- Email: [email protected]
Selective electrochemical conversion of CO2 to C2 hydrocarbons
Energy Conversion and Management 51 (2010) 30–32
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Selective electrochemical conversion of CO2 to C2 hydrocarbons M.R. Gonçalves a, A. Gomes a, J. Condeço a, R. Fernandes a, T. Pardal a,*, C.A.C. Sequeira b, J.B. Branco c a
Omnidea, Lda, Travessa António Gedeão, No. 9, 3510-017 Viseu, Portugal Instituto Superior Técnico – Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal c Unidade de Ciências Químicas e Radiofarmacêuticas, Instituto Tecnológico e Nuclear, Estrada Nacional 10, Apartado 21, 2686-953 Sacave´m, Portugal b
a r t i c l e
i n f o
Article history: Received 6 January 2009 Accepted 12 August 2009 Available online 6 October 2009 Keywords: Electrodeposits Electroreduction Carbon dioxide C2 hydrocarbons
a b s t r a c t A novel approach for the conversion of CO2 by an electrochemical process has been achieved using copper electrodes modified with electrodeposits. The presence of the electrodeposits changes the catalytic behaviour of the substrates, since a hydrocarbons mixture with higher energetic density per volume is achieved when compared with the fuel resulting from CO2 reduction without electrodeposits. In these electrodes, ethylene is selectively produced in detriment of methane. Additionally, the formation of a mixture of C2 hydrocarbons (C2H4 and C2H6) without C1 hydrocarbons is successfully obtained on specific higher surface area electrodeposits. Furthermore, it is confirmed that the presence of the electrodeposits lead to steadiness of the CO2 reduction process, since it is established the stability of hydrocarbons production during 4 h. Ó 2009 Published by Elsevier Ltd.
1. Introduction Clean power can be generated from various renewable sources such as solar, wind and hydro. However, the energy from many of these sources cannot be fully utilized because it is available intermittently and not necessarily at the time or place of the energy demand. Thus, the ability to store this renewable energy in a portable form would allow it to be used when and where it is needed. One approach to utilise this renewable energy is to employ it to synthesize fuels from captured CO2 [1–3]. This energy can then be used within the existing infrastructures to displace fossil fuels and result in a net decrease of CO2 emissions. This is especially interesting because it allows renewable energy to be applied to terrestrial transportation applications, which represents a large and growing source of CO2 emissions. Additionally, this approach is relevant for space applications, namely Mars Exploration Missions, since CO2 is nearly 95% of the Martian Atmosphere. For a return mission to Mars an In Situ Propellant Production (ISPP) facility is a key technology for manufacturing the fuel and the oxygen required by a Mars ascent vehicle also, such a system could also be adapted for providing surface power and life support. The ideal system for the conversion of CO2 to fuel would have few process stages, be easily scalable and, for terrestrial applications, could operate at ambient temperature [4]. The electrochemical process appears to be a capable alternative to achieve this aim. Furthermore, electrochemical processes can be started and
* Corresponding author. Tel.: +351 936265296; fax: +351 213964224. E-mail address: [email protected] (T. Pardal). 0196-8904/$ - see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.enconman.2009.08.002
stopped relatively easily, which is important if the process is to make use of off-peak energy. The most interesting result in the electrochemistry area is the electrochemical reduction of CO2 to a mixture of methane and ethylene at a copper electrode [5]. The reduction of CO2 to hydrocarbon products at copper appears to be a complex multi-step reaction involving shared intermediates and multiple reaction pathways. The faradaic efficiencies of the processes are strongly sensitive to the electrode surface structure and local conditions such as pH, KHCO3 and CO2 concentration and electrolysis time [6]. According to the literature, the mixture of hydrocarbons produced by electroreduction of CO2 is composed mostly of methane and ethylene, obtained with different current efficiencies. In general the faradaic efficiency of CH4 is commonly higher than that for C2H4 [5,6]. Unfortunately, all the faradaic efficiencies fall suddenly after short periods of electrochemical reduction, and the copper electrode loses its high catalytic activity toward the conversion of CO2 [7–9]. In order to improve the catalytic activity of the copper electrode and the selectivity of the hydrocarbons production, we propose the use of copper electrodes modified with electrodeposits with different specific surface areas in order to improve the catalytic activity and selectivity of the electrode. 2. Experimental Potentiostatic reduction of CO2 was performed in a flat cell at room temperature and atmospheric pressure. Electroreduction tests were performed under conditions of continuous CO2 flow (14.78 cm3 min 1). The electrolyte solution used was 0.1 mol dm 3
31
M.R. Gonçalves et al. / Energy Conversion and Management 51 (2010) 30–32
and structure does not result in the production of methane, but yields a mixture of only C2 hydrocarbons (C2H4 (10.7%) and C2H6 (3.7%)). According to the literature, the mechanism for the electroreduction of carbon dioxide has not yet been fully elucidated. The first step is probably the reduction of CO2 to form chemisorbed CO. The CO is then reduced in the presence of protons to give the radical CH2. This species has to remain adsorbed on electrode surface so that the required transfer of electrons and protons happens, producing the hydrocarbons. The CH2 radicals either undergo dimerisation to form ethylene or react with hydrogen to form methane [6]. Based on our results, the rates of formation of ethylene and
60
Current efficiency (%)
KHCO3 (Merck, p.a.). A cation exchange membrane separated the catholyte and anolyte compartments. The anode was Pt mesh (Goodfellow Metals). The electrode potential was measured using an Ag/AgCl reference electrode. A copper mesh (Goodfellow Metals) and modified copper electrodes (Omnidea, Lda) were used as cathodes (geometric areas, c.a. 2.47 cm2). An assessment of the real surface area of the copper electrodes was possible by estimating the double layer capacitance of the electrodes. To reach this, cyclic voltammograms were recorded in the double layer region of potentials at different scan rates between 10 and 100 mV s 1, in 0.1 mol dm 3 HClO4 solution. The double layer capacitance was then calculated from the slope of the current (i) vs scan rate (v) line, under the assumption that the faradaic contribution is negligible [10]. The outlet gas composition was analyzed on-line by gas chromatography using a Restek ShinCarbon ST micropacked column (L = 2.0 m, U = 1/8 in., ID = 1 mm, 100/200 mesh) and an Agilent 4890D GC equipped with a thermal conductivity detector (TCD) and a 6-port gas sampling valve with a 0.250 mL loop. The faradaic efficiencies of the products were calculated on the basis of the number of electrons required for the formation of one molecule of the products from CO2 and H2O; 8 for CH4, 12 for C2H4, 14 for C2H6, 2 for CO and 2 for H2. Only gaseous products were analysed.
A
50 40 30 20 10
3. Results and discussion
0 0
1
2
3
4
5
3
4
5
3
4
5
t (h) 60
B 50
Current efficiency (%)
The effectiveness of the modified copper electrodes on CO2 electroreduction was evaluated with two distinct modified copper electrodes (B and C). For comparison, studies with the copper mesh (A) were also performed. Based on double layer capacitance values and taking into consideration the copper mesh electrode as the reference, the real surface area of electrodes B and C is, respectively, 7 and 19 times larger than that of electrode A. The performance of the three copper electrodes on the reduction of CO2, after 1 h 15 min of reduction process, is shown in Fig. 1. Electrode A, characterised by a smoothly polished copper surface gives rise to the production of methane and ethylene with faradaic efficiencies of 19.4% and 18.7%, respectively. These percentages are of the same order of magnitude of values in the literature [11,12]. Electrode B provides for the same current efficiency an almost selective production of ethylene (33.3%) with a corresponding decrease of methane (3.6%). Electrode C gives rise to the production of a new component, ethane (C2H6), and a decrease of the current efficiency. Surprisingly, this composition
40 30 20 10 0 0
1
2
t (h) 60
C Current Efficiency (%)
45 40 35
C2H4
30 25 20 15
C2H4 CH4
C2H6
10 5
CO
0 A
CH4 CO
B Electrode
Fig. 1. Current efficiency for CO2 electroreduction at 15 min, at electrodes A–C.
Current efficiency (%)
50 40 30 20 10
C2H4 CO
0 0
1
2
C H2 1.9 V vs Ag/AgCl after 1 h
CO
t (h) CH4
C2H4
Fig. 2. CO2 electroreduction as a function of time at A–C.
C2H6
1.9 V vs Ag/AgCl on electrodes
32
M.R. Gonçalves et al. / Energy Conversion and Management 51 (2010) 30–32
methane probably depend essentially upon the surface morphology/surface area. The change of selectivity of the electrode B compared to electrode A due to the presence of the electrodeposit supports this hypothesis. Further studies are necessary to better understand and explain the decrease of the current efficiency for hydrocarbons and the formation of ethane when using electrode C. Furthermore, it is noted that changes in the composition of the cathode surface may also justify the existence of different adsorption abilities between electrodes. Hori et al. showed that, the distribution product at copper foil is highly affected by the electrode potential [13]. To investigate if the same applies to the electroreduction of CO2 on electrode C, tests were performed at different potentials. For all the potentials tested, methane was not detected and, on the contrary, ethane is always present. The electroreduction of CO2 at Cu mesh with and without electrodeposits was investigated over a period of 4 h and 15 min, Fig. 2. A delay of detection of the gaseous products occurs due to the unused volume from the electrolysis cell to the gas sampling valve. Thus the current efficiencies of gaseous products are relatively low at the early stage of the electrolysis, i.e., at 15 min as shown in Fig. 2. For electrode A without any electrodeposit, the production of hydrocarbons decreased with time, being more pronounced for periods longer than 2 h. On the contrary, in the presence of electrodeposits (electrodes B and C), the activity towards the CO2 conversion is practically unchanged. Moreover, Fig. 2 shows that hydrogen discharge is highest for electrode C. The hydrogen evolution reaction always competes with the electroreduction of CO2 in aqueous solution. As expected, this reaction is very sensitive to the relative concentration of protons and CO2. For a cathode higher real surface area, the CO2 amount at its surface is not high enough and the hydrogen evolution is promoted in favour of C2 hydrocarbons production. 4. Conclusions The presence of the electrodeposits at the electrode surface modifies strongly the catalytic behaviour of the copper electrodes for the conversion of CO2. This catalytic activity mostly depends of the electrodeposit characteristics, namely the surface area. It was revealed that ethylene can be selectively produced instead of methane. The use of specific high surface area electrodeposits in copper cathodes promotes the formation of a C2 hydrocarbons
mixture (C2H4 and C2H6) and is not selective for C1 hydrocarbon (methane) production. Furthermore, it was confirmed that the presence of electrodeposits leads to steadiness of the CO2 reduction process, since it was established the stability of hydrocarbons production during 4 h. Acknowledgements This research was financial supported by the European Space Agency – ESA (Project No: 19594/06/NL/PA). One of us (J.B. Branco) acknowledge the financial support of the Portuguese ‘‘Fundação para a Ciência e a Tecnologia”, FCT/MCTES, under program POCTI/ QUI/35394/2000. References [1] Kleiner GN, Cusick RJ. Development of an advanced Sabatier CO2 reduction subsystem. In: Intersociety conference on environmental systems. San Francisco, United States: American Society of Mechanical Engineers; 13–15 July, 1981. [2] Weimer T, Schaber K, Specht M, Bandi A. Methanol from atmospheric carbon dioxide: A liquid zero emission fuel for the future. Energy Convers Manage 1996;37:1351–6. [3] Mignard D, Sahibzada M, Duthie JM, Whittington HW. Methanol synthesis from flue-gas CO2 and renewable electricity: a feasibility study. Int J Hydrogen Energy 2003;28:455–64. [4] Gattrell M, Gupta N, Co A. Electrochemical reduction of CO2 to hydrocarbons to store renewable electrical energy and upgrade biogas. Energy Convers Manage 2007;48:1255–65. [5] Hori Y, Konishi H, Murata A, Koga O, Sakurai H, Oguma K. Deactivation of copper electrode in electrochemical reduction of CO2. Electrochim Acta 2005;50:5354–69. [6] Gattrell M, Gupta N, Co A. A review of the aqueous electrochemical reduction of CO2 to hydrocarbons at copper. J Electroanal Chem 2006;594:1–19. [7] Yano J, Morita T, Shimano K, Nagami Y, Yamasaki S. Selective ethylene formation by pulse-mode electrochemical reduction of carbon dioxide using copper and copper-oxide electrodes. J Solid State Electrochem 2007;11:554–7. [8] Friebe P, Bogdanoff P, Alonso-Vante N, Tributsch H. A real-time mass spectroscopy study of the (electro)chemical factors affecting CO2 reduction at copper. J Catal 1997;168:374–85. [9] Shiratsuchi Y, Aikoh G, Nogami. Pulsed electroreduction of CO2 on copper electrodes. J Electrochem Soc 1993;140:3479–82. [10] Waszczuk P, Zelenay P, Sobkowski J. Surface interaction of benzoic acid with a copper electrode. Electrochim Acta 1995;40:1717–21. [11] Kyriacou G, Anagnostopoulos A. Electroreduction of CO2 on differently prepared copper electrodes. The influence of electrode treatment on the current efficiencies. J Electroanal Chem 1992;322:233–46. [12] Kyriacou GZ, Anagnostopoulos AK. Influence CO2 partial pressure and the supporting electrolyte cation on the product distribution in CO2 electroreduction. J Appl Electrochem 1993;23:483–6. [13] Hori Y, Murata A, Takahashi R. Formation of hydrocarbons in the electrochemical reduction of carbon dioxide at a copper electrode. J Chem Soc, Faraday Trans I 1989;85–88:2309–26.
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Selective electrochemical conversion of CO2 to C2 hydrocarbons M.R. Gonçalves a, A. Gomes a, J. Condeço a, R. Fernandes a, T. Pardal a,*, C.A.C. Sequeira b, J.B. Branco c a
Omnidea, Lda, Travessa António Gedeão, No. 9, 3510-017 Viseu, Portugal Instituto Superior Técnico – Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal c Unidade de Ciências Químicas e Radiofarmacêuticas, Instituto Tecnológico e Nuclear, Estrada Nacional 10, Apartado 21, 2686-953 Sacave´m, Portugal b
a r t i c l e
i n f o
Article history: Received 6 January 2009 Accepted 12 August 2009 Available online 6 October 2009 Keywords: Electrodeposits Electroreduction Carbon dioxide C2 hydrocarbons
a b s t r a c t A novel approach for the conversion of CO2 by an electrochemical process has been achieved using copper electrodes modified with electrodeposits. The presence of the electrodeposits changes the catalytic behaviour of the substrates, since a hydrocarbons mixture with higher energetic density per volume is achieved when compared with the fuel resulting from CO2 reduction without electrodeposits. In these electrodes, ethylene is selectively produced in detriment of methane. Additionally, the formation of a mixture of C2 hydrocarbons (C2H4 and C2H6) without C1 hydrocarbons is successfully obtained on specific higher surface area electrodeposits. Furthermore, it is confirmed that the presence of the electrodeposits lead to steadiness of the CO2 reduction process, since it is established the stability of hydrocarbons production during 4 h. Ó 2009 Published by Elsevier Ltd.
1. Introduction Clean power can be generated from various renewable sources such as solar, wind and hydro. However, the energy from many of these sources cannot be fully utilized because it is available intermittently and not necessarily at the time or place of the energy demand. Thus, the ability to store this renewable energy in a portable form would allow it to be used when and where it is needed. One approach to utilise this renewable energy is to employ it to synthesize fuels from captured CO2 [1–3]. This energy can then be used within the existing infrastructures to displace fossil fuels and result in a net decrease of CO2 emissions. This is especially interesting because it allows renewable energy to be applied to terrestrial transportation applications, which represents a large and growing source of CO2 emissions. Additionally, this approach is relevant for space applications, namely Mars Exploration Missions, since CO2 is nearly 95% of the Martian Atmosphere. For a return mission to Mars an In Situ Propellant Production (ISPP) facility is a key technology for manufacturing the fuel and the oxygen required by a Mars ascent vehicle also, such a system could also be adapted for providing surface power and life support. The ideal system for the conversion of CO2 to fuel would have few process stages, be easily scalable and, for terrestrial applications, could operate at ambient temperature [4]. The electrochemical process appears to be a capable alternative to achieve this aim. Furthermore, electrochemical processes can be started and
* Corresponding author. Tel.: +351 936265296; fax: +351 213964224. E-mail address: [email protected] (T. Pardal). 0196-8904/$ - see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.enconman.2009.08.002
stopped relatively easily, which is important if the process is to make use of off-peak energy. The most interesting result in the electrochemistry area is the electrochemical reduction of CO2 to a mixture of methane and ethylene at a copper electrode [5]. The reduction of CO2 to hydrocarbon products at copper appears to be a complex multi-step reaction involving shared intermediates and multiple reaction pathways. The faradaic efficiencies of the processes are strongly sensitive to the electrode surface structure and local conditions such as pH, KHCO3 and CO2 concentration and electrolysis time [6]. According to the literature, the mixture of hydrocarbons produced by electroreduction of CO2 is composed mostly of methane and ethylene, obtained with different current efficiencies. In general the faradaic efficiency of CH4 is commonly higher than that for C2H4 [5,6]. Unfortunately, all the faradaic efficiencies fall suddenly after short periods of electrochemical reduction, and the copper electrode loses its high catalytic activity toward the conversion of CO2 [7–9]. In order to improve the catalytic activity of the copper electrode and the selectivity of the hydrocarbons production, we propose the use of copper electrodes modified with electrodeposits with different specific surface areas in order to improve the catalytic activity and selectivity of the electrode. 2. Experimental Potentiostatic reduction of CO2 was performed in a flat cell at room temperature and atmospheric pressure. Electroreduction tests were performed under conditions of continuous CO2 flow (14.78 cm3 min 1). The electrolyte solution used was 0.1 mol dm 3
31
M.R. Gonçalves et al. / Energy Conversion and Management 51 (2010) 30–32
and structure does not result in the production of methane, but yields a mixture of only C2 hydrocarbons (C2H4 (10.7%) and C2H6 (3.7%)). According to the literature, the mechanism for the electroreduction of carbon dioxide has not yet been fully elucidated. The first step is probably the reduction of CO2 to form chemisorbed CO. The CO is then reduced in the presence of protons to give the radical CH2. This species has to remain adsorbed on electrode surface so that the required transfer of electrons and protons happens, producing the hydrocarbons. The CH2 radicals either undergo dimerisation to form ethylene or react with hydrogen to form methane [6]. Based on our results, the rates of formation of ethylene and
60
Current efficiency (%)
KHCO3 (Merck, p.a.). A cation exchange membrane separated the catholyte and anolyte compartments. The anode was Pt mesh (Goodfellow Metals). The electrode potential was measured using an Ag/AgCl reference electrode. A copper mesh (Goodfellow Metals) and modified copper electrodes (Omnidea, Lda) were used as cathodes (geometric areas, c.a. 2.47 cm2). An assessment of the real surface area of the copper electrodes was possible by estimating the double layer capacitance of the electrodes. To reach this, cyclic voltammograms were recorded in the double layer region of potentials at different scan rates between 10 and 100 mV s 1, in 0.1 mol dm 3 HClO4 solution. The double layer capacitance was then calculated from the slope of the current (i) vs scan rate (v) line, under the assumption that the faradaic contribution is negligible [10]. The outlet gas composition was analyzed on-line by gas chromatography using a Restek ShinCarbon ST micropacked column (L = 2.0 m, U = 1/8 in., ID = 1 mm, 100/200 mesh) and an Agilent 4890D GC equipped with a thermal conductivity detector (TCD) and a 6-port gas sampling valve with a 0.250 mL loop. The faradaic efficiencies of the products were calculated on the basis of the number of electrons required for the formation of one molecule of the products from CO2 and H2O; 8 for CH4, 12 for C2H4, 14 for C2H6, 2 for CO and 2 for H2. Only gaseous products were analysed.
A
50 40 30 20 10
3. Results and discussion
0 0
1
2
3
4
5
3
4
5
3
4
5
t (h) 60
B 50
Current efficiency (%)
The effectiveness of the modified copper electrodes on CO2 electroreduction was evaluated with two distinct modified copper electrodes (B and C). For comparison, studies with the copper mesh (A) were also performed. Based on double layer capacitance values and taking into consideration the copper mesh electrode as the reference, the real surface area of electrodes B and C is, respectively, 7 and 19 times larger than that of electrode A. The performance of the three copper electrodes on the reduction of CO2, after 1 h 15 min of reduction process, is shown in Fig. 1. Electrode A, characterised by a smoothly polished copper surface gives rise to the production of methane and ethylene with faradaic efficiencies of 19.4% and 18.7%, respectively. These percentages are of the same order of magnitude of values in the literature [11,12]. Electrode B provides for the same current efficiency an almost selective production of ethylene (33.3%) with a corresponding decrease of methane (3.6%). Electrode C gives rise to the production of a new component, ethane (C2H6), and a decrease of the current efficiency. Surprisingly, this composition
40 30 20 10 0 0
1
2
t (h) 60
C Current Efficiency (%)
45 40 35
C2H4
30 25 20 15
C2H4 CH4
C2H6
10 5
CO
0 A
CH4 CO
B Electrode
Fig. 1. Current efficiency for CO2 electroreduction at 15 min, at electrodes A–C.
Current efficiency (%)
50 40 30 20 10
C2H4 CO
0 0
1
2
C H2 1.9 V vs Ag/AgCl after 1 h
CO
t (h) CH4
C2H4
Fig. 2. CO2 electroreduction as a function of time at A–C.
C2H6
1.9 V vs Ag/AgCl on electrodes
32
M.R. Gonçalves et al. / Energy Conversion and Management 51 (2010) 30–32
methane probably depend essentially upon the surface morphology/surface area. The change of selectivity of the electrode B compared to electrode A due to the presence of the electrodeposit supports this hypothesis. Further studies are necessary to better understand and explain the decrease of the current efficiency for hydrocarbons and the formation of ethane when using electrode C. Furthermore, it is noted that changes in the composition of the cathode surface may also justify the existence of different adsorption abilities between electrodes. Hori et al. showed that, the distribution product at copper foil is highly affected by the electrode potential [13]. To investigate if the same applies to the electroreduction of CO2 on electrode C, tests were performed at different potentials. For all the potentials tested, methane was not detected and, on the contrary, ethane is always present. The electroreduction of CO2 at Cu mesh with and without electrodeposits was investigated over a period of 4 h and 15 min, Fig. 2. A delay of detection of the gaseous products occurs due to the unused volume from the electrolysis cell to the gas sampling valve. Thus the current efficiencies of gaseous products are relatively low at the early stage of the electrolysis, i.e., at 15 min as shown in Fig. 2. For electrode A without any electrodeposit, the production of hydrocarbons decreased with time, being more pronounced for periods longer than 2 h. On the contrary, in the presence of electrodeposits (electrodes B and C), the activity towards the CO2 conversion is practically unchanged. Moreover, Fig. 2 shows that hydrogen discharge is highest for electrode C. The hydrogen evolution reaction always competes with the electroreduction of CO2 in aqueous solution. As expected, this reaction is very sensitive to the relative concentration of protons and CO2. For a cathode higher real surface area, the CO2 amount at its surface is not high enough and the hydrogen evolution is promoted in favour of C2 hydrocarbons production. 4. Conclusions The presence of the electrodeposits at the electrode surface modifies strongly the catalytic behaviour of the copper electrodes for the conversion of CO2. This catalytic activity mostly depends of the electrodeposit characteristics, namely the surface area. It was revealed that ethylene can be selectively produced instead of methane. The use of specific high surface area electrodeposits in copper cathodes promotes the formation of a C2 hydrocarbons
mixture (C2H4 and C2H6) and is not selective for C1 hydrocarbon (methane) production. Furthermore, it was confirmed that the presence of electrodeposits leads to steadiness of the CO2 reduction process, since it was established the stability of hydrocarbons production during 4 h. Acknowledgements This research was financial supported by the European Space Agency – ESA (Project No: 19594/06/NL/PA). One of us (J.B. Branco) acknowledge the financial support of the Portuguese ‘‘Fundação para a Ciência e a Tecnologia”, FCT/MCTES, under program POCTI/ QUI/35394/2000. References [1] Kleiner GN, Cusick RJ. Development of an advanced Sabatier CO2 reduction subsystem. In: Intersociety conference on environmental systems. San Francisco, United States: American Society of Mechanical Engineers; 13–15 July, 1981. [2] Weimer T, Schaber K, Specht M, Bandi A. Methanol from atmospheric carbon dioxide: A liquid zero emission fuel for the future. Energy Convers Manage 1996;37:1351–6. [3] Mignard D, Sahibzada M, Duthie JM, Whittington HW. Methanol synthesis from flue-gas CO2 and renewable electricity: a feasibility study. Int J Hydrogen Energy 2003;28:455–64. [4] Gattrell M, Gupta N, Co A. Electrochemical reduction of CO2 to hydrocarbons to store renewable electrical energy and upgrade biogas. Energy Convers Manage 2007;48:1255–65. [5] Hori Y, Konishi H, Murata A, Koga O, Sakurai H, Oguma K. Deactivation of copper electrode in electrochemical reduction of CO2. Electrochim Acta 2005;50:5354–69. [6] Gattrell M, Gupta N, Co A. A review of the aqueous electrochemical reduction of CO2 to hydrocarbons at copper. J Electroanal Chem 2006;594:1–19. [7] Yano J, Morita T, Shimano K, Nagami Y, Yamasaki S. Selective ethylene formation by pulse-mode electrochemical reduction of carbon dioxide using copper and copper-oxide electrodes. J Solid State Electrochem 2007;11:554–7. [8] Friebe P, Bogdanoff P, Alonso-Vante N, Tributsch H. A real-time mass spectroscopy study of the (electro)chemical factors affecting CO2 reduction at copper. J Catal 1997;168:374–85. [9] Shiratsuchi Y, Aikoh G, Nogami. Pulsed electroreduction of CO2 on copper electrodes. J Electrochem Soc 1993;140:3479–82. [10] Waszczuk P, Zelenay P, Sobkowski J. Surface interaction of benzoic acid with a copper electrode. Electrochim Acta 1995;40:1717–21. [11] Kyriacou G, Anagnostopoulos A. Electroreduction of CO2 on differently prepared copper electrodes. The influence of electrode treatment on the current efficiencies. J Electroanal Chem 1992;322:233–46. [12] Kyriacou GZ, Anagnostopoulos AK. Influence CO2 partial pressure and the supporting electrolyte cation on the product distribution in CO2 electroreduction. J Appl Electrochem 1993;23:483–6. [13] Hori Y, Murata A, Takahashi R. Formation of hydrocarbons in the electrochemical reduction of carbon dioxide at a copper electrode. J Chem Soc, Faraday Trans I 1989;85–88:2309–26.