From CO2 to Value-Added Products: Comparison
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The global warming and the dangerous climate change arising from the massive emission of CO2 from the burning of fossil fuels have motivated the search for alternative clean and sustainable energy sources. However, the industrial development and population necessities make the decoupling of economic growth from fossil fuels unimaginable and, consequently, the capture and conversion of CO2 to fuels seems to be, nowadays, one of the most promising and attractive solutions in a world with high energy demand. In this respect, the electrochemical CO2 conversion using renewable electricity provides a promising solution. However, faradaic efficiency of common electro-catalysts is low, and therefore, the design of highly selective, energy-efficient, and cost-effective electrocatalysts is critical. Carbon-based materials present some advantages such as relatively low cost and renewability, excellent electrical conductivity, and tunable textural and chemical surface, which show them as competitive materials for the electro-reduction of CO2.

  • carbon dioxide
  • electro-reduction
  • carbon-based materials
  • value-added products

1. Introduction

The energy supply currently depends mostly on fossil fuels, causing a continuous accumulation and, therefore, an excess of CO2 in the atmosphere, bringning negative effects on the environment. The population and live standards growth make nor imaginable the decoupling of energy supply from fossil fuels. Faced with this situation, different altenatives have been proposed to mitagate the enviromental impact and dependence on nonrenewable energy sources. The conversion of CO2 into value-added products by chemical reactions seems to be the most promising and attractive solution since, together with the reduction of the atmospheric CO2 levels, CO2 is efficiently recycled stablishing an ideal zero-emission carbon balance. CO2 can be converted to added-value products by photochemical [1] [2] [3] [4], thermochemical [5] [6] [7] [8], radiochemical [9] [10], biochemical [11] [12] [13] [14], and electrochemical strategies [15] [16] [17] [18]. However, the most interesting alternative is the capture and use of CO2 as raw material to produce various products ( Table 1) through its electrochemical reduction since this is a flexible and controllable process with mild and safe operating conditions and low equipment cost, which also allows coupling environmentally friendly non-fossil energy from renewable sources. Taking into account these advantages, many efforts have been made worldwide in the development and improvement of the technology available for CO2 electro-conversion.

Table 1. Equilibrium potential and Gibbs free energy for CO2 reduction reactions.

However, despite the fact that electro-reduction of CO2 (CO2RR) is thermodynamically viable, its transformation presents very slow reaction kinetics and usually requires significant energy expenditure [19] due to the high stability and inertness of the CO2 molecule [20]. Therefore, an extensive research has been developed by the overall scientific community focused on the electrocatalyst design, since the efficiency and selectivity of the reduction reaction is strongly dependent on the electrode nature, properties, and configuration [21]. An ideal catalyst for CO2 electroreduction requires: (i) Being able to mediate the transfer of electrons coupled to protons, (ii) having a low over potential for the activation of the CO2 molecule, (iii) exhibiting a selectivity preferably towards a target product, and (iv) preserving structural integrity during prolonged operation.

Lately, carbon-based catalysts have attracted much attention due to their relatively low cost and renewability, good chemical stability, excellent electrical conductivity, tunable textural and chemical surface, and large surface area, containing micropores, mesopores, and macropores that favor adsorption, access, and diffusion of molecules to the internal active sites of the material [22]. Due to these particular characteristics, carbon-based materials have been extensively used as electrocatalysts for CO2 reduction either as supports to disperse different metallic particles with several sizes (single-atoms, dual-atoms, nanoparticles) or as direct catalyst by functionalization with heteroatoms to prepare economical and sustainable metal-free electro-catalysts [ 23 ][23] . ( Figurae 1 ).

Productos de valor añadido obtenidos con catalizadores a base de carbono

Figure 1. Value-added products using carbon- based catalysts

.

2. Metal-Free Carbon Materials as Catalyst

Metal-free carbon-based catalysts emerge as an alternative to overcome the difficulties that arise when using metals as catalysts, such as their limited availability and poor durability that prevent their application on large scales. However, the activity of carbon materials itself is poor, so heteroatoms (N, B, S, P, F) are introduced into the carbon structure to promote electrocatalytic activity and selectivity [24]. Carbon doping with foreign heteroatoms affects the electronic structure of carbon materials since the different size and electronegativity of such foreign atoms compared to carbon atoms lead to a charge redistribution and, consequently, modify their electrochemical catalytic properties [25]. Additionally, the covalent chemical bonds between the carbon and the doped atoms avoid segregation problems occurring in metal-based catalysts leading to better operational stability [25]. Different heteroatoms have been used to dope carbon obtaining materials with good electrochemical performance in the CO2 reduction, and among them, N and B have been the most studied.

2.1. N-Doped Carbon-Based Materials

The N atom has a similar size to the C atom but higher electronegativity [26], therefore the defects caused by nitrogen doping can break the electroneutrality of C atoms in the hexagonal carbon structure [27] leading to an enhanced electronic/ionic conductivity without distortion in the local geometry that influences the electrocatalytic activity [28]. N-doped carbons have shown to be promising candidates as catalysts for the electro-reduction of CO2 due to the low over-potentials obtained, the high activity, stability, and selectivity towards certain products ascribed to this surface properties modification. The nitrogen species are located in several places within the carbon skeleton, which results in different active sites. The electrocatalytic behavior of the N-doped carbons towards CO2RR is deeply dependent on the type of nitrogenated surface group and its content [26].

2.1.1. N-Doping Methodology and N-Doped Catalyst Active Sites

Different carbon and nitrogen precursors have been used for the synthesis of N-doped carbon electrocatalysts (Table 2). Two main doping strategies have been developed: In situ and post-doping treatments. The first consists of simultaneously perform both the synthesis and doping of carbon-based materials at the same time; while in the second, the carbon material is first synthesized and then doped in a subsequent process [22][25]. After doping, four types of nitrogen species can be identified in the carbon skeleton by XPS: Pyridinic (398.5 eV), pyrrolic/pyridonic (399.9 eV), quaternary or graphitic N (401.0 eV), and oxidized pyridinic species (403.4 eV) [29][30]. The total amount of nitrogen fixed on the carbon structure and the nature of N functionalities clearly depends on the N precursor source, doping methodology, and carbon material.

Table 2. Carbon and nitrogen precursors to construction of electrocatalysts and active sites.

Sample

Main Product

Carbon Precursor

N Precursor

Type a

Synthesis Method

Nitrogen Species (% atomic) XPS

Active site

Ref

N b

Pyri c

Pyrr d

G e

O f

NR/CS-900

CO

CS (porous carbon nanosheets)

Polymerized Aniline with Ammonium Persistence to Polyaniline (Solid)

Post

Activation (ZnCl2) and pyrolysis

5.30

1.45

1.05

2

0.8

Pyridinic

[31]

NCNTs

CO

CNTs

Acetonitrile- dicyandiamide

In situ

Liquid vapor deposition (CVD)

5.0

1.5

1.1

2.4

n.d.

Pyridinic

[32]

WNCNs-1000

CO

Coal, NaCl template method C-700

NH3

Post

Pyrolysis

4.3

2.61

1.45

0.16

0.08

Pyridinic

[33]

NCNTs-ACN-850

CO

CNTs (Acetonitrile)

Dicyandiamide

In situ

Liquid chemical vapor deposition

4.9

1

0.5

3.4

n.d.

Pyridinic

[21]

CN/MWCNT

CO

MWCNT

NaN3 reacts with C3N2Cl3 to form g-C3N4

Post

Pyrolysis

0.12

n.d.

n.d.

n.d.

n.d.

n.d.

[34]

NCNT-3-700

CO

CNTs

Poly(diallyldimethylammonium chloride) (PDDA)

Post

Pyrolysis

1.75

0.5

0.625

0.5

0.125

Graphitic

[35]

CNPC-1100

CO

Coal

NH3

Post

Ammonia etching/pyrolysis

4

1

1

1.5

0.5

n.d.

[36]

NG-800

CO

GF (Graphene foam) Ni-foam vapor deposition

gC3N4 (Solid)

Post

Pyrolysis

6.6

4.5

1.5

0.6

n.d.

Pyridinic

[37]

MNC-D

CO

ZIF-8

Dimethyformamide DMF

Post

Heat-treated

16.97

8

5

4

n.d.

Pyridinic

[38]

NPC-900

CO

Coal arantracite

Dicyandiamide (DICY)

In situ

Activation (KOH)/Pyrolysis

1.92

0.60

0.50

0.83

n.d.

n.d.

[27]

N-CWM

CO

Natural wood

Urea

Post

Pyrolysis

4.07

0.54

2.90

0.55

0.09

Pyridinic

[39]

N-graphene

Formate

Graphene oxide

Melamine (Solid)

In situ

Pyrolysis

5.5

3

0.9

1.6

n.d.

Pyridinic

[40]

PEI-NCNT/GC

Formate

Multiwalled CNT

Ammonia plasma/co-catalyst (PEI)

Post

Ammonia plasma enhanced chemical vapor deposition (PECVD)

11.3

n.d.

n.d.

n.d.

n.d.

n.d.

[41]

HNCM/CNT

Formate

CNT

Dimethyformamide DMF

In situ

Pyrolysis

8.26

3.5

n.d.

4.8

n.d.

Pyridinic

[42]

N-C61-800

Formate

Fullerene PC61BM

Urea

In situ

Pyrolysis

2.57

0.39

0.54

1.64

 

Graphitic

[43]

CF-120

Syngas

CF (carbon foam)PU

NH3

Post

Ammonia etching/pyrolysis

5.5

2.5

1.8

0.6

0.6

Pyridinic/Pyrrolic

[44]

3D N-CNTs/SS-750

Syngas

Melamine

Melamine

In situ

Pyrolysis

6.8

3.90

0.50

2.50

n.d.

n.d.

[45]

c-NC

Ethanol

Resol

Dicyandiamide

In situ

Pyrolysis/Template method-mesoporous materials

7

2.6

2.8

1.6

n.d.

Pyridinic/Pyrrolic

[46]

MNCs-5

Ethanol

Resol

Dicyandiamide

In situ

Pyrolysis/triblock-copolymer-templating method.

~6.2

2.1

2.8

1.3

n.d.

Pyridinic/Pyrrolic

[47]

GO-VB6-4

Ethanol-Acetone

Graphene oxide

B6 Vitamin

Post

heat treatment

2.3

2.3

n.d.

n.d.

n.d.

Pyridinic

[48]

NGM-1/CP

CH4

Graphene

3-Pyridinecarbonitrile

In situ

Pyrolysis

6.52

1.45

3.35

1.72

n.d. 

Pyridinic/Pyrrolic

[49]

               

a N-doping type, b N total, c Pyridinic, d Pyrrolic, e Graphitic, f Oxide.

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