Machines that suck CO2 directly from the air could cut the cost of meeting global climate goals, a new study finds, but they would need as much as a quarter of global energy supplies in 2100.
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The research, published today in Nature Communications, is the first to explore the use of direct air capture (DAC) in multiple computer models. It shows that a “massive” and energy-intensive rollout of the technology could cut the cost of limiting warming to 1.5 or 2C above pre-industrial levels.
But the study also highlights the “clear risks” of assuming that DAC will be available at scale, with global temperature goals being breached by up to 0.8C if the technology then fails to deliver.
This means policymakers should not see DAC as a “panacea” that can replace immediate efforts to cut emissions, one of the study authors tells Carbon Brief, adding: “The risks of that are too high.”
DAC should be seen as a “backstop for challenging abatement” where cutting emissions is too complex or too costly, says the chief executive of a startup developing the technology. He tells Carbon Brief that his firm nevertheless “continuously push back on the ‘magic bullet’ headlines”.
The 2015 Paris Agreement set a goal of limiting human-caused warming to “well below” 2C and an ambition of staying below 1.5C. Meeting this ambition will require the use of “negative emissions technologies” to remove excess CO2 from the atmosphere, according to the Intergovernmental Panel on Climate Change (IPCC).
This catch-all term covers a wide range of approaches, including planting trees, restoring peatlands and other “natural climate solutions”. However, model pathways developed by researchers rely most heavily on bioenergy with carbon capture and storage (BECCS). This is where biomass, such as wood pellets, is burned to generate electricity and the resulting CO2 is captured and stored.
The significant potential role for BECCS raises a number of concerns, with land areas up to five times the size of India devoted to growing the biomass needed in some model pathways.
One alternative is direct air capture, where machines are used to suck CO2 out of the atmosphere. If the CO2 is then buried underground, the process is sometimes referred to as direct air carbon capture and storage (DACCS).
Today’s new study explores how DAC could help meet global climate goals with “lower costs”, using two different integrated assessment models (IAMs). Study author Dr Ajay Gambhir, senior research fellow at the Grantham Institute for Climate Change at Imperial College London, explains to Carbon Brief:
“This is the first inter-model comparison…[and] has the most detailed representation of DAC so far used in IAMs. It includes two DAC technologies, with different energy inputs and cost assumptions, and a range of energy inputs including waste heat. The study uses an extensive sensitivity analysis [to test the impact of varying our assumptions]. It also includes initial analysis of the broader impacts of DAC technology development, in terms of material, land and water use.”
The two DAC technologies included in the study are based on different ways to adsorb CO2 from the air, which are being developed by a number of startup companies around the world.
One, typically used in larger industrial-scale facilities such as those being piloted by Canadian firm Carbon Engineering, uses a solution of hydroxide to capture CO2. This mixture must then be heated to high temperatures to release the CO2 so it can be stored and the hydroxide reused. The process uses existing technology and is currently thought to have the lower cost of the two alternatives.
The second technology uses amine adsorbents in small, modular reactors such as those being developed by Swiss firm Climeworks. Costs are currently higher, but the potential for savings is thought to be greater, the paper suggests. This is due to the modular design that could be made on an industrial production line, along with lower temperatures needed to release CO2 for storage, meaning waste heat could be used.
Overall, despite “huge uncertainty” around the cost of DAC, the study suggests its use could allow early cuts in global greenhouse gas emissions to be somewhat delayed, “significantly reduc[ing] climate policy costs” to meet stringent temperature limits.
Using DAC means that global emissions in 2030 could remain at higher levels, the study says, with much larger use of negative emissions later in the century. This is shown in the charts, below, for scenarios staying below 1.5C (left panel, shades of blue) and 2C (right, green).
Pathways without DAC are shown in darker shades. For example, the solid dark blue line shows results from the “TIAM” model, with emissions peaking around 2020 and falling rapidly to below zero around 2050.
In contrast, the light blue solid line shows a pathway where DAC allows a more gradual decline, reaching zero in the 2060s and with negative emissions of around 30 billion tonnes per year (Gt/yr) by the 2080s. This is close to today’s annual global emissions of around 40GtCO2/yr.
“The results of both models are surprisingly similar,” says Dr Nico Bauer, a scientist at the Potsdam Institute for Climate Impacts Research (PIK), who was not involved in the study. He tells Carbon Brief: “This increases the credibility about the main conclusions that the DACCS technology can play an important role in a long-term climate change mitigation strategy.”
The use of DAC in some of the modelled pathways delays the need to cut emissions in certain areas. The paper explains: “DACCS allows a reduction in near term mitigation effort in some energy-intensive sectors that are difficult to decarbonise, such as transport and industry.”
Steve Oldham, chief executive of DAC startup Carbon Engineering says he sees this as the key purpose of CO2 removal technologies, which he likens to other “essential infrastructure” such as waste disposal or sewage treatment.
Oldham tells Carbon Brief that while standard approaches to cutting CO2 remain essential for the majority of global emissions, the challenge and cost may prove too great in some sectors. He says:
“DAC and other negative emissions technologies are the right solution once the cost and feasibility becomes too great…I see us as the backstop for challenging abatement.”
Even though DAC may be relatively expensive, the model pathways in today’s study still see it as much cheaper than cutting emissions from these hard-to tackle sectors. This means the models deploy large amounts of DAC, even if its costs are at the high end of current estimates.
It also means the models see pathways to meeting climate goals that include DAC as having lower costs overall (“reduce[d]… by between 60 to more than 90%”). Gambhir tells Carbon Brief:
“Deploying DAC means less of a steep mitigation pathway in the near-term, and lowers policy costs, according to the modelled scenarios we use in this study.”
However, the paper also points to the significant challenges associated with such a large-scale, rapid deployment of DAC, in terms of energy use and the need for raw materials.
The energy needed to run direct air capture machines in 2100 is up to 300 exajoules each year, according to the paper. This is more than half of overall global demand today, from all sources, and despite rising demand this century, it would still be a quarter of expected demand in 2100.
To put it another way, it would be equivalent to the current annual energy demand of China, the US, the EU and Japan combined – or the global supply of energy from coal and gas in 2018.
Gambhir tells Carbon Brief:
“Large-scale deployment of DAC in below-2C scenarios will require a lot of heat and electricity and a major manufacturing effort for production of CO2 sorbent. Although DAC will use less resources such as water and land than other NETs [such as BECCS], a proper full life-cycle assessment needs to be carried out to understand all resource implications.”
There are also questions as to whether this new technology could be rolled out at the speed and scale envisaged, with expansion at up to 30% each year and deployment reaching 30GtCO2/yr towards the end of the century. This is a “huge pace and scale”, Gambhir says, with the rate of deployment being a “key sensitivity” in the study results.
Prof Jennifer Wilcox, professor of chemical engineering at Worcester Polytechnic Institute, who was not involved with the research, says that this rate of scale-up warrants caution. She tells Carbon Brief:
“Is the rate of scale-up even feasible? Typical rules of thumb are increase by an order of magnitude per decade [growth of around 25-30% per year]. [Solar] PV scale-up was higher than this, but mostly due to government incentives…rather than technological advances.”
Reaching 30GtCO2/yr of CO2 capture – a similar scale to current global emissions – would mean building some 30,000 large-scale DAC factories, the paper says. For comparison, there are fewer than 10,000 coal-fired power stations in the world today.
If DAC were to be carried out using small modular systems, then as many as 30m might be needed by 2100, the paper says. It compares this number to the 73m light vehicles that are built each year.
The study argues that expanding DAC at such a rapid rate is comparable to the speed with which newer electricity generation technologies such as nuclear, wind and solar have been deployed.
The modelled rate of DAC growth is “breathtaking” but “not in contradiction with the historical experience”, Bauer says. This rapid scale-up is also far from the only barrier to DAC adoption.
The paper explains: “[P]olicy instruments and financial incentives supporting negative emission technologies are almost absent at the global scale, though essential to make NET deployment attractive.”
Carbon Engineering’s Oldham agrees that there is a need for policy to recognise negative emissions as unique and different from standard mitigation. But he tells Carbon Brief that he remains “very very confident” in his company’s ability to scale up rapidly.
(Today’s study includes consideration of the space available to store CO2 underground, finding this not to be a limiting factor for DAC deployment.)
The paper says that the challenges to scale-up and deployment on a huge scale bring significant risks, if DAC does not deliver as anticipated in the models. Committing to ramping up DAC rather than cutting emissions could mean locking the energy system into fossil fuels, the authors warn.
This could risk breaching the Paris temperature limits, the study explains:
“The risk of assuming that DACCS can be deployed at scale, and finding it to be subsequently unavailable, leads to a global temperature overshoot of up to 0.8C.”
Gambhir says the risks of such an approach are “too high”:
“Inappropriate interpretations [of our findings] would be that DAC is a panacea and that we should ease near-term mitigation efforts because we can use it later in the century.”
Bauer agrees:
“Policymakers should not make the mistake to believe that carbon removals could ever neutralise all future emissions that could be produced from fossil fuels that are still underground. Even under pessimistic assumptions about fossil fuel availability, carbon removal cannot and will not fix the problem. There is simply too much low-cost fossil carbon that we could burn.”
Nonetheless, Prof Massimo Tavoni, one of the paper’s authors and the director of the European Institute on Economics and the Environment (EIEE), tells Carbon Brief that “it is still important to show the potential of DAC – which the models certainly highlight – but also the many challenges of deploying at the scale required”.
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The global carbon cycle poses one final – and underappreciated – challenge to the large-scale use of negative emissions technologies such as DAC: ocean rebound. This is because the amount of CO2 in the world’s oceans and atmosphere is in a dynamic and constantly shifting equilibrium.
This equilibrium means that, at present, oceans absorb a significant proportion of human-caused CO2 emissions each year, reducing the amount staying in the atmosphere. If DAC is used to turn global emissions net-negative, as in today’s study, then that equilibrium will also go into reverse.
As a result, the paper says as much as a fifth of the CO2 removed using DAC or other negative emissions technologies could be offset by the oceans releasing CO2 back into the atmosphere, reducing their supposed efficacy.
Realmonte, G. et al. (2019) An inter-model assessment of the role of direct air capture in deep mitigation pathways, Nature Communications, http://dx.doi.org/10.1038/s41467-019-10842-5
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Direct CO2 capture machines could use ‘a quarter of global energy’ in 2100
Here we compare the operation of existing gas-fed CO2 electrolysers with future integrated electrolysers. We discuss the performance metrics for both conversion processes in-depth to provide a perspective on the comparative energy consumption of each route under different scenarios. We propose to gauge these two electrolyser types using the energy required to electrochemically convert one mole CO2, which can be calculated from Eq. (6). The calculated energy is independent of the current densities, which allows us to compare these two electrolysers despite the levels of current densities achieved in prior literature.
As shown in Fig. 2a, the blue region highlights the energy requirement to produce CO with varied Faradaic efficiencies and cell voltages49. Overlayed within Fig. 2a are the existing state-of-the-art Faradaic efficiencies and current densities for the gas-fed electrolysis (blue circles). The integrated electrolysis (red circles) is relevant to the integrated route described in Fig. 1. For context, Fig. 2b communicates that product Faradaic efficiency has a more profound impact on energy consumption toward target CO than the cell voltages.
As a more advanced reaction, the gas-fed electrolyser outperforms the integrated electrolyser in product selectivity, current densities, and energy efficiency22,43. (see Fig. 2a) The state-of-the-art gas-fed CO2 electrolysers can be operated at >100 mA cm−2 with a cell voltage below 3–3.5 V and a product Faradaic efficiency of 80–90% (e.g., CO), as seen in Supplementary Table 3. When converting these performance metrics into the energy required to convert CO2 into CO, we can estimate the benchmark gas-fed electrolyser to be in the range of 600–800 kJ molCO2-converted−1. Our analysis uses only near-room-temperature flow cells and membrane-electrode assemblies (MEAs) for CO2-to-CO as the model for the sequential route because this technology has a relatively high level of technical readiness49,50,51.
In gas-fed CO2 electroreduction, the dissolved CO2 in water is the main catalytically reactant for the conversion, with CO2 transported from a nearby gas phase52,53. High rates (up to 1 A cm−2) are achieved by applying gas-diffusion electrodes8,9,10, where the gases are transported from the gas channel to the catalyst facing the liquid electrolyte. Therefore, maintaining a stable electrode wettability is challenging for long-term operation21.
In these gas-fed systems, the CO2 utilisation efficiency is usually low (e.g., capped at 50% if producing CO) due to carbonation between CO2 and hydroxide ions (OH-) at the cathode interface18,34,35. In an MEA configuration using an anion-exchange membrane, the (bi)carbonates migrated to the anode are reported to evolve back to CO236,37,38. Such CO2 evolution should occur at the cost of increasing anode overpotentials by negatively affecting the anode reaction environment and the anode catalysts54. When the carbonate requires regeneration into CO2, an energy penalty of at least 254 kJ molCO2-converted−1 is associated with it in the case of 50% CO2 utilisation efficiency. Our analysis then takes this energy penalty into account.
We acknowledge the recent efforts that attempt to remove the energy penalty associated with carbonate formation and low CO2 utilisations, but these have not been demonstrated substantial overall performance metrics as compared to those presented in Fig. 2a. Such strategies use acidic environments and bipolar membranes to introduce protons to regenerate carbonate17,55 or optimize local reaction environments or operating conditions20,56. For simplicity of this analysis, however, our analysis assumes a gas-fed CO2 conversion of 50% with additional steps for product separation and carbonate regeneration processes.
In contrast to the gas-fed system above, reported electroreduction of captured CO2 in monoethanolamine solutions presently has a higher energy requirement at low current densities (Fig. 2a and Supplementary Tables 1–2). The higher energy requirement of the integrated system is a result of the lower CO selectivity than the gas-fed systems. With further research efforts, these metrics are expected to improve.
In these systems, most cell potentials were unreported as it was not a primary part of the analysis. Hence, in order to populate Fig. 2a for the integrated case, we estimated the potentials associated with the anode, membrane, and electrolytes to perform a parameter sweep and evaluate the energy consumption for conversion (see Supplementary Note 1). Taking Lee et al.’s result as an example, the estimated cell voltage is 3 V to achieve 100 mA cm−2 assuming the amine solution has the same ionic conductivity of 1 M KOH aqueous solution (21.5 S m−1 for 1 M KOH solution22,57). The amine aqueous solution has a lower ionic conductivity than inorganic electrolyte (i.e. 3.7 S m−1 for 5 M monoethanolamine solutions with about 0.4 molCO2 molamine−158 as compared to 21.5 S m−1 for 1 M KOH solution57). The ionic conductivity of the capture media can be effectively improved by including inorganic salts, such as K2SO4 and KCl22,58. As a result, the ohmic loss from the capture solvent can be significantly reduced, which is shown in Supplementary Fig. 1.
Further, The halide ions can serve as inhibitors to prevent oxidative degradation of amines59,60, and the alkali cations are effective in promoting CO2 electrochemical conversion22,58,61. Buvik et al.60 also reported that the NaCl and KI salts show negligible impacts on the CO2 capture capacity of the 30 wt% monoethanolamine solution. Nevertheless, further research efforts are needed to investigate the impacts of other inorganic salts on the properties of the capture media and the CO2 absorption performance.
Due to the low CO Faradaic efficiency, the electrolysis of the existing early reports for captured CO2 are at an energy consumption of 800 – 104 kJ molCO2−1, as compared to the 600–800 kJ molCO2−1 for the gas-fed system. From a state-of-the-art perspective, substantial energy reductions in the integrated electrolysis process are needed to make the overall integrated route more energetically favourable. The most straightforward path to reduce the energy load is through an increase in Faradaic efficiency for CO, which requires an understanding of the underlying mechanisms and catalytically active species (e.g., carbamate ions, bicarbonate, or CO2) dominating the conversion process. The outlook at the end of this article provides a detailed discussion of the mechanism for electrochemical CO2 conversion and its challenges. To continue the analysis in Fig. 3, we put aside the performance metrics achieved in existing integrated reports and instead use three performance cases to see the energy comparison versus the sequential route.
Fig. 3: Energy comparison between sequential and integrated routes in different scenarios.Scenario analysis of (a) overall energy consumption, (b) thermal energy and electricity consumption, and (c) energy cost for sequential and integrated routes. In the sequential route, the CO2 electrolyser includes state-of-the-art gas-fed electrolysers that show 50% CO2 utilisation or future scenarios with 100% CO2 utilisation. The optimistic, baseline and pessimistic electrolysis cases for the integrated routes are compared against the sequential route.
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With the conversion processes described for the sequential and integrated routes, we can compare the expected energy requirement for both routes shown in Fig. 1 through a mass and energy balance. A detailed description of the models can be found in Supplementary Note 2, Supplementary Fig. 2, Supplementary Fig. 3 and Supplementary Table 4.
Here Fig. 3 explores the potential energy advantages of the integrated route under optimistic, baseline, and pessimistic performance metric scenarios for the electrolysis processes. Detailed conditions for these scenarios are summarized in Table 1 using the two most critical parameters for the integrated electrolysis process: CO Faradaic efficiency and cell voltage. The sequential route cases assume the gas-fed electrolyser to be operated at 3 V, 90% CO Faradaic efficiency, and 50% single-pass conversion. The 50%-CO2-utilisation case assumes 50% of the reacted CO2 convert to (bi)carbonate, while the 100% case assumes all the reacted CO2 convert to CO molecules. It is important to note that the current density is not considered in the energy analysis, because current density predetermines the size and capital expense of the electrolysers, which is outside the scope of this work.
Table 1 Summary of CO Faradaic efficiency and cell voltages for the integrated electrolyser in different scenariosFull size table
Our baseline condition is based on Lee et al.’s report that the Ag-coated ePTFE electrode can achieve 72% CO Faradaic efficiency at −0.8 V vs. reversible hydrogen electrode in monoethanolamine aqueous solutions. We believe the current densities can be further improved if applying hydrophilic 3D porous flow-through electrodes, as very recently reported by Zhang et al.52 for the application of direct bicarbonate electroreduction. In the optimistic case, we anticipate the integrated electrolyser can perform similarly to the current gas-fed electrolyser. The pessimistic scenario assumes the future integrated electrolyser can only achieve a 40% CO Faradaic efficiency at a relatively large cell potential. All these three electrolysers are assumed to regenerate the capture media to a CO2 loading at 0.3 molCO2 molamine−1. We compared the sequential and integrated routes in terms of total energy, thermal energy and electricity, and energy cost.
In the sequential route, the energy consumption is shown to be dominated by CO2 electrochemical conversion to produce CO, which includes CO2 electrolysis (643 kJ molCO2−1) and (bi)carbonate regeneration (254 kJ molCO2−1). The CO2 capture requires amine regeneration energy (179 kJ molCO2−1s), CO2 compression after capture (17 kJ molCO2−1), and product purification (51 kJ molCO2−1). These are all in terms of the amount of converted CO2. Here the primary energy for the CO2 electrolysis, compression, and product purification (based on pressure-swing adsorption) is electric work, but for (bi)carbonate and amine regeneration it is mainly inputted heat. The gas-fed CO2 electrolyser was assumed to operate at a cell voltage of 3 V and a CO FE of 90%, which has been demonstrated experimentally (Fig. 2a).
When comparing the sequential route to the baseline integrated route, there is no foreseen overall energy advantage between the two routes (Fig. 3). The primary reason is the high energy requirement to convert CO2, which offsets any foreseen energy benefits from process intensification. Considering the higher cost of electricity than heat, the integrated route in the baseline is in fact inferior to the sequential route due to its high electrical energy consumption (see Fig. 3b, c).
In the optimistic case, we assume the electrolysis of the captured CO2 performs the same as the gas-fed electrolysis. In this scenario, the integrated route can save up to 44% of total energy due to a low cell voltage, high CO Faradaic efficiency, and no thermal energy associated with regeneration of amines (179 kJ molCO2−1) and (bi)carbonate (254 kJ molCO2−1), and electricity associated with CO2 compression (17 kJ molCO2−1), and product purification (51 kJ molCO2−1). (Fig. 3b) The integrated route could save 22% energy cost over the sequential route. Such reduction in energy consumption renders the integrated route a more attractive option. Our results suggest most future research emphasis is placed on enhancing the Faradaic efficiency and cell voltages at industrially applicable current densities in order to reduce the energy of the overall process. Without these conditions, the sequential route remains favourable.
In the pessimistic case, if the integrated route has a poor CO Faradaic efficiency (40%) and large cell voltage (5 V), however, the energy to drive integrated conversion is far higher (2412 kJ molCO2−1) than the gas-fed electrolyser, diminishing all the energy benefits from the process intensification. This scenario emphasises the importance of maximizing the two noted performance metrics.
Lastly, we assessed the energy consumption of the sequential route based on future CO2 gas-fed electrolysis with 100% CO2 utilization efficiency, meaning that no CO2 gas will be lost into (bi)carbonate during gas-fed conversion. Very recent reports demonstrated the potential to improve CO2 utilisation efficiency53 by developing catalyst-membrane interface44,54, optimising cell operating conditions (e.g., reducing CO2 flow rates, increasing current densities, and optimising anolyte compositions and ionic strength)46, or supplying protons towards the cathode to regenerate CO2 from the (bi)carbonates, e.g., flowing strong acidic catholyte22,55, applying cation-exchange membranes44 or bipolar membrane54 in a reverse mode. The single-pass conversion rate remains 50% in this optimistic sequential model, meaning that 50% of the inputted CO2 feed converts to CO product and reduces the required pressure-swing absorption separation energy consumption. The total energy of such a sequential route is 864 kJ mol−1CO2 (see Fig. 3). Here the integrated optimistic case then only maintains a maximum overall energy advantage of 26% and energy cost benefit of 11%. We then conclude that if the energy penalty associated with (bi)carbonate regeneration is solved, there would be substantially lower energy gain possible by integrating capture and conversion even in the most optimistic scenario as described in this article.
Overall, our comparison highlights that energy benefits brought by the integrated route strongly depend on the progress in enhancing the energy efficiencies of the CO2 electrolysis process. This trend makes sense because the CO2 electrochemical conversion is the dominant contributor to the overall energy consumption, which is the primary reason preventing straightforward CO2 capture and utilisation at a low cost.
In the analyses above, we assumed that the integrated electrolyser could recover the capture media to a lean loading state where it is directly recycled to the absorber (see Fig. 1b). If the electrolyser is unable to achieve the proposed lean state of 0.3 molCO2 molamine−1, the high CO2 loading (X > 0.3) in the lean amine stream will decrease the CO2 absorption rate in the absorber unit. To maintain the overall CO2 capture and conversion capacity of the process, adjustments to the process in Fig. 1b would then be needed. Here we discuss two possibilities, both of which will incur either additional capital or energy costs for the process.
One possible adjustment to account for lower conversions in the integrated electrolyzer is to increase the size of the absorber unit (Supplementary Fig. 4). A smaller difference between the low and high CO2 loading states will then be present and a larger absorber allows for the same CO2 capture capacity. Previous reports analysing the impacts on absorber size and capture costs of higher lean loading states indicate that an increase in lean loading from 0.3 to >0.4 molCO2 molamine−1 would require 20–38% more capture costs62,63. With the electrolyser unit dominating the energy costs, however, these increased capture costs would be less substantial when considering the complete process. This option is also at a high technology readiness level.
A second option to maintain CO2 capture and conversion capacity would be to add a secondary step after the integrated electrolyzer, which is a smaller version of the stripper and gas-fed electrolyzer unit from the sequential process (Fig. 4a). The energy implications of this option have yet to be explored in literature and will be examined within this section. In essence, this analysis examines the role of the single-pass conversion of the integrated electrolyser.
Fig. 4: Effect of the single-pass conversion of the integrated electrolyser on the overall energy efficiency.a A schematic illustration of the integrated route where the electrolyser is unable to recover the capture media to the lean loading state. The separation and electrolysis process is symbolic process highlighted with a dashed box to regenerate the capture medium to the lean loading state. X represents the CO2 loading in the capture medium, with a unit of mol CO2 per mol amine molecule. b The energy comparison of the integrated route based on baseline (green solid line), pessimistic (grey), and optimistic (red) integrated electrolyser as a function of the electrolyser single-pass conversion. The grey dashed line represents the energy consumption of the sequential route based on state-of-the-art gas-fed CO2 electrolysers. The blue region means that the integrated route is more energy-efficient than the sequential route, while the orange region indicates vice-versa.
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In the model, we included a symbolic process (including amine regeneration, gas-fed CO2 electrolysis, product separation, and (bi)carbonate regeneration, shown in Fig. 4a) to regenerate the capture medium to the lean loading state and convert the rest of captured CO2 to CO. In this case, the captured CO2 in the effluent stream of the integrated electrolyser needs to be recovered to pure CO2 gas from the regeneration unit and then fed into the gas-fed electrolyser for conversion.
We find that the role of the single-pass conversion efficiency is highly dependent on the performance of the integrated electrolyser. When the electrolyser operates at the baseline conditions, the capability of the integrated electrolyser to regenerate the capture medium becomes insignificant to the energy advantage of the integrated route. In contrast, if the electrolyser operates under either optimistic or pessimistic conditions, the single-pass conversion is essential for the overall energy consumption of the integrated route. The overall energy will benefit from an efficient electrolyser with high single-pass conversion. In contrast, a poorly performing electrolyser causes a significant overall energy penalty by increasing the single-pass conversion. This observation arises from the dominant role of the electrolysis in the overall energy of the capture and conversion process.
Here we briefly highlight how varied performance metrics of Faradaic efficiency and cell voltage impact the overall energy requirements for the integrated route. This analysis assumes the electrolyser can recover the capture medium to the lean loading state. Such an analysis provides a deeper context than the described optimistic, baseline and pessimistic scenarios above. Supplementary Fig. 5a shows that the energy advantage from the integrated route plummets linearly with the energy consumption of the integrated electrolysis. This trend highlights the core role of the electrolyser in determining the overall energy efficiency. The breakeven point for the integrated route is at the energy consumption of 1143 kJ mol−1 for the integrated electrolyser (see Supplementary Fig. 5a). The value of the breakeven point should vary with the energy efficiency of the gas-fed electrolyser and the operating conditions, such as the single-pass conversion of the gas-fed electrolyser, energies to regenerate (bi)carbonate, amines, and to separate CO2 and product. (see Supplementary Fig. 6).
The role of CO Faradaic efficiency and cell voltages were examined individually in influencing the energy gain from an integrated route. Supplementary Fig. 5b shows the breakeven point for CO Faradaic efficiency with varied cell voltages: the breakeven Faradaic efficiency is 51% at 3 V, 67% at 4 V, and 84% at 5 V. The impact from the Faradaic efficiency is more significant than from the cell voltages, as shown in Supplementary Fig. 5b, c. The energy advantage from the integrated route decreases linearly with an increase of cell voltages and diminishes at 4.1 V when the Faradaic efficiency is 70%. Similarly, the breakeven cell voltages increase if the CO Faradaic efficiency could be further enhanced. Our analysis result indicates that the integrated CO2 conversion as reported by Lee et al.22, as shown in Fig. 2a, has the potential to achieve a more energy-efficient integrated route. Our model did not consider the cost associated with the current densities, which predetermine the capital cost of the electrolysers. Like the gas-fed CO2 electrolysers, we believe operating at more than 200 mA cm−2 with a high product selectivity is a prerequisite for an industrially relevant integrated system64.
Our results identified that the electrochemical CO2 conversion is the primary energy contributor for both sequential and integrated CO2 capture and electrochemical conversion process. The reported energy efficiency of the integrated electrolyser is generally lower than the gas-fed CO2 electrolysis. Such limitation originates from (1) the low surface coverage of reactants at the catalyst surface at industrially relevant rates and (2) the limited number of active sites the medium can reach over the hydrophobic gas-diffusion electrodes22. Therefore, the following research questions should be answered to advance the integrated electrolysers.
It has been reported recently that the catalysts for gas-fed CO2 electroreduction are selective to reduce CO2 captured by amine-based capture media (RNH2)22,43,44,58. In the CO2-rich amines, the zwitterions ions including RNHCO2- and RNH3+ are the major CO2 species in the case of 30 wt% monoethanolamine aqueous solution when the CO2 loading is below 0.4–0.6 mol CO2 per mol amine32,33. Further increase of CO2 loadings could promote carbamate hydrolysis to produce (bi)carbonates. Therefore, the CO2 associated species should include carbamate ions, (bi)carbonate ions, and minor free dissolved CO2, all may contribute to the CO2 conversion.
However, there are still debates on the primary catalytically active species for the conversion in the amine (particularly for monoethanolamine) solutions. (see Fig. 5a) An early report by Chen et al.43 claims that the free CO2 dissolved in water can be the primary active species for the conversion, with nearly 100% Faradaic efficiency of hydrogen evolution regardless of the carbamate concentrations. In contrast, recent reports argued the possibility to reduce the carbamate ions as the main active reagent22,61. The claimed mechanisms for the direct carbamate reduction are different from the reduction mechanisms in CO2 electrolysis52 and direct bicarbonate reduction65,66. Interestingly, these recent reports also show an improvement of CO2 conversion selectivity by increasing operating temperatures22,45, which help release free CO2. Therefore, the primary catalytically reactant for CO2 conversion still remains a mystery but is paramount for the rational development of an efficient electrochemical system for integration.
Fig. 5: Speciation of amine-based capture media in sequential and integrated routes and their impacts on CO2 electrochemical conversion.a Proposed integrated CO2 absorption and electrolysis routes in amine-based solvents. b Schematic illustration of the role of alkali cations which promote interfacial charge transfer from the catalyst surface to the carbamate ions.
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In the CO2 capture step based on 30 wt% monoethanolamine solutions, the CO2 loadings are usually at 0.3–0.5 mole CO2 per mole amine, meaning that the concentrations of the (bi)carbonate and free CO2 are negligible. If the free CO2 is the primary active reagent, regenerating and concentrating free CO2 from carbamate and bicarbonate should be the key step to improving the integrated CO2 conversion. Meanwhile, this strategy could adversely impact CO2 capture. If the carbamate ions are the primary catalytically active species, they could be repelled by the negatively charged cathode surface, which might limit the coverage of reactants, especially at high overpotentials. Additionally, the active species need to diffuse to the negatively charged electrode through a thick hydrodynamic boundary layer usually >40 µm, if the integrated reactor configuration is similar to a CO2-fed aqueous H-cell electrolyser (see Supplementary Fig. 7 for a comparison of aqueous versus gas-fed mass transport in CO2 electrolysis)67,68. Efforts to improve integrated conversion at elevated current densities should then take such transport into consideration when designing such systems.
The results of our energy analysis indicate that the capture media for the integrated route could be designed to favour CO2 conversion at a reasonable cost on CO2 absorption. Therefore, an interdisciplinary collaboration between CO2 capture and electrolysis is highly important to advance the integrated route.
Complex homogenous equilibrium reactions often take place in the CO2-capture medium system. In the sequential route, heating is required to drive the reactions towards the recovery of capture media and CO2. Whereas the integrated route, as shown in Fig. 1b, uses electrochemical reactions to regenerate the capture medium via reduction of absorbed CO2 and chemical-induced equilibria shift to the original states of the capture medium (see an example in Fig. 5a). Therefore, understanding the reaction equilibria under CO2 electroreduction conditions is vital to the identification of chemical pathways to recover capture media inside the integrated electrolyser.
Similar to the gas-fed CO2 electroreduction, hydroxide ions should also be produced at the catalyst surface as a by-product of water reduction and increase the pH locally around the electrode69. A prior report70 has shown that the addition of a strong base (e.g., sodium hydroxide) to the CO2-amine system could result in the formation of free amines and carbonate at the end equivalent points. As such, we could anticipate the formation of carbonate ions close to the electrode surface from the reactions between the hydroxide ions and unreacted CO2 species. These carbonate ions could either reverse back to carbamate, free CO2, or bicarbonate by reacting with the protons from the membrane70,71 or stay as carbonate if additional cations are introduced into the cathode channel. The latter situation may cause operational issues for the integrated route such as inefficient CO2 conversion, alteration of solvent chemistry, and potential carbonate salt precipitation from the solvent. Hence a dedicated control and balance of ions within the electrolyser also become critical in achieving an efficient amine recovery when using electrochemical CO2 reduction as a regeneration step.
Including alkali cations such as potassium ions (K+) or caesium ions (Cs+) in the amine capture medium has shown its potential to improve CO2 conversion efficiency22. As proposed by Lee et al.22, the carbamate reduction can occur through an interfacial charge transfer mechanism, where the alkali cations can be packed (instead of protonated amines) at the electrode surface and facilitate charge transfer from the electrode surface to the carbamate ions, as illustrated in Fig. 5b. Meanwhile, an increasing number of reports also highlighted the essential role of alkali cations in activating gas-fed CO2 electrochemical conversion36,72,73. Hence, the cations could synergistically minimise the surface coverage of protonated amines and activate CO2 electroreduction. Nevertheless, the electrochemical reduction of the captured CO2 is still low in CO Faradaic efficiency at >200 mA cm−2, which could also be partially related to the limited electrochemical area due to the use of planar metal electrodes43 or the hydrophobic nature of the gas-diffusion electrodes that are frequently used for gas-fed electrolysis22.
We anticipate a significant improvement in CO2 conversion rates (>200 mA cm−2) by implementing new electrode structures such as hydrophilic 3D structured flow-through electrodes and optimised capture media74,75. The required diffusion distances of active species to achieve industrially applicable current densities are highly dependent on the concentrations and diffusion coefficients of the active species76. Therefore, understanding the primary active species and tailoring the local reaction environment could be effective in enhancing the CO2 conversion rate in the integrated electrolysers.
Further, the desired wetting condition for the CO2 conversion should have maximized solid-liquid interfaces with a minimal contact area of the gas bubble with the electrode surface. This means that the electrode surface should be hydrophilic, which is different from the desired wettability of gas-diffusion electrodes. Using metallic porous flow through electrodes is expected to achieve a high rate of CO2 conversion by maximizing the electrochemical surface area, reducing the thickness of the boundary layer, and accelerating the detachment of gas products. On the other hand, more experimental and theoretical efforts are also essential to understand the potential catalyst surface restructuring, local reaction environment (e.g., pH and local concentration of amine species), and multiphase and ion transports in the cells, which have been demonstrated important for the stability and efficiency of the gas-fed CO2 electrolysis77,78,79.
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