Review www.acsami.org
Electrochemistry of Carbon Dioxide on Carbon Electrodes Nianjun Yang,*,† Siegfried R. Waldvogel,‡ and Xin Jiang*,† †
Institute of Materials Engineering, University of Siegen, 57076 Siegen, Germany Institute of Organic Chemistry, Johannes Gutenberg-University Mainz, 55128 Mainz, Germany
‡
ABSTRACT: Carbon electrodes have the advantages of being chemically inert at negative potential ranges in all media and high offset potentials for hydrogen evolution in comparison to metal electrodes, and therefore are the most suitable electrodes for electrochemistry and electrochemical conversion of CO2 into valuable chemicals. Herein we summarize on carbon electrodes the voltammetry, electrochemical and electrocatalytic CO2 reduction, as well as electron synthesis using CO2 and carbon electrodes. The electrocatalytic CO2 reduction using carbocatalyts and the future activities about electrochemical CO2 conversion are highlighted. KEYWORDS: carbon dioxide, carbon electrodes, electrochemical conversion, metal-free catalysts, electrochemical reduction, electrocatalytic reduction, electrosynthesis, nanocarbons
1. INTRODUCTION Sabatier developed in 1902 a chemical scheme of converting CO2 and H2 to methane and later to related liquid fuels.1 Since then, the utilization of CO2 as the C1 feedstock for production of synthesis gas (e.g., CO/H2) and valuable organic compounds (e.g., methanol, methane, ethylene, formic acid) have been triggered with various chemical, photochemical, biochemical, and electrochemical approaches.2−4 As one of the most promising approaches, electrochemical CO2 reduction at metal (e.g., Hg, Cu, etc.) and related electrodes as well as electrocatalytic CO2 reduction have been extensively investigated in aqueous and nonaqueous (e.g., organic, ionic liquid, the mixture of organic and ionic liquid) media during the past decades.5−11 Depending on the half-reactions involved, various chemicals can be generated at different electrode potentials. These potentials can be estimated from the standard Gibbs energies of these reactants involved in the reactions. Some example reactions occurring in aqueous solutions and their related electrode potentials are listed in Table 1.9 Carbon materials, including glassy carbon, graphite, borondoped diamond (BDD), carbon black, carbon nanofiber, carbon tubes (CNTs), and recently graphene, have been extensively applied as the electrodes in the fundamental innovations and application aspects of electrochemistry and electroanalysis for years.12,13 They offer distinct properties over other solid metal electrodes, such as long-term stability, diverse © 2015 American Chemical Society
surface chemistry, and stable bonds of carbon with various surface modifier.13 For instance, on carbon electrodes the adsorption or the covalent grafting of organic molecules and the loading metal-contained catalysts have been realized for electrocatalytic CO2 reduction.9,10 Moreover, carbon electrode exhibts a long lifetime during electrochemical experiments and a higher overpotential for hydrogen evolution reaction than most metal electrodes, leading to the suppression of this competing reaction for CO2 reduction.9−12 Namely carbon electrodes have higher Faradaic efficiency toward electrochemical CO2 reduction, and thus they are the perfect electrodes for electrochemical and electrocatalytic CO 2 reduction. Despite the initial work about voltammetry of CO2 at carbon electrodes was conducted several decades ago,14 a review on the topic about electrochemistry of CO2 on carbon electrodes is still a gap in the literature up to date. Therefore, we survey in the first part of this review both historical and current data about voltammetry of CO2 on carbon electrodes. The second Special Issue: Electrochemical Applications of Carbon Nanomaterials and Interfaces Received: October 15, 2015 Accepted: December 18, 2015 Published: December 18, 2015 28357
DOI: 10.1021/acsami.5b09825 ACS Appl. Mater. Interfaces 2016, 8, 28357−28371
Review
ACS Applied Materials & Interfaces Table 1. Examples of Selected Half-Electrode Reactions and the Related Electrode Potentials in Aqueous Solutions at 1.0 atm and 25 °Ca half-electrochemical thermodynamic reactions CO2(g) + 4H+ + 4e− = C(s) + 2H2O(l) CO2(g) + 2H2O(l) + 4e− = C(s) + 4OH− CO2(g) + 2H+ + 2e− = HCOOH(l) CO2(g) + 2H2O(l) + 2e− = HCOO−(aq) + OH− CO2(g) + 2H+ + 2e− = CO(g) + H2O(l) CO2 (g) + 2H2O(l) + 2e− = CO(g) + 2OH− CO2(g) + 4H+ + 4e− = CH2O(l) + H2O(l) CO2(g) + 3H2O(l) + 4e− = CH2O(l) + 4OH− CO2(g) + 6H+ + 6e− = CH3OH(l) + H2O(l) CO2(g) + 5H2O(l) + 6e− = CH3OH(l) + 6OH− CO2(g) + 8H+ + 8e− = CH4(g) + 2H2O(l) CO2(g) + 6H2O(l) + 8e− = CH4(g) + 8OH− 2CO2(g) + 2H+ + 2e− = H2C2O4(aq) 2CO2(g) + 2e− = C2O42−(aq) 2CO2(g) + 12H+ + 12e− = CH2CH2(g) + 4H2O(l) 2CO2(g) + 8H2O(l) + 12e− = CH2CH2(g) + 12OH− 2CO2(g) + 12H+ + 12e− = CH3CH2OH(l) + 3H2O(l) 2CO2(g) + 9H2O(l) + 12e− = CH3CH2OH(l) + 12OH−
electrode potential (V) (vs SHE)b 0.210 −0.627 −0.250 −1.078 −0.106 −0.934 −0.070 −0.898 0.016 −0.812
Figure 1. Voltammograms of CO2 on a ratating glassy carbon disk electrode in 0.1 M pH 10 tetramethylammonium chloride at the ratating speed of (A) 40, (B) 30 and (C) 20, (D) 10 rev s−l. Reprinted with permission from ref 14. Copyright 1980 Royal Society of Chemistry.
0.169 −0.659 −0.500 −0.590 0.064
process. A mass-transport-controlled process was noticed as well when the time scale of the rotating disc electrode became shorter or when the rotating speed was faster. Bulk electrolysis at controlled potential (e.g., the potential of the first wave) in pH 9 solutions led to production of oxalate. When the concentration of hydroxide concentration was not higher than for example, 0.5 M, the major reduction product was formate. Subsequently, they reported the utilization of linear sweep voltammetry to study CO2 voltammetry in the similar electrolytes but with different anions (chloride, perchlorate).15 In those two electrolytes, similar voltammetric behavior was observed. In other words, two successive waves were noticed. These irreversible waves were reproducible within a range from 0.004 to 0.480 V s−1. For these two waves, only one electron was involved for each wave. The waves shifted to more negative potentials from −0.64 to −0.74 V with an increase in pH value of the electrolytes. The ratio of the difference of the potentials to that of pH values (dEp/dpH) was 0.049 V pH−1 and 0.090 V pH−1 for these waves, respectively. The size difference of ionic radii (e.g., tetraethylammonium cation) from that of other cations was considered to rationalize the variation of peak potentials in both electrolytes. The release of CO2 molecules from aqueous bicarbonate was believed to be the rate-limiting step. Equilibrium constants were estimated for this process using rotating disc electrode voltammetry. The calculated values were 0.069 and 0.076 when the pH value was 9.1 and 10, respectively. The rate of the formation of CO2 molecules was further calculated. However, the calculated rate constant (e.g., 1.6 s−1 obtained in pH 10 electrolyte) conflicted with that for a homogeneously kinetics-controlled process. Later Eggins and co-worker studied the voltammetry of CO2 in different media, including water, dimethyl sulfoxide, acetonitrile, and propylene carbonate.16 The supporting electrolyte was 0.1 M tetramethylammonium bromide. In water, a reduction wave appeared at −2.21 V (vs SCE). The peak potential was different from those obtained on metal electrodes. Deep analysis of current function from chronoamperometric curves and diffusion-controlled tail of reduction waves were conducted. In this way, the number of electrons (napp) involved during reduction was calculated. napp was 2.11, corresponding to the formation of formic acid in nonbuffered neutral electrolyte. The diffusion coefficient was 1.07 × 10−5 cm2 s−1. In dimethyl sulfoxide and in acetonitrile, only a one-
−0.764 0.084 −0.744
a
Reprinted with permission from reference 9. Copyright 2014 Royal Society Chemistry. bThe electrode potentials are calculated according to the standard Gibbs energies of these reactants involved in the reactions.
part deals with electrochemical CO2 reduction using directly carbon electrodes. Electrocatalytic CO2 reduction is summarized in the third part and there divided into two sections: devoted to metal-containing and metal-free catalytic CO2 reduction, respectively. Electrochemical organic synthesis using CO2 and at carbon electrodes is briefly presented in the fifth part. Throughout the presentation and comparison of older works together with new ones, the challenges and future tendencies for electrochemical and electrocatalytic CO2 reduction using carbon electrodes are concluded and presented as the summary and outlook in the final part of this review. In this way, we hope to provide the reader an idea of where and how electrochemistry of CO2 started and varied and will go in future.
2. VOLTAMMETRY OF CO2 The glassy carbon electrodes,14−16 graphite,17 and novel carbons like BDD,18 nitrogen-doped nanodiamond,19 carbon nanotubes (CNTs),20 and pristine CNT arrays21 have been applied for the investigation of the voltammetry of CO2 in different media. For example, Eggins and co-worker made the first discussion about the equilibrium and kinetic effects of CO2 reduction on a carbon electrode.14 Figure 1 shows the related voltammograms of CO2 in 0.1 M aqueous tetramethylammonium chloride solution where a rotating glassy carbon disc electrode was employed. The pH value was adjusted from 8 to 10 with tetramethylammonium hydroxide. The voltammograms showed two one-electron reduction waves with equal magnitude of peak currents. The variation of reduced currents with an increase in sweep rates indicated a kinetic-controlled 28358
DOI: 10.1021/acsami.5b09825 ACS Appl. Mater. Interfaces 2016, 8, 28357−28371
Review
ACS Applied Materials & Interfaces
ratios of these reduction products and Faradaic efficiency were dependent on the applied potential (Table 2). An electrolysis
electron reduction wave was found. It resulted from the disproportionation of radical anions which were initially formed, leading to the production of CO and CO32−. In propylene carbonate, the clear cathodic waves were not observed. It was assigned to unpurified solvent. The diffusion coefficients for CO2 in dimethyl sulfoxide, acetonitrile, and propylene carbonate were 6.15 × 10−6, 3.05 × 10−5, and 6.04 × 10−6 cm2 s−1, respectively.16 Benneu et al. employed the graphite electrode to check the voltammetry of CO2 in 0.1 M tetramethylammonium chloride (pH 9) aqueous solution.17 The voltammogram showed two successive one-electron waves, corresponding to one reduction step at a potential of −0.9 V (vs Ag/AgCl) to yield oxalic acid and another one at a potential of −1.8 V (vs Ag/AgCl) to glyoxylic acid.17 Einaga and co-workers investigated for the first time the voltammetry of CO2 on BDD in a methanol solution containing tetrabutylammonium perchlorate (Figure 2).18 The
Table 2. Electrochemical CO2 Reduction on Graphite Electrode at Different Potentialsa electrode
E/V
oxalate
formate
glyoxylate
Faradaic efficiency
graphite
−0.9 −1.05 −1.26 −1.7 −1.88
100 15 17 15 6
0 72 74 72 65
0 12 7 0 28
78 58 59 84b 93
a
Reprinted in part with permission from ref 17. Copyright 1988 Elsevier. b0.5 M supporting electrolyte.
conducted at a potential of −1.88 V (vs Ag/AgCl) produced glyoxylic acid (28%) together with oxalic acid (6%), formic acid (65%), and tartaric acid (trace). The overall Faradaic efficiency was 93%. At a glassy carbon electrode, electrochemical CO2 reduction in acetonitrile was observed only at potentials below −2.2 (V vs SCE).22 CO was the major product and accompanied by carbonate. At a high pressure (e.g., 30 atm of CO2), the selectivity for reduction products on a glassy carbon electrode was determined mainly by the applied current density.23 An increase of current density led to a drastical reduction of the Faradaic efficiency for the CO2 reduction. In 0.1 M KHCO3 aqueous electrolyte, Faradaic efficiencies of 44% for CO and 30% for formic acid were obtained at a constant current density of 50 mA cm−2. Moreover, hydrocarbons were formed, such as CH4, C2H6, C3H8, and C4H10. When small current densities were applied, C3 and C4 hydrocarbons (e.g., C3H8 and C4H10) were initially formed during electrolysis. On BDD, the bulk electrolysis experiments performed at ambient conditions showed that the reaction products were HCHO, HCOOH, and H2.18 At a potential of −1.7 (V vs Ag/ Ag+), the Faradaic efficiency for the production of HCHO was the highest (74%). The highest Faradaic efficiency for HCOOH was 15%, obtained at a potential of −1.5 V (vs Ag/Ag+). Regarding the H2 formation, the Faradaic efficiency was less than 1.1% when the applied potential was lower than −1.7 V (vs Ag/Ag+). Under identified conditions, only a Faradaic efficiency of 15% was achieved for the HCHO generation on low-quality diamond electrodes, namely BDD with higher contents of sp2-bonded carbon. Through the comparison of these Faradaic efficiencies using diamond electrodes with different sp2/sp3 ratios, Einaga et al. concluded that the high and stable Faradaic efficiency was attributed to the sp3-bonded carbon of BDD. They found out further CH3OH, aqueous NaCl, and seawater were suitable electrolytes for electrochemical CO2 reduction on diamond electrode. An electrolyte yielded efficiency of 36% was obtained in seawater for the selective formation of formaldehyde.18 CNTs have been coated on the glassy carbon electrode and employed for electrochemical CO2 reduction. The reduction products were mainly H2 (>90%), HCOOH (about 5%), and trace amounts of CO (
