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A membrane electrode assembly for the electrochemical synthesis of hydrocarbons from CO2(g) and H2O(g)
Electrochemistry Communications 50 (2015) 64–68
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
A membrane electrode assembly for the electrochemical synthesis of hydrocarbons from CO2(g) and H2O(g) Stefanie M.A. Kriescher a, Kurt Kugler a, Seyed S. Hosseiny a, Youri Gendel b, Matthias Wessling a,b,⁎ a b
RWTH Aachen University, Aachener Verfahrenstechnik — Chemical Process Engineering, Turmstr. 46, 52064 Aachen, Germany DWI-Leibniz Institute for Interactive Materials, Forckenbeckstr. 50, 52056 Aachen, Germany
a r t i c l e
i n f o
Article history: Received 9 October 2014 Received in revised form 6 November 2014 Accepted 17 November 2014 Available online 24 November 2014 Keywords: CO2 reduction Electrochemical membrane reactor Membrane electrode assembly
a b s t r a c t We report the electrochemical reduction of CO2 into hydrocarbons using a new electrochemical membrane reactor holding a yet unreported membrane electrode assembly comprising a copper mesh cathode and a Ti felt coated with mixed metal oxide (MMO) catalyst anode separated by a proton conductive membrane. CO2(g) was supplied to the cathodic reduction compartment, whilst humidified N2 was supplied to the anodic oxidation compartment. The MMO anode produces protons transported across the proton exchange membrane and electrons transported via the external circuit to the copper cathode to reduce CO2(g). Production rates of methane, propane, propene, iso-butane and n-butane were determined as a function of cell potential at temperatures between 30 and 70 °C and relative humidity between ca. 25% and 75%. Maximum methane concentration and the current efficiency for production of hydrocarbons were 3.29 ppm and 0.12%, respectively. Whilst the observed product spectrum is desirable, such low current efficiencies require systematic optimization of the catalytic membrane system, in particular an improved cathode with an optimum contact between proton conducting membrane, electrode and catalyst is desired. © 2014 Elsevier B.V. All rights reserved.
Burning of fossil fuel delivers 80–85% of the total world energy use today. However combusting one ton of carbon in fossil fuels releases more than 3.5 t of carbon dioxide into the atmosphere [1]. It would be highly desirable to reverse this reaction: producing hydrocarbons from the combustion product CO2. Assuming the availability of efficient CO2 capture, it would slow down fossil fuel consumption as well as reducing CO2 emission. In fact, electrochemical reduction of CO2 may be a promising technique for the storage of renewable energy in the form of hydrocarbons. Some studies have been performed over the last two decades on electrochemical conversion of CO2 into valuable fuels using electrochemical membrane reactors (ecMRs). Their stoichiometric anodic and cathodic reactions of CO2 reduction with protons originating from anodic decomposition of water are listed below Eqs. (1)–(5). At cathode þ
þ
−
þ
−
CO2 þ 2H þ 2e
1. Introduction
−
CO2 þ 2H þ 2e
→ CO þ H2 O
ð1Þ
* Corresponding author at: RWTH Aachen University, Aachener Verfahrenstechnik — Chemical Process Engineering, Turmstr. 46, 52064 Aachen, Germany. E-mail address: [email protected] (M. Wessling).
http://dx.doi.org/10.1016/j.elecom.2014.11.014 1388-2481/© 2014 Elsevier B.V. All rights reserved.
CO2 þ 8H þ 8e þ
→ HCOOH → CH4 þ 2H2 O
−
xCO2 þ yH þ ye
→ Cx Hy−2z O2x−z þ zH2 O:
ð2Þ ð3Þ ð4Þ
At anode 2H2 O→ 4H þ 4e þ O2 : þ
−
ð5Þ
For the oxidation of water at the anode it is important that production of protons as well as their transport through the membrane to the cathode side are effective. Most effective catalysts for the oxygen evolution reaction are the so-called mixed metal oxides of the platinum group metals [2]. At the cathode, the catalyst should show a high activity for the CO2 reduction reaction to hydrocarbons and a high overpotential for the hydrogen evolution. It is known from literature that copper is a promising catalyst for the production of hydrocarbons from CO2 [3]. It would also be desirable to reduce the amount of oxygenated products, since today's value chain of the chemical industry is mostly starting from aliphatic hydrocarbons. Electrochemical membrane reactors (ecMRs) studied so far for CO2 reduction comprise an anode and cathode separated by a polyelectrolyte membrane. They can be subdivided into three major types.
S.M.A. Kriescher et al. / Electrochemistry Communications 50 (2015) 64–68
1. Within the first “liquid–liquid” (L–L) type, CO2 gas is absorbed into the aqueous [4] or non-aqueous [5] electrolyte in the cathodic compartment and a liquid electrolyte, such as KHCO3 [6] and KOH [7], is applied in the anodic chamber. Carbon monoxide, hydrocarbons, alcohols, acids and aldehydes were reported to be the major valuable products of this type of reactor [8,9]. Distribution of products strongly depends on the applied catalyst [8]. 2. A second, “gas–liquid” (G–L) type ecMR utilises gaseous CO2 in the cathodic compartment and a liquid electrolyte in the anodic compartment [3,10,11]. The advantage of this system over the L–L is believed to be higher mass transport of CO2 in the electrode material. Main products of CO2 reduction in the G–L ecMR are comparable to those formed in L–L type reactors [3,8,10–14]. 3. A third type of ecMR is a “gas–gas” (G–G) system: it utilises gaseous phases in both half cells. Reduction of CO2 with copper as cathode and with hydrogen at the anode side was studied by Cook et al. [10, 15] at ambient temperature. The maximum achieved Faradaic efficiency (Total cell current 40.4 mA) was 0.11, 0.168 and 0.22% for methane, ethene and ethane, respectively. The production of hydrogen requires a separate process and the required catalyst to produce protons relies on precious platinum. An integration of water splitting directly into the reactor would make the process independent of a platinum based catalyst and is therefore highly desirable. In this communication we report, for the first time, the electrochemical reduction of CO2 in G–G ecMR whilst both, CO2 and pure water, are supplied into the ecMR as gas phases. A schematic representation of the process is shown in Fig. 1. 2. Material and methods
H2 O N2
MFC 100ml/min
MEA
LFC MFC 100ml/min
CO2 + e-
BPC 2 bar (g)
HCs
BPC 2 bar (g)
O2
CxH(y-2z)O(2x-z) + zH2O
H+
H2O(g)
membrane and cathode material, respectively. In the electrochemical membrane reactor cell, the MEA was placed between two titanium end plates with engraved serpentine flow channels (length 913.8 mm, hydraulic diameter 1 mm) and pressed using screws and nuts. The flow rates of CO2 (100 ml/min, purity 99.995%) and N2_(g) (100 ml/min, purity 99.999%) , were controlled with two mass flow controllers whilst N2 was applied as a carrier of water. The flow rate of deionised water was controlled with a liquid flow meter. Temperature of the CO2 stream was controlled with a Controlled Evaporating and Mixing (CEM, Bronkhorst) device. The N2_(g) and water stream were mixed in the CEM to achieve desired humidification and temperature. Back pressure of anodic and cathodic gas streams was maintained constant at 2 bars using two backpressure controllers. The oxygen produced at the anode side leaves the ecMR and was released into the atmosphere without analysis. (In a real process, this oxygen is of course a valuable product as well.) One litre samples were collected in Tedlar gas sampling bags from the product stream of the cathode side and analysed later for methane, propane, propene, iso-butane and n-butane using an Agilent GC–MS. Table 1 lists the experimental parameters of five sets of experiments that were performed during this study. Electrochemical reduction of CO2 was investigated at different temperatures (30, 50 and 70 °C) and different levels of relative humidity of the N2_(g) (app. 25, 50 and 75%). Polarisation curves were recorded using a linear sweep scan with a potential step of 0.1 V and a duration of 120 s. Chronoamperometry measurements were carried out at three different potentials for 10 min and the gas samples were collected during the last 5 min of the chronoamperometry. 3. Results and discussion
The heart of the ecMR is the Membrane Electrode Assembly (MEA) (Fig. 2c) that was prepared by hot-pressing a proton conductive membrane (Fumatech, Fumapem F-14100, diameter 50 mm) between the anode and cathode. Titanium felt (Bekaert, fibre diameter 20 μm, thickness 400 μm, porosity 40%, diameter 50 mm) coated with iridium based mixed metal oxide catalysts (Fig. 2d) (Magneto Special Anodes B.V.) was applied as anode and a copper felt (Bekaert, fibre diameter 12 μm, thickness 100 μm, porosity 40%, diameter 50 mm) (Fig. 2a) was used as cathode. The accompanying SEM images of the electrodes are shown in Fig. 2b and e for cathode and anode, respectively. The hotpressing was carried out at 95 °C at 5 kN/cm2 for 15 min followed by a cooling step (10 bars, 10 min). The MEA was stable during the whole set of experiments as long as it is stored in the ecMR in humid environment and under cathodic potential in order to protect the
CO2
65
O2 + e-
Fig. 1. Schematic representation of the electrochemical reduction of CO2. (MFC represents the mass flow controller, LFC is a liquid flow controller, and BPC is the back pressure controller).
3.1. Polarisation curve The current density increases with an increase of relative humidity and offset-potentials decrease with increasing relative humidity. This however does not mean an increase in current efficiency for the desired products but might be a result of lower resistance of the membrane at higher humidity [16]. Nonetheless, application of water vapour as a proton source in the G–G ecMR has an advantage of controllable water flux towards the cathode: controlling the water vapour pressure controls the water transmission rate and, consequently, might be used to minimise undesired formation of hydrogen. Fig. 3a represents the polarisation curves of the G–G ecMR collected at 30, 50 and 70 °C and ca. 75% relative humidity. The offset-potentials of the cell were 2.5 V, 2.4 V and 2.2 V for 30, 50 and 70 °C, respectively. From the offset-potential on, current densities increase with higher voltage. Higher current densities were observed at higher temperature: these may be a result of higher permeation rates of water from the anodic compartment to the cathode through the proton conductive membrane and to higher production rates of undesired hydrogen rather than the formation of hydrocarbons (see Section 3.2) [17]. The decrease of the off-set potential and the increase of the current with increasing temperature might be a result of the fact that the conductivity of the membrane increases, so the membrane resistance decreases, with increasing temperature [18]. Fig. 3b shows the polarisation curve recorded during the CO2 reduction on copper felt at different levels of relative humidity (25.3, 49.3 and 73.7%) at 70 °C. The offset-potentials decrease and the current density increases with an increase in relative humidity. This however does not mean an increase in current efficiency for the desired products but might be a result of lower resistance of the membrane at higher humidity [18]. Nonetheless, application of water vapour as a proton source in the G–G ecMR has the advantage of controllable water flux towards the cathode: controlling the water vapour pressure controls the water transmission rate and, consequently, might be used to minimise undesired formation of hydrogen.
66
S.M.A. Kriescher et al. / Electrochemistry Communications 50 (2015) 64–68
a
b
c Cu electrode Membrane IrMMO electrode
50mm 500µm d
500µm
e
f
50mm 500µm Fig. 2. (a) — Copper felt used as cathode (b) — SEM image of the cathode (magnification: 100×) (c) — SEM image of the MEA (magnification: 100×) (d) — IrMMO coated on titanium felt used as anode (e) — SEM image of the anode (magnification: 100×) (f) — EDX spectrum of the anode.
In Fig. 3a and b limiting current densities are reached. The origin of the limiting current density is the flux of water and/or protons that limits the current. The limiting current density is higher at higher temperature and higher relative humidity, since the conductivity of the membrane increases with increasing temperature and increasing relative humidity. 3.2. Product analysis Three potentials of every polarisation curve (Fig. 3a and 3b) were chosen as working points for the characterisation of electrochemical products: (A) the offset-potential, (B) the middle of the increase in current density and (C) at the limiting current density region. For each potential the system was operated for five minutes before sample collection. Results of the product analysis are listed in Table 1. No other species such as oxygenates could be identified next to hydrocarbons. Higher hydrocarbons as observed by Centi et al. were also not identified [19]. According to Table 1 operation at 30 °C (1A, 1B and 1C) and 72.5% relative humidity shows an increase in concentrations of all analysed products at higher potentials. The same is true for the observed current efficiency for the overall fuel production, defined as the sum of all current efficiencies of the quantified products. The same
behaviour was observed for the experiments at 50 °C (2A, 2A and 2C in Table 1). At 70 °C and 73.7% relative humidity (3A, 3B and 3C in Table 1) a rise in concentration of methane with increasing potential can be observed. Other products have rather low concentrations but do not show a relation to the change in potential even as the fuel current efficiency. This observation is also correct for the experiments at lower relative humidity (4 and 5). A decline in concentration of n-butane and iso-butane was observed at higher humidity of the nitrogen stream. The methane concentration increased with increasing humidity. No clear trend could be observed for change in fuel efficiency with change in humidity of the nitrogen stream. The current efficiency achieved in this study is in the same order of magnitude as that obtained by results of Cook et al. [10]. The main difference between both studies is the proton source employed; Cook et al. [10] used hydrogen whereas water is used in this study. The current efficiencies for CO2 reduction are at present still very low and require significant improvements before any practical application can be envisaged. However, this study presents an environmentally greener approach compared to existing ones. Whilst we are demonstrating technical feasibility with respect to the use of a MEA
Table 1 Experimental settings and results of the gas analysis. Experiment #
Relative humidity [%]
Temperature [°C]
Potential [V]
Methane [ppm]
Propane [ppm]
Propene [ppm]
Iso-butane [ppm]
n-Butane [ppm]
Fuel current efficiency [%]
A B C A B C A B C A B C A B C
72.5 72.5 72.5 73.3 73.3 73.3 73.7 73.7 73.7 49.3 49.3 49.3 25.3 25.3 25.3
30 30 30 50 50 50 70 70 70 70 70 70 70 70 70
2.7 3.4 6.0 2.7 3.9 6.0 2.4 3.3 6.0 3.0 4.0 6.0 3.9 4.3 6.0
– 0.73 1.31 0.16 1.35 2.59 1.30 1.49 3.29 0.39 1.71 2.27 0.91 0.88 1.63
0.01 0.01 0.26 0.01 0.01 0.02 0.02 0.07 0.03 0.03 0.03 0.02 0.09 0.06 0.06
– – 0.14 – – – 0.07 0.06 0.05 0.01 0.03 0.01 0.05 0.04 0.03
– – 0.15 – – – 0.02 0.02 0.01 0.03 0.03 0.02 0.08 0.07 0.06
– – 0.21 – – – 0.02 0.01 0.01 0.03 0.04 0.03 0.12 0.08 0.07
0.00 0.04 0.12 0.01 0.07 0.08 0.11 0.05 0.05 0.03 0.07 0.06 0.12 0.07 0.10
S.M.A. Kriescher et al. / Electrochemistry Communications 50 (2015) 64–68
67
Fig. 3. (a) — Polarisation curves of G–G ecMR at different temperatures (30, 50 and 70 °C) and ca. 75% relative humidity. And (b) — polarisation curves obtained at different levels of relative humidity of the nitrogen (25.3, 49.3 and 73.7%) at 70 °C. Two repetitions are shown for each experiment (solid and open symbol).
configuration, it is difficult to deconvolute the different contributions to the low efficiency. The major undesired cathodic reaction is the proton reduction to hydrogen. Its extent is not known at this stage but is currently under investigation. To improve the current efficiency for fuel production electrodes with high overpotential for hydrogen production and high catalytic activity for CO2 reduction are required. Application of a water-free proton conducting membrane is also possibly desirable. Currently, we develop a generic process model for the co-generation of hydrogen and CO2 reduction products, comparable to the work we recently published [20]. 4. Conclusions For the first time electrochemical reduction of CO2 was studied in a “gas–gas” electrochemical membrane reactor operated with pure CO2 and H2O gases at cathodic and anodic compartments. Major products of CO2 reduction are methane, propane, propene, iso-butane and nbutane. The maximum methane concentration was 3.29 ppm at applied voltage of 6 V, 70 °C and 73.7% relative humidity. At both, 30 °C (72.5%, 6 V) and at 70 °C (25.3%, 2.4 V), the maximum fuel efficiency of 0.12% was achieved. The electrochemical reduction of CO2 in an ecMR is in general possible with water as a proton source. Operation of the G–G type ecMR with water vapour has an advantage of controllable water flux into the cathodic chamber, moreover no addition of inert electrolytes is required for anodic reaction. Concentrations of the produced hydrocarbons and current efficiency are still very low. Systematic studies are required to investigate the production rates for hydrogen for instance. The system requires improvements by: (a) tailoring and optimising the reduction catalyst system (b) identifying a proton conductor with low water transmission rates (c) optimising the three phase surface area between catalyst, proton conductor and electrode (d) increasing the residence time of the CO2 in the ecMR, by decreasing the volume flow.
Conflicts of interest None.
Acknowledgement The project “Sustainable Chemical Synthesis (SusChemSys)” is cofinanced by the European Regional Development Fund (ERDF) and the state of North Rhine-Westphalia, Germany, under the Operational Programme “Regional Competitiveness and Employment” 2007–2013. M. Wessling appreciates the support from the Alexander-vonHumboldt Foundation. References [1] Gabriele Centi, Siglinda Perathoner, Gauthier Winè, Miriam Gangeri, Electrocatalytic conversion of CO2 to long carbon-chain hydrocarbons, Green Chem. 9 (6) (2007) 671. [2] Ronald L. Cook, Ambient temperature gas phase CO2 reduction to hydrocarbons at solid polymer electrolyte cells, J. Electrochem. Soc. 135 (6) (1988) 1470–1471. [3] Ronald L. Cook, Gas-phase CO2 reduction to hydrocarbons at metal/solid polymer electrolyte interface, J. Electrochem. Soc. 137 (1) (1990) 187. [4] D.W. Dewulf, A.J. Bard, The electrochemical reduction of CO2 to CH4 and C2H4 at Cu/ Nafion electrodes (solid polymer electrolyte structures), Catal. Lett. 1 (1988) 73–79. [5] M. Gattrell, N. Gupta, A. Co, Electrochemical reduction of CO2 to hydrocarbons to store renewable electrical energy and upgrade biogas, Energy Convers. Manag. 48 (4) (April 2007) 1255–1265. [6] Y. Hori, H. Ito, K. Okano, K. Nagasu, S. Sato, Silver-coated ion exchange membrane electrode applied to electrochemical reduction of carbon dioxide, Electrochim. Acta 48 (2003) 2651–2657. [7] Z. Jiang, T. Xiao, V.L. Kuznetsov, P.P. Edwards, Turning carbon dioxide into fuel, Philos. Trans. R. Soc. A Math. Phys. Eng. Sci. 368 (1923) (July 2010) 3343–3364. [8] M. Jitaru, D.A. Lowy, M. Toma, Electrochemical reduction of carbon dioxide on flat metallic cathodes, J. Appl. Electrochem. 27 (45) (1997) 875–889. [9] S. Kaneco, Nobu-hide Hiei, Yue Xing, Hideyuki Katsumata, Hisanori Ohnishi, Tohru Suzuki, Kiyohisa Ohta, Electrochemical conversion of carbon dioxide to methane in aqueous NaHCO3 solution at less than 273 K, Electrochim. Acta 48 (November 2002) 51–55. [10] Seiji Komatsu, Michie Tanaka, Akira Okumura, Akira Kungi, Preparation of cu-solid polymer electrolyte composite electrodes and application to gas-phase electrochemical reduction of CO2, Electrochim. Acta 40 (6) (April 1995) 745–753. [11] K. Kugler, B. Ohs, M. Scholz, M. Wessling, Towards a carbon independent and CO2free electrochemical membrane process for NH3 synthesis, Phys. Chem. Chem. Phys. 16 (13) (April 2014) 6129–6138. [12] Hui Li, Colin Oloman, The electro-reduction of carbon dioxide in a continuous reactor, J. Appl. Electrochem. 35 (10) (October 2005) 955–965. [13] Masunobu Maeda, Yukio Kitaguchi, Shoichiro Ikeda, Kaname Ito, Reduction of carbon dioxide on partially-immersed Au plate electrode and Au-SPE electrode, J. Electroanal. Chem. 238 (1987) 247–258. [14] Paul W. Majsztrik, M. Barclay Satterfield, Andrew B. Bocarsly, Jay B. Benziger, Water sorption, desorption and transport in Nafion membranes, J. Membr. Sci. 301 (1-2) (September 2007) 93–106. [15] M.H. Miles, E.A. Klaus, B.P. Gunn, J.R. Locker, The oxygen evolution reaction on platinum, iridium, ruthenium and their alloys at 80° C in acid solutions, Electrochim. Acta 23 (1978) 521–526. [16] Kiyomi Ohkawa, Yoshikazu Noguchi, Sumie Nakayama, Kazuhito Hashimoto, Akira Fujishima, Electrochemical reduction of carbon dioxide on hydrogen-storing materials: Part 4. Electrochemical behavior of the Pd electrode in aqueous and nonaqueous electrolyte, J. Electroanal. Chem. 369 (1–2) (May 1994) 247–250.
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[17] Shinya Ohya, Satoshi Kaneco, Hideyuki Katsumata, Tohru Suzuki, Kiyohisa Ohta, Electrochemical reduction of CO2 in methanol with aid of CuO and Cu2O, Catal. Today 148 (3–4) (November 2009) 329–334. [18] Yoshitsugu Sone, Per Ekdunge, Daniel Simonsson, Proton conductivity of Nafion 117 as measured by a four-electrode ac impedance method, J. Electrochem. Soc. 143 (4) (1996) 1254–1259.
[19] Gregory B. Stevens, Torsten Reda, Burkhard Raguse, Energy storage by the electrochemical reduction of CO2 to CO at a porous Au film, J. Electroanal. Chem. 526 (1–2) (May 2002) 125–133. [20] R. Yadav, P.S. Fedkiw, Analysis of EIS technique and Nafion 117 conductivity as a function of temperature and relative humidity, J. Electrochem. Soc. 159 (3) (January 2012) B340–B346.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
A membrane electrode assembly for the electrochemical synthesis of hydrocarbons from CO2(g) and H2O(g) Stefanie M.A. Kriescher a, Kurt Kugler a, Seyed S. Hosseiny a, Youri Gendel b, Matthias Wessling a,b,⁎ a b
RWTH Aachen University, Aachener Verfahrenstechnik — Chemical Process Engineering, Turmstr. 46, 52064 Aachen, Germany DWI-Leibniz Institute for Interactive Materials, Forckenbeckstr. 50, 52056 Aachen, Germany
a r t i c l e
i n f o
Article history: Received 9 October 2014 Received in revised form 6 November 2014 Accepted 17 November 2014 Available online 24 November 2014 Keywords: CO2 reduction Electrochemical membrane reactor Membrane electrode assembly
a b s t r a c t We report the electrochemical reduction of CO2 into hydrocarbons using a new electrochemical membrane reactor holding a yet unreported membrane electrode assembly comprising a copper mesh cathode and a Ti felt coated with mixed metal oxide (MMO) catalyst anode separated by a proton conductive membrane. CO2(g) was supplied to the cathodic reduction compartment, whilst humidified N2 was supplied to the anodic oxidation compartment. The MMO anode produces protons transported across the proton exchange membrane and electrons transported via the external circuit to the copper cathode to reduce CO2(g). Production rates of methane, propane, propene, iso-butane and n-butane were determined as a function of cell potential at temperatures between 30 and 70 °C and relative humidity between ca. 25% and 75%. Maximum methane concentration and the current efficiency for production of hydrocarbons were 3.29 ppm and 0.12%, respectively. Whilst the observed product spectrum is desirable, such low current efficiencies require systematic optimization of the catalytic membrane system, in particular an improved cathode with an optimum contact between proton conducting membrane, electrode and catalyst is desired. © 2014 Elsevier B.V. All rights reserved.
Burning of fossil fuel delivers 80–85% of the total world energy use today. However combusting one ton of carbon in fossil fuels releases more than 3.5 t of carbon dioxide into the atmosphere [1]. It would be highly desirable to reverse this reaction: producing hydrocarbons from the combustion product CO2. Assuming the availability of efficient CO2 capture, it would slow down fossil fuel consumption as well as reducing CO2 emission. In fact, electrochemical reduction of CO2 may be a promising technique for the storage of renewable energy in the form of hydrocarbons. Some studies have been performed over the last two decades on electrochemical conversion of CO2 into valuable fuels using electrochemical membrane reactors (ecMRs). Their stoichiometric anodic and cathodic reactions of CO2 reduction with protons originating from anodic decomposition of water are listed below Eqs. (1)–(5). At cathode þ
þ
−
þ
−
CO2 þ 2H þ 2e
1. Introduction
−
CO2 þ 2H þ 2e
→ CO þ H2 O
ð1Þ
* Corresponding author at: RWTH Aachen University, Aachener Verfahrenstechnik — Chemical Process Engineering, Turmstr. 46, 52064 Aachen, Germany. E-mail address: [email protected] (M. Wessling).
http://dx.doi.org/10.1016/j.elecom.2014.11.014 1388-2481/© 2014 Elsevier B.V. All rights reserved.
CO2 þ 8H þ 8e þ
→ HCOOH → CH4 þ 2H2 O
−
xCO2 þ yH þ ye
→ Cx Hy−2z O2x−z þ zH2 O:
ð2Þ ð3Þ ð4Þ
At anode 2H2 O→ 4H þ 4e þ O2 : þ
−
ð5Þ
For the oxidation of water at the anode it is important that production of protons as well as their transport through the membrane to the cathode side are effective. Most effective catalysts for the oxygen evolution reaction are the so-called mixed metal oxides of the platinum group metals [2]. At the cathode, the catalyst should show a high activity for the CO2 reduction reaction to hydrocarbons and a high overpotential for the hydrogen evolution. It is known from literature that copper is a promising catalyst for the production of hydrocarbons from CO2 [3]. It would also be desirable to reduce the amount of oxygenated products, since today's value chain of the chemical industry is mostly starting from aliphatic hydrocarbons. Electrochemical membrane reactors (ecMRs) studied so far for CO2 reduction comprise an anode and cathode separated by a polyelectrolyte membrane. They can be subdivided into three major types.
S.M.A. Kriescher et al. / Electrochemistry Communications 50 (2015) 64–68
1. Within the first “liquid–liquid” (L–L) type, CO2 gas is absorbed into the aqueous [4] or non-aqueous [5] electrolyte in the cathodic compartment and a liquid electrolyte, such as KHCO3 [6] and KOH [7], is applied in the anodic chamber. Carbon monoxide, hydrocarbons, alcohols, acids and aldehydes were reported to be the major valuable products of this type of reactor [8,9]. Distribution of products strongly depends on the applied catalyst [8]. 2. A second, “gas–liquid” (G–L) type ecMR utilises gaseous CO2 in the cathodic compartment and a liquid electrolyte in the anodic compartment [3,10,11]. The advantage of this system over the L–L is believed to be higher mass transport of CO2 in the electrode material. Main products of CO2 reduction in the G–L ecMR are comparable to those formed in L–L type reactors [3,8,10–14]. 3. A third type of ecMR is a “gas–gas” (G–G) system: it utilises gaseous phases in both half cells. Reduction of CO2 with copper as cathode and with hydrogen at the anode side was studied by Cook et al. [10, 15] at ambient temperature. The maximum achieved Faradaic efficiency (Total cell current 40.4 mA) was 0.11, 0.168 and 0.22% for methane, ethene and ethane, respectively. The production of hydrogen requires a separate process and the required catalyst to produce protons relies on precious platinum. An integration of water splitting directly into the reactor would make the process independent of a platinum based catalyst and is therefore highly desirable. In this communication we report, for the first time, the electrochemical reduction of CO2 in G–G ecMR whilst both, CO2 and pure water, are supplied into the ecMR as gas phases. A schematic representation of the process is shown in Fig. 1. 2. Material and methods
H2 O N2
MFC 100ml/min
MEA
LFC MFC 100ml/min
CO2 + e-
BPC 2 bar (g)
HCs
BPC 2 bar (g)
O2
CxH(y-2z)O(2x-z) + zH2O
H+
H2O(g)
membrane and cathode material, respectively. In the electrochemical membrane reactor cell, the MEA was placed between two titanium end plates with engraved serpentine flow channels (length 913.8 mm, hydraulic diameter 1 mm) and pressed using screws and nuts. The flow rates of CO2 (100 ml/min, purity 99.995%) and N2_(g) (100 ml/min, purity 99.999%) , were controlled with two mass flow controllers whilst N2 was applied as a carrier of water. The flow rate of deionised water was controlled with a liquid flow meter. Temperature of the CO2 stream was controlled with a Controlled Evaporating and Mixing (CEM, Bronkhorst) device. The N2_(g) and water stream were mixed in the CEM to achieve desired humidification and temperature. Back pressure of anodic and cathodic gas streams was maintained constant at 2 bars using two backpressure controllers. The oxygen produced at the anode side leaves the ecMR and was released into the atmosphere without analysis. (In a real process, this oxygen is of course a valuable product as well.) One litre samples were collected in Tedlar gas sampling bags from the product stream of the cathode side and analysed later for methane, propane, propene, iso-butane and n-butane using an Agilent GC–MS. Table 1 lists the experimental parameters of five sets of experiments that were performed during this study. Electrochemical reduction of CO2 was investigated at different temperatures (30, 50 and 70 °C) and different levels of relative humidity of the N2_(g) (app. 25, 50 and 75%). Polarisation curves were recorded using a linear sweep scan with a potential step of 0.1 V and a duration of 120 s. Chronoamperometry measurements were carried out at three different potentials for 10 min and the gas samples were collected during the last 5 min of the chronoamperometry. 3. Results and discussion
The heart of the ecMR is the Membrane Electrode Assembly (MEA) (Fig. 2c) that was prepared by hot-pressing a proton conductive membrane (Fumatech, Fumapem F-14100, diameter 50 mm) between the anode and cathode. Titanium felt (Bekaert, fibre diameter 20 μm, thickness 400 μm, porosity 40%, diameter 50 mm) coated with iridium based mixed metal oxide catalysts (Fig. 2d) (Magneto Special Anodes B.V.) was applied as anode and a copper felt (Bekaert, fibre diameter 12 μm, thickness 100 μm, porosity 40%, diameter 50 mm) (Fig. 2a) was used as cathode. The accompanying SEM images of the electrodes are shown in Fig. 2b and e for cathode and anode, respectively. The hotpressing was carried out at 95 °C at 5 kN/cm2 for 15 min followed by a cooling step (10 bars, 10 min). The MEA was stable during the whole set of experiments as long as it is stored in the ecMR in humid environment and under cathodic potential in order to protect the
CO2
65
O2 + e-
Fig. 1. Schematic representation of the electrochemical reduction of CO2. (MFC represents the mass flow controller, LFC is a liquid flow controller, and BPC is the back pressure controller).
3.1. Polarisation curve The current density increases with an increase of relative humidity and offset-potentials decrease with increasing relative humidity. This however does not mean an increase in current efficiency for the desired products but might be a result of lower resistance of the membrane at higher humidity [16]. Nonetheless, application of water vapour as a proton source in the G–G ecMR has an advantage of controllable water flux towards the cathode: controlling the water vapour pressure controls the water transmission rate and, consequently, might be used to minimise undesired formation of hydrogen. Fig. 3a represents the polarisation curves of the G–G ecMR collected at 30, 50 and 70 °C and ca. 75% relative humidity. The offset-potentials of the cell were 2.5 V, 2.4 V and 2.2 V for 30, 50 and 70 °C, respectively. From the offset-potential on, current densities increase with higher voltage. Higher current densities were observed at higher temperature: these may be a result of higher permeation rates of water from the anodic compartment to the cathode through the proton conductive membrane and to higher production rates of undesired hydrogen rather than the formation of hydrocarbons (see Section 3.2) [17]. The decrease of the off-set potential and the increase of the current with increasing temperature might be a result of the fact that the conductivity of the membrane increases, so the membrane resistance decreases, with increasing temperature [18]. Fig. 3b shows the polarisation curve recorded during the CO2 reduction on copper felt at different levels of relative humidity (25.3, 49.3 and 73.7%) at 70 °C. The offset-potentials decrease and the current density increases with an increase in relative humidity. This however does not mean an increase in current efficiency for the desired products but might be a result of lower resistance of the membrane at higher humidity [18]. Nonetheless, application of water vapour as a proton source in the G–G ecMR has the advantage of controllable water flux towards the cathode: controlling the water vapour pressure controls the water transmission rate and, consequently, might be used to minimise undesired formation of hydrogen.
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a
b
c Cu electrode Membrane IrMMO electrode
50mm 500µm d
500µm
e
f
50mm 500µm Fig. 2. (a) — Copper felt used as cathode (b) — SEM image of the cathode (magnification: 100×) (c) — SEM image of the MEA (magnification: 100×) (d) — IrMMO coated on titanium felt used as anode (e) — SEM image of the anode (magnification: 100×) (f) — EDX spectrum of the anode.
In Fig. 3a and b limiting current densities are reached. The origin of the limiting current density is the flux of water and/or protons that limits the current. The limiting current density is higher at higher temperature and higher relative humidity, since the conductivity of the membrane increases with increasing temperature and increasing relative humidity. 3.2. Product analysis Three potentials of every polarisation curve (Fig. 3a and 3b) were chosen as working points for the characterisation of electrochemical products: (A) the offset-potential, (B) the middle of the increase in current density and (C) at the limiting current density region. For each potential the system was operated for five minutes before sample collection. Results of the product analysis are listed in Table 1. No other species such as oxygenates could be identified next to hydrocarbons. Higher hydrocarbons as observed by Centi et al. were also not identified [19]. According to Table 1 operation at 30 °C (1A, 1B and 1C) and 72.5% relative humidity shows an increase in concentrations of all analysed products at higher potentials. The same is true for the observed current efficiency for the overall fuel production, defined as the sum of all current efficiencies of the quantified products. The same
behaviour was observed for the experiments at 50 °C (2A, 2A and 2C in Table 1). At 70 °C and 73.7% relative humidity (3A, 3B and 3C in Table 1) a rise in concentration of methane with increasing potential can be observed. Other products have rather low concentrations but do not show a relation to the change in potential even as the fuel current efficiency. This observation is also correct for the experiments at lower relative humidity (4 and 5). A decline in concentration of n-butane and iso-butane was observed at higher humidity of the nitrogen stream. The methane concentration increased with increasing humidity. No clear trend could be observed for change in fuel efficiency with change in humidity of the nitrogen stream. The current efficiency achieved in this study is in the same order of magnitude as that obtained by results of Cook et al. [10]. The main difference between both studies is the proton source employed; Cook et al. [10] used hydrogen whereas water is used in this study. The current efficiencies for CO2 reduction are at present still very low and require significant improvements before any practical application can be envisaged. However, this study presents an environmentally greener approach compared to existing ones. Whilst we are demonstrating technical feasibility with respect to the use of a MEA
Table 1 Experimental settings and results of the gas analysis. Experiment #
Relative humidity [%]
Temperature [°C]
Potential [V]
Methane [ppm]
Propane [ppm]
Propene [ppm]
Iso-butane [ppm]
n-Butane [ppm]
Fuel current efficiency [%]
A B C A B C A B C A B C A B C
72.5 72.5 72.5 73.3 73.3 73.3 73.7 73.7 73.7 49.3 49.3 49.3 25.3 25.3 25.3
30 30 30 50 50 50 70 70 70 70 70 70 70 70 70
2.7 3.4 6.0 2.7 3.9 6.0 2.4 3.3 6.0 3.0 4.0 6.0 3.9 4.3 6.0
– 0.73 1.31 0.16 1.35 2.59 1.30 1.49 3.29 0.39 1.71 2.27 0.91 0.88 1.63
0.01 0.01 0.26 0.01 0.01 0.02 0.02 0.07 0.03 0.03 0.03 0.02 0.09 0.06 0.06
– – 0.14 – – – 0.07 0.06 0.05 0.01 0.03 0.01 0.05 0.04 0.03
– – 0.15 – – – 0.02 0.02 0.01 0.03 0.03 0.02 0.08 0.07 0.06
– – 0.21 – – – 0.02 0.01 0.01 0.03 0.04 0.03 0.12 0.08 0.07
0.00 0.04 0.12 0.01 0.07 0.08 0.11 0.05 0.05 0.03 0.07 0.06 0.12 0.07 0.10
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Fig. 3. (a) — Polarisation curves of G–G ecMR at different temperatures (30, 50 and 70 °C) and ca. 75% relative humidity. And (b) — polarisation curves obtained at different levels of relative humidity of the nitrogen (25.3, 49.3 and 73.7%) at 70 °C. Two repetitions are shown for each experiment (solid and open symbol).
configuration, it is difficult to deconvolute the different contributions to the low efficiency. The major undesired cathodic reaction is the proton reduction to hydrogen. Its extent is not known at this stage but is currently under investigation. To improve the current efficiency for fuel production electrodes with high overpotential for hydrogen production and high catalytic activity for CO2 reduction are required. Application of a water-free proton conducting membrane is also possibly desirable. Currently, we develop a generic process model for the co-generation of hydrogen and CO2 reduction products, comparable to the work we recently published [20]. 4. Conclusions For the first time electrochemical reduction of CO2 was studied in a “gas–gas” electrochemical membrane reactor operated with pure CO2 and H2O gases at cathodic and anodic compartments. Major products of CO2 reduction are methane, propane, propene, iso-butane and nbutane. The maximum methane concentration was 3.29 ppm at applied voltage of 6 V, 70 °C and 73.7% relative humidity. At both, 30 °C (72.5%, 6 V) and at 70 °C (25.3%, 2.4 V), the maximum fuel efficiency of 0.12% was achieved. The electrochemical reduction of CO2 in an ecMR is in general possible with water as a proton source. Operation of the G–G type ecMR with water vapour has an advantage of controllable water flux into the cathodic chamber, moreover no addition of inert electrolytes is required for anodic reaction. Concentrations of the produced hydrocarbons and current efficiency are still very low. Systematic studies are required to investigate the production rates for hydrogen for instance. The system requires improvements by: (a) tailoring and optimising the reduction catalyst system (b) identifying a proton conductor with low water transmission rates (c) optimising the three phase surface area between catalyst, proton conductor and electrode (d) increasing the residence time of the CO2 in the ecMR, by decreasing the volume flow.
Conflicts of interest None.
Acknowledgement The project “Sustainable Chemical Synthesis (SusChemSys)” is cofinanced by the European Regional Development Fund (ERDF) and the state of North Rhine-Westphalia, Germany, under the Operational Programme “Regional Competitiveness and Employment” 2007–2013. M. Wessling appreciates the support from the Alexander-vonHumboldt Foundation. References [1] Gabriele Centi, Siglinda Perathoner, Gauthier Winè, Miriam Gangeri, Electrocatalytic conversion of CO2 to long carbon-chain hydrocarbons, Green Chem. 9 (6) (2007) 671. [2] Ronald L. Cook, Ambient temperature gas phase CO2 reduction to hydrocarbons at solid polymer electrolyte cells, J. Electrochem. Soc. 135 (6) (1988) 1470–1471. [3] Ronald L. Cook, Gas-phase CO2 reduction to hydrocarbons at metal/solid polymer electrolyte interface, J. Electrochem. Soc. 137 (1) (1990) 187. [4] D.W. Dewulf, A.J. Bard, The electrochemical reduction of CO2 to CH4 and C2H4 at Cu/ Nafion electrodes (solid polymer electrolyte structures), Catal. Lett. 1 (1988) 73–79. [5] M. Gattrell, N. Gupta, A. Co, Electrochemical reduction of CO2 to hydrocarbons to store renewable electrical energy and upgrade biogas, Energy Convers. Manag. 48 (4) (April 2007) 1255–1265. [6] Y. Hori, H. Ito, K. Okano, K. Nagasu, S. Sato, Silver-coated ion exchange membrane electrode applied to electrochemical reduction of carbon dioxide, Electrochim. Acta 48 (2003) 2651–2657. [7] Z. Jiang, T. Xiao, V.L. Kuznetsov, P.P. Edwards, Turning carbon dioxide into fuel, Philos. Trans. R. Soc. A Math. Phys. Eng. Sci. 368 (1923) (July 2010) 3343–3364. [8] M. Jitaru, D.A. Lowy, M. Toma, Electrochemical reduction of carbon dioxide on flat metallic cathodes, J. Appl. Electrochem. 27 (45) (1997) 875–889. [9] S. Kaneco, Nobu-hide Hiei, Yue Xing, Hideyuki Katsumata, Hisanori Ohnishi, Tohru Suzuki, Kiyohisa Ohta, Electrochemical conversion of carbon dioxide to methane in aqueous NaHCO3 solution at less than 273 K, Electrochim. Acta 48 (November 2002) 51–55. [10] Seiji Komatsu, Michie Tanaka, Akira Okumura, Akira Kungi, Preparation of cu-solid polymer electrolyte composite electrodes and application to gas-phase electrochemical reduction of CO2, Electrochim. Acta 40 (6) (April 1995) 745–753. [11] K. Kugler, B. Ohs, M. Scholz, M. Wessling, Towards a carbon independent and CO2free electrochemical membrane process for NH3 synthesis, Phys. Chem. Chem. Phys. 16 (13) (April 2014) 6129–6138. [12] Hui Li, Colin Oloman, The electro-reduction of carbon dioxide in a continuous reactor, J. Appl. Electrochem. 35 (10) (October 2005) 955–965. [13] Masunobu Maeda, Yukio Kitaguchi, Shoichiro Ikeda, Kaname Ito, Reduction of carbon dioxide on partially-immersed Au plate electrode and Au-SPE electrode, J. Electroanal. Chem. 238 (1987) 247–258. [14] Paul W. Majsztrik, M. Barclay Satterfield, Andrew B. Bocarsly, Jay B. Benziger, Water sorption, desorption and transport in Nafion membranes, J. Membr. Sci. 301 (1-2) (September 2007) 93–106. [15] M.H. Miles, E.A. Klaus, B.P. Gunn, J.R. Locker, The oxygen evolution reaction on platinum, iridium, ruthenium and their alloys at 80° C in acid solutions, Electrochim. Acta 23 (1978) 521–526. [16] Kiyomi Ohkawa, Yoshikazu Noguchi, Sumie Nakayama, Kazuhito Hashimoto, Akira Fujishima, Electrochemical reduction of carbon dioxide on hydrogen-storing materials: Part 4. Electrochemical behavior of the Pd electrode in aqueous and nonaqueous electrolyte, J. Electroanal. Chem. 369 (1–2) (May 1994) 247–250.
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