Electrochemical carbon dioxide capture to close the carbon cycle

Abstract

Electrochemical CO₂ capture technologies are gaining attention due to their flexibility, their ability to address decentralized emissions (e.g., ocean and atmosphere) and their fit in an electrified industry. In the present work, recent progress made in electrochemical CO₂ capture is reviewed. The majority of these methods rely on the concept of “pH-swing” and the effect it has on the CO₂ hydration/dehydration equilibrium. Through a pH-swing, CO₂ can be captured and recovered by shifting the pH of a working fluid between acidic and basic pH. Such swing can be applied electrochemically through electrolysis, bipolar membrane electrodialysis, reversible redox reactions and capacitive deionization. In this review, we summarize main parameters governing these electrochemical pH-swing processes and put the concept in the framework of available worldwide capture technologies. We analyse the energy efficiency and consumption of such systems, and provide recommendations for further improvements. Although electrochemical CO₂ capture technologies are rather costly compared to the amine based capture, they can be particularly interesting if more affordable renewable electricity and materials (e.g., electrode and membranes) become widely available. Furthermore, electrochemical methods have the ability to (directly) convert the captured CO₂ to value added chemicals and fuels, and hence prepare for a fully electrified circular carbon economy.

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R. Sharifian

Ir. Rezvan Sharifian is PhD candidate in the Vermaas group, Chemical Engineering department at Delft University of technology, and Wetsus, Centre of Excellence for Sustainable Water Technology, in the Netherlands. Her research focuses on new technologies for electrochemical CO2 capture via pH swing. As a recipient of the full TU Delft Energy Scholarship, she graduated her MSc with cum laude in petroleum engineering in the applied earth sciences faculty, in 2016.

R. M. Wagterveld

Dr Ir. R. Martijn Wagterveld is scientific project manager and theme coordinator Sensoring at Wetsus, European Centre of Excellence for Sustainable Water Technology, Netherlands. In this position, Dr Wagterveld is responsible for novel project definitions and guidance of academic research by PhD and postdoctoral researchers. His expertise is on water technology, primarily in the field of calcium carbonate crystallization, ion-exchange membranes, salinity gradient energy, separation technology, water quality sensors and data fusion.

I. A. Digdaya

Dr Ibadillah A. Digdaya received his BS in Engineering Physics from Bandung Institute of Technology in 2009 and completed a PhD from the Department of Chemical Engineering at Delft University of Technology with Professor Wilson A. Smith in 2018. He then joined the group of Dr Chengxiang Xiang as a research scientist in Joint Center for Artificial Synthesis at California Institute of Technology. Currently, his research interest includes design, fabrication, optimization of devices for CO2 capture and conversion using electrochemical and thermochemical processes.

C. Xiang

Dr Chengxiang (“CX”) Xiang is a Principal Investigator and Staff Scientist in Liquid Sunlight Alliance (LiSA) at California Institute of Technology. Dr Xiang leads a team of chemists, engineers and material scientists working on photoelectrochemical hydrogen generation, catalytic CO2 conversion and electrochemical CO2 capture from dilute sources. Dr Xiang received his PhD in Physics from University of California, Irvine in 2009. Dr Xiang research interests include design and fabrication of high efficiency solar-fuel prototypes, opto-electronic-catalytic modeling of advanced micro/nanostructured photoelectrochemical systems and multi-ion transport modeling in solution and polymer electrolytes.

D. A. Vermaas

David is an assistant professor at Delft University of Technology. He obtained his PhD in Chemical Engineering at University of Twente and Wetsus (2014), and started a research group on electrochemical flow systems at Delft in 2016. David's group currently hosts 15 researchers focusing on mass transport and membrane applications in electrochemical conversion, and collaborating in the e-refinery platform with industry and academics to innovate renewable energy technologies. Current projects include (bipolar) membranes for electrolysis, electrochemical CO2 capture, H2–Br2 redox flow batteries, and local transport near gas bubbles and membrane electrode assemblies.

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Broader context

The necessity of taking actions to achieve a net zero CO₂ emission has brought up the question “how to achieve a circular carbon economy?”. To close the carbon cycle, sustainable CO₂ capture and utilization are indispensable. Current technologies for CO₂ capture rely dominantly on absorption of CO₂ from flue gas, regenerated via energy-intensive temperature swings, which imply a huge energy consumption when expanding capture towards a net zero CO₂ emission. When also realizing that approximately 40% of the CO₂ emission is decentralized, which may even increase when power plants and industry are transitioning to renewable energy, we are facing a massive challenge for closing the carbon cycle. In that framework, electrochemical technologies for CO₂ capture, from the atmosphere, ocean, and flue gas, can play an important role. The installed renewable electricity generation capacity is increasing each year, with more than 2300 GW in 2018. However, currently, most electrochemical methods are at early stages of development and are still energy intensive. In this contribution, we evaluate each of these technologies while focusing on the pH-swing approach. We establish the current state of the art, describe the major technical barriers and R&D needs, and discuss opportunities for improving electrochemical CO₂ capture.

1 Introduction

To honour the Paris Agreement, a global net zero greenhouse gasses (GHG) emission needs to be realized by the end of this century.^1–3 Among all GHG's, CO₂(g) is accepted to be the main anthropogenic control knob on the climate and has been the focus of many studies.^4,5 To achieve a net zero emission,⁶ reducing the total CO₂ emission obtains most impact in the short term.⁷ The largest CO₂ emission is due to combustion of fossil fuels and thus can be reduced through fossil fuels phase out. However, although ultimately fossil fuels phase out might be possible for the power industry and transportation sector (through substitution with renewable energy), many sectors (e.g., steel/cement production, intercontinental air transport or non electrical trains) do not have a suitable alternative for carbon based fuels. In addition, due to the current agriculture techniques, the net global deforestation and the growing demand for carbon as a resource, CO₂ will be emitted due to many anthropogenic activities. Therefore, to achieve a net zero GHG's emission, in addition to emission reduction, CO₂ capture and utilization is required in the long term. Also, as the long term CO₂ emitters include decentralized emitters, direct capture from CO₂ sinks (e.g., atmosphere and ocean, see Fig. 1 is necessary for effectively addressing the anthropogenic CO₂ emissions.^8,9

Fig. 1 Schematic of the carbon cycle. The total carbon reserve is shown in gigatonnes of carbon (GtC). The global GHG emissions in CO₂(eq) by economic sectors: 25% electricity and heat production, 24% agriculture, forestry and land use, 6% buildings, 14% transportation, 21% industry and 10% other energy usages. Data extracted from ref. 10–12.

The main challenge that currently available CO₂ capture methods face is a high energy consumption.^13–15 A few mature, non-electrochemical CO₂ capture technologies e.g., absorption, adsorption, membrane separation and cryogenic capture are already available in industrial scales,^16–25 Section 4. These methods often depend on the availability of thermal energy, which makes them less well suited for CO₂ capture from dilute sources e.g., air and ocean. Furthermore, the cost of direct air capture via traditional technologies has been estimated in the literature from $100 to $1000 per tonne of captured CO₂.^3,26–28

Electrochemical processes have the potential to be rather energy efficient as they can target molecules directly (instead of the medium surrounding them).^29,30 Using electrochemistry for CO₂ capture dates back to the late 1960s, when molten carbonates as electrolyte were used for CO₂ control in a manned spacecraft.^31–34 Nowadays, electrochemical CO₂ capture methods can be applied to all CO₂ containing streams with any concentration. Direct capture from air,³⁵ ocean^8,36 and flue gas^37–39 have been reported. Such capture units can be retrofitted as plug-and-play processes, allow small footprints and are geometrically flexible.^39,40 They do not require external sources of heat or high pressures/vacuum for operation, nor degradation of sorbent material is expected.⁴⁰ Although heat integration can be beneficial for power plants in reducing the need for energy, other industries (e.g., food and water sector) cannot integrate high heat, rising the need for technologies that can perform under low heat conditions from centralized emitters as well.¹⁴ In electrochemical systems, the electric potential gradient is the main driving force which can be controlled precisely to drive chemical reactions isothermally.^29,30,41 A number of bench-scale capture demonstrations via electrochemical pH-swing have obtained CO₂ capture and release at the promising value of ∼100 kJ mol⁻¹ CO₂.^42,43

The added advantage of the electrochemical methods is their ability to integrate CO₂ capture and utilization⁴⁴ (Section 5). An example is the modular, sequential CO₂ capture and conversion system that uses the pH-swing concept to produce CO electrochemically.^45,46 In addition, reactive CO₂ capture, in which the CO₂ capturing medium pre-concentrates the dilute feed and produces favorable local micro-environments, has become an emerging field for integrated CO₂ capture and conversion using electrochemical techniques.^47,48

A wide variety of efforts to electrochemically capture CO₂ has been demonstrated in the past 50 years, with an intensification of research activities in the last two decades. In this work we give an overview of such advances. The center of our scope are the electrochemical methods that use a pH-swing concept, as shown in Fig. 2a and b. The swing refers to (electrochemically) shifting the pH of a working fluid (continuously) between basic and acidic pH to influence the CO₂ equilibrium to capture and recover CO₂.

Fig. 2 (a) Effect of pH on the CO₂ equilibrium (for a closed system at temperature of 25 °C and salinity of 35 ppt). The solution is buffered around two pK_a values. (b) Schematic of the pH-swing concept.

The pH-swing allows absorption and desorption at ambient temperature and does not require use of any special chemicals. Widely available, inexpensive, non toxic/non corrosive/non volatile salt solutions, such as NaCl, KCl, KHCO₃ or even seawater can be used for the process. The pH-swing can be applied electrochemically using electrolysis, bipolar membrane electrodialysis (BPMED), capacitive deionization and reversible PCET agents as demonstrated in Fig. 3. In this work, after providing the required theoretical background in Section 2, each pH-swing route is explained and the research of this route is comprehensive reviewed in Section 3. In addition to a pH-swing approach, other electrochemical technologies involve using redox-active carriers⁴⁹ (Section 3.3), molten carbonate cells and hybrid methods (Section 3.4) such as electrochemically enhanced absorption/adsorption.^29,30 To enable comparison, we also briefly describe the main conventional capture methods, their drawbacks and advantages, and opportunities to improve these technologies with electrochemical methods, in Section 4. Finally, CO₂ utilization and storage is discussed briefly in section to close the carbon cycle (Section 5).

Fig. 3 Summary of electrochemical CO₂ capture methods.

2 Theory: pH-swing concept and involved reactions

The concept of CO₂ capture via pH-swing leverages the responsiveness of the thermodynamic equilibrium of CO₂ to pH changes. A pH-swing is also used for other resource recovery applications than CO₂ capture, but the method is not always explicitly referred to as “pH-swing”. For instance, pH-swing is employed to recover ammonia electrochemically from urine^50,51 and to remove phosphate from waste water streams.⁵²

2.1 Carbonate equilibrium

The carbonate equilibrium can be described as open or closed based on the contact with an overlying gas and is explained in detail in ref. 53–56. In an open system, the total concentration of the dissolved inorganic carbon varies by changing in the pH; acidification results in CO₂(g) out-gassing, while basification lead to more CO₂(g) absorption, increasing the DIC. In a closed system (e.g., inside of an electrochemical cell), the total DIC remains constant regardless of any pH changes. If so, the dominant carbonic species alter by changing the pH as demonstrated in Fig. 2a. The equilibrium can be shown through reactions (1)–(4):

CO₂(g) ⇌ CO₂(aq) (1)

Reaction (1) is dependent on the fugacity of CO₂(g) and often described through Henry's law (see Section 2.3). It is often assumed to be really fast (i.e., 10¹⁰ s⁻¹ in both directions) in a well-mixed scenario (i.e., only mass-transport limited).^55,57,58 For experimental values of k₁, k₋₁, k₂, k₋₂ see ref. 57, 59 and 60. As the pH increases, reaction (3) pushes the equilibrium towards HCO₃⁻, and it predominates reaction (2) above pH 8.5.⁵⁹ See Section 2.2 for more discussion on the kinetics of the reactions. The total dissolved inorganic carbon, DIC, can be described as the summation of the concentration of all present carbonic species:

DIC = [H₂CO₃*] + [HCO₃⁻] + [CO₃²⁻] (5)

where H₂CO₃* = H₂CO₃ + CO₂(aq). In multi-ionic systems, additional carbonate species or complexes (e.g., NaCO₃⁻ and NaHCO₃(aq)) are present in the definition of DIC, as well.^61,62

2.2 Reactions kinetics and how to improve it

To capture and release CO₂(g) through a pH-swing, both thermodynamics and kinetics play a role.^57,59 Assuming thermodynamic equilibrium, the equilibrium ratio of changes by a factor of more than 100 over when using a small pH-swing of only 2 pH-units, see Fig. 2a. However, although such a narrow pH-swing is thermodynamically favourable and enables lower energy consumption, it suffers from slow kinetics. At near neutral pH, the CO₂ hydration rate is ∼2.9 × 10⁻² s⁻¹ and the (HCO₃⁻) dehydration rate is ∼2 × 10⁻⁴ s⁻¹.^42,60 These reaction rates are a function of e.g. temperature and ionic strength of the system.^63–65 To overcome the slow kinetics, (1) a wider range of pH-swing or (2) a reaction catalyst need to be applied. As for a wider pH-swing range, the base promoted hydration rate of the CO₂/HCO₃⁻ system is reported to be ∼6 × 10³ M⁻¹ s⁻¹ and the acid-promoted dehydration rate is increased to ∼4.1 × 10⁴ M⁻¹ s⁻¹.^42,59,60,65 Alternatively, the use of a catalyst such as the carbonic anhydrase (CA) enzyme⁶⁶ is suggested for improving the kinetics.^42,67,68 CA is reported to enhance both the CO₂ hydration and dehydration reaction kinetics; hydration rate constant ∼10⁵ s⁻¹ and dehydration rate constant ∼10⁶ s⁻¹ near a neutral pH of 7 are reported.⁶⁶ However, although initially effective, such enzyme is not stable, and is prone to enzyme loss, deactivation, or degradation. Investigation on improved catalysts is recommended, but until such catalysts are developed, to effectively capture and recover CO₂, a wider range of pH-swing (pH < pK_a1 and pK_a2 < pH) is needed for capture application.

2.3 Inorganic carbon solubility

In equilibrium, the concentration of free CO₂ in water (i.e., [CO₂(aq)]) is proportional to its partial pressure in the gas phase; according to the Henrys law, [CO₂(aq)] = K₀ × f_CO2(g) where K₀ (in mol L⁻¹ atm⁻¹) is the solubility coefficient of CO₂(g) and f_CO2(g) stands for the gas fugacity (in atm), which is close to the partial pressure of CO₂(g) (within 1%). Fig. 4a shows the solubility of CO₂ as a function of temperature and CO₂ partial pressure. At elevated alkalinity, the total solubility of CO₂(g), and hence DIC (in eqn (5)) increases due to reaction (3) route. Fig. 4b shows the solubility of DIC as a function of the water pH and the partial pressure of CO₂. See the ESI† for the used equations and references.

Fig. 4 (a) CO₂ solubility in water as a function of the temperature and CO₂ partial pressure. (b) Logarithmic solubility of dissolved inorganic carbon (DIC) in water at 25 °C as a function of the water pH and CO₂ partial pressure.

2.4 Inputs and metrics

Fig. 5 demonstrates input parameters and the metrics of electrochemical CO₂ capture based on a pH-swing. The feed, cell, process and kinetics can be leveraged for the product/output. The main challenge such capture technologies are facing is their (estimated) high Capex, resulted from a high energy consumption and the immaturity of the technology.³ Data on Capex of electrochemical methods is still scarce, making the energy consumption (at high current density and capture efficiency) the most practical metric of comparison. Thankfully, despite the currently high energy consumption, optimization of the process and cell parameters can significantly decrease the energy loss involved in such processes as explained in Sections 3.2.1 and 3.5.

Fig. 5 Input and metrics of electrochemical pH-swing based CO₂ capture and the overall process concept.

3 Electrochemical pH-swing concepts

Electrochemically induced pH-swings for CO₂ capture have been demonstrated through (membrane) electrolysis, bipolar membrane electrodialysis, reversible redox couples, capacitive deionization and hybrid processes that combine two or more methods as shown previously in Fig. 3. In this section we introduce each method separately and compare them in terms of feasibility, energy consumption, energy efficiency and technology readiness level (TRL).

3.1 Electrolysis

Electrolysis can enable the pH-swing in the vicinity of (two) electrodes as shown in Fig. 6. (Membrane) electrolysis for CO₂ capture is used for alkali absorbent (re)generation^38,69–73 or simultaneous H₂ production.^36,74–76 H₂ production can (partially) offset the cost of CO₂ capture and is possible when water electrolysis takes place. In the earliest work done by Stucki et al., CO₂ is absorbed from a flue gas in an KOH absorbent, the resulted (bi)carbonate solution is fed into the electrolyser (for alkaline regeneration), where CO₂ is recovered and H₂ is produced via the following reaction:

2H₂O + K₂CO₃(aq) → 2KOH(aq) + CO₂(g) + H₂(g) + 0.5O₂(g) (6)

The KOH can be used as absorbent for capturing CO₂, turning into K₂CO₃(aq), which can be fed again to the electrolysis system.

Fig. 6 Schematic of water electrolysis. Depending on the catholyte/anolyte (air, water, etc.), applied voltage and the pH of the medium, different half reactions take place.

One downside in such systems is that the cell voltage has, in addition to 1.23 V for water splitting, a contribution from the pH difference between the anolyte and the catholyte, represented by Nernstian potential of Φ = 0.059ΔpH (in volts) at 25 °C, as shown in the Pourbaix diagram (Fig. 7).⁷⁴ When using an extreme pH gradient (pH 0 at anode, pH 14 at cathode), this would imply a 2.06 V for balancing the free enthalpy of the reaction.

Fig. 7 Pourbaix diagram of water electrolysis.⁷⁷

Using the pH-swing in electrolysis, the CO₂ can be either released as (purified) gas (e.g., viareaction (6)), or as (bi)carbonate products in the work of Rau and Park et al.^70,71,75,78 The latter has been demonstrated for Ca(HCO₃)₂(aq) (Fig. 8a) and^71,75 solid carbonate mineral as shown in Fig. 8b.⁷⁰ Natural (mined and crushed) carbonate minerals are used to provide Ca²⁺ in Fig. 8a. Through reaction (7), CO₂ can be removed from an overlying gas mixture (e.g., air or flue gas). The produced Ca(HCO₃)₂(aq) can be diluted and stored in the ocean, water reservoirs or underground:

2H₂O + CaCO₃(s) + CO₂(g) → Ca(HCO₃)₂(aq) + H₂(g) + 0.5O₂(g) (7)

Capture of 1 mole of CO₂(g) for each mole of CaCO₃viareaction (7) is possible, while 22 tonnes of CO₂ can be captured per tonne H₂ generated (assuming a 1 : 1 molar ratio).⁷¹ This implies that, depending on CO₂ emission involved in the operation (e.g., for supplying electricity and for limestone/carbonate mining, crushing and transport), the system has the potential of producing carbon-negative hydrogen.⁷⁹

Fig. 8 (a) H⁺ ions at the anode dissolve CaCO₃(s), the resulted Ca²⁺ ions move towards the cathode, forming Ca(HCO₃)₂(aq). Reprinted (adapted) with permission from ref. 71; Copyright (2008) American Chemical Society. (b) NaCl electrolysis, where NaOH is used as the CO₂(g) absorbent and CaCO₃(s) is the final capture product, figure from ref. 70. Both approaches use the concept of placing the anode inside of a porous container.

The process efficiency in Fig. 8a can be enhanced e.g., by:

• Improving the mass transfer between the overlying gas and the absorbent (to improve CO₂(g) dissolution).

• Introducing an ion selective membrane between the electrodes (to avoid unwanted secondary reactions).

Alternative to water electrolysis, salt electrolysis, such as NaCl⁶⁹ and KCl³⁸ can also be used for CO₂ capture. If so, only an alkaline pH (at the cathode) is enabled, because at the anode e.g., the reaction 2Cl⁻ → Cl₂(g) + 2e⁻ takes place instead of the oxygen evolution reaction (OER).⁷⁰ The produced alkalinity at the cathode can then be utilized to sequestrate CO₂ in the form of carbonate salts. When the capture product is CaCO₃(s), no CO₂(g) desorption step is required. Furthermore, CaCO₃ is easier to transport and store compared to CO₂(g). NaCl electrolysis for alkaline absorbent (re)generation follows reaction (8):⁶⁹

2H₂O + 2NaCl(aq) → 2NaOH(aq) + Cl₂(g) + H₂(g) (8)

Using salt electrolysis for CO₂ capture is especially interesting due to salt availability in the seawater, leveraging possible substitution of the electrolyte with abundant seawater.⁶⁹ The produced (toxic) Cl₂(g) at the anode can be treated by the produced NaOH solution using a scrubber, or can alternatively be utilized as a feedstock for the synthesis of HCl, Cl containing polymers and bleaching agents.⁶⁹ Alternatively, using water electrolysis and by inserting two ion exchange membranes (IEMs) between the electrodes (Fig. 9), produces H₂ simultaneous acidifies seawater to recover CO₂(g).^36,80,81 The use of two IEMs avoids the production of Cl₂(g) and electrode contamination.

Fig. 9 CO₂ capture and H₂ production via sea water acidification and water electrolysis. In the middle compartment, cation exchange resins are used. Reprinted (adapted) with permission from ref. 81; Copyright (2014) American Chemical Society.

When applying electrolysis using a reversible redox reaction – thus no net gas production – saves energy. For example, the production of O₂(g) (or Cl₂(g)) can be avoided by recirculating the produced H₂(g) from the cathode to the anode compartment.^72,73 Alternatively, H₂ production can be avoided if O₂ reduction takes place on the cathode, instead of the H₂O reduction.⁸² At the cathode, depending on availability of O₂ or H₂, two possible reactions may exist for OH⁻ production:

2H₂O + O₂ + 4e⁻ → 4OH⁻ (E⁰ = 0.401 V vs. SHE) (9)

4H₂O + 4e⁻ → 2H₂(g) + 4OH⁻ (E⁰ = −0.83 V vs. SHE) (10)

When sufficient O₂ is available at the cathode, or sufficient H₂ at the anode, the H⁺/OH⁻ generation can take place in a practical cell voltage range of 1.3 V to 2.2 V through reaction (9). The supply of sufficient gas to the opposing electrode can be assisted by using gas diffusion electrodes.⁸³ Without recirculating the gaseous products, the cell voltage is typically >2.2 V.⁸²

From a thermodynamic point of view, the use of (net) water electrolysis requires a significant thermodynamic minimum energy (1.23 V for water splitting), in addition to the potential required for generating a pH difference (see Fig. 7).^77,84 Considering the necessity of a wide pH-swing to enhance the carbon equilibrium kinetics, and using pH 0 and 14 to favour electrolyte conductivity, a minimum of 2.06 V is required, corresponding to 199 kJ mol⁻¹ CO₂. Furthermore, in almost all demonstrated devices/systems, the rate of CO₂ capture is locked with the rate of H₂ or O₂ generation, which sometimes may not be desirable and present additional challenges for multi-stack development of the system. These drawbacks can be addressed using bipolar membrane electrodialysis as explained in the following section.

3.2 Bipolar membrane electrodialysis (BPMED)

A bipolar membrane (BPM) consists of an anion (AEL) and a cation (CEL) exchange layers, laminated together. When a sufficient electric field is applied, the BPM dissociates water into OH⁻ and H⁺, producing a controllable ΔpH over the membrane as shown in Fig. 10a and b.^85–90 Using a bipolar membrane, the thermodynamic minimum voltage required for this water dissociation is 0.829 volts for a produced ΔpH = 14. That is 2.5 times lower than that of water electrolysis at the same ΔpH = 14 (1.23 + 0.829 at minimum), as no gas evolution takes place using a bipolar membrane.⁸⁵ The thermodynamic voltages over the BPM are even lower for smaller ΔpH over the membrane.^84,88 The feasibility of using bipolar membrane electrodialysis for pH-swing based CO₂ capture is shown in early studies in 1995;⁹¹ alkaline KOH and acidic H₂SO₄ were regenerated in a two compartment BPMED cell, containing a BPM and a cation exchange membrane. After CO₂ from air is captured in KOH absorbent, it can be recovered through acidification. The produced K₂SO₄ is treated in the BPMED cell to regenerate the desired acid and base again. CO₂ capture via BPMED has been further explored by others, and these works are addressed in more details in next sections.

Fig. 10 (a) BPMED schematic. (b) BPMED for CO₂ recovery in ref. 92. (ES) electrode solution = KOH, (AS) acid solution of KH₂PO₄ + H₃PO₄, (BS) base solution of six different mixtures of KHCO₃, K₂CO₃ and KOH. Figure adapted from ref. 92.

3.2.1 Energy consumption and capture efficiency. In BPMED, a trade off between the energy consumption and the process rate, determined by the applied current density, exists. Operating at very low current densities is not effective due to the higher salt ion leakage through the BPM and hence the lower water dissociation rate.⁹² On the other hand, at high current densities, the ohmic voltage losses increase, leading to an undesirable higher energy consumption (see ESI†). As an example, Fig. 11a shows the minimum energy consumption for CO₂(g) recovery from 0.125 M K₂CO₃-rich solution via BPMED at a current density around 10 mA cm⁻².³ However, although the energy consumption is the lowest at that current density, production rate favours higher current densities as shown in Fig. 11c. The experimentally measured current efficiency (i.e., faradaic efficiency) in Fig. 11b shows – in addition to its low values at i < 10 mA cm⁻² due to salt cross-over – a slight decreases at i > 20 mA cm⁻². It is hypothesized that current densities above the optimum value, reduce the BPM's permselectivity⁹³ or enhance the water splitting reaction in the AEM's,^94,95 leading to a lower efficiency.⁹² The current efficiency also decreases if the current is carried by an undesired ion through the membranes. For example, in Fig. 10b, the (bi)carbonate ions are the desired current carriers. However, if (as a result of high pH) OH⁻ ions become the main charge carrier through the AEM, the current efficiency decreases. On the other hand, when capturing CO₂(g) from flue gas or in DAC, a high pH (i.e., high [OH⁻]) is favoured in the outlet of the same compartment, because CO₂(g) absorption capacity and rate increase in a solution with high pH. Adopting monovalent-ion-selective AEM's that favour the transport of HCO₃⁻ ions over OH⁻ ions then could improve the efficiency when such process is intended.^3,96

Fig. 11 Trend of energy consumption, faradaic efficiency and CO₂ production rate vs. current density in CO₂(g) recovery from 0.125 M K₂CO₃-rich solution via BPMED. The black squares are experimentally measured data by ref. 92 while the red lines show the calculated values performed by ref. 3. Reprinted (adapted) with permission from ref. 3; Copyright (2020) American Chemical Society.

As opposed to what is shown in Fig. 11b, the current efficiency of BPMED can reach as high as 95% if e.g., a 0.5 M KHCO₃ is used instead of the 0.125 M K₂CO₃ for CO₂(g) recovery through acidification. In fact, both in BPMED and (membrane) electrolysis, in addition to the current density, the anolyte and catholyte concentrations and the operation temperature affect the current efficiency, as demonstrated in Fig. 12³⁸ and summarized in Table 1.

Fig. 12 Change of the measured current density (blue diamond line) and faradaic efficiency (red circle line) with regards to the change in catholyte concentration, anolyte concentration and the applied temperature in an electrochemical KOH (absorbent) production via KCl electrolyser. Reprinted from ref. 38, Copyright (2019), with permission from Elsevier.

Table 1 Electrochemical CO₂ capture

Capture method	Feed	Energy-consumption^a (kJ mol^−1 CO2)	Current-efficiency	Product	Current-density (mA cm^−2)	Remarks
a Only for the electrochemical step, defined as in eqn (11). b For KOH electrochemical regeneration. c Although the theoretical work requirement for the reaction is only 266 kJ mol^−1 CO2. d Calculated from data on energy consumption for KOH production, assuming 2 moles KOH delivers 1 mole CaCO3. e Plus 200 for CO2(g) capture into hydroxide solutions.^193 f Only for the production of HCl and NaOH, not for CO2 capture and recovery. g Calculated based on data from ref. 76: R = 0.5–1.8 Ω, i = 20.4–61.2 mA cm^−2 and recovered CO2 rate of 0.004 mol min^−1. h Only for the CO2 desorption step.
Fuel-cell ^35	Air + H2	350	23%	CO2(g)	0.5	Upscaling unfeasible (low current-density)
Electrolysis^74	Air	290–350 (kJ mol^−1 KOH)	>95%^b	CO2(g) + H2(g)	100	KOH/K2CO3 electrolysis at 70 °C
Electrolysis ^71	Air + mined CaCO3(s)	1 267 200^c	Not reported	H2(g) + CO2(g)	Not reported	Water electrolysis. Fig. 8a
Electrolysis ^38	Synthetic flue gas + cement kiln dust	>634–1276 (kJ mol^−1 CaCO3)^d	60–90%^b	CaCO3(s)	100–200	KCl electrolysis. Fig. 12
BPMED ^91	K2SO4	∼308	Not reported	CO2(g)	50	Produces KOH + H2SO4
BPMED ^106	NaHCO3/NaOH	160–500	65–80%	CO2(g)	5–20
BPMED ^92	KHCO3/K2CO3	100–450^e	95% (KHCO3), 50% (K2CO3)	CO2(g)	5–100	Fig. 10b for cell design & Fig. 11 for cell performance
BPMED ^43	KHCO3	200–500	70–90%	CO2(g)	22–139	High pressure BPMED enables 30% energy win. See Section 3.2.3
BPMED ^113	Artificial seawater	250–400	<70%	CO2(g)	1–3	59% of DIC from seawater was removed using ca. 242 kJ mol^−1 CO2
BPMED ^8	Artificial seawater + NaCl	390–640	60–95%^f	CO2(g) or CaCO3(s)	100	Cost of membrane contactors is avoided via the base route
BPMED ^107	Artificial seawater + CO2(g)	1080–2880 (kJ mol^−1 CaCO3)	Not reported	CaCO3(s)	Not reported	CO2(g) absorption in basified seawater followed by crystallization
EDI + BPMED ^42	Synthetic flue gas	Not reported	Not reported	CO2(g)	2–16	At near neutral pH, the slow kinetics can be enhanced using CA enzyme
EDI + electrolysis ^36,76,81,194	Natural seawater	2775–6940^g	Not reported	CO2(g)/H2(g)	20–61	Fig. 9
MCDI ^156	Synthetic flue gas	40–50	60–80%	CO2(g)	0.02–0.06	Using IEM's improve efficiency^37
Redox-active carriers + pH-swing ^140	Synthetic flue gas	106^h	90%	CO2(g)	18	Aqueous tiron (Na2Q) is used as pH mediator/active carrier to capture CO2
Redox-active carriers ^147	Synthetic flue gas	56	>75%	CO2(g)	0.5

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The (slightly) positive effect of temperature on improving the faradaic efficiency of electrochemical methods has been reported.^38,69 Higher temperatures improve the electrochemical reaction rates. In the case of BPMED, the kinetics of water dissociation in BPM is also enhanced at elevated temperatures.^97,98 However, in addition to the extra energy needed for heating up the electrolytes, the thermal stability of the ion exchange membranes poses a limitation on high temperature processes. For instance, the commercial bipolar membranes are cannot withstand temperatures higher than 40–60 °C for a long duration.⁹⁹ Moreover, CO₂ solubility decrease at higher temperatures (up to 100 °C).^100–102 This means that operating at high temperature is not yet practical.

A breakdown of energy consumption and comparison for (membrane) electrolysis and BPMED is provided in Section 3.5 and the ESI.†

3.2.2 Cell configuration. The cell configuration defines the number of membranes, membrane cell pair arrangement, electrolyte flow path and the thickness of the compartment/membrane. The design of the cell is the most important factor to minimize the cell resistance.⁷⁴ As opposed to the BPMED for traditional acid and base production (i.e., BPM–CEM–AEM–BPM), the pH-swing process for CO₂ capture allows a simplified membrane sequence due to the limited pH-range (i.e., pH 3–9 instead of 0–14).

In choosing the optimised configuration, the (co-)ionic leakage of the membranes is crucial. When using multi-ionic CO₂ feed, such as seawater, carbonic species should be rejected to obtain a maximum capture efficiency. Carbonic species are neutral (e.g., CO₂(aq) and H₂CO₃) or negatively charged and hence are rejected better by a CEM than by an AEM. In addition to higher selectivity, the use of CEM's instead of AEM's have proven to increase conductivity and mechanical stability in a BPMED cell for CO₂ capture⁷²).¹⁰³

For further reducing co-ionic cross-over, a trade off between the permselectivity of IEM's and their resistance should be considered, which is controlled by the membrane thickness. In general, thicker membranes have higher resistivity but show a better selectivity.⁷² Applying a reinforced structure to a thinner membrane can improve its selectivity while maintaining its resistivity to a certain extent.^72,104

Depending on the charge carriers in the cell, the flow mode (i.e., one-way pass vs. batch mode) and feed concentration, each BPMED process asks for a different cell configuration. While BPM–CEM might be the choice of some researchers for CO₂ recovery from carbonate solutions,^105,106 others chose a BPM–AEM^43,92 for the same purpose. That is while novel configurations as BPM–AEM–AEM are also emerging for minimizing BPM fouling when extracting CaCO₃ from seawater,¹⁰⁷ see Section 3.2.4.

3.2.3 High pressure BPMED. In a closed system (such as the one created inside of an electrochemical cell), acidification increases the concentration of dissolved CO₂. This concentration can lead to the formation of CO₂(g) bubbles inside of the cell. Theoretical models, suggest the trapped CO₂ in the acid solution dominate the cell resistance and increase the energy consumption.¹⁰⁸ Trapped gas bubbles lower the effective membrane surface area, cause high voltages and shorten the membrane lifetime (due to localized “hot spots” of high current density). To avoid gas production inside of the cell, Eisaman et al.⁴³ proposed a high pressure BPMED process using a similar cell as shown in Fig. 10b where the entire stack is kept at high pressure, i.e., there is no pressure difference across the membranes in the stack. The authors observed that by increasing the pressure, the total cell voltage decreases due to the avoided CO₂ gas bubbles production in the cell; the electrochemical energy required at 6 atm (333 kJ mol⁻¹ CO₂) was seen to be 29% less than that of 1.5 atm (471 kJ mol⁻¹ CO₂) at a current density of 139 mA cm⁻².

3.2.4 BPMED for CO₂ extraction from seawater. The ocean is a massive sink for CO₂.^11,109,110 The higher carbon content of the oceans compared to that of the atmosphere^8,54,111 makes the ocean an interesting source for CO₂ capture.¹⁰⁹ Moreover, as opposed to the separation of CO₂ from a gas mixture that involves two steps (i.e., capture and release), in CO₂ capture from seawater, a separate CO₂(g) adsorption/absorption step is not required, because the ocean already acts as CO₂ absorbent leveraging its gigantic surface. The ocean contain DIC of ∼2.3–2.5 mM, mainly in the form of HCO₃⁻ ions, in normal seawater of pH ∼ 8.1.

The product of seawater CO₂ capture can be (1) gaseous CO₂,⁸ (2) solid carbonates,¹⁰⁷ or (3) dissolved bicarbonates and carbonates ions (to be subsequently stored in the ocean).^71,75,78,112 To extract the CO₂ as gas, the bi-/carbonate ions present in the seawater can be converted into H₂CO₃* in the acidic compartments adjacent to the BPM.¹¹³ Subsequently, through vacuum stripping of the acidified stream using membrane contactors, CO₂(g) can then be produced.^8,113 Alternatively, at alkaline pH > pK_a2, HCO₃⁻ converts to CO₃²⁻ and can subsequently precipitate through reaction with dissolved Mg²⁺ or Ca²⁺.^8,107 When the partial pressure of CO₂ remains unchanged, the pH determines which mineral is obtained e.g., the precipitation of CaCO₃ (in synthetic seawater) is favoured in 9.3 < pH < 9.6.^8,114 Considering that the Ca²⁺ and Mg²⁺ concentrations in seawater are respectively ×4 and ×25 higher than that of DIC, mineralization has the potential to remove all DIC, theoretically.¹¹⁵ In addition, mineralization eliminates the expenses of using a membrane contactor.^116,117 In the third option, converting CO₂ to dissolved alkaline bicarbonates and carbonates enables carbon sequestration. This is a long-term carbon storage which also helps against ocean acidification by increasing the ocean alkalinity.^71,75,78,112 However, further research is required for understanding the full range and capacity of such approach.

The main bottlenecks in using in situ BPMED for this mineral crystallization are the risk of membrane contamination (i.e., fouling) in the stack and the slow kinetics of the carbonate precipitation. To avoid the fouling, pure NaCl (instead of seawater) can be used in the BPMED for NaOH and HCl production.⁸ The produced NaOH is then added to the seawater stream in a controlled crystallizer to initiate the precipitation. In order to improve the kinetics of the precipitation, use of a seeded crystallizer unit is suggested.^107,118 Although seawater CO₂ capture through mineralization is shown feasible, debates on the environmental impacts of reducing Ca²⁺/Mg²⁺ concentration of the ocean are in progress.^115,119

3.2.5 Electrodeionization (EDI). Electrodeionization (EDI) combines ion exchange membranes technology with ion exchange resins.^120–125 Ion exchange resins are solid cross-linked polymers that contain fixed charged groups typically based on acrylic or styrene monomers. When filling the flow compartments with beads or 3D structures of ion exchange resins, the polymer–electrolyte interface area is increased.¹²⁶ The combination can help overcome the concentration polarization losses associated with electrodialysis and electrolysis. The major application of electrodeionization (EDI) includes the removal/recovery of heavy metals and organic acids for pure water production.^127–131 The combination of BPMED with resin wafer electrodeionization (RW-EDI) is demonstrated in the lab-scale, where porous, solid matrix of ion-exchange resin beads are incorporated in between of the CEM's and BPM's in the cell.⁴² However, unfortunately, data on the energy consumption and current efficiency of the system is not reported. The combination of electrodeionization (EDI) with electrolysis is shown feasible as demonstrated in Fig. 9.^76,81 However, as shown in Table 1, the process still requires further optimization (in terms of production of H₂(g) and cost/stability of resins) to decrease its significant energy consumption. Furthermore, the (bio)fouling of the resin beads due to the contact with seawater decreases the overall efficiency of the method and needs yet to be addressed.

The pH-swing based EDI application for CO₂ capture has only been explored at a basic level.¹²⁷ More work has been done on the use of amine based ion exchange resins as adsorbent for CO₂ capture, through thermal swing. Such resins are reported to show good stability after repetitive adsorption–desorption cycles with only a small reduction in capture capacity, and require relatively mild desorption conditions.^126,132,133 Recently, wafer enhanced electrodeionization for conversion of CO₂ into HCO₃⁻ feed for algae cultured photobioreactors is reported.¹³⁴ Suggestions for further research on EDI technology are producing inexpensive ion exchange resins, reducing possibility of resin fouling (e.g., for the case of seawater feed⁷⁶), and maintaining long-term resin stability.¹²⁷

3.3 Redox-active carriers and electrode reactions

An alternative to BPMED and electrolysis for CO₂ capture, is the more classical electrochemically-mediated separation strategies that perform absorber/desorber cycles using specific redox-active sorbent carriers.^39,49 These redox-active carriers can be used for separation of CO₂(g) from a gas mixture through (1) binding route Fig. 13a and (2) pH-swing route Fig. 13b. Both routes have been demonstrated feasible in the lab-scale.^135,136

Fig. 13 (a) Electrochemical CO₂ separation using gas diffusion electrodes (GDE) through binding with quinone redox-active carrier (i.e., no pH-swing is created). Reprinted (adapted) with permission from ref. 144; Copyright (2015) American Chemical Society. (b) Combination of pH-swing with the chemistry of redox active carriers through (PCET) reaction using mixture of hydroquinone, quinone, and sodium bicarbonate. Reprinted (adapted) with permission from ref. 136; Copyright (2015) American Chemical Society.

As for the “binding route”, the suitable carrier is activated at the cathode and can bind with the target species at its reduced state. The target species in this case is the CO₂ molecule. Subsequently, the captured CO₂ can be released at the anode through oxidation of the carrier while the carrier is regenerated. This process is also referred to as “electrochemical CO₂ pumping”,^135,137 see Fig. 13a. The cycle can be broken down into four steps:⁴¹

• Sorbent activation through oxidation or reduction.

• CO₂(g) capture on the activated sorbent.

• Sorbent deactivation through the reverse electrochemical process.

• CO₂(g) release.

Alternatively, in order to decrease the required electrical energy, these steps could be integrated in a two or three-stage process^40,41 by e.g., enabling CO₂ capture or release to be performed simultaneously with electrochemical reduction or oxidation of the carrier.

In order to improve the kinetics of CO₂ capture and release, the “pH-swing route” can be integrated, where the chemistry of redox-active carriers are designed to undergo proton coupled electron transfer (PCET) reactions,^{135,136,138,139} as shown in Fig. 13b.^135,136,140 If so, an “electrochemical H⁺ pumping” takes place that enables an acidic and a basic pH on the anode and the cathode, respectively.

Although redox-active systems have yet to achieve industrial utility, they have the potential of producing a pure CO₂ stream even from dilute gas mixtures, such as air.¹⁴¹ Among different classes of redox-active compounds that have been explored, such as bipyridines,^142,143 disulfides⁴⁹ and copper/amine systems,^29,30 the quinone species^{141,144–146} are of particular interest,⁴⁰ owing their strong binding affinity for CO₂ in their reduced form compared to that of their neutral state.^39,147 Quinones are organic compounds derived from aromatics, through conversion of an even number of –CH= groups into –C(=O)– groups.¹⁴⁸ Quinones have also gained great interest as potential ideal candidates for PCET mechanism^{138,139,149–153} inspired by their role in biological systems.¹⁵⁴ These redox reactions are (ideally) reversible, and can operate within the water splitting window.¹³⁵

The choice of catalysts on the surface of the electrodes to facilitate the electron transfer together with the type and concentration of the electrolyte are important in determining the process efficiency in terms of capture, transport and release of CO₂, and kinetics.¹³⁶ Furthermore, the local pH shown in Fig. 13b is not the same as the bulk pH.¹⁵⁵ While the high pH at the cathode determines the CO₂ absorption and the lower pH at the anode drives CO₂ desorption, only the bulk pH can be controlled in a practical process. That makes the choice of an appropriate average pH that enhances both reactions, challenging. Alternatively, the local pH can be maintained by inserting (1) an anion-exchange membrane (AEM) or (2) an extra salt compartment sandwiched between an AEM and a cation-exchange membrane (CEM) between the electrodes.¹⁴⁷

Despite its progress in the lab-scale, CO₂ separation using redox active carriers is not yet practically implemented because of the limitation it faces in terms of both solvents and carriers. It is difficult to find a solvent that is inexpensive, safe, electrochemically stable and allows high solubility of the redox species.^49,147 Possible improvements in terms of solvents can be achieved by using ionic liquids as electrolytes^39,144 or using salt-concentrated aqueous electrolytes.¹⁴⁷ As for the carriers, the solubility, chemical stability and kinetics of the redox molecule can pose limitations on the process.⁴⁹ To address these drawbacks, electrochemically mediated amine regeneration can be employed, using a similar concept, but by employing amines rather than quinones as described in Section 4.1.3. Alternative to using dissolved carriers in an electrolyte – where the transport of both the electrolyte and the carrier molecules between the two electrodes is required, the active carriers can be immobilized between the opposite electrodes.³⁹ The latter is explained further in Section 4.2.1. Recently, solid polymerized quinone (formed into a composite with carbon nanotubes to confer electronic conductivity) is employed.³⁹ This so called “electro-swing” process exhibits a high faradaic efficiency and a low energy consumption (40–90 kJ mol⁻¹ CO₂ captured) compared to the thermal or pressure swings for sorbent regeneration. However, it needs to be improved in terms of capacity and kinetics.

3.3.1 Electrode induced pH-swing. Another electrochemical based CO₂ capture concept utilizing local pH near electrodes is the (membrane) capacitive deionization (MCDI) method.^37,156 Capacitive deionization is mainly used for water treatment, but its application has recently expanded to energy harvesting and CO₂ capture.^157–165 The motivation to capture CO₂via such system is that deionized water can be used without the need of any other chemicals, using inexpensive carbon based electrodes.^37,156

MCDI cells consist of activated carbon electrodes and ion-exchange membranes. When a current is applied in the charging step of MCDI, HCO₃⁻ and H⁺ ions are adsorbed into the porous electrodes inside of the cell (causing a local low pH at the cathode and increasing pH in the bulk). As the electrolyte is being depleted from ions, more CO₂(g) can be absorbed in the deionized water due to the shift in the CO₂ equilibrium, to make up for the depletion. It has been proposed to use a gas–liquid contactor spiral glass tube outside of the cell for in situ absorption. When the current direction is reversed (i.e., discharging step), the subsequent desorption of H⁺ and HCO₃⁻ (plus a small amount of CO₃²⁻) ions from the carbon electrodes drives the chemical equilibrium in the opposite direction. As the concentration of H₂CO₃* will exceed the solubility, CO₂(g) is formed from the electrolyte. CO₂(g) absorption and desorption can be controlled through shifting the current direction. CDI can also be used in combination with NH₃-based CO₂ capture.¹⁶⁶

In addition to the carbon based capacitive electrodes, other metals can also be used. For example, CO₂ can be captured through intercalation/deintercalation of protons on the manganese dioxide (MnO₂) based electrodes. The electrodes can host protons (i.e., intercalation) during reduction and release them (i.e., deintercalation) during oxidation, creating the required pH-swing for CO₂ absorption and desorption, respectively.¹⁶⁷ In such methods, periodic electrode polarity and the switching fluid flows ensure a continuous process.

3.4 Molten carbonate cells and hybrid electrochemical capture methods

Electrochemical methods can benefit from a pH-swing approach but are not limited to it. Examples are using (high temperature) molten carbonate cells or the hybrid methods that integrate CO₂ capture and conversion.

High-temperature molten carbonate cells are early electrochemical CO₂ capture examples.^32,168 In such process, a CO₂ containing (flue) gas is fed to the cathode side of an electrochemical cell, where electricity is used to drive CO₂ and O₂ (in form of CO₃²⁻) across a molten carbonate salts electrolyte (sandwiched between ceramic membranes in contact with the electrodes). At the anode, carbonate ion will be reduced to CO₂ and O₂ again. An alternative design enables a molten carbonate fuel cell, where CO₂ is captured while H₂ is produced.^169–171 Involved challenges are difficult operating conditions due to the high temperatures, corrosion and sensitivity to the presence of SO_x in the gaseous mixture.^20,24,172 Research for developing dense molten carbonate CO₂ selective membranes at high temperatures (>723 K) is still in progress.^173–175

Examples of electrochemical hybrid capture and utilization is the electrochemical seawater battery system,¹⁷⁶ the alkali metal-based CO₂ batteries (e.g., lithium–CO₂ batteries^177,178) and electrochemical CO₂ capture and conversion combinations.^25,179–183 The absence of pH-swings, and the lack of further development of these proposed electrochemical capture routes, categorizes these concepts beyond the scope of this review. The broader context in terms of CO₂ utilization is discussed in Section 5.

3.5 Which electrochemical method to use?

A comparison of the metrics of electrochemical pH-swing based CO₂ capture methods is given in Table 1. These capture methods have often energy consumption >300 kJ mol⁻¹ CO₂. As a comparison with conventional methods, the energy consumption of CO₂ capture (from flue gas) via aqueous monoethanolamine (MEA) using a thermal swing, currently the most mature capture method, is between ∼170–300 kJ mol⁻¹ CO₂.^184–190 Approximately ∼80% of this energy is the contribution of the thermal regeneration,¹³ included in the reboiler heat duty.¹⁹¹ Most conventional and electrochemical captures are energy intensive, when compared to the combustion energy of various fuels and the emitted CO₂ per mole of the fuel. From an economic point of view, CO₂ capture is only interesting if the energy consumption of the capture is <66 kJ mol⁻¹ CO₂.^42,192

Renewable sources would be preferred over fossil fuels for driving electrochemical CO₂ capture to (1) maximize the reduction in net carbon emission and (2) leverage the advantage of electrifying the CO₂ capture process. At the same time, the absence of flue gas from power plants in renewable sources would make diffused CO₂ sources (e.g., atmosphere and seawater) the most logical feed for electrochemical CO₂ capture technologies. This is also reflected in Table 1, where most research has focused on capture from air, seawater or (low concentrated) bi(carbonate) solutions.

At present, (membrane) electrolysis and BPMED are the most studied electrochemical capture approaches. Fig. 14a shows the estimated energy consumption of both methods (see ESI† for calculations). This electrical energy consumption, E (in kJ mol⁻¹ CO₂), is calculated via:

where i is the current density (A m⁻²), A is effective area of the electrodes (m²), V is the total cell voltage (volts) and r_CO2 is the recovery rate of the captured CO₂(g) in mol s⁻¹. To put the energy consumption of Fig. 14a in perspective, the energy produced per mole of fuel (through combustion) and the associated emitted CO₂ are shown in Fig. 14b. This emphasizes the energy-intensive nature of the capture process: capturing 1 mole of CO₂via BPMED consumes 25–60% of the energy obtained from combustion of hydrocarbon fuel per mole of CO₂, dependent on the fuel type. For conventional methods and electrochemical capture via electrolysis, this figure is even larger.

Fig. 14 (a) Electrical energy consumption for direct air capture (DAC) through (membrane) electrolysis. Current density of 20 mA cm⁻² and ΔpH = 14 is assumed for both cases. See Section S2 of the ESI† for detailed calculations. The thermodynamic work required for DAC (assuming 400 ppm CO₂(g)) is 20 kJ mol⁻¹ CO₂.¹⁵ (b) The combustion heat of common fuels and their CO₂(g) emission, obtained from ref. 195 and stoichiometric combustion reactions.

Fig. 14a shows that the ohmic losses cover a significant part of the energy losses,⁷⁷ and bring in a lever to reduce E. In electrochemical cells, such losses can appear as e.g., heat and/or unwanted chemical byproducts. For example, losses caused by product recombination, such as recombination of produced H⁺ and OH⁻ in BPMED or the recombination of O₂(g) and H₂(g) in water electrolysis to form water again. Membrane co-ion leakage is another common loss often involved in membrane based electrochemical processes.⁹³ The limitations and areas of improvements of each electrochemical CO₂ capture method can be summarized as:

(1) It is obvious from Fig. 14a that the energy consumption for BPMED is lower than that of electrolysis when targeting CO₂ capture. In case hydrogen is an aimed product as well, the energy difference between these two methods (which is equivalent to 1.23 V) can be justified, and can be lower than the energy for two separate systems making acid/base and hydrogen. However, in more detail, the combination of these two products in electrolysis complicates the optimal current density, which is not necessarily the same for hydrogen and acid/base production. Moreover, most hydrogen catalysts are geared towards acidic environments, and earth abundant oxygen catalyst are available for alkaline environments, while the opposite environments are present in the combined electrolysis/acid–base production.¹⁹⁶ That limits the options for electrocatalytic material (e.g., platinum and ruthenium), which can pose resource limitations as a relatively large electrode area is required. On the contrary, up scaling can be done easily for BPMED capture method by repeating multiple cell pairs within a single electrode pair.

(2) When using BPMED, despite its uncomplicated up scaling, the typical energy consumption in Table 1 is still 2–3 times more than theoretically expected in Fig. 14a. These losses are due to the involved non-idealities in charged membranes (e.g., high resistivity, co-ion leakage, low chemical stability), the high over potential of water dissociation in BPM, or carbonate feed (requiring 2 H⁺ per molecule to acidify to H₂CO₃) instead of bicarbonate. Thankfully, achieving a lower energy consumption is possible both through process optimization and material engineering. Process optimizations can be done by e.g., controlling the applied current density (i), feed flow rate (residence time in the cell), electrolyte conductivity/pH, cell configuration, flow path/mode, and operation pressure/temperature, while water dissociation can be improved through material engineering.^197,198 The current minimum economic cost of BPMED is at least twice of the cost of the wet-scrubbing rival.³ However, the cost can significantly decrease if improvements in the cost of renewable energies, cost and availability of ion exchange membrane, membrane life time and membrane selectivity is achieved. Finally, the total capture cost does not only depend on the method, but also on the source of the capture. For example, depending on the pumping facilities and plant location, indirect ocean capture (IOC) can be economically favored relative to direct air capture (DAC), or vice versa.¹⁹⁹

(3) The reversible free enthalpy in Fig. 14a is based on a ΔpH = 14. Using lower ΔpH, a lower reversible free enthalpy is involved.⁸⁴ Although it is theoretically possible to perform a pH swing between pH 4–7, potentially reducing the energy consumption to values close to the thermodynamic limits, this low energy consumption is not obtained in practical BPMED at medium to high current density.

(4) Membrane capacitive deionization (MCDI) is recently demonstrated to capture CO₂ from a CO₂(g) + N₂(g) mixture, using only demineralized water^37,156 with the lowest capture energy among other methods (∼40–50 kJ mol⁻¹ CO₂), see Table 1. However, the obtained current density is extremely low (∼0.02–0.06 mA cm⁻²) and there is a long way to go before (M)CDI becomes a competitive means of CO₂ capture as the stability and performance of the large-scale CDI applications are yet unknown. Future work is suggested to investigate the physical and chemical effects of weak electrolyte solutions in CDI.¹⁵⁶

(5) Only a few works on CO₂ capture through electrodeionization (EDI) show lab scale feasibility^76,121,200 as summarized in Table 1. However, no data on the energy consumption of those systems is available. CO₂ capture through EDI is limited due to the cost of ion exchange resins, their poor stability and their sensitivity to fouling.¹²⁷

(6) Electrochemical methods that use redox-active carriers are shown to be less energy intensive (∼100 kJ mol⁻¹ CO₂). However, these results have been only obtained using synthetic flue gas. Quinones, used as binding agents for CO₂, are highly sensitive to water and oxygen, making the applications for real flue gas (or direct air capture) impractical. Moreover, most redox carriers need organic solvents electrolytes that suffer from a low ionic conductivity which limit the current density. When using quinones as carriers, the total CO₂ carrying capacity is limited by the solubility of quinone in the solvent, the applied electrode potentials, evaporative solvent losses and consequent drying of the electrodes.¹⁴⁴ Luckily, the efficiency of the CO₂ absorption and desorption in such systems can be increased by combining a pH-swing through electrochemical reactions,¹³⁶ where proton coupled electron transfer (PCET) takes place. However, the practicality of this approach still awaits the improvement of electrochemical redox kinetics. The low solubility of PCET organics limits its capture capacity.¹³⁵ Furthermore, PCET carriers are also very sensitive to gasses such as O₂ and sulfur, posing again challenges for (real) gas CO₂ capture application.¹³⁵ Upscaling can be done by using a larger electrode surface area.

All electrochemical CO₂ capture methods are still under development (TRL 5 to 6), although they enable high CO₂ recovery rate (>80%). In order to provide a framework of comparison between these methods and the conventional (non-electrochemical) processes – which have already been commercialized – the conventional processes are described further in the next section.

4 Combining electrochemical methods with conventional capture technologies

Conventionally, the capture industry only targeted CO₂ capture from centralized CO₂ emitters such as fossil fueled power plants, iron, steel, and cement industries. However, this approach is impotent to achieve a net zero CO₂ emission because, as shown in Table 2, decentralized sources still constitute a large part of the total emission (total GHG emission of 55 Gt CO_2eq in 2019 ²⁰¹). Therefore, currently DAC and IOC are gaining attention (Table 3). For achieving climate targets, CO₂ removal within a gigaton order of magnitude is needed.^202,203

Table 2 Thermodynamically required energy, properties and the scale of main capture feeds

Source	Thermodynamic properties & required energy	Emission & capture scale
Centralized	• Flue gas:	• >58% of total emission, see Fig. 1
	– 150 < T < 1200 °C, pCO2 ∼ 0.03–0.15 atm^204,205	• Current post-combustion capture >2.4 Mt CO2 per year^207
	– 7 kJ mol^−1 CO2 for 13% flue gas^206
Decentralized	• Direct air capture (DAC):	• <42% of total emission, see Fig. 1
	– Ambient T, pCO2 ∼ 0.0004 atm (∼400 ppm)	• Current DAC capture shown in Table 3, but no large scale IOC, yet^209
	– 20 kJ mol^−1 CO2^15,206,208
	• Indirect ocean capture (IOC):
	– 5 < T < 35 °C, pCO2 ∼ 0.072 atm (∼2.5 mM DIC)
	– Same thermodynamically required energy as DAC^199

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Table 3 Large scale DAC companies

Company	Process mechanism	Capacity (tonne CO2 per year)
Carbon Engineering ^210	(KOH) absorption + high temperature calcination	1 000 000
Global Thermostat ^211	(Amine based) adsorption + thermal & pressure swing regeneration	4000
Climeworks ^212,213	(Amine based) adsorption + thermal swing regeneration	900

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The (centralized) capture technologies are often grouped in main categories of;¹⁸ (1) oxy-fuel combustion, (2) pre-combustion, (3) post-combustion and (4) chemical looping (combustion) as discussed in Table 4. In addition, capture by algae,^214–220 biochar^221–226 and charcoal²²⁷ are recently demonstrated. While there are many CO₂ capture methods, introducing all of them are out of scope of this work and we refer the reader to various available literature on this topic.^{13,17–25,228,229} For an overview of the current state-of-the-art of CO₂ capture, transport, utilisation and storage see ref. 230. Among the available methods, most research focus has been given to the post-combustion methods as they can be retrofitted more easily to the existing industrial units, compared to the oxy-fuel and pre-combustion methods.²²⁹ Traditional post-combustion capture methods are absorption,^{18,19,231–236} adsorption,^{13,18,237–239} membrane gas separation,^240–246 calcium looping^19,247–249 and mineral carbonation,^{110,250–260} see Table 4. These methods are combined with thermal swing, pressure swing or vacuum swing for CO₂ desorption.

Table 4 Dominant non-electrochemical CO₂ capture methods (continued)

Capture method	Process mechanism	Challenges
Oxy-fuel ^261–263	Instead of air, pure O2(g) or a CO2(g)/O2(g) mixture is used for fuel combustion. 75–80% CO2 is produced as opposed to combustion in normal air where 3–15% CO2 is produced	• High oxygen production energy costs (ca. 200–220 kW h per tonne of oxygen generated by cryogenic air separation^263)
		• High sensitivity to air leakage into the system
		• Although an efficient capture method,^264 difficult to retrofit compared to post-combustion method
		• Special materials are needed to resist the high flame temperature (ca. 3500 °C). Although, the recycled CO2 can be used to moderate this temperature^18
Pre-combustion ^265–268	CO2 is separated from H2-rich fuel before combustion. For example, synthetic gas is produced from fossil fuel by adding steam or pure oxygen at high temperature and pressure (1400 °C and 25–55 atm) and a subsequent water–gas shift reaction.	• Retrofit to existing plants is costly and more difficult compared to oxy-fuel and post-combustion
		• Reaction CO + steam ↔ CO2 + H2 produces 15–40% CO2 at 14–40 atm. The produced H2 can be used for power generation, but CO2 needs to be removed using a subsequent separation technique
Post-combustion:
Absorption^18,19,231–236	CO2(g) (from flue gas or air) is absorbed (i.e., chemically or physically) in an absorbent. Pure CO2(g) is subsequently recovered through temperature swing desorption while the absorbent will be regenerated. Currently, amine based capture is the most mature method for CO2 capture	• Limited CO2 absorbing capacity resulted from the reaction stoichiometry and dependent on the absorbent type (e.g., low capacity as 0.4 kg CO2 per kg-MEA or higher as 1.2 kg CO2 per kg-NH3^18,269)
		• High absorbent regeneration (i.e., CO2 desorption) energy
		• Solvent losses caused by volatility or thermal/chemical degradation, the subsequent equipment corrosion & negative environmental impacts of solvent emissions
		• High sensitivity to flue gas temperature, pressure and presence of impurities, such as NOx, SOx and oxygen (i.e., oxidative degradation)^270
Adsorption^13,18,237–239	CO2(g) is adsorbed on solid materials and will be recovered through temperature, pressure or vacuum swing desorption	• When used for flue gas, pre-treatments to remove impurities, such as NOx, SOx and H2O and to decrease gas temperature are needed
		• Possible loss in the adsorption capacity of the adsorbent after desorption step (e.g., 4–9% loss for amine immobilized onto solid silica)
		• Sorbent degradation in cyclic operation
Membrane gas separation^13,240,241,244,246,271,272	Process is driven by partial pressure difference of the gas molecules to be separated. Gas molecules permeate according to their size, diffusivity, or solubility through the membrane.	• Not feasible for streams with low CO2 partial pressure and concentration
		• To accommodate the high flow rate of industrial flue gas, high membrane surface area is required
		• Sensitivity to presence of moisture (i.e., lower selectivity)
		• On-going research on new membranes with high selectivity and permeability^273–275
Calcium looping^13,248,276	A variant of chemical looping (combustion) that involves carbonation and calcination. Metal oxides e.g., CuO, Mn2O3, NiO, and Fe2O3 are used instead of O2(g) in oxy-fuel combustion	• Rapid decrease in the limestone (i.e., sorbent) capacity after a number of cycles of reaction with CO2
		• Environmental concerns caused by limestone mining, the waste from Ca-looping (i.e., the spent CaO) and the need for high temperatures for the operation
		• Need for air separation unit to obtain pure O2 for calcination

---

The conventional absorption and adsorption based captures can be combined with electrochemical methods to decrease (or eliminate) the required thermal energy for the regeneration step as discussed in Sections 4.1.3 and 4.2.1. No available work have been found on electrochemical enhancement of the other conventional methods.

4.1 Absorption (wet scrubbing)

CO₂ is a weak acid. Therefore, substrates that contain basic moieties such as amine groups (e.g., alkanolamines) are efficient absorbents for CO₂ capture.²⁷⁷ Amines are derivatives of ammonia (NH₃), containing a basic nitrogen atom, where one (R–NH₂) or more hydrogen atoms have been replaced by a substituent. Amines act as a nucleophiles (i.e., electron pair donors also known as Lewis bases), reacting with CO₂ at the electrophilic (i.e., electron pair acceptor) carbon center to form a carbamate (derivatives of carbamic acid H₂NCOOH where one or more hydrogens are replaced by other organic functional groups).^49,167,278 Although the amine capture processes are extensively studied, the reaction mechanism is not yet fully understood. For a detailed description see ref. 279.

The post-combustion CO₂ capture is dominated by amine-based absorption, using aqueous solutions of mono-, di-, tri-ethanolamine or hindered amines to absorb CO₂ in gas form.^{232,278,280–283} Aqueous solutions are often used in order to control the density, viscosity, surface tension, and the thermal expansion coefficient of the pure amines.^284–286 This is while. the energy consumption for solvent regeneration decreases substantially with increasing the amine concentration (due to an improved CO₂ reaction rate and absorption capacity²⁸⁷). However, such high concentrations also increase the degradation rate, viscosity and the involved environmental concerns, posing challenges on the overall capture process.^285,287 In addition to aqueous solutions, both non- and low-aqueous solvents (i.e., water–lean solvent) are viable.^284,288,289 These alternatives are not well studied yet although they have gained interests recently. All currently assessed water–lean solvents have shown lower CO₂ solubilities than aqueous monoethanolamine (MEA). However, because the heat of absorption is not much affected, solvents with the lower volatility than water could potentially offer opportunities for processes with overall less reboiler heat duties than that of ordinary aqueous MEA.²⁸⁴

Amines have been used for CO₂ capture since 1930.²⁷⁸ Monoethanolamine (MEA) scrubbing technology is seen as a benchmark technology for CO₂ capture from flue gas of large-scale power plants.^{187,190,290,291} In this method, CO₂(g) is chemically absorbed at low temperatures (∼40 to 60 °C) in the absorbent and is extracted in a desorber column later, generally via a temperature swing at high temperatures (∼120 °C), where the absorbent is also regenerated.^186,292 The energy consumption of CO₂ capture (from flue gas) via aqueous monoethanolamine (MEA) thermal swing is between ∼170–300 kJ mol⁻¹ CO₂.^184–190

To address the drawbacks discussed in Table 4, for CO₂ capture via chemical absorption, other inorganic solvents, such as aqueous potassium and sodium carbonate,¹⁶⁷ ammonia solution and alkali hydroxide solution have been investigated.^293,294 When using alkaline or carbonate based solution, hydration of CO₂ takes place rather than carbamate formation.¹⁶⁷ However, CO₂ absorption in carbonate is very slow compared to that of amines.²⁷⁰ Increasing the absorbent pH (i.e., applying a wide range pH-swing), can enhance the kinetics substantially.

4.1.1 pH-swing vs. thermal swing. All available large-scale CO₂ capture processes rely on heating or using a combination of heat and vacuum to release the captured CO₂.^27,294 Given the initial focus was to capture CO₂ from flue gas of fossil fuel power stations, heat integration is relatively straightforward. However, as an alternative to this thermal swing, an (electrochemical) pH-swing approach can be applied for CO₂ recovery and absorbent regeneration.³

In a thermal swing absorption process, the energy performance is dominated by (1) absorbent absorption capacity, (2) absorption rate, (3) heat of absorption and (4) thermal degradation.²⁷⁰ The required thermal energy is often exacerbated due to use of aqueous solutions in which the capturing agent is contained (e.g., water for the case of MEA²⁷⁰).⁴⁹ Furthermore, the required heat is normally generated from combustion of fossil fuels, decreasing the net captured CO₂ in the conventional processes.¹⁰⁸

Alternatively a pH-swing can be used, to absorb and desorb CO₂. The cost of absorbent regeneration through pH-swing via BPMED in wet scrubbing (using KOH absorbent) is estimated to be ∼773 $ per tonne CO₂. That is more than three times of the cost of the thermal swing rival. However, the cost can significantly decrease if the cost of renewable energy decreases (e.g., from 0.06 to 0.018 $ per kW h), cost of membrane decreases (to lower than 100 $ m⁻²), membrane life time increases from around 3 years²⁹⁵ to 15 years and the process is optimized.³

4.1.2 Physical absorption. The advantage of the physical absorption to chemical absorption explained above is its lower heat consumption in the solvent regeneration step.^21,22 Solvents, such as methanol, poly(ethylene glycol) and dimethyl ether can be used to absorb CO₂(g) physically. Recently, ionic liquids have been proposed as alternatives to the conventional absorption solvents.^296–298 Ionic liquids are molten salts that exist as liquids near room temperature, often composed of an organic cation with an inorganic or organic anion and featuring polar properties.²⁵ Ionic liquids are referred to as green solvents due to their low volatility, exceptional thermal stability, non-flammability and environmentally benign character.²⁹⁹ The capture using ionic liquids is often based on physisorption although some ionic liquids react with CO₂ in a chemisorption mechanism.²²

4.1.3 Electrochemical enhancement of amine based absorption. Electrochemical methods provide alternative routes to the conventional thermal regeneration step in absorption based capture. Such electrochemical enhancements are shown feasible through (1) pre-concentrating the CO₂ rich amine stream, (2) substituting CO₂ with suitable metallic species or (3) pH-swing. These three routes are explained below.

For pre-concentrating the CO₂ rich amine stream, a capacitive deionization unit (CDI) can be used. Inside the CDI cell, when current is applied, ionic species (i.e., MEAH⁺ and MEACOO⁻ in case of monoethanolamine absorption) are adsorbed at the electrodes, creating an ion-free solution (mainly water²⁷⁰) that can be sent back to the absorber column without the need to undergo the thermal desorption step. Subsequently, when power is switched off or reversed, the adsorbed ions will be released back from the porous electrodes, creating a carbon rich stream that can then be sent to the stripper column. When applying thermal regeneration, the concentrated solution from the CDI unit then requires 50% lower solvent regeneration heat energy because of its high CO₂ loading.³⁰⁰

As an alternative approach, the conventional temperature swing step in the amine absorption process can be replaced by metal ion substitution in an electrochemical cell.^29,30,301 Such a cell consists of multiple anode and cathode chambers made of copper. At the anode, Cu²⁺ reacts with the amines, displacing the CO₂ as shown in Fig. 15. CO₂ is subsequently removed in flash tanks after the anode chambers and the amines are regenerated by subsequent reduction of the Cu²⁺ to Cu in the cathodes. The process allows for higher CO₂ desorption pressures, smaller absorber columns and lower energy demands.

Fig. 15 Electrochemically-mediated amine regeneration (EMAR) using copper. Reprinted from ref. 302, Copyright (2019), with permission from Elsevier. The conventional thermal swing is replaced with an electrochemical step. In addition to Cu/Cu²⁺, many possible chemistries can be utilized in such separation processes as discussed further in ref. 29, 30 and 301.

There are few works available on using a pH-swing to regenerate amine based absorbents; The early work of Zabolotskii et al. shows the feasibility of a low-temperature BPMED for regeneration of aqueous monoethanolamine (MEA) sorbent.³⁰³ Huang et al. demonstrated a pH-swing for amine based absorbents, doing so electrochemically (for flue gas desulfurization)³⁰⁴ and Feng et al. by simply adding a (weak) acid to the rich amine stream (for CO₂ capture).²³⁶ Such addition of acid is reported to have the potential to increase the volume of the released CO₂ and to decrease the absorbent regeneration heat energy. Both contributing to a higher energy efficiency.²³⁶

4.2 Adsorption

CO₂ can be adsorbed on solid porous materials, where CO₂(g) is subsequently recovered and the adsorbent is regenerated through a temperature, pressure,³⁰⁵ vacuum^306–315 or electric swing desorption.³¹⁶ Previous work has shown CO₂ adsorption on metal–organic frameworks (MOF's), silica, zeolites, immobilized amine, alumina, polymeric resins, molecular sieves and activated carbon. Adsorption is possible through both physical (e.g., on zeolite, graphene, MOF's, silica) and chemical (e.g., amine and calcium based materials) bonding with CO₂.

An advantage of adsorption based capture is that using solid adsorbents (instead of the mature aqueous monoethanolamine (MEA) technology) in capturing systems reduce the regeneration heat (due to the much lower heat capacity of solid adsorbents and the avoidance of water evaporation in the regenerator).¹⁸⁷ The (calculated) regeneration heat for polyethyleneimine (PEI)/silica adsorbent based capture is reported to be around 2.46 GJ per tonne CO₂, which is much lower than the value of 3.3–3.9 GJ per tonne CO₂ for a typical aqueous MEA system.¹⁸⁷ In general, adsorption also has higher CO₂ adsorption capacity compared to that of absorption e.g., ca. 88–176 kg CO₂ per kg adsorbent³⁰⁵vs. 0.4–1.2 kg CO₂ per kg absorbent.^18,269,317

4.2.1 Electrochemical enhancement of adsorption. Similar to absorbents, the CO₂ capture and recovery of recent adsorbents has been assessed via electrochemical swings instead of the conventional pressure or temperature swings.^39,318 As an example, the redox active carriers described in Section 3.3 can be employed as solid adsorbents, immobilized on surface of electrodes. Such electro-swing systems normally operate in charge/discharge cycles where changing the cell polarity regulates the activation and deactivation of the carrier.³⁹ The significant advantage of the electro-swing process with respect to pressure swing adsorption (PSA) and temperature swing adsorption (TSA) is that the CO₂ capacity of these solid adsorbents does not depend on the feed concentration, making them suitable for CO₂ capture even from very dilute streams.³⁹ Moreover, up-scaling is easily achievable in electro-swing adsorption by using multiple anode and cathode chambers repeating within one cell.

Alternative to the electrochemical electro swing approach described above and in ref. 39, the electrochemical process can induce a temperature swing. In that case, a low voltage passes through a conductor to change the sorbent temperature via Joule heating (i.e., resistance turning electric energy into heat).^318–322

5 CO₂ utilization

After CO₂ is captured, it can be stored or utilized, see Fig. 16. CO₂ is an inexpensive, non-toxic, renewable commodity.^323,324 The market for CO₂ use is projected to grow from 0.23 gigatonnes (Gt) per year today to 7 Gt per year by 2030.³²⁵

Fig. 16 Schematic of direct and indirect (i.e., via conversion) utilization of CO₂. Alternative to gaseous CO₂, carbonate minerals (e.g., CaCO₃) can be utilized³²⁶ in paper industry, coating, plastics,³²⁷ paints, adhesive/sealants, rubber, cement and construction materials.³²⁸

In addition to fuels, chemicals such as ethylene, alcohols, formic acid (or formate), syngas, urea and other organic materials can be produced from CO₂, electrochemically, thermo-chemically or by other approaches.^{323,329–337} Such organic chemicals are often more expensive than fuels and may offer advantages in the techno-economic analysis. However, the global demand for them are much lower than fuels as shown in Fig. 17. For instance, the global demand for one of the largest chemical markets, ethylene, is 184 Mt per year.³³⁸ Ethylene manufacture also stands as the prime contributor for CO₂ emissions among carbon-based chemicals, with a CO₂ emissions rate of 184.3–213 Mt in 2015, which could reach 1.34 Gt per year by 2030.³³⁹

Fig. 17 Average price of CO₂ utilization products as a function of their market size. Error bars indicate anomalies in some regions (see the ESI,† Table S1 for values). CO₂ and H₂ are included for reference.

Production of synthetic hydrocarbon fuels and chemicals often requires high purity CO₂ feeds. Many CO₂ capture facilities using centralized sources simultaneously capture sulfur containing compounds, such as hydrogen sulfide (H₂S) that must be removed upstream to avoid catalyst poisoning.³⁴⁰ The process of desulfurization typically involves liquid phase oxidation to promote H₂S oxidation to sulfur, followed by sulfur separation from the gas stream.³⁴¹ Chemical adsorption using high surface area ZnO-based adsorbents may be used in sequence to further eliminate H₂S prior to pipeline transmission.^342,343 Alternatively, CO₂ conversion can take place via biocatalytic routes with both natural and engineered microorganisms^344,345 that generally have higher tolerance for impurities.³⁴⁶ CO₂ capture from dilute sources, such as from air and seawater, usually contains non-negligible amount of O₂ and N₂.^28,45 While N₂ is an inert gas and may not negatively impact the CO₂ conversion, a trace amount of O₂ may lower the catalytic turnover through the unwanted reaction with hydrogen in the thermochemical process,³⁴⁷ or through the preferential reaction of oxygen reduction in the electrochemical process.⁴⁵ Strategies for O₂ removal from CO₂ stream include catalytic oxidation of hydrogen³⁴⁸ or methane³⁴⁹ on noble metal catalysts such as Pt or Pd,³⁵⁰ chemical adsorption of O₂ on Cu surfaces at elevated temperatures,³⁴⁸ and electrochemical reduction of O₂ on Ag catalysts.⁴⁵

A pressurized CO₂(aq) stream is beneficial for a subsequent CO₂ conversion step. In a CO₂ electrolyser for methanol production, when the produced CO₂ remains dissolved due to an applied pressure, the avoided typical gas regeneration step and CO(g) compression step save more than 150 kJ mol⁻¹ CO₂.⁴⁶ High pressure electrolyser (up to 40 bar) is also reported to increase the current efficiency of CO₂ reduction to formic acid/formate.^351,352 Moreover, a high pressure CO₂(g) is required for the transportation in the pipes, enhanced oil recovery or geological CO₂ sequestration for underground injection. Supercritical CO₂ (above its critical temperature 31.0 °C and pressure 72.8 atm) with its low density and viscosity plays a significant role in the extraction of oil.^353–355 Furthermore, supercritical CO₂ can be used in the enhanced geothermal system for heat exchange.^247,356

The electrochemical CO₂ conversion lacks studies that demonstrate the capability of the technology at scales large enough for industrial implementation.³⁵⁷ As an alternative, CO₂ can be converted biologically to organic carbon in plants or microalgae, through photosynthesis, resulting in various products, such as biofuel or animal feed.^358,359

Gaseous CO₂ can be stored (i.e., sequestrated) in deep geological layers or in the ocean.^19,360–362 In 2019, from the 19 in operation large-scale carbon capture & storage (CCS) facilities, 33.2 Mt per year of CO₂ were captured and stored³⁶³ (i.e., less than 0.1% of the total global emissions).³⁶⁴ By addition of the already in constructions facilities, this value is estimated to be doubled, within the coming years. Most of this storage is done through enhanced oil recovery (EOR) application (i.e., over 260 Mt of anthropogenic CO₂ by 2019).³⁶³

As an alternative to sequestration of gaseous CO₂, CO₂(g) can first be converted to bicarbonate ion and, subsequently, be stored as already abundant forms of ocean alkalinity, through electrogeochemistry.^71,75,78,112

There are social concerns associated with the gas sequestration including fear of CO₂ leakages, lack of suitable locations, required site monitoring, unknown impacts on living organisms and limitations involved with CO₂ transportation and injection to storage sites. Alternatively, solid (carbonates) storage can be used. Such mineral carbonate sequestration is permanent, safe, has a large worldwide storage capacity and is less subject to social opposition.^328,365,366

6 Conclusions

Electrochemical CO₂ capture methods are undergoing a renaissance as their applications expand due to their higher energy efficiency, flexibility and sustainability compared to the conventional approaches. Electrochemical CO₂ capture is classified into four categories; (1) methods that apply a pH-swing to capture and recover the CO₂, (2) methods that rely on the binding affinity of CO₂ molecules to redox-active species, (3) molten carbonate cells and (4) hybrid electrochemical processes that combine CO₂ capture and e.g., direct conversion. Among the electrochemical capture methods, pH-swing based approaches, leveraging the carbonate equilibrium, are most widely studied, due to their straightforward operation and the absence of toxic or expensive chemicals. In theory, a mild pH-swing over ca. 2–3 pH units would allow to capture >98% of the CO₂. However, in practice, to improve the slow kinetics associated with such a mild swing, either a wider pH range (ca. 5–6 pH units) or catalytic enzymes (e.g., carbonic anhydrase) need to be applied.

An electrochemical pH-swing is induced via electrolysis, bipolar membrane electrodialysis (BPMED), redox active molecules that undergo proton coupled electron transfer (PCET) or capacitive deionization. Among all, electrolysis is the earliest method (used for alkaline absorbent regeneration), but is still rather energy intensive due to the intrinsic irreversibility of the gas evolution redox reactions. However, the produced H₂(g) (through water electrolysis) can be used to offset the total energy consumption. Capture via PCET active agents enable high current efficiencies, but are at the moment limited by slow electrode kinetics, low solubility of PCET organics and the sensitivity of the process to impurities in the flue gas such as O₂, water and sulfur. Using (membrane) capacitive deionization (MCDI) enables capture with an electrical energy consumption as low as 40 kJ mol⁻¹ using only deionized water. However, (M)CDI capture is so far only applied at very low current densities, is still in the very early stage of the lab-scale research (TRL of 3) and its large scale applications await more performance studies. All electrochemical capture methods can be used easily as plug-and-play units. Among the four technology for electrochemical pH-swing, BPMED has the advantage of keeping a small footprint upon up scaling. At present, the cost of using BPMED for CO₂ desorption and alkaline (re)generation in alkaline wet scrubbing is estimated to be 2–3 times more than the conventional thermal swing desorption rival. However, the cost can significantly decrease if improvements in the cost of renewable energies and ion exchange membranes, membrane life time and their permselectivity and resistivity are achieved.

Despite of the successful lab-scale demonstrations, both CO₂ capture via electrolysis and BPMED have achieved TRL of 5–6 and are not yet commercialized. This is while, the conventional post-combustion CO₂ capture methods, such as absorption, adsorption, membrane separation and chemical looping have found their way to industrial applications. As opposed to the electrochemical captures, conventional methods are mainly designed to capture CO₂ from centralized emitters such as fossil fueled power plants. These methods have a larger footprint and are not geared for efficient decentralised emitted CO₂. However, in the last decade, to benefit from the high efficiency and selectivity of the electrochemical processes, conventional methods are also promoted to be combined with electrochemical process. Substituting the temperature or pressure swings with an electrochemical swing in an amine based absorption capture is shown to halve the energy consumption of the CO₂ desorption step while decreasing the footprint of the unit. The framework established in this paper can be the basis for future studies on the energetics of electrochemical CO₂ capture processes, not only for flue gas separations, but also for a range of other applications, such as seawater CO₂ capture and direct air capture.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was performed in the NWO-cooperation framework of Wetsus, Centre of Excellence for Sustainable Water Technology (http://www.wetsus.nl). Wetsus is funded by the Dutch Ministry of Economic Affairs, the European Union Regional Development Fund, the Province of Fryslân, the City of Leeuwarden and the EZ/Kompas program of the “Samenwerkingsverband Noord-Nederland”. The authors like to thank the participants of the research theme “Concentrates” in Wetsus and research group “Transport phenomena” in faculty of applied sciences at TU Delft for the discussions and their (financial) support. This research received funding from the Netherlands Organization for Scientific Research (NWO) in the framework of the project ALW.2016.004. The work on aqueous carbonate equilibrium and CO₂ utilization is based on work performed by the Liquid Sunlight Alliance, which is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Fuels from Sunlight Hub under Award Number DE-SC0021266. The authors also thank the support from SoCalGas.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ee03382k

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