Designing Carbon-Based Materials for Efficient Electrochemical

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Kinetics, Catalysis, and Reaction Engineering

Designing Carbon-Based Materials for Efficient Electrochemical Reduction of CO2 Samira Siahrostami Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05253 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 19, 2018

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Industrial & Engineering Chemistry Research

Designing Carbon-Based Materials for Efficient Electrochemical Reduction of CO2 Samira Siahrostami* Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4 * Corresponding author: [email protected]

Abstract Carbon-based materials have attracted a great deal of attention in recent years for electrochemical CO2 reduction reaction (CO2RR). This contribution gives an overview of recent experimental and theoretical efforts in designing carbon-based materials for CO2RR and provides hints for further tuning the electronic structure of carbon-based materials for highly efficient CO2RR catalysis. Using combined density functional theory (DFT) calculations and experimental evidence, I show that graphene-based materials without embedded metal atoms, exclusively reduce CO2 to CO. I also show that in order to see any post CO reduced product, the presence of metal atoms is essential. By examining variety of metal-doped at single vacancy of graphene I demonstrate facile reduction of CO to methane.

Keywords: Density functional theory (DFT) calculations, calculated limiting potential, hydrogen evolution reaction (HER), scaling relation, free energy diagram, single atom catalyst (SAC).

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Introduction Electrochemical conversion and reduction of CO2 coupled with a renewable energy source (such as wind or solar) is an appealing approach to generate carbon-neutral fuels or industrial chemicals. Carbon is at the core of potentially applicable earth-abundant materials for CO2RR. Porous carbon materials have shown great promise in this regard owing to their excellent stability, high surface area, tailorable porosity and excellent electronic conductivity. Hierarcheical porous carbon includes various porosities from micro-/meso-/macroporous network, which provide high surface area and facile mass transport as well as suitable ground for fabrication of active materials. It is known that the electronic properties of these materials can be further tailored by introducing dopants,1–3 and generating defects. Nitrogen doped porous carbon, for instance, has been shown to highly enhance the CO2RR performance.3–11 The addition of nitrogen results in more disordered carbon structure with more defects acting as potential active sites. We have recently studied a wide range of possible defects in graphene with and without N-doped and shown that a wide range of activities can be expected for CO2RR.12 Most importantly we demonstrated that CO does not bind to almost all the possible defect configurations and N-doped carbon motifs, hence the major product of CO2RR is CO. These results were in very good agreement with previous experimental reports on carbon-based electrocatalysts for CO2RR with CO as the most common product.13–16 Recent careful experimental analysis demonstrates that many synthesis procedures for carbon-based materials will incorporate a wide range of metal impurity types and amounts.17 These metallic impurities can significantly change the electrochemical properties of the carbonbased materials and have potential applications for variety of reactions. A recent experimental study by Bell et al. demonstrates that the presence of metallic impurities, especially copper, increases the activity for CO2RR toward further reduced products.18 Metal dimers have been theoretically studied for CO2RR by Sun, et al and proven to significantly enhance the activity toward methane and methanol formation.19 These studies show that in order to capture CO and further reduce it, the presence of a metal component is necessary. To investigate these effects, we have recently considered the CORR activity in the presence of single transition metal atom embedded into carbon-based materials.20,21 Of note, single atom catalysts (SAC) are unique

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systems that have been the subject of numerous theoretical and experimental studies.22–27 The interaction between metal and carbon atoms has been broadly studied based on energy calculations28–30, molecular dynamics31,32 and experimental observation.23,33 The single metal atom trapped in the conductive network of the graphene and nanotubes shows distinctively different properties from its metal counterpart.30,34 Herein, I summarize recent theoretical and experimental results on CO2RR on carbon-based materials in the absence and presence of single metal atoms. Carbon-based Materials for CO2RR to CO We previously examined CO2RR on a wide range of possible defects as well as N-doped configurations in the graphene structure using DFT calculations and computational hydrogen electrode approach.35 In all of these calculations we assume aqueous solution. We first investigated the two-electron CO2RR mechanism, which goes through COOH* and CO* intermediates using a simple thermochemical analysis, CO2+ * + (H++ e−) → COOH*

(1)

COOH* + (H++ e−) → CO*+ H2O

(2)

CO* → CO + *

(3)

Total: CO2+ 2(H++ e−) → CO+ H2O

E0 = -0.12 V

(4)

Figure 1 displays the calculated adsorption energies of COOH* vs. CO*, mostly adapted from our previous publication.36 Included here are also the results of other doped hetero-atoms such as boron, phosphor, chlorine and sulfur (pink circles). The horizontal green dashed line displays the adsorption energy of the ideal catalyst that catalyzes the two-electron reduction of CO2 to CO with zero overpotential. The vertical distance between this line and each point in the graph can be used as a metric for activity. The blue solid line in Figure 1 represents the pure transition metals scaling line. This plot shows that on weak binding active sites (GCO > 0 eV), the CO molecule readily desorbs from the surface and the major product will be CO gas. Many of the examined active sites are in this range. Also it is interesting to note that there are several

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examined motifs (see Ref. [12] for structural details) with better activity than those of Au and Ag for selective two-electron reduction of CO2 to CO. 37 This could be the reason that carbon-based materials represent especially interesting alternatives for the two-electron reduction of CO2 to CO. On the other hand, over the few configurations that bind CO (GCO < 0 eV, blue shaded area), further reduced products past CO may be produced. The (GCO = 0 eV shows the chemical potential of CO in the gas phase, and the blue shaded area separates the region that CO can be bound to the surface and gets further reduced from no adsorption regime.

Figure 1. Calculated free adsorption energies of COOH* vs. CO* intermediates for variety of examined defects, and hetero-atom doped structures. Other dopants shown by pink circles include hetero-atoms such as boron, phosphor, chlorine and sulfur. Transition metals (111) data is adapted from Ref. [37]. Undoped and N-doped data are adapted from Ref. [36]).

Single Atom Catalysts for CORR to further Reduced Products So far, I have shown that carbon-based materials are not capable of reducing CO2 to hydrocarbons. For further reduction of CO to more valuable hydrocarbons and alcohols a different catalytic design must be implemented. One possible solution that we have recently addressed is to embed transition metal atoms into carbon-based materials in order to capture CO and further reduce it. Of note, transition metals can be trapped in the single and double

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vacancies of the graphene.23,28,30,33 Depending on the size of the vacancy, single metal atoms or clusters of several atoms can be localized in graphene layers.33 We have recently used DFT calculations in conjunction with the computational hydrogen electrode (CHE)35 model, to study a wide range of transition metals (including Cr, Mo, W, Mn, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, and Cu) embedded in single vacancy (M@SV) of graphene for the electrochemical reduction of CO. Of note, we did not consider the possibility of the transition metal in the double vacancy for simplicity. Figure 2a shows that all the transition metal dopants form strong bonding with the single vacancy of graphene. This phenomenon is known and has been reported for other transition metals in particular Au doped graphene28–30 as well as metalfunctionalized porphyrin-like graphene38. Figure 2b shows the projected density of states (PDOS) for Ni atom as an example in both Ni(111) and Ni@SV (Figure 2b). It can be seen that electronically an isolated Ni atom in the Ni@SV is noticeably different from that of in Ni(111) surface. This difference arises not only from it being a single atom but also from electronic hybridization with carbon atoms, which results in opening a gap between occupied and unoccupied states in contrast to those of Ni(111). The unoccupied states corresponding to the narrow peak at ∼1 eV above Fermi energy (EF) originate from the hybridization of the in-plane dstates of Pt with the sp2-states of its neighboring C atoms. Clearly such significant amount of unoccupied anti-bonding states reveals a strong Ni-C bonding. The most important asset of the Ni@SV system is the presence of much higher PDOS around EF than that found in Ni(111). Indeed, these states are those responsible for the high catalytic activity of M-doped graphene, as we shall elaborate in the following.

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a)

b) 0

-0.5

Formation Energy /eV

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

Cu

W

-1

Os

-1.5

Pd Pt

-2 -2.5

Ir

Ni

-3

Mo

Fe

Co

Rh

Ru Cr

-3.5

Figure 2. a) Calculated formation energies of the studied M@SV of graphene, referenced to the bulk metal and bare graphene with a single vacancy adapted from Ref. [20]. Inset shows the atomic structure of M@SV of graphene. Color code; C: gray, metal (M): pink. b) Projected density of state for d-states Ni@SV and Ni(111). Inset displays the charge density for Ni@SV obtained via ρdiff = ρall – ρNi – ρvacancy and represent the charge redistribution upon formation of Ni@SV. Thermodynamic Analysis We first study thermochemical analysis and later expand to more complex kinetic analysis of the CORR. Recent computational studies on transition metals have indicated that the high overpotential for the CORR originates from the correlation between different key intermediates.39 Moreover, similar calculations have shown that the reduction of CO* to form CHO* is the potential limiting step in CORR. Therefore, improvement in CO2RR catalytic activity has been suggested to require decoupling of the CO* and *CHO binding energies. Several strategies to circumvent the scaling relation between *CO and *CHO have been proposed and applied in the past.40 Figure 3 displays the binding energies of CHO* vs. CO*. The dashed green line in Figure 3 represents the pure transition metals scaling line. The black dotted line represents the ideal case where both CO* and CHO* energies are equal and therefore formation of CHO* from CO* requires zero overpotential. Figure 3 shows that the binding energies of CO* and CHO* for M@SVs deviate significantly from the scaling relation for transition metals in the direction of improved activity.

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1.5 Ag Cu

1

Au

Pd

GCHO* / eV

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


Cu

0.5 Ni

Fe

Pd

Ru

0 Pt Rh

Os

-0.5

Ni Cr

Pt

Rh Mn Mo Co

Ir

W M@SV M(111)

-1

-1.5 -1.5

-1

-0.5

0

GCO* / eV

0.5

1

1.5

Figure 3. Adsorption energies of CHO* vs. CO* for different M@SV of graphene (circles) and pure transition metals (triangles). The green dashed line shows the linear fit, showing the scaling between CO* and CHO* binding energies for transition metals. The black dotted line shows the diagonal where the binding energies for CO* and CHO* are equal. The dashed vertical line denotes the chemical potential of CO in the gas phase, above which CO does not bind to the surface. By closer inspection of Figure 3 we note that the outliers from the scaling line belong to the cases of the oxophilic transition metals such as Mn, Co, W, Os, Ir, Cr, and Mo, where *CHO binds to metal center in a bidentate form (through both the C and O atoms). It is therefore stabilized compared to CO* (Figure 4). We note that for all the outliers (in particular W and Mo), very small overpotentials are associated with converting CO* to CHO*. However, although the oxophilic nature of the elements such as W and Mo improves the CO reduction catalytic activity of M@SVs, it could bring another problem, that is, facilitating water decomposition and subsequent OH* poisoning of the surface. In other words, if OH* binds too strongly to the active site, reducing it off the surface may become the potential limiting step rather than CO* reduction to CHO*. Moreover, OH* is also one of the possible intermediates in the CORR pathway39, in which case reduction of OH* becomes the thermodynamic bottleneck. This issue becomes more pronounced when we take into account the fact that if some of the intermediates that may or may not be

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directly involved in CORR path bind strongly, they would poison the active site and thus terminate the reaction.38

(a)

(b)

Figure 4. (a) and (b) display CO* and CHO* adsorbates configurations (respectively) on W@SV. Color code, red: oxygen; white: hydrogen; gray: carbon; blue: tungsten.

To assess the possibility of OH* poisoning, Figure 5 shows the limiting potential for the two most likely thermodynamic bottlenecks for CORR i.e., either reduction of CO* to CHO* (CO*→CHO*) or reduction of OH* to H2O (OH*→H2O) against each other. The dotted diagonal line indicates the same limiting potential for both steps. For the M@SVs above diagonal such as Ir, Os, Mn, Mo, and W@SV, the potential limiting step is determined by OH*→H2O. For Cr and Co, the limiting potentials for CO* and OH* reduction are equal, which indicate both CO* and OH* are reduced at the same applied potential. On the other hand, for the M@SVs below the diagonal such as Pd, Pt, Cu, Ni, Fe, Ru and Rh@SV, the CO*→CHO* step remains thermodynamically limiting. This indicates that less negative potentials are required to reduce CO* than that of OH* reduction. Therefore, it is less likely for OH* to block the active site under CORR potentials on these M@SVs. These six candidate M@SVs are taken into account in the next section to further investigate the CORR kinetics. 41

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0.4 M@SV 0.2

UL(CO*CHO*) / V

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


W

0

Mo

Cr

-0.2 -0.4

Mn Os

-0.6

Ir

Co Fe

Rh Ru Ni

-0.2

0

-0.8

Pd

Pt Cu

-1 -1.2 -1.4 -1.4

-1.2

-1

-0.8

-0.6

-0.4

UL(OH*H2O) / V

0.2

0.4

Figure 5. Competition between CO reduction and facilitated OH* reduction: for the surfaces that are above the diagonal (dashed line), OH* poisoning will prevail over CO reduction. The surfaces corresponding to the points below the diagonal the reduction of CO* and OH* occur simultaneously.

Kinetic Analysis A previous study on transition metals (111) and (211) showed that electrochemical barriers for reduction of CO* to CHO* scale with the CO binding energy.42 Therefore, electrochemical activation energies could be a good descriptor of activity. Hence, we further analyzed the electrochemical barriers for the key proton-electron transfer steps to predict the catalytic activity by taking into account energy contributions from rearrangement along the reaction pathway.41 To investigate the electrochemical barriers we used the explicit solvation model. We showed that on several single atom catalysts (such as Pt@SV shown in Figure 6), reduction of CO* to CHO* is a critical step for methane formation and that the corresponding electrochemical barriers scale more favorably with the CO* binding energy than for 211 and 111 transition metals. Hydrogen evolution reaction (HER) (Eq. 5-7) is a competing reaction in aqueous solutions under reducing potentials. Abundant solvated protons in the aqueous solution are the first species to meet the electrode surface and active sites. As soon as we start sweeping the potential

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to negative range, protons will occupy the active sites and be reduced to hydrogen gas. Therefore, HER marks a selectivity challenge for CORR. (H++ e−) + * → H*

(5)

H* + (H++ e−) → H2 + *

(6)

Total: 2(H++ e−) → H2

E0 = 0.0 V

(7)

To investigated the competition between HER and CORR we compared electrochemical barriers for these two reactions and found a higher selectivity for CORR than HER over all the M@SV systems.41 These results showed that carbon-based materials can be tuned to be both highly active for CORR to further reduced products such as methane and selective over hydrogen evolution reaction (HER). This study underlined M@SV systems as superior electrocatalysts over transition metals for CORR.41

Pt@SV *COH

H2O(l)

0 VRHE -0.5 VRHE

*CH H2O(l)

*CHOH CO(g)

*CHO

*CH2OH

*CH2O

*CH2

*CO *CH3

*CH4 (g)

Figure 6. Free energy diagram for CORR to CH4 on Pt@SV at 0 (black) and -0.5 V (red) vs. RHE, adapted from Ref. [41].

Challenges Moving Forward We note that it is quite challenging to exclusively synthesize M-doped at single vacancy (M@SV) in the experiment. In reality, a large distribution of active sites including both M-doped at single vacancy and double vacancy can be formed during synthesis. In addition, one common 10


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experimental trick to embed single metal atoms in the carbon-based material is to start with Ndoped sample which already has a high density of defects/vacancies and facilitate the inclusion of single metal atoms. These powerful techniques have enabled the synthesis of SAC however, they strongly affect the coordination chemistry of transition metal and ultimately the nature of active site. We have recently studied these effects for an example of CO2RR to CO on Ni-doped graphene. Figure 5 shows that by varying the coordination environment of transition metal atom, the binding energies of COOH* and CO* intermediates significantly change. As a result, the CO2RR mechanism and efficiency is strongly affected by the density of M@SV or M@DV active sites. It is important to note that the adsorption energies of COOH* and CO* are much weaker on the Ni@DV than those on Ni@SV. Hence, these results show that CO2 can exclusively be reduced to CO on Ni@DV.

a)

b) Ni@SV

Ni@DV

N-Ni@SV

N-Ni@DV

Figure 7. (a) Different possible SAC configurations for the example of Ni-doped graphene. (b) Calculated free energy diagram for the two-electron CO2RR at -0.12 V vs. RHE. Color code, N: blue, C: gray; Ni: red. We have recently examined several SAC systems for CO2RR experimentally and showed that on Ni single atoms in the carbon nanofiber, CO2 can be selectively reduced to CO with very high

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efficiency.43 This experimental result was interesting in that it showed comparable efficiency to Au and Ag for reduction of CO2 to CO. However, no further reduce products were detected. Based upon the theoretical analysis in Figure 7, we argue that the density of active sites with positive CO binding energy such as Ni@DV and N-Ni@DV are much higher than the density of active sites with negative CO binding energy such as Ni@SV. This is the reason we don’t observe further reduced products in the experiment. Conclusion Carbon-based materials provide a rich and highly tunable network for variety of catalytic reactions. In this work, we summarized our recent understanding on designing carbon-based materials for CO2RR. Using DFT calculations and ample evidenced experimental reports, we demonstrated that carbon-based materials with no metal components reduce CO2 exclusively to CO. In the presence of single metal atoms, CO can be reduced to hydrocarbons. Using both thermochemical and kinetic analysis we have carefully examined the effect of including variety of single metal atoms in the single vacancies of the graphene for CORR. These results show that methane formation is facile on many of the metal-doped at single vacancy of graphene. There are however, challenges to experimentally synthesize the single metal atoms with exclusively M@SV. We showed that how coordination chemistry of the single metal atom affects the energetics of reaction intermediates. Therefore, future research direction should focus on more rigorous synthesis methods to target a specific single atom coordination. References: (1)

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Designing Carbon-Based Materials for Efficient Electrochemical

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