A Review on Recent Advances for Electrochemical Reduction of

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A review on recent advances for electrochemical reduction of carbon dioxide to methanol using MOFs and nonMOFs catalysts; challenges and future prospects Fayez Al-Rowaili, Aqil Jamal, Mohammed S. Ba-Shammakh, and Azeem Rana ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03843 • Publication Date (Web): 24 Oct 2018 Downloaded from http://pubs.acs.org on October 25, 2018

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ACS Sustainable Chemistry & Engineering

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A review on recent advances for electrochemical reduction of

2

carbon dioxide to methanol using MOFs and non-MOFs catalysts;

3

challenges and future prospects

4

Fayez Nasir Al-Rowaili 1, 2*, Aqil Jamal 1, Mohammad S. Ba Shammakh 2, Azeem Rana 3,

5 6

1 Research 2 Chemical

and Development Center, Saudi Aramco, Dhahran, Saudi Arabia, 31311

Engineering Department, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia, 31261

7 8

3 Chemistry

Department, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia, 31261

9 10 11 12 13 14

*Corresponding author email: [email protected] Phone: +966 506831223

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1 ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Contents

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Abstract .......................................................................................................................................................3

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Introduction ................................................................................................................................................4

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Challenges for electrocatalytic reduction of CO2 into methanol ............................................................9

26

Homogeneous and Heterogeneous Electrocatalysis for CO2 Electroreduction ...................................12

27

Homogeneous Catalysts ...........................................................................................................................12

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Heterogeneous catalysis ...........................................................................................................................13

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Kinetics for electroreduction of CO2 into methanol ..............................................................................14

30

Formation of CO2·- ....................................................................................................................................16

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Formation of HCOO-................................................................................................................................18

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Formation of CO.......................................................................................................................................19

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Formation of methanol.............................................................................................................................19

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Hydrogen evolution reaction (HER) .......................................................................................................20

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Benchmark non-MOF based catalysts for CO2RR ................................................................................22

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MOFs as catalysts for CO2RR .................................................................................................................31

37

Conclusion and Recommendations .........................................................................................................41

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References .................................................................................................................................................45

39 40


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Abstract

42

Transformation of carbon dioxide into various chemicals including methanol is top priority

43

field of study owing to the association of CO2 with global warming. There is a need for renewable

44

and sustainable energy sources and replacement of fossil fuel with a fuel having comparable energy

45

density. Electrochemical reduction is a unique approach to convert CO2 to methanol by employing

46

alternative energy sources where electrocatalyst plays a crucial role. A lot of efforts are made to

47

understand and increase the efficiency of electrocatalysts. Unadulterated metals, metal oxide,

48

composite materials and metal-organic frameworks (MOFs) are employed for the electrochemical

49

reduction of CO2 to methanol. However, MOFs engrossed the enormous consideration due to

50

simplicity, higher surface area, and unique structural features. In recent years MOFs and their

51

derivatives find significant applications in the electrocatalysis of oxygen and hydrogen evolution,

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oxygen, hydrogen, and CO2 reduction. The primary emphasis of the current review is the

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electroreduction of CO2 to methanol by coalescing the vantages of non-MOFs, MOFs and their

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composite materials. The challenges to achieve electrocatalyst with higher efficiency and better

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selectivity for the electroreduction of CO2 are analyzed. Several research directions are proposed

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for MOFs electrocatalysts to enhance the catalytic efficiency in methanol production. This review

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substantiates the efforts to develop new MOFs with superior efficiency, chemical stability, and

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conductivity.

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Keywords: Carbon dioxide, Methanol, Heterogeneous catalysis, Metal-organic frameworks,

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Artificial photosynthesis, Green fuel



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Introduction

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Combustion of hydrocarbons and biomass materials produce carbon dioxide (CO2) as the

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key end-product. Deforestation and human industrial activity are also among the major causes of

64

the continuous rise in CO2 concentrations 1–5. The concentration of CO2 in atmosphere rise by 30

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% since the industrial revolution starts (the 1750s) 6. CO2 is considered as a waste product due to

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its adverse effects as a greenhouse gas on the atmosphere. The greenhouse gases absorb and reemit

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the infrared radiations of sunlight and increase the temperature of the blue planet that is also called

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global warming

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community has been diverted towards the control of CO2 quantity in the atmosphere by controlling,

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capturing and consumption. To reduce CO2 emission, several governments engaged in the Kyoto

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Protocol of the United Nations Framework Convention on Climate Change 9,10. Alternatively, the

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scientific community putting efforts to develop and utilized technology to cut down the CO2

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emission. One of the best options is taking CO2 from principal sources, for instance, thermal

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power stations and convert it into valuable chemicals and fuel i.e., a carbon dioxide capturing and

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reusing (CCR) 11,12.

7,8.

Owing to its mounting production, the attention of world and scientific

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The CO2 could be exploited as a carbon source to get various critical chemical compounds

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and intermediate compounds. Consequently, a self-sustained industry can help to reduce the

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environmental CO2 concentration. Application of CO2 for production of chemicals offers attractive

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advantages such as the non-toxic nature of CO2, abundant in the atmosphere, renewable, and

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fascinating physiochemical characteristics

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numerous chemicals such as urea, salicylic acid, and polycarbonate-based plastic products

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CO2 is also used as the supercritical fluid that acts as a solvent in chemical preparation, separation,

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and purification process 16–18. CO2 has enormous potential for the application in the area of oil and

13.

Currently, CO2 is employed vastly to produce 14,15.


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gas recovery 19. The chemicals produced from CO2 along with their Gibbs free energies are shown

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in Fig. 1. Although, all the methods described above are reducing the CO2 level in the atmosphere.

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However, the annual production of CO2 is 25 gigaton 6, and still, extraordinary efforts are required

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on an urgent basis to lessen the CO2 related issues.

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Figure 1. The chemical compounds originating from CO2 reduction with Gibbs free energies.

Fossil fuels are chiefly utilized to meet the basic energy requirements of the world, and

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20,21.

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their natural reservoirs are depleting quickly

The swiftly rising population and

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industrialization also requiring the augmented energy demand. Thus, an alternative to fossil fuels

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is needed that is sustainable, carbon-based and easy to scale-up. Consumption of atmospheric CO2

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for getting methanol at large scale can reduce the CO2 emission by reducing the use of fossil fuels

95

22.

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energy sources such as wind or solar power. This process is called artificial photosynthesis 23–25.

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Methanol is a relatively environment-friendly fuel that has half of the energy density as compared

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to the energy density of gasoline fossil fuel 26–28. Methanol is among the most vital intermediate to

Conversion of CO2 into methanol requires energy that can be provided through inexhaustible



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get important chemicals such as aldehydes, olefins, and resins 18,29. The direct or blended use of

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methanol in engine provides the superior fuel that amends combustion, rise octane rating, and

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diminish CO2 emission by cleaner burning. Methanol can be stored at atmospheric pressure and

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has a high octane rating; therefore, it can be used directly without any modification in gasoline

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engines and direct methanol fuel cells (DMFC)

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conversions of CO2 into methanol is a hot area of research. Even though significant advances have

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been made in this area, nevertheless additional exertions are desired to get economically viable

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technology that can be commercialized on an industrial scale.

30,31.

Attributed to the reasons mentioned above,

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There are numerous approaches described in literature where CO2 is employed as a C1-

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carbon source for methanol production, for example, chemical, biochemical, photochemical,

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hydrothermal, and electrochemical

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striking approach, that offers several advantages such as a) Electrode potential and reaction

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temperature help to control the process; b) The utilization of chemicals is very low that make the

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recycling of supporting electrolyte possible; c) Renewable energy sources could be exploited to

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reduce CO2 electrochemically; and d) it is easy to make changes in the electrochemical cells 13.

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The electroreduction of CO2 produce various products that depend on type of catalyst and

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supporting electrolyte. The electrochemical conversion of CO2 mainly follows the reduction

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pathways involving two, four, six and eight electrons depending on gaseous, non-aqueous, aqueous

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reaction mediums and nature of electrodes employed in the process 13,38. The CO2 act as C1 or C2-

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building block source and produce various vital products during electroreduction, for example,

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carbon monoxide, formaldehyde, methanol, methane, oxalic acid, ethylene, and ethanol

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Fig. 2 is a schematic diagram that describes the utilization of CO2 to produce methanol by the

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electrochemical reduction process. Among all aforementioned chemical products of CO2

. The electrochemical reduction of CO2 is among most

32–37

39.

The


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electroreduction, production of methanol is of significant importance and involve various

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challenges.

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Figure 2. Schematic diagram of electrochemical reduction of CO2 and possible applications of fuel product.

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Highly efficient electroreduction of CO2 into methanol is very crucial and challenging. CO2

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is chemically passive and very stable compound with very low Gibbs free energy (ΔG= -394.4

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kJ/mol) and its conversion back to methanol with relatively higher Gibbs free energy (ΔG= -159.2

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kJ/mol) is an endothermic process. The electroreduction of the CO2 has the chief hurdle of the high

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overpotential of the reduction process, lower efficiency and stability of catalysts 6. In order to

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overcome the abovementioned complications, redox electrocatalysts are employed that lower the

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kinetic barriers and give the higher reaction efficiency

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have been made to improve the effectiveness of catalyst and interpreting the reaction mechanism.

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The majority of the literature related to activation of CO2 depends on metal centers of complex

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compounds. So, the developments in the electroreduction of CO2 are majorly associated with the

40,41.

Hence, considerable advancements



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progress earned in the area of metal-organic chemistry 42. Various sort of metals attracted attention

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due to their high electrocatalytic activity; however, ligands are necessary to enhance the catalytic

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performance of these metals. As a result of better understanding of the role of ligand in CO2

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activation, numerous fascinating discoveries are made for boosting the catalytic performance 43.

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Versatile materials have been reported in the literature that are used as an electrocatalyst to reduce

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CO2 such as transition metals, conducting polymers, ionic liquids, enzymes and metal-organic

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frameworks 38,44–49.

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In the last decade, metal-organic frameworks (MOFs) which belong to the larger metal-

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organic porous materials group (MOPMs) have offered the opportunity of various kind of

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applications and used as modern multifunctional materials

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porous coordinating polymers or networks. They are hybrid materials that are made up of three

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components – metallic component, pore space, and the organic linker 52. The organic linkers are

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the sites that are attributed to the catalytic activity of the MOFs 53,54. MOFs are an excellent choice

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for the adsorption, separation, and electroreduction of CO2 due to the highly porous structure,

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specific structural features, large surface area, and high conductivity 45,55–58.

50,51.

MOFs are also referred to as

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Substantial exertions have been held to convert CO2 into methanol utilizing various

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catalysts. Several reviews have been recently published covering catalytic applications of MOFs

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and their composite 38,59,60. However, a report discussing the use of MOFs for electroreduction of

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CO2 into methanol is still desired. To fill this vacant space, a review examining the fundamental

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aspects of the electrochemical conversion of CO2 into methanol, thermodynamic studies to

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comprehend the mechanism of electroreduction of CO2, critical electrocatalysts for the conversion

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of CO2 and recent advancement towards the utilization of MOFs for electroreduction of CO2 into


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methanol has been scripted. Eventually, our present effort will give the prospects of the MOF-

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based material to get better electrocatalyst.

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Challenges for electrocatalytic reduction of CO2 into methanol The conversion of CO2 in electrochemical redox system could be represented by following

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a set of equations in table 1 Table 1: The electrochemical conversion reactions of CO2 into methanol. Cathode

CO2 + 6H+ + 6e- ⇋

CH3OH + H2O

0.024 V (versus SHE)

Anode

3H2O

⇋

1.5O2 + 6H+ + 6e-


Overall Reaction

CO2 + 2H2O

⇋

CH3OH + 1.5O2


162 163

The electroreduction of the CO2 to CH3OH is a thermodynamically feasible but non23.

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spontaneous reaction

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SHE) higher in an aqueous electrolyte solution as compared to the potential for water reduction

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that causes hydrogen evolution reaction (HER) 61. That is why the applied electrolysis potentials

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are more negative as compared to the equilibrium value for the electroreduction of CO2. The

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energy requirement for the conversion of CO2 into methanol can be mentioned as in following

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equation

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CO2(g) + 2H2O(l)

⇋

Though, the equilibrium reduction potential of CO2 is 0.02 V (versus

CH3OH(l) + 1.5O2(g) Ho = -727 kJmol-1 & Go= -703 kJmol-1 (1)

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Firstly, the CO2 is activated to CO2-2 ion at -1.90V (versus SHE), due to a tremendous

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amount of energy is required to rearrange a linear CO2 molecule to the bent radical CO2-2 structure.

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The fundamental difficulty in the conversion of CO2 to methanol lie in the assemblage of the nuclei

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to convert comparatively simple molecule (CO2) into the complex active molecule (CH3OH) 62.

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The electroreduction of CO2 is significantly derived by reaction kinetics. The mechanism 9 ACS Paragon Plus Environment


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description shows that CO2 reduce to CO and HCOO- ion that needs applied potential lower than

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-1.12V (versus SHE) that can lead to the formation of CH3OH. However, around -1.35V (versus

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SHE) hydrocarbons start to appear and become dominant over CO and HCOO- 63,64. The table 2

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describes the possible electrochemical reduction pathways of CO2 into different products. Table 2: Possible electrochemical reduction pathways of CO2

CO2(g) + 2H+ + 2eCO2(g) + H2O(l) + 2eCO2(g) + 2H+ + 2eCO2(g) + H2O(l) + 2eCO2(g) + 4H+ + 4eCO2(g) + 3H2O(l) + 4eCO2(g) + 6H+ + 6eCO2(g) + 5H2O(l) + 6eCO2(g) + 8H+ + 8eCO2(g) + 6H2O(l) + 8e2CO2(g) + 2H+ + 2e2CO2(g) + 2e2CO2(g) + 12H+ + 12e2CO2(g) + 12H+ + 12e-

⇋ ⇋ ⇋ ⇋ ⇋ ⇋ ⇋ ⇋ ⇋ ⇋ ⇋ ⇋ ⇋ ⇋

HCOOH(l) HCOO-(aq) + OHCO (g) + H2O(l) CO(g) + 2OHCH2O (l) + H2O(l) CH2O(g) + 4OHCH3OH (l) + H2O(l) CH3OH(l) + 6OHCH4 (g) + 2H2O(l) CH4(g) + 8OHH2C2O4 (aq) C2O42- (aq) CH2CH2(g) + 4H2O(l) CH3CH2OH(g) + 3H2O(l)

-0.25 V (versus SHE) -1.08 V (versus SHE) -0.11 V (versus SHE) -0.93 V (versus SHE) -0.07 V (versus SHE) -0.90 V (versus SHE) +0.02 V (versus SHE) -0.81 V (versus SHE) +0.17 V (versus SHE) -0.66 V (versus SHE) -0.50 V (versus SHE) -0.59 V (versus SHE) +0.06 V (versus SHE) +0.08 V (versus SHE)

180

On the other hand, when higher -ve potential is applied to the aqueous electrolyte solution

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the HER start to compete with CO2 reduction reaction. Competition between CO2 reduction and

182

hydrogen evolution reaction (HER) is responsible for the reduced faradaic efficiencies because

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both reactions occur in the same potential range 65,66. The above discussion shows that reduction

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of CO2 into methanol face kinetic and thermodynamic challenges and such electrocatalyst is

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required that enhance the selectivity to get CH3OH and suppress the HER. The most probable

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mechanisms for electrochemical reduction of CO2 into methanol are demonstrated in Fig. 3.

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60,62,67,68.


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Figure 3. Possible mechanisms for electrochemical reduction of CO2 into methanol.

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The mechanism 1 produces dioxymethylene ion (HOCO-) and CO. The mechanism 2 forms

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formate ion and formic acid (HCOOH). Both mechanisms involved formation of HCO

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intermediate product and formaldehyde (HCHO). The types of metals present in MOFs and non-

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MOFs catalysts determine the electrochemical reduction of CO2 will follow either mechanism 1

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or 2. Such as, the Zn and Cu metal electrode supports carbon coordinated attachment of CO2 that

194

produce methanol by following mechanism 1, while oxygen coordinated attachment of CO2 on the

195

metal electrode with high hydrogen overpotential leads to mechanism 2. Methanol is formed

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during the electroreduction of CO2 along with different chemical compounds such as CO,

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HCOOH, CH4, CH2O, propanol, ethylene glycol, glyoxal, etc. 59,69–73. CO2 is a thermodynamically

198

stable molecule with very low Gibbs free energy due to very strong double bonds between carbon

199

and oxygen atoms. Conversion of the extremely non-reactive CO2 molecule to an energy-rich

200

compound such as methanol requires an external energy source e.g. heat, irradiation/photons, and

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electrons that make the reaction kinetically slower. The conversion of CO2 to CH3OH is a

202

kinetically slower process as mentioned in mechanisms (Fig. 3) that it involves six electrons 32,74.



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Therefore, electrocatalysts that display higher efficiency, better selectivity and lower the

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overpotential for the electroreduction of CO2 are needed to boost the production of CH3OH.

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Homogeneous and Heterogeneous Electrocatalysis for CO2 Electroreduction There are two types of catalytic reactions for the electrochemical reduction of CO2 into

206 207

methanol.

208



Homogeneous catalysis

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

Heterogeneous catalysis

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Homogeneous Catalysts

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Molecular compounds are intensely engaged as the homogeneous catalysts for the

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electrochemical reduction of CO2 75–77. Homogeneous catalysts act as an electron carrier between

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electrode and CO2 and cause indirect electrolysis to happen as mentioned in Fig. 4. The potential

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required by a homogeneous catalyst to reduce CO2 is not the same potential that is needed for

215

direct reduction of CO2. That is why the potential of homogeneous catalysis for the CO2 reduction

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should be less negative as compared to the potential for direct CO2 reduction 78. There are specific

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characteristics of a material to act as a homogeneous catalyst in the CO2 electroreduction reaction

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such as; the reduction potential of the catalytic material should be in an appropriate range, the

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reduced form of the catalyst should be highly stable, and the rate of catalyst derived reaction must

220

be high to get a higher yield in a short time. The catalyst should operate close to the thermodynamic

221

potential of the CO2 reduction reaction. All the features as mentioned earlier can be easily adjusted

222

by the introduction of suitable metal and specific ligands to the MOFs structure.


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Heterogeneous catalysis

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In order to overcome overpotential hurdle for electrochemical reduction of CO2, various

225

electrode materials are utilized as the catalyst that chemisorbs the CO2 on their surfaces, such as

226

Cu, Pt, and Ag 79. Nature of solvent, temperature, pH, and type of electrolyte prominently affect

227

the selectivity of the electrode materials. Besides, the chemical composition of the catalyst

228

determines the binding energy between the catalyst surface and reaction intermediates, which

229

govern the fate of reaction and end-products 49. The surface area of catalytic material proves to be

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a crucial factor that determines the quantity of CO2 binding to the surface of material and controls

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the reactivity of catalyst. The nanostructured materials have a large surface to volume ratio that

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offers a large number of coordination sites for the catalytic reaction 70,80. In order to enhance the

233

electroactivity and specificity, the composite materials are used to cause a synergetic effect for

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catalytic efficiency and improve the selectivity as compared to the single material used for the

235

electrochemical reduction of CO2 into methanol 81–83.

236 13 ACS Paragon Plus Environment


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Figure 4. the schematic diagram for homogeneous and heterogeneous catalysis.

238

The selection of the most beneficial type of catalysis for the electrochemical reduction of

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CO2 is a tough job because homogeneous and heterogeneous catalysis has several benefits and

240

shortcomings. In the case of heterogeneous catalytic reaction product isolation, waste handling

241

and reprocessing of the stream in a flow cell can be done more efficiently. In contrast,

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homogeneous catalysis can be highlighted by a high degree of interaction between the catalyst and

243

reactant molecules that enhance the selectivity and efficiency of the reaction. Degradation of the

244

catalyst is a significant issue in both kinds of catalytic reactions that limit the catalyst lifetime 84.

245

Noteworthy developments are desired to expand the features of homogeneous or heterogeneous

246

catalysts to make them appropriate for the industrial application. In this review, our core emphasis

247

will remain on the heterogeneous catalysis.

248

Kinetics for electroreduction of CO2 into methanol

249

CO2 is a thermodynamically stable molecule with very low Gibbs free energy due to robust

250

double bonds between carbon and oxygen atoms. Conversely, methanol has higher Gibbs free

251

energy (lower stability) as compared to CO2 13 as shown in Fig. 1. Conversion of the extremely

252

non-reactive CO2 molecule to an energy-rich compound such as methanol requires an external

253

energy source, e.g. heat, irradiation/photons and electrons 85. Additionally, catalysts are introduced

254

to lower down the energy barriers 59. Conversion of CO2 into methanol is an endothermic reaction

255

(ΔGo = -4.1 Kcal.Mol-1) as shown in Fig. 5.


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Figure 5. Thermodynamically favorable reduction pathway (solid line) from CO2 to CO and to CH3OH, other

257

competing pathways (dash lines) are also depicted. With permission 86.

258

The huge variance in geometries of CO2 and its CO2.- radical anion make the outer-sphere

259

electron transfer barrier very large. The electrochemical reduction of CO2 requires substantial

260

overpotentials due to the kinetic barriers and the vast difference in the HOMO and LUMO energies

261

as shown in Fig. 6 41.



Page 16 of 73

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Figure 6. Molecular orbital energy level diagram of CO2.

264

The electrochemical reduction of CO2 has been widely studied on the surface of different

265

kinds of metal electrodes. The general reaction mechanism involves the charge transfer and can be

266

divided into following steps.

267

Formation of CO2·-

268

The electroreduction reaction of amphoteric CO2 molecule begins with the chemisorption

269

on the surface of catalysts. The type of interaction majorly depends on the nature of the electrode

270

material

87.

The exact orientation of the binding of CO2 and metal is unknown, possibly the 16 ACS Paragon Plus Environment

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electrode materials bound to CO2 through a carbon atom, an oxygen atom or both

272

demonstrated in the following Fig 7.

88

as

273 274

Figure 7. Illustrative representation of CO2 interaction with the metal surface.

275

The one-electron reduction of CO2 leads to CO2·- anion radical, that is energetically highly

276

unfavorable reaction. The standard potential needed for the formation of CO2·- in aqueous media

277

is in between -1.90 V (versus SHE)with a transfer coefficient in the lower overvoltage area was

278

found to be 0.67

279

reaction of CO2·- anionic radical the carbon atom act as a reactant. The CO2·- can exist in both

280

aqueous and non-aqueous electrolytic solutions in the free state 91,92. The bounded anionic radical

281

further reduce to form carbon monoxide (CO) or formate ion (HCOO-) depending on the nature of

282

electrocatalyst employed in the reaction 93.

89,90.

The experimental and theoretical studies revealed that in the nucleophilic



Page 18 of 73

283

Formation of HCOO-

284

Formate ion (HCOO-) is produced as a primary product of electroreduction of CO2 on the

285

surface of Hg, Sn electrodes 94. The CO2 bound to the surface of the metal and produce anionic

286

radical at a negative potential of -1.6 V (versus SHE). Afterward, the carbon atom in CO2·- take a

287

proton from the water molecule and HCOO- ion is produced 95. The reactions can be described as

288

CO2.-ads + H2O

⇋

HCOO.ads + OH-

(2)

289

HCOO.ads + e-

⇋

HCOO-

(3)

290

The hydrogen atom adsorbed (intermediate of HER) on the surface of the catalyst can

291

directly react with CO2·- to produce HCOO- ion directly 96

292

CO2.-ads + Hads

⇋

HCOO-

(4)

293

Formation of HCOO- ion is favored by the oxygen coordinated attachment of CO2·- radical

294

with the electrocatalyst. The metal electrodes with high hydrogen overvoltages such as Cd, Pb, Tl,

295

and Sn demonstrate the formation of formate ion as in Fig 8.

296


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297 298 299


Figure 8. The main reaction pathways at the electrode surface, with adsorbed blocking the majority of the surface and hydrocarbon products being formed by the further reduction of adsorbed CO. with permission 97

300

Formation of CO

301

The carbon coordinated adsorption of the CO2·- on the metal surface leads to protonation

302

and reduction steps. The carbon atom of CO2·- adsorbed on the transition metal complex is

303

stabilized due to back donation, that facilitates the protonation of oxygen atom where water act as

304

an electrophile and .COOH radical is formed instead of HCOO.. The final product attained is CO

305

accompanied by OH- ion 45,98,99. As shown in following equ.

306

CO2.-ads + H2O

⇋

.COOH ads

307

.COOH ads

⇋

COads + OH-

308

+ e-

+ OH-

(5) (6)

On the other hand, the direct reaction of adsorbed hydrogen with CO2·- resulted in the

309

formation of CO.

310

CO2.-ads + Hads

⇋

COads + OH-

(7)

311

The metal electrodes such as Ag, Au, Pd, Ga, and Zn have the capability to stabilize the

312

CO2·-, so the interaction of CO with these metals is very weak. The adsorbed CO quickly desorb

313

in the form of gaseous molecule form the electrode surface. Conversely, the Cu based electrodes

314

can further reduce the CO into hydrocarbons and methanol fuel 100.

315

Formation of methanol

316

The CO and HCOO- ion display prominent yield at low overpotential (-0.9 V versus SHE),

317

while the yield of CH4 and C2H4 start to increase on relatively high potential, i.e. -1.2 V (versus

318

SHE)101,102. The above-mentioned statistics demonstrate that CO and HCOO- may act as precursors

319

for the formation of hydrocarbon and alcohols. FTIR and Raman study revealed that CO strongly

320

adsorbed on the surface of Cu electrode at -0.6 V (versus SHE), confirming the role of CO as a 19 ACS Paragon Plus Environment


Page 20 of 73

321

precursor for the useful products 103,104. The strong binding to CO to Cu surface also suppress the

322

competing HER 105,106. The adsorbed CO reduce in the presence of water molecules gives rise to

323

formyl CHO. Intermediate. The stepwise reduction of the adsorbed CO led to CHO*, CH2O* and

324

CH3O* intermediates on the surface of the electrocatalyst. The Cu metal electrode supports the

325

hydrogenation of oxygen atom in CH3O* that ends up with the formation of methanol. Fig. 9

326

demonstrates the electroreduction mechanism of CO2 on the surface of Cu 107.

327

Figure 9. A schematic representation of the reaction occurring on the surface of the Cu electrode.

328

Hydrogen evolution reaction (HER)

329

Throughout the electrochemical reduction of CO2 into methanol, HER is one of the major 60.

330

competing side reaction

331

changes of aqueous electrolyte solution in the highly acidic region and it becomes prevailing

332

process 108. However, in the slightly acidic, neutral and alkaline region, the HER is independent of

333

solution pH

334

source of hydrogen. On some metal electrodes, the H+ or Hads reduce to H2 on certain applied

335

potential; the HER reaction can be written as

66,109–111.

The reaction kinetics of HER is prominently dependent on the pH

During the HER reaction protons or adsorbed atomic hydrogen act as the


Page 21 of 73 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


336

H+ + e-

⇋

Hads

(8)

337

2Hads

⇋

H2

(9)

338

Or Hads + H+ + e-

⇋

H2

(10)

339

The pH is one of the critical factors that prominently affect the selectivity of the

340

electrochemical reduction of CO2 112,113. Hori et al. revealed that variation in surface pH has a

341

significant consequence on the selectivity of product, such as the formation of methane is

342

dependent on the pH of the electrolyte; on the other hand, the ethylene production was independent

343

of proton concentration 114,115. In the basic electrolytic solution ethylene is a vital product due to

344

the decoupled proton-electron transfer step. Conversely, methane is favored in the neutral or acidic

345

electrolytic solution. The buffer capacity also plays a crucial role in the determination of selectivity

346

of the CO2RR by controlling the pH. The electrolyte having low buffer capacity favor the

347

formation of ethylene 116.

348

Interestingly, the pH near to the electrode surface is higher as compared to the bulk

349

electrolyte due to continuous consumption of H+ ions and production of OH- ions 117,118. The local

350

pH can be up to 6 units higher as compared to the bulk electrolyte

351

affecting the selectivity of the product during electrochemical reduction of CO2 can be majorly

352

attributed to the effect of local pH. The high local pH shifts the equilibrium of the CO2RR towards

353

the carbonates/bicarbonates and hinders the mass transport of CO2. Therefore, the access of CO2

354

to the electrode surface is favored by diffusion through high pH region 117. The local pH plays a

355

complicated role in the mass transport of CO2 during electrochemical reduction reaction that needs

356

more fundamental insight for understanding and designing of the efficient and highly selective

357

electrocatalyst. The studies suggested that low local pH favors the C1 products, while the high

115.

Therefore, electrolytes



Page 22 of 73

358

local pH enhances the selectivity of the CO2 reduction reactions towards the valuable multi-carbon

359

compounds and suppress HER 111,117–119.

360

The electrochemical reduction of CO2 involves two, four, six, and eight electron reduction

361

pathways as described in table 2. Consequently, formic acid, formate ion, CO, formaldehyde,

362

methanol, methane, oxalic acid, oxalate ion, ethylene, and ethanol are generated. Unfortunately,

363

during the electroreduction of CO2, a mixture of compounds is obtained rather than a single target

364

product. Although, the mechanism of the electrochemical reduction of CO2 is briefly described in

365

the above section. However, the limited understanding of the possible reaction mechanism is the

366

major hurdle in designing of the optimum electrocatalyst. The limited knowledge of reaction

367

mechanism brings main issues such as low efficiency, poor selectivity and less stability of the

368

electrocatalyst. Due to the above-described problems, a reliable large-scale process is still absent

369

that can give satisfactory production of methanol from CO2. The quest for such electrocatalyst

370

could be aided by the fundamental insight into the reaction mechanism, modeling, and detailed

371

experimental work. The recent advances in MOFs and non-MOFs electrocatalysts for the

372

electroreduction of CO2 into methanol are discussed in detail in following sections for better

373

understanding and promotion of development in this area of research.

374

Benchmark non-MOF based catalysts for CO2RR

375

The application of the CO2 as a raw material to get chief chemicals in a CO2RR is currently 120.

376

very attractive for energy storage/conversion

The CO2RR produce a wide range of products

377

including CO, HCOOH, CH3OH, C2H5OH, CH4, C2O4H2, C2H4, etc. which highlight that

378

selectivity is crucial for getting specific product

379

because protons or water is involved in the CO2RR processes. The type of metal utilized for

380

CO2RR heterogeneous catalytic reactions determine the fate of reaction. Co, Sn, and Pb usually

74,84,121.

Hydrogen is an essential by-product


Page 23 of 73 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


381

display high Faradic efficiency (FE), turn over frequency (TOF) and selectivity to get carbon

382

monoxide or formate during CO2RR 59,74,79. Cu is a well-established catalytic material to catalyze

383

the multi-electron reduction reactions and produce CH4, C2H4, and CH3OH 62,107,122. Nevertheless,

384

it is tough to get the particular product during CO2RR in the presence of Cu. Recently, several

385

metals such as Zn, Ag, and Au are explored to catalyze the production of multi-electron reduction

386

products 123–125. Homogeneous catalysts also demonstrated comparable selectivity as compared to

387

heterogeneous catalysts. Iron complexes can show predominantly high TOF and FE for reducing

388

the CO2 to CO

389

must be introduced with the capability to reduce the overpotential required. A lot of research has

390

been put into creating homogeneous and heterogeneous catalysts for both electrochemical and

391

photochemical CO2RR.

126.

In order to drive this complex reaction with slow kinetics, an active catalyst

392

For the electrochemical reduction of CO2, Summers and coworkers employed Mo metal

393

electrode. The reaction in slightly acidic media at room temperature mainly produce methanol with

394

minor quantities of CO and CH4. The continuous use of Mo electrode for several days illustrates a

395

dramatic decrease in FE for methanol production. The decline in Mo electrode efficiency was

396

attributed to the vicissitudes in electrode topographies due to passivation by corrosion and

397

deposition of impurities such as Hg or As from the electrolyte solution 127. Jianwei and coworkers

398

utilized a highly pressurized (60 atm) supercritical CO2-water mixture for production of methanol

399

in the presence of a copper catalyst. High-pressure play a critical role in getting better current

400

efficiency 128. Shen et al. applied cobalt protoporphyrin modified pyrolytic graphite electrode for

401

the electrochemical reduction of CO2. The upshots exposed that CO was received as the chief

402

product, while CH4, HCHO, and HCOOH were present as by-products. The FE for the reaction

403

was dominated by the pH of the acidic electrolyte solution. In the highly acidic solution, the FE 23 ACS Paragon Plus Environment


Page 24 of 73

404

for CO and CH4 production was meager, and H2 was the most prominent product. As the pH of

405

the solution increase from 1.0 to 3.0, a dramatic upsurge in FE for CO, i.e., 40% was observed.

406

Increase in CO2 pressure caused the rise of CO2 to proton ratio that also positively affected FE by

407

suppressing HER 102.

408

The stainless-steel surface electrodeposited with Cu2O film display higher conversion

409

electroreduction rate of CO2 into CH3OH as compared to the anodized copper electrode. The

410

improved yield on Cu2O film was attributed to presence of copper in Cu (I) form that plays a

411

critical role in the selectivity of the reaction to convert CO2 into methanol. After 30 mins of

412

continuous reaction, the CH3OH yield started to decrease and CH4 start to appear, that was

413

accredited to the simultaneous reduction of Cu(I) to Cu (0). The appearance of Cu (0) prominently

414

disturb the selectivity of the catalyst and affect the rate and yield of reaction

415

modified the surface of carbon paper electrode with commercially available Cu2O and Cu2O-ZnO

416

mixture for uninterrupted reduction of CO2. The Cu (I) oxide alone displays better selectivity but

417

low stability towards the formation of methanol, while the combination of Cu2O-ZnO remains

418

stable for a longer duration. Even though the FE was very high, but the limited mass transfer of

419

CO2 during reaction demand further modification of the process 130. To cope with the limited mass

420

transfer, the same research group used continuous gas phase electroreduction of CO2. The gas

421

diffusion electrode conspicuously strikes the CO2 transfer rate for a constant reduction reaction.

422

The flow rate of catholyte, anolyte and CO2 were optimized to get a higher yield. The schematic

423

diagram of the filter press cell for the formation of methanol is demonstrated in Fig. 10 37. Recently,

424

Jonathan et al. utilized Cu2O/ZnO composite gas diffusion electrode in the presence of pyridine-

425

based electrolyte for the electrochemical reduction of CO2 to methanol. The pyridine-based

426

electrolyte act as the co-catalysts that prominently reduce the CO2 electrochemical reduction

129.

Jonathan et al.


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427

potential and slightly acidic pH (pH=5) of 10 mM 2-methylpyridine significantly raise the FE to

428

25.6% 131.

429 430 431 432 433 434 435 436 437 438 439 440 441 442

Fig 10. Schematic diagram of the electrolytic cell configuration for the electroreduction of CO2 supplied directly from the gas phase. with permission 37

443

A budding area of study is the reduction of CO2 using appropriate bimetallic or multi-

444

metallic catalysts. Indications are already emerging that introduction of different sort of atoms to

445

the surface of metal electrodes can cause changes in their selectivity. Theoretical studies of CO2

446

reduction have been boosting knowledge into the design of more efficient, better and stronger

447

chemically selective bimetallic catalysts. The efforts are being made to retain or reproduce the

448

distinct reducing characteristic of Cu in Cu-alloys and simultaneously either prevent the HER or

449

lower the onset potential 132. Sakata et al. carried out alloying of Sn, Ag, Zn, Ni, and Cd with the

450

Cu, which significantly change the reactions characteristics of the parent electrode. The alloy

451

formation occasioned higher selectivity and yield for methanol as compared to the single metal.

452

Such as Cu and Ni could not produce methanol individually, while Cu/Ni alloy lowered the

453

overpotential to produce the methanol and demonstrated the 5% Faradaic efficiency

133.

Popić 25

ACS Paragon Plus Environment


Page 26 of 73

454

utilized the Ru metal and further introduced adatoms such as Cu and Cd for the transformation of

455

CO2 into methanol. The stability of Ru and its oxide under vigorous hydrogen evolution and

456

capability to adsorb hydrogen at lower potentials leads to a reduction of CO2 without the

457

competition of HER 134. In order to get the highly selective production of alcohols, the nanoporous

458

copper film modified surface of the platinum electrode was electrochemically deposited with

459

copper and gold alloy. The surface characterization of the Cu-Au alloy revealed the deposition of

460

Cu (I) and Au (0). The gas chromatographic analysis of products showed that methanol and ethanol

461

are produced due to the electrocatalytic reaction at Cu-Au/NCF cathode. The comparison of the

462

established surfaces demonstrates that Cu-Au/NCF electrode shows better FE and superior

463

selectivity towards the production of methanol as compared to Cu plate and copper nanoparticle-

464

based electrode 135.

465

Composite of Ru and Ti metal oxides was acquired on the Ti sheet by thermal

466

decomposition of corresponding metal halides. The RuO2:TiO2 determine the destiny of the

467

reaction; such as, in case of an increase in the amount of TiO2 the hydrogen evolution become the

468

primary reaction. In order to improve the efficiency of electrocatalyst, the surface of the composite

469

electrode was further modified by the thin layer of Cu. Consequently, the reduction potential of

470

hydrogen become more negative while the reduction potential for CO2 remains unchanged. The

471

optimization of various factors on rotating disk electrode with the continuous purging of N2 gas

472

resulted in limited hydrogen evolution and electroreduction of CO2 became independent of

473

electrolyte pH

474

nanotubes (NTs) and nanoparticles (NPs) for electrocatalytic reduction of CO2. The composite

475

electrode having NTs display better current efficiency as compared to the electrode consisting of

476

NPs of RuO2/TiO2. The TiO2 nanotubes are famous for their low binding energy and can favor the

136.

In another work, Jianping et al. modified the Pt electrode with RuO2/TiO2


Page 27 of 73 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


477

diffusion of reacting species. On the other hand, the dispersed RuO2 particles increase the

478

electroactive area of the electrode and enhance the number active site for the CO2RR

479

functionalization of TiO2 with 3-aminopropyltriethoxysilane ligands, blatantly improve the light

480

harvesting features and conduction capabilities of the photocathode. The electrochemical

481

impedance spectra (EIS) of amino-functionalized Ni/TiO2 approves a decline in resistance as

482

compared to Ni/TiO2. Subsequently, significant number of electrons are available on the cathode

483

for the electroreduction of CO2. The mechanism proposed for this study demonstrates that

484

electrons in the ground state (HOMO) of the dye molecules move to an excited state (LUMO) by

485

absorption of photons. Afterward, the excited electron traveled to the conduction band of TiO2 and

486

transferred to the Ni foam that is available for the reduction reaction. The mechanism can be

487

described as following in Fig. 11

137.

The

138

488 489

Figure 11. Mechanism of the photocatalytic process of CO2 reduction. with permission 138



Page 28 of 73

490

An early study revealed the potential of the p-type GaP solar driven electrode for the

491

electrochemical conversion of CO2 into methanol. The result brought out that the implication of

492

pGaP electrodes has an advantage over other photo-driven electrodes that the reduction of CO2

493

does not stop after production of formic acid. However, the reduction reaction continues to yield

494

formaldehyde and methanol. The optimization of solar power wavelength revealed that the highest

495

quantum efficiency for conversion of CO2 in methanol was observed at 365 nm.

496

study, Emily and co-workers employed photoelectrochemical reduction of an aqueous solution of

497

CO2. The pGaP electrode demonstrates 100 % FE in the presence of pyridinium. The catalytic

498

action of pyridinium vividly enhances the selectivity of the photoelectrochemical CO2RR toward

499

conversion of methanol

500

catalyst for the reduction of CO2 underpotential. The presence of pyridinium facilitates the

501

homogeneous catalytic reaction, whereas heterogeneous reaction also takes place on the surface

502

of the cathode. The pyridinium along with Pd cathode act as electrocatalyst and also do the

503

poisoning of the electrode surface for the hydrogen reduction reaction

504

studied the effect of particle size of Pd nanoparticles (PdNPs) on electrochemical reduction of CO2

505

into CH3OH in acidic media. In order to mitigate the HER and stabilization of PdNPs polyaniline

506

(PANI) was introduced to the surface of nanoparticles.

507

electrocatalysts for CO2RR reported in the literature.

23.

139.

In another

Gayatri et al. evaluated the function of pyridinium as the homogeneous

141

140.

Weiran et al. briefly

Table 3. briefly describes non-MOF

508


Page 29 of 73 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


Table 3. Summary of non-MOF electrocatalysts for CO2RR Cathode

Electrocatalyst

Electrode

Electrolyte

Catalysis type

potential Molybdenum metal

Molybdenum metal

Cu

Cu

Coprotoporphyrin/ pyrolytic graphite electrode Electrodeposited Cu2O film

Coprotoporphyrin Electrodeposited Cu2O film

Carbon paper

Cu2O Cu2O/ZnO

Carbon paper

Cu2O Cu2O/ZnO

Cu2O/ZnO Ru/Cu

Cu2O/ZnO and pyridine based co-catalyst Ru/Cu

Cu-Au

Cu-Au

Ni/TiO2

Ni/TiO2

RuO2-TiO2

RuO2-TiO2

- 0.8 V (versus SCE) -1.1 V (versus SCE) -0.5 V (versus RHE)

Catalytic

Reaction

performance

Time

Remarks

0.2 M Na2SO4 (pH 4.2)

Heterogeneous

FECH3OH = 84 % j =120 µAcm−2

23.3 h

C 2H5OH-H2OLiCl

Heterogeneous

FECH3OH = 40 % j = 9 mAcm−2

8h

HClO4NaClO4-NaOHborate

Heterogeneous

FECO = 60 % FECH4 = 3 %

1h

-1.1 V (versus SCE) - 1.3 V (versus Ag/AgCl) - 1.39 V (versus Ag/AgCl)

0.5 M KHCO3

Heterogeneous

FECH3OH = 38 %

10 min

Reduction of Cu (I) into Cu (0)

129

0.5 M KHCO3

Heterogeneous

FECH3OH = 45.7 jtotal = 6.93 mAcm−2

5h

Limited mass transfer

130

0.5 M KHCO3

Heterogeneous

FECH3OH = 54.8 jtotal = 10 mAcm−2

20 h

-

- 0.8 V (versus Ag/AgCl) -0.8 V (versus SCE) -1.0 V (versus SCE)

10 mM 2-mPy

Homogeneous and heterogeneous Heterogeneous

FECH3OH = 25.6 %

5h

Change in selectivity after longer reaction time

FECH3OH = 41.3 %

4h

0.5 M KHCO3

Heterogeneous

FECH3OH = 15.9 %

Stable Ru catalyst/Catalytic poisoning Formation of side products in a higher concentration other than methanol

0.1 M KHCO3

Heterogeneous

FECH3OH = 100 %

28 h

Buffer

Heterogeneous

FECH3OH = 29.8 %

-

-0.6 V (versus SCE) - 0.95 V (versus SCE)

0.5 M Na2HCO3

Passivation of electrode during extended electrolysis reaction High pressure needed to maintain supercritical CO2 solution A mixture of products is obtained

Ref

127

128

102

37

131

134

135

138

-

136


ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

RuO2-TiO2 NTs

RuO2-TiO2 NTs

p-GaP

p-GaP

p-GaP

pyridinium

- 0.8 V (versus SCE) - 1.4 V (versus SCE)

Page 30 of 73

0.5 M NaHCO3

Heterogeneous

FECH3OH = 60.5 %

10 min

-

137

0.05 M K2HPO4/KH2PO 4 buffer solution

Heterogeneous

FECH3OH = 60% ΦMeOH = 0.61% j = 6.0 mAcm−2

24 h

Lower selectivity of electrocatalyst for methanol production

139

- 0.52 V (versus SCE)

0.1 M acetate buffer + 10 mM pyridine

Homogeneous

FECH3OH = 100 (quantum efficiency) ΦMeOH = 44% jtotal = 0.2 mAcm−2

30 h

23

Hydrogenated Pd

pyridinium

- 0.55 V (versus SCE)

0.5 M NaClO4 + 10 mM pyridine

Homogeneous and Heterogeneous

FECH3OH = 30

19 h

Limited energy conversion efficiency

PdNP-PANI

PdNP-PANI

- 0.9 V (versus Ag/AgCl)

0.5 M H2SO4

Heterogeneous

FECH3OH = 5.4 %

-

Very low FE for methanol production

140

141


Page 31 of 73 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

510


MOFs as catalysts for CO2RR

511

MOFs are produced by the interaction of metal ions (Fe, Co, Ni, Mn, Cr, Cu, etc.) with

512

organic ligands (comprising C, H, O, N), and the resultant compounds are characterized by distinct

513

crystal structure, high surface area (500 – 6240 m2 g-1), even pore size distribution (3.0 – 3.5 Ao),

514

enormously low density and substantial approachable pore volumes (0.4 – 3.6 cm3 g-1). MOFs

515

were initially elucidated by Yaghi 142 in the 1990s, and until now over 20000 types of MOFs have

516

been produced and characterized for different applications 143. The unique properties make them

517

suitable for a large number of applications such as gas capturing 144, gas purification and separation

518

145,

519

drug-carrying/delivery 150 and catalysis 151. The application of MOFs as a catalyst in conventional

520

chemical reactions are widely demonstrated and thoroughly reviewed 152,153. Metals or “open metal

521

sites” plays a crucial character in the catalytic features of MOFs. These coordinative unsaturated

522

metallic sites are the active center and work as Lewis acid in catalytic reactions.

luminescence

146,

magnetism

147,

proton and ion conduction

148,

molecular recognition

149,

523

The important application of MOFs is as an electrocatalyst for the reduction of CO2 to

524

methanol. However poor electrical conductance, low chemical, and thermal stability in highly

525

acidic or basic aqueous environments is a noteworthy hurdle. The metal centers can be substituted

526

with other metal atoms, or doping can be done to enhance the electrocatalytic behavior. Doping

527

with different heteroatoms dramatically improve the catalytic activity of MOFs. Moreover,

528

heterometallic MOFs or sophisticated nanomorphologies can also be achieved to get better control

529

over composition and regioselectivity of MOFs. In order to introduce the essential features to the

530

MOFs, the composites can be produced by functionalization of MOFs with various materials.

531

These composites display improved characteristics due to the synergetic effect of the modification

532

material and MOFs that are not possible to attain if MOFs are present in the isolated form. Various 31 ACS Paragon Plus Environment


Page 32 of 73

154,

materials are utilized to modify the MOFs such as metal nanoparticles

534

quantum dots

535

are widely employed in various applications. Due to high porosity, superior surface area and the

536

high density of the catalytic sites distributed on the surface of MOFs making them highly suitable

537

for the catalytic applications. MOFs also carry pores of specific size and shape that made them

538

highly selective for the molecules under study. On the other hand, MOFs also demonstrate high

539

activity, excellent stability, and reusability. MOFs can be tailored to single atomic active metal

540

centers which can increase the interfacial contact between electrolyte and electrode

541

Furthermore, the structure of MOFs can be modified to enhance their electrocatalytic activity, e.g.,

542

the introduction of other metals or nanoparticles. Numerous methods are reported in the literature

543

for the development of MOFs on a highly conductive surface such as carbon nanotubes, graphene

544

or other materials 161,162. The characteristics as mentioned earlier made MOFs and their composites

545

highly suitable for catalytic applications.

156,

carbon nanotubes

157,

graphene

158,

polymers

159,

ionic liquids

155,

533

etc. The composite materials

160.

546

Zhao and coworkers carbonized of the copper-based MOF (HKUST-1) to acquire oxide-

547

derived Cu/C (OD Cu/C) electrocatalyst. The OD Cu/C selectively catalyze the electrochemical

548

reduction of CO2 into methanol and ethanol at -0.1 to -0.7 V (versus RHE). The OD Cu/C

549

electrocatalyst produce ethanol at lowest overpotential (-0.1 V versus RHE) as compared to other

550

catalysts reported in the literature. The diminution in overpotential was attributed to the robust

551

binding of CO2 with the electrocatalyst surface because of the confinement effect. The soaring

552

conductivity of OD Cu/C as compared to the porous copper and porous carbon material was

553

accredited to the synergetic effect of highly dispersed copper in a porous carbon matrix. The copper

554

nanoparticles embedded into carbon matrix act as the catalytic active sites for binding the CO2.

555

The CO2 adhered to nanoparticles dissociate into CO and further protonated to alcohols. On the 32 ACS Paragon Plus Environment

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556

other hand, porous carbon structure maintains the high pressure of CO2 on the electrode solution

557

interface that facilitates C – C coupling. Additionally, a porous carbon structure also helps in the

558

efficient transfer of CO2 to the electrocatalyst and diffusion of the alcohols to the solutions. The

559

OD Cu/C render sizeable electroactive area and stable structure for prolonged electroreduction of

560

CO2 into alcohols as compared to porous copper. The stability of the OD Cu/C materials was

561

ascribed by the existence of a carbon matrix that helps to protect Cu from deactivation during

562

electrochemical reduction of CO2. The comparison of production rates and stability of OD Cu/C

563

was made with porous copper for 5 consecutive batches, each batch lasts 3h. The study of the

564

reaction capacity revealed that the production rate for the methanol and ethanol could vary between

565

5.1−12.4 and 3.7−13.4 mg L−1 h−1 correspondingly due to the different activity of the OD Cu/C

566

composite depending on different carbonization temperatures. The possible reaction path for the

567

electrochemical reduction of CO2 can be shown as follow in Fig. 12 163.

568 33 ACS Paragon Plus Environment


569

Page 34 of 73

Figure 12. The proposed reaction mechanism for CO2 electroreduction on OD Cu/C. with permission 163

570

In another study, gas diffusion electrode (GDE) containing various copper-based MOFs

571

such as (1) HKUST-1 metal-organic framework (MOF), [Cu3(µ6-C9H3O6)2]n; (2) CuAdeAce

572

MOF, [Cu3(µ3-C5H4N5)2]n; (3) CuDTA mesoporous metal-organic aerogel (MOA), [Cu(µ-

573

C2H2N2S2)]n; and (4) CuZnDTA MOA, [Cu0.6Zn0.4(µ-C2H2N2S2)]n have been used for

574

electrochemical reduction of CO2. The surface characterization and voltammetric study revealed

575

that among the materials under study HKUST-1 carry greater porosity, large surface area, and

576

highest conductivity. The HKUST-1 start electroreduction of CO2 at -1.0 V (versus Ag/AgCl) and

577

follow 6 electrons pathway to produce methanol and 12 electrons mechanism to synthesize

578

ethanol. The applied current density plays a crucial role in electrocatalytic activity and composition

579

of the product obtained, as shown in Fig. 13.


Page 35 of 73 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


580 581

Figure 13. The rate of formation and FEs for methanol and ethanol during electrocatalytic reduction of CO2 as a

582

function of the Current densities applied with a) HKUST-1, b) CuAdeAce, c) CuDTA and d) CuZnDTA. with

583

permission 57

In order to attain maximum production of the alcohols, 3 mLmin-1cm-2 and 20 mLmin-

584 585

1cm-2

586

Although, the MOF based electrodes display good electrocatalytic activity towards the reduction

587

of CO2. However, the FTIR and PXRD study revealed that the 5h reaction in GDE gives rise to

588

uninterrupted activity loss and FE drop for CuAdeAce, CuDTA, and CuZnDTA up to 65, 98, and

589

51% respectively. The decrease in activity was accredited to the leaching of the metal-organic

590

porous material from the GDE carbon support. During the preparation of MOPM-GDE defects in

were selected as the optimal value for the flow rate of electrolyte and gas respectively.



Page 36 of 73

591

the catalytic layer assist the H2 formation that is also the primary cause of the drop-in product

592

yield. On the other hand, HKUST-1 display relatively stable formation rates up to 17 h that was

593

attributed to the conservation of the MOF structure even with a noteworthy loss in crystallinity

594

during reaction time. Moreover, the poor understanding of the reaction mechanism of over

595

catalytic system, low efficiency of conversion, poor selectivity towards specific product and

596

insignificant long-term stability are the major issues to be solved before the large-scale application

597

of the electrocatalyst 57.

598

A covalent organic framework (COF) Co-Pc-PBBA has been designed to promote the high

599

concentration of CO2 around the reduction center to enhance the % yield of the reduction products.

600

The investigation of CO2 storage capacity revealed that pores of COF could hold up to 24

601

molecules of the CO2 with the maximum concentration in the middle of pores. The greater

602

gravimetric capacity of the COF at normal pressure also make it suitable for the electrochemical

603

reduction of the CO2. The relation between Gibbs free energy and concentration of CO2 can be

604

written as

605

Δ𝐺 = Δ𝐺𝜃 − 2.303 ∙ 𝑅𝑇𝑙𝑔𝑐(𝐶𝑂2),

(11)

606

Where Δ𝐺𝜃 denotes free energy obtained by the DFT calculations. The high

607

concentration of CO2 prominently lowers the reaction overpotential from 0.39 V to 0.27V (versus

608

RHE) and reaction rate increase

609

by 97.7 times as compared to the

610

aqueous

611

design

electrolyte for

the

86.

The

integrated


Page 37 of 73 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

612


electrode system is shown in the Fig. 14.

613 614 615 616 617 618 619 620

Figure 14. Suggested electrode design by deposition the COF layer onto the hollow porous metal tube. With

621

permission 86.

622

Recently, Cardoso and co-workers engrafted 50 nm zeolite imidazole framework-8 (ZIF-

623

8) nanoparticles on the Ti/TiO2 nanotubes utilizing layer by layer process. The resulted Ti/TiO2-

624

ZIF-8 electrodes display capability to absorb CO2 in the form of carbamate as shown in Fig 15.

625

Afterward, photoelectrocatalytic reduction under UV–vis irradiation at room temperature

626

produced a mixture of methanol and ethanol. The bias potential plays a crucial role in the CO2

627

conversion to value-added products on the surface of Ti/TiO2-ZIF-8 electrode. Outcomes revealed

628

the fact that the application of 0.1 V bias potential prominently enhances the yields of alcohol as

629

compared to photolysis, photocatalysis, and electrolysis. The better performance of

630

photoelectrocatalysis was accredited to better separation of electron/holes due to capturing of

631

photoelectrons by linkers and bending of conduction bands. The proposed mechanism for the

632

reaction can be shown as in Fig. 15.



Page 38 of 73

633

Figure 15. Proposed mechanism for alcohol formation using Ti/TiO2-NT-ZIF-8 electrodes operated under UV-vis

634

irradiation and with the application of the external potential. .with permission

164

635

The reaction commenced with photoactivation of Ti/TiO2-ZIF-8 electrode surface and

636

produced imidazole radicals that affirm the preconcentration of dissolved CO2 gas by the

637

establishment of carbamate. The photogenerated electrons on the electrode surface contribute to

638

the conversion of the carbamate to methanol and ethanol. The nitrogenous group on the ZIF-8

639

found responsible for the selective reduction of CO2 to methanol. On the other hand, the ZIF-8

640

coated on Ti/TiO2 shifts the conduction band to a higher energy level that promotes CO2

641

conversion to ethanol. The reaction description proposed that the CO2 produce methanol with

642

traces of side product. Furthermore, the availability of the photoelectrons reduces the methanol to

643

ethanol as an end product. Ti/TiO2-ZIF-8 demonstrate remarkable photocurrent stability for 50 h

644

during the electroreduction of CO2. The SEM image after 50 h reaction shows that the material 38 ACS Paragon Plus Environment

Page 39 of 73 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


645

structure remains intact. The IR spectra of the Ti/TiO2-ZIF-8 photoelectrode confirms the CO2

646

absorption band intensity decrease at 2338 and 2359 cm-1 while a strong band at 1727 and 3330

647

cm-1 confirms the appearance of carbamate species

648

the solventless partial doping of the Zn (II), Ru (III) and Pd (II) to an electrocatalyst HKUST-1

649

(Cu) was done. The resultant materials were employed as an electrode in a continuous flow filter

650

press electrochemical cell. The increase in dopant concentration prominently enhances the ethanol

651

selectivity as compared to pristine HKUST-1 MOFs material. The ethanol selectivity was

652

attributed to the strong interaction of reactants with dopant metals that favor C – C coupling to C2

653

(ethanol) product. Ru (III) based electrode display highest yield of methanol and ethanol with 47.2

654

% FEs as compared to other dopants. The GDE activity continually decays with the passage of

655

time that was attributed to the continuous loss of MOF particles from the electrode surface

656

Table 4. Briefly summarize the MOF-based electrocatalysts for CO2RR.

164.

In order to synthesize heterometallic MOF

165.


ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 40 of 73

Table 4. Summary of MOF-based electrocatalysts for CO2RR Cathode

Electrocatalyst

Electrode potential

Electrolyte

Catalysis type

Catalytic performance

Reactio n Time

Remarks

Ref

5h

Relatively low yield of methanol

163

17 h

Very low FE

57

24 h

Not specific for the methanol production Relatively low yield of methanol Unstable activity for longer reaction

86

Cu/C-derived from MOF

Cu/C

-0.1 V (versus RHE)

0.1 M KHCO3

Heterogeneous

HKUST-1

HKUST-1

- 1.0 V (versus Ag/AgCl)

0.5 M KHCO3

Heterogeneous

Co-Pc-PBBA

Co-Pc-PBBA

-0.27 V (versus RHE)

-

Heterogeneous

FECH3OH = 43.2 % FEC2H5OH = 34.8 % FECH3OH = 5.6 % FEC2H5OH = 10.3 % j = 10 mAcm−2 -

Ti/TiO2-ZIF-8

Ti/TiO2-ZIF-8

+0.1 V (versus Ag/AgCl)

0.1 M Na2SO4

Heterogeneous

-

50 h

Ru doped HKUST-1

Ru doped HKUST-1

-2.0 V (versus Ag/AgCl)

0.5 M KHCO3

Heterogeneous

FECH3OH + FEC2H5OH = 47.2 %

60 min

164

165

657


Page 41 of 73 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

658


Conclusion and Recommendations

659

The evolution of exceedingly effective and low-priced catalysts for the electroreduction of

660

CO2 into CH3OH is an emerging area of research. The electrochemical CO2RR required external

661

bias that can be provided by utilizing the renewable energy sources such as solar power or wind

662

power 84. The electrocatalyst that can proficiently convert the energy from the renewable energy

663

sources is of core interest. The ideal features obligatory for a material to act as catalyst includes;

664

low cost, availability, high catalytic activity, porous structure, large surface area, a gigantic number

665

of active sites and mass transport of the target compounds 74. Owing to the importance of methanol

666

as a fuel, various sort of materials can be employed for the efficient electrochemical reduction of

667

CO2 to methanol such as metal-free carbon, graphene, CNTs, metal oxides and MOFs 55,80,166–168.

668

Among numerous catalytic materials, MOFs get the better attention as a catalyst due to

669

their tailorable nature. The building units (ligands and metal centers) and reaction conditions

670

render the freedom to develop the MOF with desired characteristics. The discussion in our review

671

divulged that the MOFs could be used in the pure form or the materials can be derived from MOFs

672

that act as electrocatalysts

673

tunable and uniform pore structure, distinct confined nanopore microenvironment and existence

674

of electroactive sites make MOFs appropriate for electrocatalytic applications 169,170. The limited

675

thermal and chemical stability hinder MOFs for the full potential application as a catalyst. In order

676

to enhance the catalytic capabilities, MOFs are combined with other materials to give rise

677

composites that display better characteristics as compared to pure MOFs 164.

678

163,164.

The high surface area, well defined and designable structure,

MOFs are broadly applied for the catalytic hydrogenation of CO2 49,151,166,174,175.

171–173

and

679

electrochemical reduction of CO2

However, the product selectivity is not

680

satisfactory up to now. The MOF catalysts can make a mixture of chief products, but it is difficult 41 ACS Paragon Plus Environment


Page 42 of 73

681

to get the specific products of interest at large scale. The electrochemical reduction of CO2 into

682

methanol is barely touched area

683

review show low FE and poor selectivity for the methanol yield 57. The selectivity of MOF catalytic

684

material depends on the nature of the metal, distribution of catalytic sites, particle size, surface

685

morphology and CO2 pressure

686

electroreduction of CO2 into methanol is a challenging task and need deep insight. The intrinsic

687

conductivity of the MOFs electrocatalyst prominently affects the electrocatalytic activity, charge

688

propagation, and mass transport during the reaction. The inadequate conductivity of the pure

689

MOFs is a substantial drawback that can be resolved by growing or blending MOFs with highly

690

conductive materials (graphene), mingling with nanomaterials or deriving highly conductive

691

materials by calcination of MOFs

692

conductive, stable porous structure with numerous active sites for the catalytic reaction. Blending

693

with other materials can affect the selectivity, therefore a comparative study of electrocatalytic

694

activity of the modified materials is mandatory for the choice of best electrocatalyst. The

695

composites suppressing the competing reaction can enhance the selectivity of MOF catalyst

696

towards formation of methanol. The types of single active sites extant on MOFs may include open

697

metal sites (OMSs), metalloporphyrins, and reactive functional groups

698

can be easily set up on MOFs because of their potential for having unlimited design opportunities.

699

The single active site catalysis demonstrates the advantages of the highly selective product,

700

reusability of catalyst, the possibility of spatial and electronic environment adjustment of active

701

sites 179,180. Therefore, a better understanding of the MOFs single-site catalysis can lead the way

702

to build ideal catalyst for the sustainable conversion of CO2 into methanol with better selectivity.

57,86,163,164.

151,176,177.

The MOFs based electrocatalysts discussed in this

The development of electrocatalysts for selective

161,163,164.

These derived materials can provide a highly

178.

The single-active site


Page 43 of 73 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


703

Furthermore, theoretical and computational methodologies can also render a way to

704

comprehend and improve the catalytic action of materials. Even though the DFT methods are well

705

established for the various catalytic system, but there is negligible progress for the systems

706

involving electrocatalysis by MOFs and need exploration

707

performance of specific MOF based electrocatalyst by employing theoretical study. Solid

708

fundamental theories and optimized standard experimental systems are still absent. The production

709

of methanol from the electrocatalytic reaction of MOFs has recently discovered a field that needs

710

more exploration. When the electrochemical reduction of CO2 is exercised on the surface of MOFs

711

the methanol yield is very low. Due to multiple functionalities present on the MOFs it has potential

712

to grow in the field of electrocatalytic reduction of CO2 into methanol.

181,182.

It is complicated to predict the

713

The selection of electrolyte become very crucial due to the low solubility of the CO2 in

714

aqueous electrolytic solution. The limited supply of the CO2 effect the FE and the current density

715

of the electrochemical reduction reaction. To overcome the limitation a preconcentrated high-

716

pressure CO2 stream can be utilized, but it dramatically increases the process cost. The solid oxide

717

electrolyte can directly reduce the CO2 to syngas that acts as feedstock for the methanol synthesis.

718

The configuration of an electrochemical cell also plays a vital role in catalytic output. For instance,

719

the fuel cell like configuration is not suitable for the electrochemical reduction of CO2 because of

720

HER competing reaction. A buffer solution could be utilized to avoid excessive flux of protons

721

towards the cathode. Nevertheless, the cell conductivity inversely depends on the thickness of the

722

buffer. The utilization of anion exchange membrane arrangement allows the electroreduction of

723

CO2 on the cathode and O2 evolution reaction in alkaline medium. Gas diffusion electrodes (GDEs)

724

helped in reducing the internal resistance and enhancing the reactant mass transfer process.



Page 44 of 73

725

Though, the limitation of GDEs is the blockage of pores because of accumulation of liquid phase

726

products.

727

In conclusion, low carbon fuels from CO2 electroreduction is aimed to lessen extreme

728

environmental hazards due to CO2 release. According to the strong opinion of the authors of this

729

review paper, well-directed exertions are necessitated to improve the fundamental research about

730

the electrocatalytic reduction of CO2 into methanol on the surface of MOFs. The noticeable

731

improvements in this area of research can be accomplished by developing MOF materials with

732

specific design carrying high electroreduction capabilities and better stability. The progress in

733

electrocatalytic reduction of CO2 into methanol will originate a facile and clean recycling process

734

for CO2 that also aid to diminish the greenhouse effect.

735

Acknowledgment:

736

The corresponding author wants to acknowledge Prof. Omar Yaghi and Kyle E. Cordova for their

737

comments in this review paper.


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738

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The sustainable electrochemical conversion of CO2 into methanol by utilizing MOFs and non-MOFs electrocatalysts.

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A review on recent advances for electrochemical reduction of carbon dioxide to methanol using MOFs and non-MOFs catalysts; challenges and future prospects

Figure 1. The chemical compounds originating from CO2 reduction with Gibbs free energies.



Figure 2. Schematic diagram of electrochemical reduction of CO2 and possible applications of fuel product.


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Figure 3. Possible mechanisms for electrochemical reduction of CO2 into methanol.



Figure 4. the schematic diagram for homogeneous and heterogeneous catalysis.


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Figure 5. Thermodynamically favorable reduction pathway (solid line) from CO 2 to CO and to CH3OH, other competing pathways (dash lines) are also depicted.



Figure 6. Molecular orbital energy level diagram of CO2.


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Figure 7. Illustrative representation of CO2 interaction with the metal surface.



Figure 8. The main reaction pathways at the electrode surface, with adsorbed blocking the majority of the surface and hydrocarbon products being formed by the further reduction of adsorbed CO.


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Figure 9. A schematic representation of the reaction occurring on the surface of Cu electrode.



Fig 10. Schematic diagram of the electrolytic cell configuration for the electroreduction of CO 2 supplied directly from the gas phase.


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Figure 11. Mechanism of the photocatalytic process of CO2 reduction.



Figure 12. The proposed reaction mechanism for CO2 electroreduction on OD Cu/C.


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Figure 13. The rate of formation and FEs for methanol and ethanol during electrocatalytic reduction of CO 2 as a function of the Current densities applied with a) HKUST-1, b) CuAdeAce, c) CuDTA and d) CuZnDTA.



Figure 14. Suggested electrode design by deposition the COF layer onto the hollow porous metal tube.


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Figure 15. Proposed mechanism for alcohol fomarion using Ti/TiO 2-NT-ZIF-8 electrodes operated under UV-vis irradiation and with application of external potential. .


A Review on Recent Advances for Electrochemical Reduction of

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