisely these problems, although they solve them through different architectures.
