Master Chemistry
Science For Energy and Sustainability
Literature Thesis
Recent Insights on different Parameters influencing the
Electrochemical CO2 Reduction Pathway
by
Marit Stoop
May 28th 2021
Student number
11909145
Research institute
Van 't Hoff Institute for Molecular
Sciences
Research Group
Heterogeneous Catalysis for Sustainable
Chemistry
Supervisor
Dr. A.C. Garcia
Daily Supervisor
Drs. Connor Deacon Price
Examiner
1
Table of content
Abstract ... 2
1) Introduction ... 3
2) Mechanistic insights of CO2RR ... 4
2.1 Electrocatalysts for CO2RR reactions ... 4
2.2 Possible products of the CO2RR reactions ... 5
2.3 Half-reactions of the products ... 7
2.4 Catalytical pathway on Cu for each Cn-category ... 8
2.4.1 C1 products ... 10
2.4.2 C2 products ... 11
2.4.3 C3+ products ... 11
3) Influences on the CO2RR pathway ... 12
3.1 Initial activation of CO2RR ... 13
3.1.1 Hydride transfer... 13
2.1.2 Concerted proton-electron transfer ... 14
3.1.3 Sequential proton-electron transfer ... 15
3.2 Reaction Conditions ... 15
3.2.1 Effects of the Electrolyte selection ... 15
3.2.3 Local solvent pH effects ... 21
3.2.4 Pressure and Temperature effects ... 22
3.2.5 Potential ... 24
3.3 Electrode Morphology ... 25
3.3.1 Particle size ... 25
3.3.2 Interparticle distance ... 27
3.3.3 Crystal Facet effects ... 27
3.3.4 Roughness ... 30 3.3.5 Defects ... 30 3.3.6 Oxidation State ... 32 4) Conclusion ... 33 5) Acknowledgements ... 34 6) References ... 34
2
Abstract
Electrochemical CO2 reduction combined with renewable electricity is a promising technology as
chemical energy storage in fuels to combat Climate Change. However, the complex CO2RR pathway is
still far from being completely understood. A large variety of CO2RR pathways are currently suggested
throughout the literature with no current standard for reaction conditions or electrode morphology. Only that Cu is the most suitable electrocatalyst as it can uniquely facilitate the C-C coupling in the mechanism and that *CO is an important intermediate are widely accepted in the literature. In this review, we provide a comprehensive investigation on the current views of the effect of reaction conditions, activation mechanism and electrode morphology on the CO2RR pathway. The CO2RR
products can be classified into C1 and C2+ products as the C3 products are rarely formed and far from
understood. The C2+ products are the most favourable for further application, therefore the focus of
this research. The most optimal reaction conditions for enhanced C2+ products are high near-surface
pH, high CO2 concentration and low overpotential. The CO2 concentration is influenced by electrolyte
selection, temperatures and pressures. Furthermore, a large nano-sized Cu crystal with low particle distance, high surface roughness, a facet structure of (100) and an oxidation state of an average of 0.4 will tune the CO2RR towards C2+ products. Defects in the crystal structure can influence the electrode
morphology, thus influencing the reaction pathway as well. These influences can tailor the CO2RR
towards the most lucrative product for further application.
Keywords:
3
1) Introduction
Our current habits of living are producing CO2, which causes severe environmental issues like global
warming, environmental pollution and climate change.1–3 The current CO
2 concentration in the
atmosphere keeps rising, leading to the importance of new carbon-free or carbon-neutral technologies.4 To meet the climate goals of the Paris agreement to keep the global mean surface
temperature below 1.5-2.0°C above pre-industrial levels, carbon capture utilization and storage (CCUS) is a crucial technology in the industrial sector and power generation.5–7 The CO
2 that is captured can
be converted into hydrocarbons with high energy densities and ease for transportation and storage. Different techniques can be used to convert the captured CO2, including photochemistry, biochemical,
thermochemical, electrochemistry and various hybrid approaches.8
Electrochemical CO2 reduction reaction (CO2RR) can store energy in the form of (multi-)carbon
oxygenates and hydrocarbons for further applications.9 Possible applications are, for example,
chemical building blocks and fuels.6 Numerous different catalysts have been tested by Hori et al. on
their product selectivity of the CO2RR, all of which lead to different products.10–18 Only Copper (Cu) has
the unique ability to catalyse the CO2 into multi-carbon products by carbon coupling (C-C) with
reasonable efficiency and selectivity. The efficiency of the C2 and C3+ products is due to that the C-C
coupling in the CO2RR is competing with the C-H, C-O and H-H under the standard reaction conditions
in an aqueous solvent.19 Another problem with this reaction is the selectivity of the Copper catalyst
because it can generate 16 different C1-C3 hydrocarbons and oxygenate products.20 One of the most
debatable topics of the CO2RR is the reaction pathway and mechanism from CO2 to several products.21
One accepted step is that *CO is an important intermediate, but the rest is still under discussion. Not only the catalyst and the solvent but also the pH, cation species and other reaction conditions influence the selectivity of the CO2RR.22
Cost reduction of the electricity from renewables is increasing the intermittency and the grid balancing problem as the renewables are unpredictable.23–25 Essentially, energy storage will be a key element in
the future to enable more and more electricity generation from renewables. One manner is to convert the electrons that are generated by the renewables towards fuels.6,26 Currently, the electrochemical
reduction of CO2 to fuels and feedstock is evolving as a promising technique in the industry to solve
the renewable resources’ intermittency and close the carbon cycle if powered by renewables.27 The
CO2RR products have the advantage to be implemented into the already built infrastructure for
long-term storage of renewable energies, compared to the electrolyzation of water to hydrogen. The majority of the C2+ products possess higher energy density, market size and socioeconomic value
4 producing certain C2+ products and optimizing the efficiency with both experimental and theoretical
approaches.
Taking all these points into consideration, if the CO2RR could be tuned to a certain valuable
multi-carbon product, it would be a gigantic step for further application. Currently, the CO2RR reaction does
not have a specific product that it produces. The direction of the pathway can be tuned by varying reaction conditions. In this review, the current status of the reaction pathway and all the factors that influence the CO2RR will be discussed. Evidentially, this review will lead to a summary of the most
suitable reaction conditions and electrode morphology to increase the selectivity towards the most valuable products.
To answer our research question, we start with an overview of the mechanistic insights from the past years for the electrochemical CO2RR. The different types of catalysts for CO2RR and the possible main
products will be briefly addressed. This followed by a discussion on the different pathways suggested throughout the literature. Preceding with the influences of the selectivity and activity of the CO2RR,
which cover the reaction conditions (pH, electrolyte selection, cation/anion species, temperature, pressure) and the electrode morphology (particle size, interparticle distance, facet structure, roughness, defects and oxidation state). These influences will be described on how they change the selectivity of the CO2RR.
2) Mechanistic insights of CO
2RR
The catalytical mechanism/pathway for the CO2RR is a challenging subject,30 and is intensively
researched over the past three decades with theoretical and computational investigation methods.31
A suggested cause of this challenge is related to the fact that electrochemical reduction of CO2 involves
many different proton/electron transfer steps, which leads to the formation of many intermediates and products. For example, that there are at least 16 different products formed from the CO2RR when
copper is used as an electrocatalyst, according to Kuhl et al. (2012).32 However, to develop the most
effective catalyst, it is crucial to understand the mechanisms of all the different products during the CO2RR.31 Therefore, in this section of the review the mechanistic insights of CO2RR with Cu as a catalyst
into valuable (multi)carbon products are addressed. First, the explanation why Cu is the unique catalyst for CO2RR into multi-carbons and then the mechanistic understanding over Cu will be discussed.
2.1
Electrocatalysts for CO
2RR reactions
At the end of the 20th century and the beginning of the 21st century, Hori et al. largely contributed
towards the starting understanding of electrocatalyst for CO2RR reactions.10,13–18,33 They concluded that
5 organic products with high faraday efficiencies. However, Hori et al. noted that other catalysts can reduce CO2 and therefore four groups of metal electrodes were identified, see Figure 1. The first group
(Pb, Hg, Tl, In, Sn and Cd) mainly produce formate (HCOOH), the second (Ag, Au and Zn) produce mainly carbon monoxide (CO), the third group (Ni, Fe, Pt, Pd, Ga, Fe and Ti) reduces very little CO2 but instead
reduces water to H2, and the last group (Cu) produces products beyond *CO.10,13–18,33 The latter is the
group that this review is mostly interested in.
The difference in the classification of the groups as noted before was recently confirmed by an investigation of Bagger et al. (2017).20 Their research resulted in the conclusion that the main cause of
the difference between the product distribution is the binding energies of key CO2RR and HER
intermediates, which include *H, *OCHO, *COOH and *CO. This conclusion is in line with the Sabatier principle which states that adsorption energy is a crucial factor for the electrocatalyst choice for CO2RR
reactions.20,31,34,35 When the catalyst has too strong of a *CO adsorption energy, the catalyst can be
poisoned as *CO will be unable to desorb from the surface and no metal is available for further reactions. On the other hand, if the adsorption energy is too weak, CO will desorb from the surface and not react any further. Therefore, only a catalyst with suitable adsorption energy for *CO can catalyse CO2 into products further reduced than CO.20 In Figure 1b, the binding energies of *CO and *H
of the different catalysts are presented. From which can be concluded that Cu is a unique catalyst that is suitable for CO2RR towards products beyond *CO due to positive adsorption energy of *H and
negative adsorption energy of *CO.20 Therefore, the main emphasis of this review will be on Cu or Cu
derivatives as electrocatalysts for CO2RR.
2.2
Possible products of the CO
2RR reactions
As discussed before, there are at least 16 different products that can be detected with CO2RR on
polycrystalline Cu catalyst.32 These products, therefore, vary from C
1 to C2+ and from hydrocarbon to
ester products depending on the reduction conditions.32,35 The possible products for the Cu catalyst
according to Kuhl et al (2012) are presented in Figure 2.32 This observation was obtained by a custom Figure 1: Metal electrocatalysts for electroreduction of CO2 in a) major product classification and b)
their binding energies of ΔECO* and ΔEH* . From Bagger et al. (2017).20
6 electrochemical cell designed to maximize product concentrations coupled to gas chromatography and nuclear magnetic resonance for identification and quantification of the products in gas and liquid phase. The reason that this custom electrochemical cell is used, is because then all products will be detectable via GC chromatograms and NMR spectra. However, other studies also included formaldehyde,30 oxalic acid, oxalate as main products, depending on the electrolyte and catalyst.35
One study reported that the formaldehyde can be formed from the methanol produced during the CO2RR.36 Furthermore, oxalate has not been observed as a major product for the CO2RR on a Cu
electrode.10 These experiments lead to the exclusion of oxalate and formaldehyde in our assumption
of the products that can be formed in the CO2RR.
The products can be classified on the number of carbon atoms of the molecules into C1, C2 and C3+
products. In Figure 2, the C1 products are the products in the blue box and include formate (CHOOH),
methanol (CH3OH), carbon monoxide (CO) and methane (CH4).32 As mentioned in the previous section,
the selectivity towards C1 products largely depends on the type of catalyst that is used in the CO2RR.
When Cu is used as a catalyst, the major product is methane.9 The C
1 products all have industrial use
and therefore have a marked value.36,37 For example, formic acid can be used for organic synthesis and
preservative and antibacterial agent;34 Methanol is a gasoline additive and can produce formaldehyde
and acetic acid;16 and methane is fuel and can be reformed to syngas for further chemical
purposes.36,38 And CO can be used for methanol production and the Fischer – Tropsch synthesis. The
industrial value for CO is significantly lower as it needs reprocessing before further utilization.36
The CO2RR is highly dependent on the pH and electrolyte. In organic solvent, like acetonitrile, the C2
products that are produced from the CO2RR are glyoxal (C2H2O2), acetate (CH3COOH), glycolaldehyde
(C2H3OOH), ethylene glyoxal (C2H4(OH)2), acetaldehyde (CH3CHO), ethanol (C2H5OH) and ethylene
(C2H4).32 Whilst the major C2 products in an aqueous solvent when Cu is used as electrocatalyst in the
CO2RR are ethylene and ethanol.9,39 Similar to the C1 products, the C2 products also have significant
values in the chemical and fuel industry.36,39 For example, ethylene and ethanol can be used for the
production of chemical feedstocks to produce polyethylene, esters and other chemicals which can also
7 be used as fuel. Also, acetate can be used as feedstock and for food processing and acetaldehyde for organic synthesis.36 Ethylene is arguably the most interesting product, as it has the highest market
price and also has the highest normalized price (price per KWh).34
Finally, the last category is the C3+ products of the CO2RR reaction catalysed by Cu. The C3+ products
include hydroxyacetone (CH3COCH2OH), acetone (CH3COCH3), allyl alcohol (CH2CHCH2OH),
propionaldehyde (CH3CH2CHO) and propanol (C3H7OH).32 Mainly propionaldehyde or propanol are
produced during the CO2RR.40 All of these products are hydrocarbons and can be used as energy
vectors or in the chemical industry.36 However, the selectivity towards C
3+ products is markedly
decreased in the CO2RR.21,40
2.3
Half-reactions of the products
In an electrochemical cell, half-reactions play a crucial part in the understanding electron transfer which makes it important for mechanistic evaluation of CO2RR.41 The half-reactions that occur are
based on electron transfer on a conductive electrode, called the anode and cathode.31 In an
electrolyser, at the anode oxidation occurs and at the cathode the reduction takes place. Therefore, the reaction that we are interested in (CO2RR) takes place at the cathode. The reduction of CO2 can
happen over different electrocatalysts and with varying numbers of electron reduction pathways at different potentials.35 The difficulties behind forming multi-carbon products is that carbon-carbon
coupling competes with the formation of C-H and C-O bonds under CO2RR conditions.21
The CO2RR equation can be viewed as a multiple proton-electron reaction leading to various products
and water, see equation 1.30,31 The most typical products in aqueous media with their equilibrium
potential are shown in Table….31 However, this table is by no means exhaustive. These reactions take
place at the cathode. Another reaction that also is crucial for the CO2RR pathway is the CO reduction
reaction (CORR). In many studies, CO is identified as a key intermediate for multi-carbon products. CO can be produced from a two proton-electron transfer of CO2, see Eq. 2. Moreover, the reaction
equation of CORR into products (Eq. 3) is important for the CO2RR pathway. 30,31,41
𝑥CO2+ 𝑛H++ 𝑛e−→ 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 + 𝑦H2O Eq. 1
CO2+ 2H++ 2e−→ CO(g) + H2O Eq. 2
𝑥CO + 𝑛H++ 𝑛e− → 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 + 𝑦H
8
Table 1: Electrochemical Reactions with Equilibrium Potentials in aqueous media. Adapted from Nitopi et al. (2019).31
These reactions, when in aqueous media, are balanced with the oxygen evolution reaction (OER) at the anode side, see Eq. 4.31 With the OER, water is electrochemically converted into oxygen, four
2 H2O → O2+ 4 H++ 4 e− Eq. 4
protons and 4 electrons. Instead of the CO2RR or the COR, also hydrogen can be formed at the cathode
when in aqueous media (Eq. 5). This reaction is a competitor at the electrode as it takes place in the same range of potentials as CO2RR. Therefore, it is not favourable for hydrogen to evolve in the CO2RR
reactions as then less metal is available for the CO2RR. 30,31,41
2.4 Catalytical pathway on Cu for each C
n-category
As noted before, the catalytical pathway for CO2RR is a challenging topic in the electrochemistry sector
and there have been published several different pathways throughout history.17,18,30,32,39,42–48 The most
comprehendible pathways of CO2 reduction to C1 and C2 products on polycrystalline copper are
presented in Figure 3. This figure illustrates that all of the products that were introduced by Kuhl et al. (2012) were formed,32 and consists of multiple different pathways to achieve some of the
products.17,18,30,32,39,42–48
(Product) Name,
abbreviation Molecule Reaction
E0/ [V vs
RHE]
Formic acid CO2+ 2H++ 2e−→ HCOOH (aq) -0.12
Carbon monoxide CO2+ 2H++ 2e−→ CO(g) + H2O -0.10
Methanol (MeOH) CO2+ 6H++ 6e−→ CH3OH (aq) + H2O 0.03
Methane CO2RR: CO2+ 8H++ 8e−→ CH4(g) + 2H2O
CORR: CO + 6H++ 6e−→ CH
4(g) + H2O
0.17 0.26
Acetic acid 2CO2+ 8H++ 8e−→ CH3COOH (aq) + 2H2O
0.11
Acetaldehyde 2CO2+ 10H++ 10e−→ CH3CHO(aq) + 3H2O 0.06
Ethanol (EtOH) CO2RR: 2CO2+ 12H++ 12e−→ C2H5OH(aq) + 3H2O
CORR: 2CO + 8H++ 8e−→ CH
3CH2OH(aq) + H2O
0.09 0.19 Ethylene CO2RR: 2CO2+ 12H++ 12e−→ C2H4(g) + 4H2O
CORR: 2CO + 8H++ 8e−→ C
2H4(g) + 2H2O
0.08 0.17
Propionaldehyde 3CO2+ 16H++ 16e−→ C2H5CHO(aq) + 5H2O 0.09
Propanol (PrOH) 3CO2+ 18H++ 18e−→ C3H7OH(aq) + 5H2O 0.10
9 The reason that the CO2RR pathway is so difficult to define is because the pathway is influenced by the
initial reaction mechanism, reaction conditions such as electrolyte species, local pH near the electrode surface, pressure, temperature, potential, and the electrode morphology. Thus, the pathway varies enormously in different conditions.31 There are no standard conditions in the electrochemistry section,
therefore it is impossible to determine the initial CO2RR pathway.49 In this section, we will emphasise
and explain a few of the main pathways to extend the knowledge on a suggested pathway. These pathways are for C1 products, the suggestion of Kortlever et al. (2015), and for C2 products, one of the
most recent proposed pathways of Garza et al. (2018) will be reviewed.30,39
10
2.4.1 C
1products
Firstly, we start with discussing the proposed pathway of the C1 products by Kortlever et al. (2015), see
Figure 4.30 The pathway of C
1 products has been investigated extensively because the pathway contains
less complex and fewer possible reactions compared to the multi-carbon products.39,47,50 The C 1
pathway towards the *CO intermediate is the starting route of the entire CO2RR catalytical pathway
towards multi-carbons, as the electroreduction of CO results in the same product distribution as that of CO2.21,31,39,40
Figure 4: Possible pathway of CO2RR towards C1 products.
The first step of the CO2RR reaction towards a C1 product is a single-electron transfer to form CO2
-radicals.1,2,15,19 These radicals are unstable and are thus quickly protonated to form (1) a bidentate
intermediate (*OCHO) or (2) an adsorbed carboxyl (*COOH). Of course, this only happens when the solvent is aqueous, in section 2.2.1 the influence of the solvent will be discussed. The *OCHO can then be reduced by protonation and desorption from the surface of the catalyst to form formic acid. The other intermediate, *COOH, is further reduced by the removal of water to a *CO intermediate. As previously mentioned, this intermediate is an important intermediate in the CO2RR because the
adsorption energy of the *CO intermediate and the catalyst is related to the selectivity towards different products. The *CO is protonated to form formyl (*CHO) or hydroxymethylidyne (*COH) intermediates, which are further reduced to methanol and methane by six or eight protons and electrons.30 This proposed pathway produces all 4 of the C
1 products that were previously detected in
11
2.4.2 C
2products
Secondly, the pathway to produce C2 products will be discussed in this section. In Garza et al. (2018), a pathway that
produces all of the observed C2 products of CO2RR was proposed.39 Together with a recent study of Todorova et al. (2020)
on CO2RR towards multi-carbon products with Copper catalysts,51 a possible mechanism is suggested here. C2 products will
not be formed under just any conditions, for example, the facet (100) of the Cu electrocatalyst for C2 is most suitable.39 The
influences on the selectivity of the pathway will be discussed extensively in section 2. However, the C-C coupling mechanism is one of the most debated topics of the CO2RR.52 Especially, the disagreement is surrounding whether the
process happens over a proton and/or electron transfer.39,51,53,54
Figure 5: Possible pathway of CO2RR towards C2 products.
The mechanism starts with a *CO intermediate as this is accepted as the key starting intermediate.39,51
The mechanism towards *CO is the same as in the pathway of the C1 product until the *CO
intermediate is produced. Furthermore, two of the *CO intermediate will reduce together with an electron and proton to form a glyoxal intermediate, see Figure 5. This intermediate is further reduced to a single glyoxal molecule or a *COCH2O* intermediate. The glyoxal intermediate can then be
reduced to a glyoxal molecule, which can itself be further reduced with numerous electrons and protons to produce either ethylene glyoxal, glycolaldehyde, ethanol or acetaldehyde. The *COCH2O*
intermediate can be reduced to a *COHCH2O* intermediate and eventually with different amounts of
electrons and protons, ethylene or acetic acid will be formed.
2.4.3 C
3+products
Finally, the mechanistic understanding in the formation of C3+ products in CO2RR and CORR are poorly
understood, as they are rarely formed.40,55,56 As seen in Figure 3, the main emphasis in mechanistic
understanding of CO2RR and COR reactions are on C1 and C2 products. The expectations are that the
C3+ products are formed from C2 intermediates which undergo further coupling with CO.40,56 However,
this estimation is still under debate and no direct pathway has been suggested. Propanol and propionaldehyde are the main C3+ products and hydroxyacetone, acetone and allyl alcohol are
12 produced in small amounts.40 It is possible to form C
4 products in CO2RR reactions, but are scarce and
have been limited to FE <7% Due to these reasons, we will focus solely on the formation of propanol and propionaldehyde.
Figure 6: Possible pathway of CO2RR towards C3 products.
Currently, there are only two main suggestions for the pathway on of C3+ product formation.56–58 First,
a study of showed that n-propanol is formed on the agglomerates of the oxide derived copper nanocrystals by the C-C coupling of CO and C2H4, see Figure 6a.57 Oxide derived copper nanocrystals is
a catalyst with a higher electrocatalytic active surface due to the quick removal of the oxygen during the reduction of oxidized Cu.31 In this pathway, the O atom is attached to the Cu surface. On the
contrary, more recent studies have shown that the C-C coupling between CO and acetaldehyde can result in C3+ products, see Figure 6b.56,58 Here, one of the C atoms is attached to the Cu surface. Even
though, the CO insertion reaction seems most likely to be the cause of the C3+ products, more
mechanistic work is necessary to fully understand the formation of C3+ products.40
3) Influences on the CO
2RR pathway
All of the suggested CO2RR reaction pathways from Figure 3 are proposed under different conditions.31
The usual CO2RR process can be divided into three different steps, 1) the chemisorption of the CO2 on
the surface of the electrocatalyst; 2) the transfer of the electrons and protons to break the C=O bonds and/or produce the C-H, C-O and C-C bonds; and 3) the desorption from the catalyst surface of the products.59 These steps are influenced by the activation mechanism of the CO
2RR as that includes the
way the CO2 is bounded to the surface, reaction conditions and the electrode morphology as it
influences the chemisorption of CO2 and desorption of the products. In Figure 7, a schematic overview
of the influences on the CO2RR reaction is presented.40 Theses factors all will be discussed thoroughly
13
3.1 Initial activation of CO
2RR
First, the initial activation concerns how the CO2 is attached to the surface of the electrocatalyst.
Whether the C atom or one of both of the O atoms is attached to Cu, influences the direction of the pathway.40 There are three different types of activation possible in the CO
2RR with Cu: concerted
proton-electron transfer, sequential proton-electron transfer, or hydride transfer.60–68 The activation
of CO2RR starts with an activation redox reaction.40 In the previous section, the catalytic pathway that
we chose solely focussed on that the activation would happen by sequential proton-electron transfer. The activation mechanism depends on several reaction conditions and the electrocatalyst.66 There are
several controversial publications about the initial activation mechanism of the CO2RR reaction,40
however, in this section we will discuss a few of activation and how the initial activation influences the direction of the reaction pathway of the CO2RR.
3.1.1 Hydride transfer
Firstly, the hydride transfer as initiation reaction for the CO2RR reaction involves an anionic hydride
that is attached to the surface of the electrocatalyst, see Eq. 3.1 and Figure 8.40,62–64 Then, there are
two pathways that the anionic hydride can follow. The first pathway is that the hydride then reacts to the carbon atom of the CO2 molecule by a nucleophilic attack. With the exchange of numerous protons
and electrons, formic acid is then formed. The second pathway with the anionic hydride is that it will combine with a proton towards hydrogen. Clearly from this activation mechanism can be seen that the HER is the competing reaction with the CO2RR. This initiation reaction results solely in the production
of formic acid and hydrogen.40
∗ + H++ 2e−→ ∗ H− Eq. 3.1
Figure 7: : Influences on the CO2RR pathway, a) intitial activation, b) electrode morpology, c) the overal reaction pathway and d) reaction and process conditions. From Birdja et al. (2019). 40
14
Figure 8: Activation of CO2RR by Hydride transfer and its pathway
2.1.2 Concerted proton-electron transfer
Secondly, in the concerted proton-electron transfer (CPET) the reaction is initiated by the reduction of CO2 by a proton and electron at the same time towards *OCHO, *OCO*, *COOH or *HCOO-, see Eq.
3.2-3.5 and Figure 9.67
The *COOH intermediate will most likely be the beginning of the *CO intermediate, which is assumed to be a crucial part of the C-C bond formation in CO2RR. The other intermediates will result in formic
acid or formate as the main product. These assumptions can be confirmed by the DFT calculated binding energies. These calculations prove that selectivity towards formic acid or formate is higher when the surface is bonded to the oxygen or hydrogen atom of the intermediates.67 These
intermediates are mostly proven by the computational community and is doubted by the molecular electrocatalyst to be the main activation mechanism.40
Figure 9: Activation of CO2RR by concerted electron-proton transfer and its direction of pathway
∗ + CO2+ H++ e−→ ∗ CHO ∗ Eq. 3.2
∗ + CO2+ H++ e−→ ∗ OCHO− Eq. 3.3
∗ + CO2+ H++ e−→ ∗ HCOO− Eq. 3.4
15
3.1.3 Sequential proton-electron transfer
Lastly, the molecular electrocatalyst community assumes that the initial activation is mainly by the binding of CO2 to the electrocatalyst by an electron transfer binding step, see Eq. 3.4 and Figure 10. 40
The CO2 will then bind to the electrocatalyst surface as presented in Eq. 3.5-3.6 and then can further
react to the crucial intermediate of *CO to further react and participate in the CO2RR pathway.60,61,65,68
The CO2- anionic adduct is bound to the metal center of the catalyst. For example, with a Cu catalyst,
the CO2 binding takes place when the oxidation state of the Cu center reduces from Cu(ii) to Cu(i).40
Afterwards, subsequent protonation steps will generate *COOH or *COOH- intermediates.60,61,65,68
However, if the formation CO2 anionic adduct is the rate-determining or potential determining, the pH
of the electrolyte will play a crucial role in the rest of the pathway, which will be discussed in the next section. 40
Figure 10: Activation of CO2RR by sequential proton-electron transfer.
3.2 Reaction Conditions
Multiple reaction conditions influence the CO2RR activity and selectivity, including the nature of the
solvent, the local pH of the solvent, the identity of ionic species, overpotential, temperature and pressure.21,40,41 These physical and/or chemical factors change the thermodynamic adsorption energies
of the intermediates and the kinetic barriers of the reactions, leading to alternative reaction pathways.21 To fully understand and to be able to tune the reaction in a certain direction,
understanding the affectability of all the different reaction conditions is crucial.
3.2.1 Effects of the Electrolyte selection
In an electrochemical cell, the electrolyte has the primary function to conduct the electric charge between the electrodes. The electrolyte consists of three different components: an inert electrolyte, solvent, and the electroactive species.41 The electrolyte influence the catalyst performance by the
concentration, different species, buffer capacity and pH value.40,69 The CO
2 molecules are continuously
supplied to the catholyte to dissolve until the CO2 is saturated in the electrolyte by the chemical
equilibrium between the liquid and gas phases (Henry’s law):70
∗ + CO2+ e−→ ∗ CO2 Eq. 3.4
𝐶𝑢(𝐼𝐼) + 𝑒−→ 𝐶𝑢 (𝐼) Eq. 3.5
16 The amount of CO2 dissolved in the electrolyte depends on the pressure, temperature and salinity of
the electrolyte.70 The CO
2 that is dissolved in the electrolyte will be transported in the medium to the
cathode surface by convection and diffusion to further react to the final products. The electrolytes also contain a ionic component that conducts the electrons of the electrochemical cell. The catalyst-electrolyte interface is constructed as an electric double layer, which is called the Helmholtz plane, see Figure 11.71 In the inner Helmholtz plane (IHP), the reaction intermediates and other covalently bonded
species are present, and in the outer Helmholtz plane (OHP) the hydrated ions are present. The Helmholtz plane is determined by the type of ions and the applied electrode potential.
For the CO2RR, the electrolyte must have certain properties to be suitable for the electrochemical CO2
reduction towards (multi-)carbon products. The first requirement is that CO2 needs to be dissolved in
the solvent. Another requirement is that the solvent needs to be stable and chemically compatible with the electrode materials and the active catalyst. Moreover, the solvent needs to have a low viscosity, leading to good CO2 mass transfer from the bulk electrolyte to the electrode surface.41
Momentarily, the electrolyte types that could be utilized in CO2RR reactions are: aqueous solvents of
inorganic salts, organic solvent electrolytes and Ionic liquids, which will be discussed here with their influence on the CO2RR pathway.
CO2(g) → CO2(dissolved) Eq.
Figure 11: (a) Schematic illustration of the electric double layer composed of the inner Helmholtz plane (IHP) and outer Helmholtz plane (OHP) with chemical equilibria involved. (b) Schematic illustration of possible effects of the interfacial ions on the catalyst surface or the electrocatalytic process under the CO2RR conditions with the negative potential applied. From Sa et al. (2020).71
17
3.2.1.1 Aqueous solvent of inorganic salts
The most common solvent for CO2RR is water and most of the studies are performed in an aqueous
environment with an inorganic salt to conduct the electrons.72 Unfortunately, there are challenges
with water as a solvent for the CO2RR. To start off, the solubility of CO2 in pure water is significantly
low, 34 mM at standard conditions, and when salts are added it will cause the salting out effect, lowering the solubility even more.73 The solution to the low solubility is to use a gas-diffusion based
setups or a membrane electrode assembly reactors with a gas phase. In these situation the gaseous CO2 is in contact with an electrolyte close to the electrocatalytic surface.74 Another downside to water
as solvent in the electrolyte is that the HER reaction, than, is a competing reaction with CO2RR, leading
to lower Faradai efficiencies.30 The selectivity and activity of the CO
2RR reaction depends on the choice
of electrolyte (anion/cation) and the pH of the solution. Cationic effects
To allow for a conducting medium, salts are added to the aqueous medium.21,72 However, the choice
of cations in the salts will influence the selectivity of the CO2RR reaction, which has already been
proven by Murata and Hori in 1991.75 They concluded that when increasing the Stokes Radii, from Li+
to Cs+, the selectivity towards C2 products increases, see Figure 12a-b. Wherewith Li+ as an
electrocatalyst, the HER was more favourable. The explanation Murata and Hori provided for this phenomenon is that the potential of the OHP, which has been discussed before, is induced by changing the cation size. Moreover, Resasco et al. (2017) discovered that the change of the OHP potential stabilizes the intermediates bound to the surface of the electrocatalyst.76 Then, when Ringe et al.
(2019) found the same connection between the cation effect on CO2RR on Cu surfaces and the changes
in the electric field.77
Figure 12: Explanations for the cation effect on CO2RR: a) Change in outer Helmholtz plane (OHP) potential; b) Hydrolysis of water molecules comprising the hydration shell of the cation; c) The cation acts as a catalytic promotor. From Moura et al. (2019).72
18 However, the influence of the cation in the electrolyte for CO2RR on the selectivity of hydrocarbons is
still under debate. The CO coverage on the electrode is sensitive to the choice of cation, which directly influences the CO2RR and the HER.78 One other explanation was introduced by a study of Pérez-Gallent
et al. (2017), which included that the cation effect is more due to a catalytic promotor effect, see Figure 12c.79 With the promotor effect the cations stabilize the intermediates, leading to a change in free
energy landscapes of the reduction. The catalytic promotor effect was supported by a study by Akhade et al. (2018) and Sato et al. (2020).80,81 The main observation is that the K+ stabilizes the CO
2 adduct on
the metal center. The study of the effects how the cation influences the product distribution is an essential step to tune the selectivity and activity of the CO2RR.78
Anionic effects
Not only the cations of the salt will affect the CO2RR, but also the anion will affect the selectivity and
activity of the CO2RR. 21,72 Hori et al. (1989) already discovered that the non-buffering anions (Cl-, ClO4
-, SO4-) provide for higher selectivity towards C2H4 and CH3CH2OH, which means also a lower selectivity
towards CH4 and H2, compared to bicarbonate anions (HCO3- ). Moreover, the results also concluded
that the phosphate anions (H2PO4-) lead to a higher selectivity towards H2 and CH4.15 However, in a
more recent study by Resasco et al. (2018) the composition and concentration of the anions only has an effect on the formation of H2 and CH4, but only has a small effect on the formation of CO, HCOO-,
C2H4, and CH3CH2OH.82 The explanation for this was the difference is due to the ability of the buffering
anions to donate hydrogen directly to the electrode surface which competes with water. This was supported by Jackson et al. (2019) that showed that phosphate can outcompete water as a proton donor in the HER on a gold electrode.83 Furthermore, Hashiba et al. (2018) resulted in a similar
conclusion that the buffer capacity influences the CO2RR selectivity.84
The effect of halide anions on CO2RR is the main focus of most studies, including the study of Varela et
al. (2016) who studied the effect of the halide anions by adding the salts to a 0.1 M KHCO3 electrolyte.85
The nature and the concentration of the halide anions (Cl-, Br- and I-) determines the activity and
selectivity of the CO2RR. The main product when Br- and Cl- is the anion in the electrolyte is CO, while
when I- is the anion methane is the main product. Another result is that the surface morphology
changes by the type of anion,85 which was confirmed by the fact that the type of adsorption of the
anions on the catalytic surface is different for each anion.86 However, when no HCO
3- is present in the
electrolyte, the product selectivity increases to C2 products.87,88 The halide anions change the
coordination environment of the surface-bound CO and then increasing the C2 product formation,
19
3.2.1.2 Non Aqueous solvents
One of the main bottlenecks of electrochemical reduction of CO2 is its competition with the hydrogen
evolution reaction (HER).21,40,69 The use of organic solvents or ionic liquids can be an alternative to
minimize the competition with H2 evolution during the reaction since it is possible to apply a higher
cathodic potential window due to the difficulty of breaking the C-H bonds when compared with O-H bonds from water. In this way, higher overpotentials can be applied, and more selective reduction products can be obtained because of the suppression of hydrogen evolution. However, to be efficient and to reaction to proceed, it also requires free or easily reducible protons in solution. Moreover, organic solvents are a suitable electrolyte solvent as they have higher solubility of CO2 compared to
water. For example, the solubility of CO2 in acetonitrile is about 8 times that in water.89 The
non-aqueous solvents that can be used are classified in protic organic (methanol), aprotic organic (acetonitrile, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), hexamethyl phosphoramide (HMPA), propylene carbonate (PC), tetrahydrofuran (THF)),69,90 and multiple ionic solvents.91
Protic organic solvents
One polar protic solvent that has been extensively investigated on its effect on the CO2RR reaction is
methanol.92–96 The reaction products of CO
2RR on Cu as electrocatalyst with methanol are similar to
those in an aqueous solvent only with higher Faradaic efficiencies, which are hydrocarbons and alcohols such as methane, ethylene and ethanol. The reaction pathway either happens with a proton transfer from the solvent to the surface intermediate or an oxygen transfer from the surface intermediate to the solvent. A proton transfer from the solvent to the surface intermediate results in the formation of hydrocarbons, formate, methanol or ethanol, whereas the oxygen transfer occurs in the formation of carbon monoxide. We can conclude that protic organic solvents have similar pathways as in aqueous solvents, as discussed above. 56
Aprotic organic solvents
Over the past five decades, numerous reports about aprotic organic solvent electrolytes have been published on their effect on the CO2RR reaction activity and selectivity.69,72,97,98 One of the most
researched organic solvents is acetonitrile.89,97,98 For example Tomita et al. (2000), who studied the
CO2RR reaction at platinum electrodes in acetonitrile-water mixtures. The Pt can reduce CO2 to oxalic
acid in acetonitrile, and with increasing concentration of water the selectivity is towards formic acid.89
Similar observations are presented by Figueirdo et al. (2016), as Cu electrodes in acetonitrile with small amounts of water influences the selectivity towards carbonate, bicarbonate and copper.98 The reaction
is highly sensitive to water, leading to the formation of carbonate or bicarbonate. Whenever acetonitrile is not dried properly, it can act as a weak protic solvent.98
20 The reaction mechanism is different in an aprotic organic solvent as there are three different pathways, see Figure 13. The three different pathways are the formation of oxalic acid via dimerization of two CO2*- molecules; 2) disproportionation reaction where CO2*- and CO2 form CO and CO32-; and 3) in the
presence of water the formation of formic acid due to protonation of a CO2*- radical followed by an
electron transfer.99–101 One of the influences that the authors noticed was an increased oxalate
formation with a decreasing electron capability of the aprotic solvent.69
Figure 13: The three different pathways in aprotic solvents. Based on Amatore et al. (1981), Constentin et al. (2013) and Singh et al. (2016).99–101
Ionic liquids
Recently, ionic liquids (IL) have gained attention in the electrolyte choice in CO2RR due to their high
CO2 adsorption capacity, high selectivity and low energy consumption.91 Zhou et al. (2014) studied the
comparison between the CO2RR in a variety of ionic liquids with different concentrations of water (20,
40, 60 and 80 wt%) and Ag, Au, Cu and Pt as electrodes.102 An Ag electrode in BMIM-Cl electrolyte with
20 wt% water was the most suitable with a current density of 2.4 mA cm-2 and a selectivity of >99%.
The results where explained by the assumption that the Cl- strongly bonded to the hydrogen atoms,
21
Figure 14: The 'Carbene' pathway in IL solvents to ethanol, propanol and ethylene. Based on Zhang et al. (2020).103
More recently, Zhang et al. (2020) discovered that the CO2RR reduction with different IL suppresses
the formation of propanol, ethanol and ethylene through the ‘carbene’ (*CH2) mechanism without
influencing the other products.103 In the ‘carbene’ mechanism, see Figure 14, the *CO intermediate is
reduced to a *CH2 intermediate and then can or be coupled with a CO intermediate to *COCH2 or
dimerised to ethylene. The *COCH2 intermediate can follow three different pathways. The first
pathway is the reduction to ethylene as well. Secondly, a protonation to *COHCH2 with a further
reduced towards n-propanol, or lastly reduced to a *CH2OHHC*, resulting in ethanol. This research
showed the promise of the tunability of the CO2RR product spectrum with the IL modification effect.103
3.2.3 Local solvent pH effects
The pH is an important factor for the activity and selectivity of the CO2RR as the protonation process
is involved in every step of the CO2RR.21,72 Additionally, it influences on the CO2/bicarbonate
equilibrium, see Eq. 3.7-3.9.104 The CO
2 molecules have complex acid-base equilibrium in an aqueous
electrolyte, but can mostly be seen as CO2 (aq) as only 1/1000 will be hydrated to carbonic acid.70 The
equilibrium is different for each pH, for example in Eq. 3.8 which is in a basic environment and can further neutralize the OH- at the electrode surface (Eq. 3.9). The electrochemical selectivity of CO
2RR
can be tuned by the local pH at the electrode interface, which is different from the pH of the bulk electrolyte. 21,72 The influences on the local pH at the electrode surface is dependent on the current
density, the buffering strength of the electrolyte and the mass-transport of OH-, CO
2, HCO3-, and CO3
2-.104
CO2(aq) + 𝐻2O ↔ HCO3−+ 𝐻+ Eq. 3.7
CO2(aq) + OH− ↔ HCO3− Eq. 3.8
22 The surface pH affects the HER reaction on the electrochemical surface because the HER is pH-dependent.21,72 A previous research of Hori et al. (1997) reported the importance of the local pH on
the CO2RR, resulting in a pH-dependent pathway towards methane, while ethylene is pH insensitive.17
More research on the pH influences on the CO2RR pathway has been investigated by the Koper group
which confirms these claims that the pH-dependent C1 pathway that primarily produces methane and
forms ethylene by dimerization of intermediates and a pH-independent C2 pathway that produces
ethylene via the formation of a CO dimer intermediate.53,105,106 In Figure 15, a suggested pH-dependent
mechanism is presented based on the publication of Schouten et al. (2014).106
Figure 15: pH dependent reaction pathways on the Cu(111) surface. Based on Schouten et al. (2014). 106
All of these studies and their research led to the conclusion of the tunability of the CO2RR selectivity
by the pH according to the studies of Varela et al. (2016; 2016; 2020).85,107,108 The main cause is that
the pH near the surface is expected to increase due to proton consumption. And when the pH (alkaline) is high near the surface, the HER is suppressed and the selectivity is towards the C2 products.85 By
stirring an alkaline solvent, the mass transfer is increased and the pH near the surface will be more similar to the bulk pH and then leads to a larger selectivity toward CO compared to CH4.109The
near-surface pH depends on the current density, thickness of the diffusion layer and the buffer capacity of the electrolyte. To tailor the selectivity by altering the pH, these parameters all can/need to be considered to achieve the tailoring of the CO2RR.107
3.2.4 Pressure and Temperature effects
As previously mentioned, the CO2 dissolved in the electrolyte depends on the pressure and
temperature and salinity of the electrolyte.70 The CO
2 that is dissolved in the electrolyte will be
transported in the medium to the cathode surface by convection and diffusion to further react to the final products. According to Henry’s law and the acid/base buffer equilibria (CO2/HCO3-/CO32-), the
23 concentration of CO2 in the solvent will be increased by lowering the temperature or increasing the
pressure. Multiple studies investigated the influence of the temperature and pressure on the CO2RR
or COR reactions, but a direct comparison between these studies is difficult to draw. In Figure 16, the concentration of dissolved CO2 in the electrolyte as a function of the pressure and pH with different
temperatures is displayed.31
Trends between most reaction products and temperature or pressure were not clear.31 Except for CH 4,
multiple studies have reported that with decreasing temperature the selectivity of CH4 increases. 13,110– 115 A few other studies showed that the production rate of C
2H4, 13,112 and CO, 13,110 increases at higher
temperatures. The overall CO2RR rate only shows minimal changes by temperature, however, the HER
is favourable at higher temperatures.116 This leads to an overall higher selectivity for CO
2RR at lower
temperatures and lower selectivity at higher temperatures where the HER is dominant.31 In a more
recent study by Sargeant et al. (2020), the electrochemical conversion of CO2 at subzero temperatures
resulted in similar conclusions as a decade before: the combination of the high CO2 concentration in
the electrolyte and the decrease in the competitive HER due to the low temperatures leads to a change in the product selectivity.115
A similar relation is found for the pressure, as the CO2 or CO pressure increases, the total CO2RR rate
increases as well.116–119 For pressures below 1 atm, the HER is dominant and suppressing the CO 2RR
reaction. Attributing by an increase in pressure, an increase in local CO concentration and surface coverage will lead to the enhancement of a C2H4, according to Kas et al. (2015).118 The assumption is
that these trends are most likely due to the changes in the dissolved concentration of CO2 in the
electrolyte. However, systematic studies about the impact of temperature or pressure alterations over a broader range of potentials and conditions will benefit the mechanistic understanding of the CO2RR.31
Figure 16: Effect of pressure and temperature on the pH and concentration of dissolved CO2 in 0.1 M KHCO3 electrolyte. From Nitopi et al. (2019). 31
24
3.2.5 Potential
In an electrochemical system, the potential is the energy that is needed for the reaction to happen. The start of the C1 or C2 pathways from Figure 3 all occur at different ranges of applied overpotentials,
leading to a dissimilar start of the CO2RR. The selectivity of the CO2RR from the studies of Wang et al.
(2018) and Hahn et al. (2017) show a clear ratio of oxygenate/hydrocarbon products for the potential-dependent trend, see Figure 17a).120,121 When dealing with lower overpotentials the selectivity for
oxygenates is higher, and at higher overpotentials the hydrocarbons are dominant. Both studies
suggest different expected causes for this ratio: Wang et al. (2018) suggested that this trend could be the result of the smaller driving force for the C-O bond scission due to the less polarized reaction intermediates at more positive potentials, forcing in a higher selectivity in the production of oxygenates.122 Whereas Hahn et al. (2017) suggested that the higher selectivity of oxygenate at
more-positive intermediates is due to the lower surface coverage of *H.121
Multiple studies showed that the *CO-CO dimerization pathway is energetically more favourable at low overpotentials and *CO-COH is dominant at high overpotentials.54,123 Wang et al. (2018) reported
a similar relation between the C1/C2+ product distribution as the C2+ is enhanced at more positive
potentials at both COR and CO2RR, see Figure 17b). The expectation is that the lower *CO surface
barrier compared to the *CO hydrogenation results in this product distribution.122 Due to these
expectations, the reaction pathway is different at each potential. One suggested influence on the pathway is published by Schouten et al. (2011;2013) and more clarified by Zheng et al. (2019), see Figure 18.21,50,124 This pathway is based on a DFT calculation on the CO
2RR where *CO is a key
intermediate. At an overpotential of U > -0.6 V, two *CO intermediates couple to form *COOC* and then is protonated to *COHOC* and reduced to form ethylene. Whereas, for overpotentials U < -0.8 V, the *CO intermediate is protonated to *CHO, which can form methane or can couple with *CO towards ethylene as well. The overpotentials in the middle (U = -0.6 to -0.8 V) favour the HER.21
Figure 17: Product selectivity CO2RR and COR on Cu catalyst as a function of the potential: (a) oxygenate/hydrocarbon ratios, and (b) Faradaic efficiencies for C1, C2, and C3 products. From Wang et al. (2018). 122
25
Figure 18: Calculated potential dependent reaction pathways on the Cu(100) surface. Based on Schouten et al. (2011; 2013) and Zheng et al. (2019). 21,50,124
3.3
Electrode Morphology
As aforementioned, the CO2RR is a structure sensitive process as the electrode morphology can
influence the CO2RR performance.40,90 Both nanostructured and mesostructured electrocatalysts have
been shown to influence the CO2RR in a particular manner.40 On the nanoscale, the crystal facets,
defects and oxidation state play a huge role. Whereas, the particle size, interparticle distance and surface roughness affect the CO2RR on a mesoscale level.90 Tailoring the shape of the particles can lead
to materials with superior activity and selectivity. Consequently, the causes of the effects by the electrode morphology will be discussed in the following section.
3.3.1 Particle size
The particle size of the electrocatalyst has a crucial effect on the reactivity of the CO2RR reaction.31,40,90
The main effect of the particle size is when nanocrystals are used for the electrocatalyst, which is therefore the emphasis of this section.125 The cause of the effect is due to the increase of surface to
bulk atom ratio as the size is decreased, which can be the result of different possible effects.31 First,
the d-band of the catalysts is shifted by the induced strain on the small particles of the surface and thus influences the reactivity.126 Secondly, the curvature of the surface of the particle increases and
the coordination of the surface atoms is lower when the particle size decreases, due to a high occurrence of uncoordinated sites.127 Both of these possible effects depend on the process and the
26 Due to a multi-electron transfer process in the CO2RR, the prediction of the effect of change in the
electronic structure is too difficult to determine.31 However, the difficulty of the prediction did not
stop a few groups to investigate the effects of the nanoparticle size of the CO2RR electrocatalyst.128–130
All of these studies resulted in different trends as they use different catalyst synthesis methods, leading to differences in size, morphology and stability. Moreover, clustering of the Cu particles on the surface occurs under the reaction conditions, leading to change in size, shape and distribution of the electrocatalyst.131,132 Therefore, the results are not in line with each other and show different trends
concerning the effect of particle size on the CO2RR.128–130
Firstly, Reske et al. (2014) reported that reducing the particle size to smaller than 6 nm increased the current density at a fixed potential, leading to increased product rates of CO and H2, see Figure 19a.
The increased selectivity towards CO and H2 is due to the increase of surface area and the decrease of
the binding energy of CO and Cu. Moreover, compared to polycrystalline foils, the nanoparticles exhibit higher selectivity towards CO and H2 than towards hydrocarbons.128 On the contrary, later that year
Manthiram et al. (2014) discovered that when performing the CO2RR with a potential of 0.95 and
-1.45 V RHE and Cu nanoparticle size of 7 nm the selectivity is orientated towards methane.130 Due to
these contradicting results, Loiudice et al. (2016) investigated the effect of the size and the shape of the nanocrystals. The focus of this research was on larger cubic nanocrystals of 24-63 nm, which showed that for ethylene, a nanoparticle of 44 nm at -1.1 V vs RHE has the highest Faradaic efficiency and selectivity.129
More recently, an investigation about the size effects on the selectivity and activity of CO2RR of Cu
nanoclusters by Rong et al. (2021) resulted in the conclusion that the larger the nanocubes, the higher the selectivity and activity towards C2+ products.125 Taking all these research points into considerations,
the research shows that the increase in size of the nanosized catalyst will lead to an increase in activity and selectivity towards C2+ products. As Reske et al. (2014) showed that small nanocrystals will result
in a higher selectivity towards CO and H2 as products.128 Then, when combining with the results of the
latter two research, the multi-carbon products will be formed by larger nano-sized copper catalysts.125,129
Figure 19: Cu nanoparticle size dependence of (a) the composition of gaseous reaction products (balance is CO2) during catalytic CO2RR, (b) the faradic selectivity’s of reaction products during the CO2RR. From Reske et al. (2014).128
27
3.3.2 Interparticle distance
Not only the particle size but also the loading of the number of particles (also known as the interparticle distance influences on the reactivity of the catalyst on the CO2RR. The interparticle distance is a
mesoscale phenomenon.31,40,90 Mistry et al. (2016) investigated the tuning of the selectivity at the
mesoscale via interparticle interactions on the CO2RR reactions. The results show that re-adsorption
of the CO intermediates followed by further reduction towards other products was more likely to happen with small interparticle distances and larger nanoparticle sizes.133 In Figure 20, the results
presented by Mistry et al. (2016) are presented to provide for a more in-depth understanding.
Moreover, the interparticle distance is directly related to the loading of the catalyst onto the surface, which was studied by Kim et al. (2017).134 An ensemble of densely packed copper nanoparticles enables
selective conversion of CO2RR reaction towards multi-carbon products. Which was supported by Wang
et al. (2017) that investigated the catalyst particle density of copper oxide nanoparticles’ influence on the product formation of the CO2RR.135 Recently, the influence of the atomic spacing between two Cu
was investigated computationally on the CO2RR by Jeong et al. (2020). The most optimal spacing is
5-6 Å between two Cu facets, leading to a Faradaic efficiency of approximately 80% for C2+ products.136
All of these studies argue that the increase in multi-carbon products while increasing the density or decreasing the interparticle distance is due to the ability of re-adsorption of the intermediates by the other crystals close to the reduction place.133–136
3.3.3 Crystal Facet effects
Tailoring the orientation (facets) of the Cu crystalline leads to electrocatalysts with superior activity and selectivity towards a singular product.21,22,31,40,90The facets on the surface influence the
physicochemical property of the electrocatalyst. Mostly the adsorption configuration and energy of the intermediates are affected, resulting in different main products for each facet.137 Copper can be
classified in the face-centered cubic system, see Figure 21.138 As seen in the system, there is a relation Figure 20: CO2RR faradaic selectivity as a function of interparticle distance at E = −1.1 V vs RHE over (a) 1.5 nm,
28 between the facet orientation and the shape of the nanocrystal, for example, a perfect nanocube has a (100) facet.139,140 The classification of the facets is based on the Miller indices, with the identities of
Cu(hkl) on a planar surface.31,138 The higher-index facets are a combination of the lower-index facets
with steps of the other facets on the same plane, already altering the activity and selectivity of the low-index facets.138
Starting from the beginning, Hori et al. (1995; 2002; 2003) discovered that the low-indexes (111) mainly form CH4, (100) produce C2H4 and (110) yields into CH3COOH and aldehydes and alcohols.10–12,16
The highest ethylene selectivity was formed with the facet indexes 711 and 810. With the result of Hori et al., Bagger et al. (2019) published the enlargement of the catalyst vs binding energies towards the influence of the facet and the binding energies of CO and H, see Figure 22.141 As can be confirmed from
this Figure, the selectivity of the facets on the CO2RR depends on the binding energies of the CO, H and
the intermediates as well.141 However, Hori et al. never published any contribution to the molecular
mechanism which would contribute to the CO2RR research on tailoring the catalyst towards a certain
product.
Further research on the facet selectivity confirmed the findings of Hori et al. on the low-index facets,
53,124,140–143but proposed new high-index facets for enhanced C
2+ selectivity. One of these facets is the
Cu(751) film as the hydride transfer barriers are expected to be lower than for CEPT.142 Schouten et al. Figure 22: a) Binding energies of ΔECO* and ΔEH* for different products. b) Zoom-in of a focussed on Cu and different facets of Cu. From Bagger et al. (2019).141
Figure 21: (a) Unit stereographic triangle of face-centred cubic metal single-crystals. (b) Unit stereographic triangle of polyhedral nanocrystals exclusively exposing single crystal planes/facets from (a). From Strasser (2018).138
29 (2013) suggested Cu(911) would be a suitable facet for ethylene selectivity at low overpotentials.124
However, according to Bagger et al. (2019) proposed classification system of the Cu facets and the face-centred cubic system, these proposed facets all originate from the Cu(100) facet. 138,141
Moreover, several mechanistic studies have investigated the low-index facet influence on the CO2RR
pathways on the Cu electrodes.47,50,53,105,144 The different facets both stabilize different intermediate
steps, for example (100) favours CO dimerization and (111) CO protonation. Each step determines the pathway of the CO2RR reaction, and therefore also the selectivity.144 In the beginning of 2021, Xiang et
al. proposed a pathway based on the Marcus Theory of charge transfer or the thermodynamics control for the selectivity of the Cu facets, see Figure 23.19 The stabilities and activation barriers of the
intermediates influence the reaction pathway. Furthermore, the *COH and *CHO intermediates not only determine the catalytic selectivity, but also the activity of the CO2RR reaction. These recent
findings were in line with the proposed mechanisms in the early 2010s. 47,50,53,105
Current research is focusing on dual-facet 3D structures of nanocubes to produce C2+ products. 143,145– 150 One manner to alter the nanocrystals is by etching, then high-energy facets on the surface are
created which forms (110) facets on the side edges of the Cu atom.146 Mangione et al. (2019)
investigated why cube-shaped Cu catalyst with a combination of (100), (110) and (111) facets enhances the selectivity towards ethylene and other C2 products.147 Previous research has already reported the
degradation of the facets for (100) (110) facets.148 Due to previous research on Cu octahedral and their
size-dependent catalytic behaviour resulted in the enhanced selectivity towards methane of the dual-facet combination (111)-(110) and (111)-(100), the research was orientated towards the dual-dual-facet influence of (110)-(100).149On the contrary, recent literature presented that Cu
2O nanoparticles with
the facets (100) and (111) enhance the selectivity and activity towards ethylene.150 These contradicting
results emphasise the importance of a deeper understanding of the synergy effects of combining facets that would largely benefit the CO2RR research by the tunability of the activity and selectivity.
Figure 23: Proposed Electrocatalytic Selectivity of C1 and C2 Products on Different Copper Facets. From Xiang et al. (2021).19
30
3.3.4 Roughness
The roughness of the electrode surface area has been reported as an influence on the selectivity of the CO2RR reaction,9,40,90 and not on the total CO2RR electrochemical active surface area current density.9
Tang et al. (2012) first concluded that the most roughened copper surface possesses higher selectivity’s towards hydrocarbons, resulting from the growth in undercoordinated sites on the surface.151 More research showed that not only the population of uncoordinated sites but also the high
pH environment within mesopores and adjacent to the surface influence the C2+ product distribution
of the CO2RR.152,153
One manner to influence the surface topography is by plasma pretreatment,152,154 however, according
to Ebiad et al. (2020) the manner of surface roughening does not matter.152 A recent study by Jiang et
al. (2020) investigated the role of the surface roughening on the CO2RR with copper as an
electrocatalyst.154 The results are presented in Figure 24 and show a clear relation between the surface
roughness and the selectivity of the CO2RR. From this research can be concluded that at a high surface
roughness, the HER reaction is repressed and the CO2RR reaction will favour multiple carbon products
(Figure 24A-B). Moreover, by increasing the surface roughness the product distribution favours the oxygenates compared to hydrocarbons (Figure 24C).154
3.3.5 Defects
A catalyst’s performance is directly affected by defects. Defects and interfaces of an electrode have the potential to change the adsorption behaviours and chemical activities of the reactants on the surface, leading to a change in the CO2RR pathway.59 There are different kinds of defects such as grain
boundaries, dopants and atom vacancies, see Figure 25. In this section, a few different kinds of defects will be discussed on their influence on the CO2RR pathway.
Figure 24: CO2RR product distribution, A-B) as a function of the O2 plasma treated time, and C) oxygenate/hydrocarbon distribution. From Jiang et al. (2020).154
