Competitive advantages of low-temperature electrolysis of CO2 to CO

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MSc Chemistry

Track: Science for Energy and Sustainability

Literature Thesis

Identifying competitive advantages of

low-temperature electrocatalytic reduction of

CO

2

to CO at an industrial scale

Anna Butter

10784632

October 2020

12 ECTS

Supervisor:

dr. B. van den Bosch

Examiner:

dr. K.J.P. Schouten

Second examiner:

prof. dr. G.J.M. Gruter

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1 Abstract

CO is a chemical building block used for many applications. Currently, the production of CO is centralized and fossil-based, resulting in a green-house gas emitting industry. The production of CO via electrochemical reduction uses CO2 as feedstock, which results in reduced dependence

on fossil feedstocks compared to current commercial methods. Besides this, the electrochemical CO2 reduction reactions (CO2RR) can be fueled by electricity, which can originate from

zero-carbon sources. However, CO2RR are currently not competitive. This literature study aims to

find opportunities to improve performance of low-temperature (LT) CO2RR compared to

high-temperature (HT) electrochemical CO2RR for the production of CO. It is found that the major

challenges that prevent scale-up of LT- CO2RR electrolyzers are a low current density and low

energy efficiency. This is caused by the low availability of CO2 at the cathode surface, lack of

research in cell designs that could potentially be scaled up and selectivity towards CO over the hydrogen evolution reaction. This thesis shows that by applying a high pressure in combination with methanol as electrolyte, the current density as well as the overall efficiency and thus the likelihood of commercialisation of this technology can be improved. This way, new opportunities arise to establish the competitive position of LT-CO2RR.

2 Introduction

2.1 Current industrial production and use of CO

CO is a useful chemical building-block and has a wide range of applications: it is used as a feedstock for the water-gas shift reaction (WGSR), as a reducing agent to produce high-purity metals and is used for carbonylation reactions in the fine-chemicals industry.1 One of the most notable carbony-lation reactions is the carbonycarbony-lation of methanol to acetic acid, of which around 6 million tonnes is produced annually with CO.2 Besides this, it is also used in combination with H2 as a feedstock in

the Fischer-Tropsch reaction to form hydrocarbons that are used as fuel.3Lastly, almost half of the world’s formic acid is produced using methylformate, which is synthesized by reacting methanol with CO.4

CO is produced when carbon is partially oxidized with a restricted supply of oxygen and is obtained on an industrial scale by gasification of coal, steam reforming or partial oxidations of hydrocarbons such as natural gas or naphtha.1,5 All of these processes require a high energy input. For instance, the gasification requires a pressure of up to 8 MPa, and both steam reforming and partial oxidation operate at temperatures above 1000 K. Another contribution to the high energy demand arises from separation steps that have to be implemented to obtain a pure product, as none of these industrial methods can produce CO with 100% purity.1

In the (chemical) engineering field it is generally known that larger plants operate more effi-ciently than smaller plants.6,7Because the energy input for CO production is high and to keep costs as low as possible, it is essential to optimize the efficiency. This results in large-scale plants and a centralized production.8 When the CO is purified, it is stored and transported in large quantities, which could be regarded as unsafe.

Assessing these fossil-based production processes from a sustainability point of view, not only the choice of feedstock should be improved, but also the CO2 emissions from CO production should

be addressed. Emissions are caused by both the heat supply for the reaction and direct emissions of the reaction.1 Transportation of the final CO product also contributes to emissions. Even though

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the centralized large-scale production is in general more efficient than small-scale processes, the contribution of transport emissions could rise significantly due to longer travel distances. Finally, the compounds made from CO are burned at the end of their life-time, releasing CO2 into the

atmosphere.

2.2 Electrolysis reduction of CO2 to CO

An alternative to the current situation is to produce CO via electrochemical reduction of CO2.

Electroreduction has the advantage that lower temperatures or pressures can be used compared to the fossil-based production methods. A process with a lower temperature and lower pressure creates the opportunity to produce CO on a smaller scale, thereby enabling decentralization of CO production, which results in a decrease of transport emissions and costs. Besides this, it is less costly to interrupt the reaction, enabling a non-continuous process that can run on demand. On-demand and on-site production of CO reduces risk of hazardous situations, as no large amounts of CO have to be stored.

It also promises a more sustainable process on a large scale. Reducing industrial heat demand has a substantial impact on CO2emissions in the Netherlands and is crucial to reaching an

environ-mentally friendly economy.9 Reports by both Berenschot and McKinsey show that decarbonizing the dutch economy requires effort in multiple areas, of which energy efficiency, electrifying heat demand, and carbon capture utilisation (CCU) are elements to which electrolytic CO2RR will

contribute.9,10

A situation in which the energy required for low-temperature electrolysis is supplied by carbon-neutral renewable electricity sources, such as hydropower, solar energy, and wind energy, would be a significant contribution to the transition to a more sustainable economy. Therefore, the combination of a low heat demand and CO2as a feedstock are essential elements of low-temperature

electrochemical CO2 reduction reactions (CO2RR), which show that efficient development and

implementation of this will result in a decrease in fossil fuel demand and CO2 emissions. Hence,

this process would contribute to the decarbonization of our economy. Realizing CO2RR on an

industrial scale will enable circular carbon use as seen in Figure 1, as opposed to our current linear situation.

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On top of this, lower temperatures allow the use of less complex and more abundant materials in the electrolyzer. This could improve degradation rates, or improve the recycle-ability of components of the cell after degradation. In addition, the input for electrochemical CO production is CO2 gas

instead of a fossil-based feedstock. CO2can be obtained from industrial exhaust gases, fermentation

of biological matter, or carbon capture from the air.

In this thesis both sustainability and economics are taken into consideration. The Atom Econ-omy is a parameter that is often used to assess the economics of a chemical reaction (Equation 1). A reaction is valued based on the amount of mass that is added to a reactant to form the favored product. Therefore, when comparing the atom economy of CO production from fossil feedstock or from CO2 (Table 1), CO2 does not seem like the favored choice, as the mass decreases during the

process. In the case of fossil-based CO production, mass is added, and the by-product, hydrogen gas, is valuable as well. However, the method to obtain the fossil-based sources, and the impact this has on the environment are not integrated into the formula of atom efficiency. This means that when comparing fossil-based CO production with electrochemical CO2RR, it is necessary to

evaluate the process from cradle to grave and consider both environmental impact, as well as pro-duction costs. This way, a complete comparison can be made, and exploitable advantages can be identified.

Atom Economy = Mass of desired product

Mass of reactants × 100% (1)

Source Reaction Atom Economy Fossil Based C + H2O −−→←−− CO + H2 233.4%

CO2RR CO2 −−→←−− CO + H2O 63.7%

Table 1: Atom Economy comparison between fossil based and electrochemical CO production. Water is assumed to be free.

2.3 Goal and outline of this thesis

Currently, high-temperature CO2RR are more mature, and is considered the more energy efficient

method.8 Additionally, electrochemical technologies for CO2RR at lower temperatures have been

less extensively explored compared to HT technologies, therefore overlooked opportunities might exist. This thesis aims to identify the competitive advantages of low-temperature electrolysis of CO2 towards CO.

This thesis explores the reasons why high-temperature is more energy efficient, and whether and how low-temperature electrolysis could achieve or surpass this on an industrial scale. Therefore, the advantages that are identified should be scalable and relevant for operation beyond lab-scale. Profitability is crucial for this, in the sense that the technology should be competitive with large-scale production, or could offer a solution to applications that are less explored in industry.

To find these advantages, we review the most mature technologies for HT electrolysis and inves-tigate why these are considered more energy efficient. This provides a benchmark for LT electrolysis. After this, the current state of the art for low-temperature is investigated and suggestions are made on how LT technologies can achieve a competitive position.

Theoretical background on mechanistic and thermodynamic aspects of electrochemical CO2RR

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electrolysis. The current state of affairs of high-temperature electrolysis of CO2 to CO will be

treated in Chapter 3, with the aim to gain insight into why this is considered to be more energy efficient. In Chapter 4 the working principle and state of affairs in the field of low-temperature is discussed and is compared to the benchmark set by HT technologies. We finish with a discussion and recommendation on where LT could enforce its competitive position towards HT. We discuss aspects such as energy efficiency, costs and fields/applications where LT electrolysis has potential applications and advantages to HT electrolysis.

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3 Theoretical background of electrochemical reduction of CO

2

to

CO

3.1 Working principles and thermodynamics of an electrochemical cell

CO2RRs are executed in an electrolyzer which consists of at least a cathode, an anode, electrolyte

and a voltage source. Reduction of CO2 occurs at the cathode, and is fueled by the electrons and

protons that are produced with an oxidation reaction that occurs at the anode. The electrodes can be separated from each other with a porous diaphragm or ion-exchange membrane, which is surrounded by an electrolyte. The function of the electrolyte is to facilitate the transport of ions and CO2 to and from the electrode. Electrolyzer designs vary strongly between HT and LT, and

depend on the phase of the input, reaction conditions and desired product.

CO2 can be reduced to several products. The reactions are summarized in Equation 2.

Half-reactions and their standard reduction potentials to the main products can be seen in Table 2.12In aqueous environments, P is either carbon monoxide, formic acid, formaldehyde, methanol, methane, ethanol or ethylene. Which product is dominant depends on the amount of electrons and protons available, the catalyst and the applied potential.

k CO2+ n(H++ e-) −−→←−− P + mH2O (2) Half-reaction E0 (V vs RHE) CO2+ 2 H++ 2 e– −−→ CO + H2O -0.10 CO2+ 2 H++ 2 e– −−→ HCOOH -0.20 CO2+ 4 H++ 4 e– −−→ HCHO + H2O -0.07 CO2+ 6 H++ 6 e– −−→ CH3OH + H2O 0.02 CO2+ 8 H++ 8 e– −−→ CH4+ 2 H2O 0.17 CO2+ 12 H++ 12 e– −−→ C2H4+ 4 H2O 0.08 CO2+ 12 H++ 12 e– −−→ C2H5OH + 3 H2O 0.09

Hydrogen Evolution Reaction

2 H++ 2 e −−→ H2 0.0

Table 2: Most common CO2 reduction half-reactions with their equilibrium potential

The electrode potential of a half-reaction is obtained via the standard Gibbs free energies of the reactants. For the formation of CO this is −0.10 V vs. Reversible Hydrogen Electrode (RHE).12 A more positive potential indicates that the molecule is reduced more easily. Since the HER and CO2RR can occur at similar potentials, it would imply that both reactions have a comparable

thermodynamic barrier and are therefore competitive. This is why for the production of CO the catalyst has to be selective towards CO2RR, and not the HER. Hydrogen as by-product causes

extra process costs because it causes the need for a purification step to be implemented. This can be prevented by the use of a catalyst which has a higher selectivity towards CO than H2, or by

tuning the reaction conditions.8

The cell potential is given by the electrode potentials plus the overpotential η, as in Equation 3.

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Ecell= E0cathode+ E0anode+ η (3)

∆G0= −nFE0_cell (4)

The cell is in equilibrium at the equilibrium potential E0_cell, which is lower than Ecell. The

equilibrium potential is obtained via (Equation 3), in which n is the amount of electrons transferred, F is the faradaic constant and Ecellis the potential of the cell. On top of E0cell, an overpotential η has

to be applied to drive the reaction forward. The overpotential can add substantially to the costs of the process: A higher overpotential means more kWh are needed, and thus the costs will be higher: 1 volt overpotential will cost almost e0.29 per kg CO extra (Appendix 1), if the price of 1 kWh is e0.15. Compared to the market price of CO, this is a significant amount: In 2018 the market price of CO was e0.51 per kg.13 Minimizing the electrode potential and resistance is therefore essential to design an economically feasible process, and emphasizes the importance of selecting a catalyst with a low overpotential towards CO. The right catalyst can stabilize the intermediate during the reduction reaction, thereby lowering the overpotential that is needed. The energetic effect of the catalyst is envisioned in Figure 2.

Figure 2: Reaction profile of CO2 reduction with and without catalyst, in the case where the . The

overpotential is denoted as η. With the right catalyst, the overpotential will be smaller (red).14

3.2 Mechanism of electrochemical CO2 reduction

The reduction reaction consists of several steps (Figure 3). First, CO2 has to adsorb on an active

site at the surface of the cathode, which is located at the interface of the electrolyte and cathode. When CO2 is adsorbed, a one-electron reduction reaction will take place, creating CO2–. This

reaction has a high activation barrier that is caused by the symmetry of CO2. Therefore, breaking

the symmetry to form intermediate CO2– requires an energy input. As a result, a large overpotential

compared to the standard reduction potential is needed, increasing the energy demand of the reaction.16 Adsorption to the catalyst is essential to form CO, because non-coordinated CO2 will

create another intermediate, CO2• –, that will be reduced to formate.17 The pH has a significant

influence on which product will be formed: A lack of CO2 or excess of protons will lead to more

H2 formation, therefore reduction to CO is often performed in basic environments.

After a second one-electron reduction of the intermediate, CO is formed on the catalyst. The product has to desorp from the cathode, and diffuse towards either bulk gas or liquid phases to

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Figure 3: Mechanism for CO2RR on metal surfaces in aqueous environments.15

exit the system. It is important that the catalyst can thus bind the intermediate, but is not able to reduce CO further. In Scheme 3 the catalysts used for CO2RR are divided into three groups:

the first does not bind the intermediate, the second binds the intermediate and can’t reduce CO, and the third binds the intermediate and is able to reduce CO even further. For reduction to CO, the second group is most suitable, of which Au and Ag are most often chosen.8,18 Similar to the cathode, the material of the anode also depends on the reaction that has to take place. In aqueous cells, IrO2 is most often used for an anode that performs the oxygen evolution reaction (OER).19

3.3 Influence of electrolyzer design

During the reduction, the availability of CO2 is a limiting factor. Due to the low its low solubility in

water, additional measures have to be taken to increase the concentration of CO2 near the cathode.

The design of the electrolyzer plays an important role in this.20A good electrolyzer enables optimal transport of CO2 to the electrode, and shortest diffusion length with minimal ionic and electronic

resistance.

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of the least amount of operational steps. However, to test a catalytic system in a lab, a batch or semi-batch cell is commonly used. The H-Cell (Figure 4) is an example of this. In this cell, CO2

gas is fed to the liquid electrolyte on the side of the cathode, called the catholyte. The gas stream is applied until the electrolyte is saturated. Then, CO2 has to transfer to the electrode surface to

be reduced. Nevertheless, even in the case where the catholyte is stirred well, the mass transfer of dissolved CO2 towards the electrode is limiting the H-cell to reduce large amounts of CO2. These

limitations restrict the experiments to current densities lower than 100 mA/cm2.21,22 However, due to its low costs and ease of assembly, the H-cell is often chosen to screen electrolyte and electrode efficiency on a lab-scale.

Figure 4: Schematic view of an H-cell used for CO2RR.18

The H-cell is thus a simplified cell, that will differ greatly in kinetics compared to a flow reactor as factors such as flow rate are not taken into account. Therefore the performance in a pilot-plant will be different.21

Which cell type is used, depends on the goal of the cell: to test cathode selectivity an H-cell is convenient, but to achieve high current densities and test conditions with the goal of industrial application a flow cell can be more adequate. A flow cell can operate continuously, and the elec-trolyte can either be a liquid or solid, depending on the temperature and material. Regardless of the phase, the flow cell overcomes the mass transfer limitations that are encountered with the H-cell. Cell configurations will be discussed in more detail in Chapters 3 and 4.

3.4 Desired achievements

Even though research is performed to understand and build electrolyzers to reduce CO2, there

are still barriers that need to be overcome in order to reach optimal performance and realize the step towards industrial electrochemical CO2RR. Currently, the main obstacles to achieving a high

efficiency are the activation energy to reduce CO2, mass transfer limitations and the conductivity of

the electrode and electrolyte. These factors are overcome by applying a high overpotential, which affects the economics of the process, and therefore the competitive position. Concluding, a high selectivity, stability and activity are crucial to realize industrial scale electrolysis. The aim is to reach the highest possible energy efficiency at high catalytic rates. In this thesis, only technologies that (could) perform at an overpotential of 2.5V at a current density of 150 mA/cm2 are taken into consideration.

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3.5 Parameters to assess performance

To assess the viability of an electrochemical process, several parameters are used. This section serves as a summary of the terms used to assess the performance of electrocatalytic systems.

1. Faradaic efficiency

The Faradaic efficiency (FE) indicates the percentage of the charge transfer that is used towards the desired product and is therefore a measure of selectivity. A low FE could be caused by the formation of side reactions or product recombination.

2. Current density

The current density is the current divided by the surface area of the working electrode, and is denoted as j in mA/cm2_{. It is a measure of rate of conversion.}23 _{A higher surface area}

corresponds to more active sites, and will result in a higher conversion rate compared to an electrode with a smaller surface area. j is dependent on the kinetics of the catalyst, as well as substrate concentration. Equilibrium of the reaction is obtained when the current density is zero. The partial current density jp for a certain product can be calculated by multiplying

j with the corresponding FE.23One of the most significant factor contributing to the current density are the mass-transport limitations. Each technique has a strategy to overcome these limitations, as it is the main challenge to overcome to achieve a high conversion.

3. Energetic efficiency

The energetic efficiency (EE) shows the overall usage of the energy towards the desired product and is therefore a measure of the efficiency of converting electrical energy into chemical energy. EE is defined as

EE =E

0_{· (FE)}

E0+ η (5)

in which E0 is the reversible cell voltage, FE the Faradaic efficiency and η is overpotential. The denominator shows the required potential for a given current density. As the EE gives a more complete picture of the efficiency losses than the FE and j, it is a useful parameter to assess scalability and financial viability.8

4. Overpotential

As mentioned previously, the overpotential is denoted as η and is defined as the difference in equilibrium cell potential E0 and the applied potential. It defines the extra energy input that is needed to complete the reaction. η consists of three parts: Ohmic, concentration and activation which is expressed in the following equation:

η−− ηohmic+ ηact+ ηconc (6)

When ions and electrons move through conductive media, they will experience resistance. This contribution is dependent on the conductivity, length and cross-section area of the conductor and is referred to as ηohmic. When the kinetic reaction or charge transfer at the electrode

is the limiting step, the activation overpotential ηact will be the main contributing factor.

In this case, the overpotential is used to overcome an activation barrier. The concentration overpotential ηconc is the contribution of resistance by the slow diffusion of reactants an

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products through the electrolyte when differences in concentration are present. Mass diffusion effects are the main cause of this to happen. When the transport of reactants and products to and from the electrode is slower than the rate the electrochemical reaction, diffusion will be the rate determining step. This rate determines the limiting current of the electrochemical process, and should therefore be as high as possible.24

5. Degradation rate

The stability of the catalyst is an important factor to decide on the industrial viability of a process. If the catalyst can not be used for a long period of time, the waste and replacement costs will rise substantially. Besides costs, environmental considerations should also be taken into account, which logically results in aiming for the least amount of waste possible.

6. Purity of the product

Separation steps increase the costs of a process, therefore the purity should be considered in assessing the economical viability.

7. Technological Readiness Level

The TRL scale is used to evaluate the maturity of a technology. The nine levels range from idea to production and can be found in Appendix A.

3.6 Objectives, boundaries and definition of industrial relevance

This thesis is written with the aim to identify competitive advantages of low-temperature elec-trocatalysis of CO2 to CO. If a technology uses temperatures above 150°C it is referred to as

high-temperature (HT), which is the case for solid oxide electrolysis cells (SOECs) and molten carbonate electrolysis cells (MCECs). Otherwise, the system is regarded as low-temperature (LT). Competitive is defined as industrially relevant, which implicates that a process or part of a pro-cess is scalable, (potentially) financially competitive and (based on) a proven or partially proven technology. A way to measure this, is the technological readiness level (TRL) of a technique (Ap-pendix A). The TRL level needs to be at least 2, and contain the potential to be scaled to TRL 5. More specifically, it has to be relevant for Avantium, whose aim it is to target niche-applications. Processes should have a current density of at least 150 mA/cm2, the catalyst should be (or have the potential to be) stable for weeks, and the overpotential not higher than 2.5 V. Most favorable are processes which produce the least amount of waste, and use earth-abundant materials.

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4 High-temperature electrolysis

4.1 Introduction

At this moment, the highest current densities with the lowest overpotential for electrocatalytic CO2RRs to CO are performed at a high temperature (Figure 5). The highest energy efficiency is

achieved at a high temperature as well (Figure 6). This chapter elaborates on the reasons why the performance of high-temperature technologies such as MCEC and SOEC perform is higher than for LT technologies, and what limitations and challenges influence their performance.

Figure 5: Comparison of low-temperature and high temperature performance: Current density and potential.8

In this chapter, different aspects of the best performing cells will be summarized with the aim to give a picture of the state of the art in the field, and to set a standard for the performance that low-temperature electrolysis should have to be competitive. Besides this, some promising research will be highlighted.

4.2 Considerations specific for high-temperature electrolysis

For all CO2RRs, the total energy demand ∆H consists of an electrical energy fraction ∆G, and heat

fraction T∆S. The relation between these factors for the reduction of CO2 to CO is envisioned in

Figure 7, which shows that at a higher temperature, the reaction becomes increasingly endothermic. This means high temperature systems use a larger fraction of heat energy compared to electrical energy.

Besides this, at high temperatures the system itself will create heat as a result of resistance. The heat that is produced by passing current is called Joule heat, and can be consumed by the reaction.25 This is said to increase the energetic efficiency of HT systems. In the case of LT systems, most of the heat will be lost to surroundings, thereby not making a significant contribution. Especially if waste-heat is present, operating at a high temperature becomes attractive, as heat is less costly as energy source than electricity.

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Figure 6: Comparison of low-temperature and high temperature performance: Energy efficiency and current density.8

4.3 Solid oxide electrolysis cells (SOECs)

4.3.1 Working principles of SOECs

SOECs are electrolysis cells based on the concept of solid oxide fuel cells (SOFCs). The SOEC performs the reverse reaction of the SOFC and can therefore consist of the same materials.8 Com-parable with LT electrolysis, the cell consists of at least a cathode to execute the CO2RR reaction,

an anode to perform the OER, and an electrolyte to conduct ions.27 However, the electrolyte is in this case a solid, that is sandwiched between the electrodes.

The electrolyte dictates the type of reactions that occur at the electrodes, as it can either be oxygen-ion conducting or proton conducting. The oxygen-ion conducting type allows oxygen anions to move from the cathode to the anode, where the ion is oxidized to molecular oxygen (Figure 8a). With this electrolyte, the feedstock should contain water as well as CO2. The protonic electrolyte

facilitates the oxidation of water at the anode, which produces oxygen gas and protons. Oxygen gas will leave the system, while the protons will travel to the cathode, where they can reduce adsorbed CO2 (Figure 8b). One of the advantages of a SOEC with a proton-conducting electrolyte is that

it can use CO2 without water as input. As a result, the output will not contain any water or H2,

which reduces purification steps.27

Note that the proton source is in this case water from the inlet stream, as opposed to low-temperature aqueous techniques, in which the electrolyte supplies the protons.

The reduction reaction in the SOEC takes place between CO2-gas, the ion-conducting

elec-trolyte, and the electron conducting electrode, which is defined as a triple phase boundary (TPB). At the TPB all aspects for electrocatalysis are present:, O2 – or H+-ions, electrons, and gaseous CO2.28 Because CO2 is fed as a gas at the electrode, the mass-transfer of CO2 is not limited to its

solubility, which is the case for some aqueous electrolyzers. As a result, higher current densities are achieved compared to electrolysis in aqueous electrolytes.27 The limiting factor is in this case

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Figure 7: Energy demand for CO2 reduction to CO26

Figure 8: Reduction of humified CO2 in SOECs with a) oxygen-conducting electrolyte, and b)

proton-conducting electrolyte.27 The cathode is depicted in green, the anode in orange.

thought to be the catalytic kinetics, and more specifically the bending of adsorbed CO2.29 The

barrier for this activation is easier to overcome at a high temperature, which explains why the SOEC operates at temperatures above 600°C.

4.3.2 Electrode materials

As the electroreduction takes place at the triple boundary point at the electrode surface, the amount of TBPs determines the rate of electrolysis and thus the activity. Therefore the cathode should be a good electron conductor, and has to be able to withstand high gaseous CO2 and H2O

concentrations. A composite of nickel with yttria-stabilised zirconia (YSZ) is most commonly used for the cathode as it has proven to be stable and affordable.27 This composite conducts both ions and electrons well, but is prone to sulfur poisoning, meaning the input of the reaction is limited to pure CO2.8 Alternatives such as perovskite-related oxides have been studied but did not show an

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improvement in stability compared to YSZ, explaining why nickel doped YSZ is still preferred as cathode material.26,29

The 3D structure of the electrode can also be tuned to improve performance or stability. For example, a nickel electrode with a high porosity can prevent the deposition of carbon (known as coking) onto the electrode surface. Coking reduces the conductivity of the electrode.30 Another aspect crucial to attain long-term stability is a matching thermal expansion coefficient of the solid electrolyte and electrodes to prevent mechanical tears.29This limits the choice of materials greatly.

For the anode, electron conducting oxides are currently considered the most promising choice, because these are less expensive than noble metals such as platinum and gold, and have comparable catalytic efficiency for the OER.29

4.3.3 Electrolytes

In principle, the solid electrolyte resembles the liquid electrolyte-membrane set-up that is used in low-temperature cells: it is able to facilitate ion transport, but not electron transport. The fundamental difference however, is that the electrolyte is solid.

To manufacture the solid electrolyte into the cell, it has to be easily shapeable into a robust thin film. For oxygen-ion conducting electrodes, stabilised zirconias such as YSZ and scandia-stabilized zirconia (ScSZ) are often chosen due to their stability in varying redox environments, low cost (YSZ), mechanical robustness and high conductivity. The conductivity of YSZ is highest for 1000°C, and decreases with a lower temperature.29 This is the same for doped ceria, which is also used because of its high conductivity. However, doped ceria is less redox stable. Doped LaGaO3 is

more suitable for temperatures in the range of 400 - 800°C, but shouldn’t be used in combination with nickel, which is often chosen as cathode material in SOECs.31Bismuth oxide is also a potential choice, but low stability and low melting point limit its application.29

4.3.4 Cell assembly in SOEC

SOEC cells often have a planar design, which means the cathode, electrolyte and anode are sand-wiched together. This cell design is also referred to as a ’zero-gap’ cell (Figure 9). A stream of CO2

is supplied at the side of the cathode and doest not come into direct contact with the electrolyte. Because the electrolyte is impermeable to gases, one outlet per electrode is required. As a result, the product from the anode does not mix with the product of the cathode. However, the outlet of the cathode contains a mix of CO with CO2 if not all CO2 is reduced. The cathode input can be

either pure CO2 or a mix of CO2 and H2.

Symmetrical SOECs use the same material for the anode and cathode, which could simplify the production of the cell. However, these are made from perovskite materials that are not yet commercially applicable due to stability issues.27 A tubular design of the electrodes is also under investigation, as this design is better in heat dissipation to reduce thermal stress, and can reduce start-up times compared to planar stacks.

In contrast with the SOFC, the SOEC reaction is endothermic, which can result in a temperature difference of 100°C lower at a distance of 10 cm from the inlet when operating in electrolysis mode.33 This difference can lead to varying degrees of expansion within the cell, causing cracks in the assembly. Therefore a good temperature distribution and moderate thermal expansion coefficients of the materials used are essential.

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Figure 9: Cross-sectional SEM image of SOEC with a NiO/YSZ support, NiO/YSZ cathode, YSZ electrolyte and LSM/YSZ + Ru2O anode.32

4.3.5 Challenges of CO production with SOEC

Even though SOECs are currently achieving the highest current densities for CO2RR, it still faces

challenges that prevent wide commercial implementation.34The main issues are coking or poisoning of the electrode and mechanical failure of the electrolyte.29 Reduction and decomposition at the cathode are observed for SOECs that use nickel. The deposition of coke on the electrode causes the electrode to degrade and eventually having to be replaced, which is unattractive for industrial processes.

The fast degradation of a Ni-YSZ cell is partly caused by the oxidation of nickel particles due to the combination of a high concentration of CO2 at a high temperature. These harsh reaction

conditions limit material choices. On top of this, the agglomeration of nickel particles on the electrode is often seen, which reduces the catalytic surface area.35

Another limitation SOECs face is caused by the large amount of time needed for heating and cooling down, making intermittent use difficult. Tubular designs have been proposed to overcome this issue, but are not yet used on large scale. One test has been performed that showed tunable selectivity for the production of syn-gas, and a stable system for 500 hours.36 This system has not been tested for the production of pure CO. Worth mentioning is that a tubular design also allows for the application of higher pressures, which has shown to influence current density and selectivity towards CO at low temperature aqueous electrolysis.37

Depending on the voltage at which the process is operated, the cell needs to be exclusively heated during start-up. If the applied potential is larger than the thermoneutral potential, the heat generated by the resistance of the current, Joule heat, will be used after the system has started.26,39 To ensure the system has enough energy input, it is therefore crucial to apply efficient insulation. If the system is operated above the thermoneutral voltage, the reaction will be exothermic, and the outlet will be hotter than the inlet. The resulting temperature gradient throughout the cell needs to be controlled, which is often done with either hot or cold air flows. Installing large blowers and heaters is a costly way of controlling the temperature of the system.25 Even though this is not represented in the EE, it does reduce the total efficiency of the production.

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Figure 10: Example of a tubular design for a SOEC. In this case used for production of syn-gas.38

4.3.6 Commercial availability

Currently, Danish chemical producer Haldor Topsoe has a commercialized small-scale SOEC-module that produces CO with a high purity of over 99.9% after purification. The input is pure CO2, and is heated to 800°C before it enters the system. Due to the endorthermic character of

the reaction, the temperature at the output is 760°C.40 It can deliver CO at quantities of 6 -200 Nm3/h.41They claim that usually, for these quantities of CO, the transport is done with trucks that have a tube-shaped gas cillinder on its trailer. These trailers are parked on-site until the tank is emptied, which means CO is stored in quantities that could be hazardous, and their small-scale SOEC will prevent this.42,43 After a patented purification step, the unit can deliver 99% pure CO. In test setting, the SOEC was stable for 7000 h when fed a pure CO2 stream.44

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4.4 Molten carbonate electrolysis cells (MCECs)

4.4.1 Working principle of MCECs

The MCEC, sometimes called molten salt CO2capture and electrochemical transformation

(MSCC-ET), also derives from its fuel cell (MCFC) analogue, but has a lower TRL than SOECs. The MCEC operates in the range of 450 - 1000°C and uses a molten carbonate as electrolyte. Where the MCFC uses a nickel cathode, the MCEC for reduction to CO employs stainless steel with calcium particles or transition metals such as titanium.45 At the cathode CO32 – is reduced to CO and oxygen-ions,

while at the anode the OER is performed (Figure 11, Equation 7). The carbonate is delivered by the electrolyte, which means that after some time, more oxide-electrolye than carbonate-electrolyte is present (for instance more Li2O than Li2CO3), gaseous CO2 can be added to the system to

balance this. The cell uses a different electrolyte in electrolysis mode compared to fuel mode.8

Advantages of this technology compared to other CO2RR methods are that the input stream does

not have to be as pure as is the case for SOECs or aqueous cells: sulfur-containing streams do not have a large effect on in reducing CO2, and the output will still be pure CO.46Besides this, the fact

that the product streams are separated is convenient, because a purification step is not needed.

Figure 11: Reduction of CO2 in MCECs.8

CO32−+ 2 e−−−→ CO + 2 O2− (7)

4.4.2 Electrolytes

The unique property of a MCEC is its electrolyte, which is a molten carbonate. This is either a single alkali carbonate such as Li2CO3, Na2CO3 or K2CO3, or can be an eutectic mixture such

as Li2CO3-K2CO3, Li2CO3-Na2CO3 or Na2CO3-K2CO3 et cetera. The main advantage of this

electrolyte is that solubility of CO2 is not an issue.

4.4.3 Performance of MCEC for CO production

At the moment of writing, no MCECs for CO2 to CO are available on a commercial scale. On

lab-scale the results seem promising, but the longest running tests do not exceed 100 hours.47This is caused by the highly corrosive nature of the electrolyte melt.

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The best performance to CO was achieved by Kaplan et al. with a titanium cathode, graphite anode and Li2CO3 electrolyte and operated it for 100 hours at 900°C, at a current density of

100 mA/cm2 and a FE of 96%.47

4.4.4 Challenges of MCECs

Side reactions that can occur at the cathode are shown in Equations 8 and 9. Reaction 8 shows that solid carbon can form, which will deposit onto the electrode. Reaction 9 is a result of the usage of a K-Li-Na electrolyte, and can happen together with 8 causing a mix of both products to precipitate onto the cathode.47

CO32−+ 4 e−−−→ C + 3 O2− (8)

Li+or K+or Na++ e− −−→ Li or K or Na (9) Using temperatures higher than 900°C can result in formation of CO2from CO via the Boudouard

reaction (Equation 10).47,48 This is an unwanted side reaction, and will cause the output to consist of CO with CO2 instead of pure CO. However, the MCEC has to employ this high temperature

to produce CO.46,47 A lower temperature is not suitable for producing CO, implying that MCEC might be a better fit for other electrolysis processes that can operate under lower temperatures.49

2 CO −−→_{←−− CO}2+ C (10)

The materials for the cell should be chosen carefully because of this high temperature and corrosive environment. For MCECs especially the container is a point of attention, as at the high temperatures used, the oxygen ions could accumulate and dissolve alumina and zirconia. This can be avoided by choosing a titanium container.47

Lastly, a high performing anode with low overpotential has thus far remained elusive.45

4.5 Discussion

SOECs are the first electrochemical cells to be commercially implemented for CO2RR. However, it

is not yet a replacement for large-scale CO production. This is mainly due to the stability of the systems: The high temperature CO2 creates harsh conditions that limit the lifetime of the SOEC.

The set-ups that have been proposed so far are not facile to disassemble, which would imply that the whole system is rendered useless when the electrodes are damaged. This is costly, as well as wasteful.

Besides this, the input of the system needs to be pure CO2, to prevent poisoning of the electrode.

Even though this could result in a product that does not contain by-products, a purification step needs to be done before the CO2RR. Overall, the purification step is only moved up in the process,

while eliminating this step would be better. Additionally, the purest form of industrially available CO2 is usually obtained from the production of ammonia or hydrogen. The feedstock for these

processes is natural gas. It would be favorable to use industrial flue gases as feedstock, which is one of the possibilities that MCECs offer. However, MCECs have not been tested on long time scales, so no statements can be made regarding long-term stability. It can be concluded that MCECs are not yet industrially relevant, and will not be further discussed in this thesis.

Regarding fossil feedstocks, not only the input is of concern. Even though heating is only needed during start-up of an SOEC if it is operated above its thermoneutral voltage, a high temperature

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needs to be generated. Heating on industrial scale is often fossil-based, which implicates that reduction of CO2 generates CO2 as well.10 A solution to this could be using waste-heat, but the

temperatures that are needed are not easily available. As a result, decentralisation is hard to realise, and large-scale production could still support the consumption of fossil resources. From a sustainability point of view the heat is a concern. Utilizing, and thereby funding, fossil resources seems a waste of opportunity, since electrochemical CO2RR actually opens possibilities to move

our industries away from fossil based chemicals. Even though the energy efficiency of SOECs is currently higher than LT systems, the use of fossil resources thwarts the realisation of efficient circular carbon use. Therefore, looking for ways to use CO2input that originates from fermentation

or other biological processes, should have priority. This means systems should be able to withstand impurities, which is not the case right now.

To summarize, while the energy efficiency and current density are impressive, SOECs need to improve regarding stability, sustainability, and robustness.

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5 Low-temperature electrolysis

First of all, why do we explore possibilities at low-temperatures? Intuitively, it would seem that operating at low temperature would be less costly, and creates the possibility of using water as electrolyte. This would overcome the mechanical issues that a solid electrolyzer faces.

Also, operating at a low temperature means that the energy input will be largely electrical, instead of thermal (See Figure 7). Even though heat is generally referred to as ’low grade’ energy and is therefore less costly, a high demand for electricity can still be advantageous. For instance, in the case where an intermittent electricity source is coupled to the system, it could perform CO2RR

when an excess of electricity is generated as a way of chemical energy storage. Currently, industrial heat generation is one of the main focus points to achieve decarbonisation of the Dutch economy. Therefore, electrifying the energy demand by operating at low temperatures could therefore be a selling point for LT-CO2RR.

As shown in Figure 12 (also shown in the previous chapter), LT has so far reached a lower current density and lower EE compared to HT techniques. The inferior performance of LT is mainly attributed to the diffusion limitations that occur when an aqueous electrolyte is used, which causes high overpotentials.

Figure 12: Comparison of low-temperature and high temperature performance: Energy efficiency and current density.8

This chapter explores the state-of-the-art of low temperature electrolysis to explain the differ-ences in performance with HT, and serves as a base to identify what advantages can be exploited. For this, the working principles of the cell are explained, after which the current performance and challenges are discussed.

5.1 Working principles and assemblies of LT electrochemical CO2RR cells

A fundamental difference between with high temperature systems is that low-temperature CO2RR

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into protons and oxygen ions during electrolysis, the electrolyte acts as the proton source for the reduction reaction. Besides this, it is the transporting medium for ions.

The pathway of the feed towards the electrodes can differ as well. CO2 can be supplied and

transported to the electrode via solvation in the electrolyte (Figure 13, left), which is similar to MCECs, as in both techniques the feed is present in the liquid phase. For LT techniques this is referred to as liquid-fed electrolyzers. Another route is via a gas diffusion layer (GDL)(Figure 13, right).12The feed reaches the electrode by diffusing through the layer, which facilitates the transfer of CO2 gas to the electrolyte-cathode interface.

Figure 13: Schematic view of low-temperature flow cells. On the left CO2 is supplied via the

electrolyte. On the right a GDL is applied.22

Opposed to this is a vapor-fed electrolyzer. In this case, humidified CO2 is supplied to a cell

in which no catholyte is present. The electrodes are sandwiched together with an ion exchange membrane in between (Figure 14). These cells are also known as membrane electrode assemblies (MEA) and have the advantage that less ohmic losses occur, because the travelling distance of ions is decreased due to the membrane being directly attached to the electrode.39 Additionally, it eliminates the risk of cathode degradation that occurs when impurities are present in the catholyte. However, if liquid products are also present, they can obstruct the pores.18 _{In the case of CO}

production with a high selectivity, this is less relevant.

All three mentioned cell designs contain a membrane, either attached to the electrode, or be-tween the catholyte and anolyte. It allows the movement of ions to and from the electrodes, while the exchange of products is restricted, which prevents re-oxidation.20,21 Either a cation exchange membrane (CEM), anion exchange membrane (AEM) or bipolar membrane (BPM) can be used (Figure 15). The CEM transports cations from the anode to the cathode. The AEM allows for transport of OH–, HCO3– and CO32 –. Lastly, the bipolar membrane facilitates movement of both

H+ to the cathode, and OH– to the anode. The BPM allows a more constant pH throughout the cell compared to CEM and AEM, but is more challenging to produce.21

The membrane has to be wet at all times to operate properly. In the case of an MEA, this is not guaranteed. As a solution, humidified CO2 is used as feed, but this does not eliminate the

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Figure 14: Schematic view of a vapor-fed cell with a MEA assembly.18

Figure 15: Transport of ions with a cation exchange membrane, anion exchange membrane or bipolar membrane.21

risk completely. Therefore the anolyte is essential to ensure the wetting of the MEA. The water management can be seen as a challenging factor when scaling up the system.50

5.2 Electrode materials

Silver and gold are most often chosen as catalyst, due to their high selectivity towards CO. These are noble metals, which means costs should be taken into account. Some sources state silver and gold are less expensive than other noble metals, but the price fluctuates significantly over time. However, silver has been signficantly less expensive over the past decade and is therefore considered an economical choice (Appendix B).51

Still, efforts are made to see how costs can be reduced while increasing the selectivity and activity. Ways to achieve this are for instance the use of nanoparticles on a support.50

Apart from silver and gold, other non-noble metals have been explored. For example, studies with zinc have been performed, but the performance is inferior to gold and silver. Also, stability tests longer than 10 hours have not been performed.52,53

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5.3 Electrolytes

5.3.1 Aqueous electrolytes

As mentioned, one of the reasons to operate at low temperatures is the ability to use an aqueous electrolyte. Regardless of the low solubility of CO2 in water, which is just 0.034 M under standard

conditions, it is often chosen due to its abundance and price.54

Compared to SOECs, the role of the electrolyte is similar. Both serve the transport of ions between the cathode to the anode. However, in the case of aqueous electrolyzers, the consumption of protons and generation of hydroxide-ions near the cathode will cause a difference in pH between the layer on the cathode, and the bulk electrolyte. These pH differences influence the product selectivity, and should be minimized.55This is done by increasing the flow or stirring the electrolyte. Besides selectivity, the pH also impacts the reaction rate, as more acid/base reactions will lead to less available CO2, and therefore less product. It can be concluded that an increased flow is

beneficial for the availability of CO2 near the cathode-electrolyte interface as well as reducing pH

effects.

Reducing pH effects can also be achieved by employing electrolytes that act as a buffer.56CO2

itself can also act as a buffer in aqueous media as seen in equations 11 and 12, and can neutralize OH– at the electrode surface.

CO2(aq) + H2O −−→←−− HCO3−+ H+ (11)

HCO3−+ OH−−−→←−− CO32−+ H2O (12)

Lastly, aqueous electrolytes consists of inorganic salts such as KHCO3 and KOH dissolved in

water. These ions help stabilize the CO2• – intermediate.57 The size of the cation that is dissolved

can direct the selectivity: a larger cation will result in a higher FE for CO.58

5.3.2 Non-aqueous electrolytes

Solvents such as MeCN, DMF, DMSO, and methanol have been explored as well, the main reason being the high solubility of CO2 in these solvents.59However, the reduction in aprotic solvents likely

follows a different reaction pathway, as there is no proton available. In this case, the intermediate is CO2• –, after which an oxygen ion is transferred from a surface intermediate to the solution (Figure

16).

Using an aprotic electrolyte creates the opportunity of avoiding the HER to occur, as no water is present at all, and it has different operational temperature ranges compared to water. However, the lack of a standard reference electrode is inconvenient. Besides this, if a flow cell set-up is used in which large quantities of electrolyte is pumped around, it is economically favorable to use water.

Ionic liquids can also be effective, as they can operate at an overpotential of just 0.2V with high FE.61 However, it is claimed that they are not yet economically attractive compared to water and organic solvents. Besides this, the production of this electrolyte is an energy intensive and polluting process.39

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Figure 16: Comparison of proposed mechanism of CO2 reduction to CO in aqueous electrolyte

(top) and aprotic electrolyte (bottom)59,60

5.4 Performance of low-temperature cells

5.4.1 Small-scale systems

An LT cell with a flat electrode in a CO2 saturated aqueous electrolyte at standard conditions is

limited to a performance of 20 mA/cm2 due to the low solubility of CO2. At current densities above

this limiting current, the mass transfer limitations of the product into the bulk electrolyte are of greatest influence. Both contribute to the fact that the energy efficiency is lower than current HT systems.62 To achieve a high EE, the overpotential should be as low as possible, while the FE as high as possible.

However, the combination of both a high FE and high current density is often a challenge. One example of this is the work by Haas et al. in 2018. This aqueous electrolyzer achieved a good FE by operating a GDE with a slightly elevated temperature.63 The elevated temperature caused a higher conductivity of the electrolyte, leading to a higher current density. Nevertheless, at both 30 and 60°C, the FE decreased from approximately 87% at 20 mA/cm2 to 40% at 200 mA/cm2.

Regarding cell potential, the most efficient small-scale systems operate at cell potentials in ranges of 1.8 to 2.0 V, whereas SOECs achieve high current densities with just 1.2 to 1.4 V. This results in a significantly lower EE for LT systems, which needs to be increased to be competitive. To increase the EE and current density, the availability of CO2 near the cathode is therefore key

point that needs to be improved.

5.5 Strategies to increase performance

Several tools are implemented to avoid CO2 mass transport limitations, increase selectivity, and

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5.5.1 Gas diffusion electrodes (GDE)

A gas diffusion electrode is an electrode that contains a gas diffusion layer. CO2 gas is purged

through the electrode, to create a triple boundary phase (TBP) on the electrolyte-cathode interface. This way the GDE circumvents challenges related to transport and solvation, as in this case the feed does not need to be dissolved. This, in combination with a shorter diffusion length, will lead to a higher current density.64 The selectivity is increased as well due to the formation of a saturated layer of CO2 on the catalyst, which has shown to prevent HER.65

Because the gas has to be able to diffuse through the electrode, it is by definition a porous electrode. It is built up from porous layers that are stacked onto eachother.66First, a gas diffusion layer is needed, followed by a microporous support on top of which a thin catalytic layer is placed. The catalytic layer consists often of nanoparticles or a very thin layer of catalyst (Figure 17). Because the electrode is porous, they are prone to blocking. Therefore the system is sensitive to impurities.18

For this reason, the absence of CO2 in the electrolyte is favorable, as no carbonate can react

with the hydroxide ions in the electrolyte to form salts, which is often an issue at high current densities.67 The crystallisation of hydroxides and bicarbonate salts block the active sites in the porous electrode and reduces the performance of the cell.

Figure 17: A TEM image of a cathodic catalyst layer of a GDE. The active sites are red, and are located at the TBP. In this case, the nafion membrane (CEM) is the ion conductor, and is seen in white at the top. CO2 diffuses through the pores, while electrons are supplied through the

support.66

Note that the difference between the liquid-phase GDE and MEA is that the MEA does not contain any liquid electrolyte at the cathode side of the cell. The membrane is attached directly on the electrode, which is not necessarily the case for all GDEs.

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5.5.2 High-pressure systems

The use of a GDE also allows working with a high pressure. This increases the availability of CO2

near the cathode surface.

Besides the availability of CO2, it was discovered in 1995 that the selectivity will be different

at a pressure of 30 atm compared to ambient pressure.37 The mechanism behind this is unclear, but it is suggested that the change in partial pressures of different products changes the activation barrier towards a certain product. However, there is no evidence for this. It is mention worthy that the greatest share of literature on the subject of high pressure CO2 reduction was published

at the end of the nineties. After 2002, a lot less is published regarding this subject.

Currently, combining a high pressure with high alkalinity achieves the best results for LT sys-tems. The results of this study are not included in Figure 12. This research was performed in 2018, making it one of the most recent works to make use of a high pressure for CO production.68 This study from Gabardo et al. shows that using a GDE in combination with a high pressure in alkaline media works nicely: at 300 mA/cm2, an overpotential of just 300 mV and EE of 81.5% were reported.

Combining a slightly higher temperature with a high pressure is also possible. Dufek designed a continuous system in which he not only uses a high pressure, but also applies the highest possible temperature with an aqueous electrolyte.69 At 90°C and 18.5 atm, FEs of 92% at current densities of 225 mA/cm2 _{were observed, with an overpotential of 2.9V. The FE reduces with increasing}

current density and temperature.

A high pressure combined with the use of an electrolyte such as methanol showed that the FE can also increase with increasing pressure.70 _{With reagent grade methanol at 40 bar and a Cu}

electrode, Saeki et al. obtained a current density towards CO2 reduction products of 426 mA/cm2

at a potential of 2.3 V. Note that this is not towards CO only, but this could possible be improved by using an Ag or Au catalyst.37_{Regardless of the low selectivity, the partial current density towards}

CO was 234 mA/cm2.

Lastly, even though the potential is not mentioned, the system of Hara et al. is very impressive. They reported in 1996 a a system that achieved 3000 mA/cm2_{at 70% current efficiency at 30 bar.}71

While it is not entirely clear why a high pressure increases the selectivity, there are several reasons why it increases the efficiency. Firstly, gas bubbles that can occur in aqueous cells are reduced. These gas bubbles reduce the number of available sites on the electrode, thereby decreasing conductivity, as gas has no electrical conductivity. By applying a high pressure, the bubbles are prevented, a higher conductivity is achieved, and therefore a lower overpotential.26 Secondly, the pressure also increases the kinetics of the system.59 It is also speculated that the increased mass transfer due to a higher concentration of dissolved CO2 pushes the reaction towards the product.

5.6 Efforts of scaling up and commercial systems

Krause et al. managed to scale up a low-temperature GDE cell, which consisted only of commer-cially available materials. With this set-up, they found that adding a pressure of 30 bar resulted in a less efficient and more costly conversion, which is contrary to what is expected. This is likely to be caused by the decreased conductivity of the electrolyte at this pressure. Without adding the pressure, the system worked best at 60°C.

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OPUS-12, a startup (2015) from the US produces MEA electrolyzers that can perform LT-CO2RR

to CO. Originally their focus was on the production of syn-gas, methylene and ethylene,72_{but they}

announced the start of a new project in July this year that combines a PEM with a biorefinery to produce CO from at least 150.000 tonnes of CO2 annually.73 They state that the system is ready

for scale-up, and the TRL is approximately 5/6. It is unclear whether this includes their patented electrolyzer, and if their set-up can produce pure CO. Nevertheless, it seems that their output contains at least reactant and one product, implicating a purification step is needed to obtain pure CO.74 The figure is not included here due to low resolution.

Another effort is made by Dioxide Materials, who designed a lab-scale set-up that uses a mem-brane with the cathode and anode painted on each side. This project is currently at TRL 4, and achieves an FE of 95% with a cell potential 3V. However, the current density is only 100 mA/cm2.50,75

5.7 Challenges of CO production with LT CO2RR

While for LT systems high FEs for CO are reported, the EE is still limited due to large overpotentials at high current densities.68 The lower EE is often explained by the fact that HT can use the joule heat from the reaction as energy input, which is not the case for LT systems. Also, the pH is a factor that influences the solubility of CO2, as well as the selectivity and thermodynamics. Besides

this, the type and concentration of salts in the electrolyte influence the local environment and can decrease the performance.59

Challenges related to pH arise mostly from the difference in bulk and local pH. Therefore the regulation of the pH throughout the cell is important, and is often done by employing a buffering electrolyte, or ensuring a flow. While the latter also aids the availability of CO2 at the cathode, it

adds a component to the cell, as well as a process to monitor. This could increase both operating and capital costs.

The combination of a flowing electrolyte with a GDE could be implemented to increase avail-ability of CO2, but this complicates the structure of the cell and thus production could become

more challenging. The rate of production of CO should reach hundreds N m3h−1 for the produc-tion of specialty chemicals, and in the ten-thousands for fuels.76 _{This means that to increase the}

total amount of CO that is produced in a certain time span, either the electrode surface could be increased or the current density. In the case of increasing the electrode size, the entire cell should match this, including the membrane. However, membranes are often produced on a small scale as larger scale membranes are more difficult and costly, partly because of the amount of waste that is produced in the process. Due to this, the diversity in supply of large membranes is limited, and processes now mostly use commercially available types such as Nafion.77 Another solution to circumvent this challenge is to stack multiple smaller cells, instead of increasing the size of a single cell.

In addition to implementing a GDE or MEA to increase the current density, the temperature, pressure and the amount of dissolved salts in the electrolyte can also have a positive impact on the performance.78

It can be concluded that in general the most influential factors on performance are

1. Cell design, to overcome mass transport limitations and to prevent mixing of input and output;

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3. Pressure, to increase mass transport and selectivity;

4. Temperature, to increase current density. While the temperature can increase the conductiv-ity, it can decrease the solubility of CO2 in aqueous electrolytes due to enhanced diffusion.

There are still combinations that are not tested. It would be helpful to investigate the perfor-mance of a cell in a more structured way. Ideally, the last three parameters are only tested in cell designs that are suitable for scaling up.

Regarding cell design, it seems obvious that the GDE is almost an essential addition to LT aqueous cells. Problems regarding solvation are avoided, and the highest performances mentioned in literature are achieved with GDEs. To summarize, a strategy to increase the availability of CO2

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6 Discussion and recommendation

6.1 Present advantageous aspects LT CO2RR

Without improving the current performance, several selling points already exist for LT that should be acknowledged:

1. Sustainability: Energy use

Firstly, the main source of energy for LT systems is electricity (Figure 7), and this source can be chosen to be carbon-neutral. For SOECs, the temperature needed is around 800°C. Even though it is possible that the heat is only necessary during the start-up, it should be available if the system has to operate intermittently. It is hard to find waste-heat at this temperature, and heat is often obtained from processing fossil-resources. In the case that heat is generated with electricity, an extra step is included in the process. This seems unnecessary and inefficient, as energy will be lost during conversion of electricity to heat. Using electricity directly as input is therefore a more sustainable choice.

2. Sustainability: Wastefulness

Besides the source of energy, the wastefulness of the process should also be evaluated. The degradation rate for SOECs is expected to be higher than low-temperature systems, due to harsher conditions. If, for instance, the cathode has to be replaced due to degradation, it is easier to remove from an aqueous alkaline cell, than a tubular SOEC, due to the fabrication method. However, it is unclear if cathodes at LT are stable for a longer period of time, due to the scarcity of long-term research of LT systems. Regardless, modularity often reduces waste, and is better achieved in aqueous cells.

3. Broader material choices due to less harsh conditions

A lower temperature, even with a higher pressure, could mean that the degradation rate will be lower. Also, this allows for a larger range of materials that can be used in the cell. 4. No thermal challenges

No challenges regarding heat distribution occur, which is an aspect SOECs currently struggle with. The temperature difference between input and output can differ up to 100°C because of the endothermic reaction. This causes uneven expansion and affects the mechanical ro-bustness. With LT systems, this problem is not present.

6.2 How to improve the competitive position of LT CO2RR

6.2.1 Energy Efficiency and competitiveness

How do we define competitive? The current parameters energetic efficiency and electric power consumption fail to give a complete picture of a process from feedstock to application of the product. Judging the efficiency should be done starting from the input to the final product. The final product is commonly regarded to be CO, but this is often processed to the actual commercial product. One example is the production of formic acid. The energy that is used to produce the compound that is made from CO should be incorporated in an efficiency parameter as well, as this enables a more precise judgement of which CO2RR technique suits an application best. This is envisioned in Figure

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Figure 18: Proposing the efficiency to application parameter, which includes the energy use from feed towards the application that is made from CO

Especially in a multi-step process, it is most efficient to maintain a constant pressure and tem-perature. Capturing CO2, reducing it to CO and reacting CO towards a product with commercial

application can be seen as such a multi-step process.

Focussing on processes that use CO and high pressures are one way to increase this overall efficiency. Looking broader than solely the production CO, and keeping the end-goal in mind could accelerate the commercial implementation of LT-CO2 techniques. For instance, CO is used in large

quantities to produce formic acid via methyl formate hydrolysis. This process used to be executed at pressures around 45 bar to obtain up to 95% conversion of CO, but newer plants operate at lower pressures and higher heat.79 Using pressurized CO and low heat would contribute to electrifying this industrial process, and would preserve the energy that is put into pressurizing CO2 during or

before electrolysis.

In a situation where flue gas is used, energy will be put into pressurizing the gas.80 From that point on, a high pressure could be maintained up until the production formic acid. It would be interesting to compare this to a SOEC, in which the temperature and pressure both fluctuate throughout the process.

Figure 19: The fluctuation of conditions of a process should be minimized, also when approaching the process as more than just the reduction in the electrolyzer.