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Production of syngas plus oxygen from CO2 in a gas-diffusion electrode-based electrolytic cell
Electrochimica Acta 47 (2002) 3327 /3334 www.elsevier.com/locate/electacta
Production of syngas plus oxygen from CO2 in a gas-diffusion electrode-based electrolytic cell Toshio Yamamoto a, Donald A. Tryk a,1, Akira Fujishima1 a,*, Hiroshi Ohata b a
Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan b Electric Power Development Co., Ltd., 6-15-1 Ginza, Chuo-ku, Tokyo 104-8165, Japan Received 16 January 2002; received in revised form 8 April 2002
Abstract Most of the research published in electrochemical CO2 reduction has been reported for half-cells, with little consideration of the overall system. However, it is necessary to consider the eventual involvement of full cells. We conducted CO2 reduction and water oxidation in a CO2-reducing full cell with larger geometric surface area (2 /2 cm2) and with a relatively small inter-electrode gap (1 /2 mm) in order to minimize ohmic losses. The result was an ca. 1:1 CO/H2 (v/v) gas ratio at a current density of 10 mA cm 2 and a cell voltage of 3.05 V, producing O2 at the counter electrode. Based on an enthalpic voltage of 1.36 V, this constitutes an overall energy efficiency of 44.6%. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: CO2 reduction; Full cell; Gas diffusion electrode
1. Introduction Recently the buildup of greenhouse gases such as CO2 has become increasingly linked with global warming. There has been much interest in developing ways to convert CO2 into other forms, including fuels, rather than releasing it directly into the atmosphere. Of the various types of chemical approaches that are being considered for the conversion of CO2, the electrochemical approach is attractive. It has the following advantages: (1) the possibility of using water as the proton source [1]; (2) low-temperature operation; (3) intrinsic high efficiency; and (4) the production of pure oxygen gas as a by-product. However, an important consideration is the cost and availability of the required electric power. Therefore, it is important to develop ways to electrochemically reduce CO2 with high efficiency, as well as high selectivity at high current densities (CD). One of the major techniques for achieving high CD for CO2 reduction involves the use of gas diffusion * Corresponding author. Tel.: /81-3-5841-7245; fax: /81-3-38126227 E-mail address: [email protected] (A. Fujishima1). 1 ISE member.
electrodes (GDEs). Furuya and co-workers investigated electrochemical CO2 reduction with GDEs prepared from a variety of electrocatalysts, including supported metals [2 /5] and metal phthalocyanines [6,7]. They achieved high partial CD, i.e. CD in terms of production of specific products of CO2 reduction, up to several hundreds of mA cm 2. Cook et al. [8] and Ikeda et al. [9,10] investigated electrochemical CO2 reduction with GDEs that utilized high area copper supported on carbon black. Hara et al. [11 /14] reported very high partial CD, up to 1 A cm 2, for CO2 electroreduction with GDEs under high CO2 pressure. Most of the research published in electrochemical CO2 reduction has thus far been reported for half-cells, with little consideration of the overall system, including the product at the anode and the total cell voltage and thus energy efficiency. The development of full cells has been neglected, perhaps because it has been assumed (1) that such development must wait until it has been demonstrated that it is justified, or (2) that this step is somewhat trivial. However, in order to accelerate and focus the efforts in the area of CO2 electroreduction, it is necessary to consider the eventual involvement of full cells. One of the early works that addressed this subject that of Russell et al., examined the electrochemical reduction of CO2 to formic acid on various metal
0013-4686/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 2 ) 0 0 2 5 3 - 0
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electrodes [15]. They calculated the energy efficiency as follows: o
DH (percent faradaic efficiency)
(1)
nF(Erev hc ha hIR )
where DH is 269 kJ mol 1 and Erev is 1.43 V for the reaction CO2(gas)/H2O(liquid)0/HCOOH(aq)/1/ 2O2(gas). The term (Erev/hc/ha/hIR) is simply the total cell voltage. These workers obtained a 1 mA cm 2 current density for formic acid production on mercury at a cathodic overpotential hc of 1.15 V. Assuming a value of 0.5 V for the anodic oxygen evolution overpotential ha, a value of 45% was obtained for the overall efficiency, neglecting IR losses. They also estimated a maximum real efficiency, including IR losses, of 33% with an indium electrode in 0.05 M Li2CO3 at a current density of 2.9 mA cm 2. A simpler form of Eq. (1) can be written, based on the commonly used equation used in water electrolysis [16], modified slightly to include the faradaic efficiency: o
DEH (percent faradaic efficiency)
(2)
DEexp
where DEH can be called the enthalpic voltage, and DEexp is the experimental cell voltage. This equation is also analogous to those that are used to calculate efficiencies for fuel cells [17]. Formic acid is just one of the possible products of CO2 reduction. Others include alcohols, alkanes and alkenes. The enthalpy and free energy changes, together with the corresponding enthalpic and reversible cell voltages, for the generation of various cathodic pro-
Table 1 Possible overall cell reactions for electrochemical CO2 reduction, with corresponding enthalpy changes, free energy changes, and enthalpic and reversible cell voltages Reaction
DDfH 8 DEH (kJ mol 1) (V)
CO2 0 CO1/2O2 CO2H2O0 HCOOH1/ 2O2 CO2H2O0 HCHOO2 CO22H2O0 CH3OH1/ 2O2 CO22H2O0 CH43/2O2 H2O0 H21/2O2 CO2H2O0 COH2O2
283 255
1.466 257 1.319 270
1.333 1.400
571 727
1.479 529 1.255 702
1.371 1.212
890 286 569
1.153 818 1.481 237 1.474 494
1.060 1.229 1.281
DDfG 8 DErev (kJ mol 1) (V)
These values were calculated on the basis of thermodynamic data given in the ‘Physical Chemistry’, Sixth Edition, by P.W. Atkins, Oxford University Press, Oxford (1998). The values for H2O, HCOOH, CH3COOH are for the liquid state; others: gas, 1 bar pressure. Abbreviations: DDfH 8, change in standard enthalpy of formation; DDfG 8, change in standard free energy of formation; DEH, enthalpic cell voltage; DErev, reversible cell voltage.
ducts, together with oxygen gas as the anodic product, are given in Table 1. Included is the direct production of a 1:1 CO/H2 synthesis gas or syngas. The electrochemical approach for producing syngas must be carefully compared with conventional ones such as steam reforming (SR), partial oxidation of methane (PMO) and CO2 reforming of methane (CRM). SR and CRM are quite endothermic, but the electrochemical route is even more endothermic, because it uses water rather than methane as a hydrogen source, and therefore the energy cost of producing hydrogen is included explicitly [18,19]. In our previous work, we have been examining the use of various types of electrocatalysts for CO2 reduction in GDEs and have carried out the measurements with small GDEs (effective area, 0.5 cm2) in an H-type cell [20,21]. This type of cell is not optimized for full-cell operation, because it includes high ohmic resistance. The half-cell results, however, have been encouraging enough to motivate us to examine a full cell, particularly with the electrocatalyst materials based on ultrafine nickel particles dispersed in the pores of activated carbon fibers (ACF), which contain slit-shaped pores with widths of ca. 2 nm. Therefore, we have recently scaled up to a 2 /2 cm GDE cathode and an oxygengenerating anode of a similar size, mounted in a rectangular cell with a relatively small inter-electrode gap (1 /2 mm) in order to minimize ohmic losses.
2. Experimental The ACF/Ni catalysts were prepared as previously reported [21]. ACF (type KF-1500) was obtained from Toyobo Co., Ltd. (Osaka, Japan). The fibers were placed in contact with aq. solutions of Ni(NO3)2 (0.5 M) and were stirred at room temperature for ca. 10 h. The fibers were then washed thoroughly with water, and, after air drying, the adsorbed metal ions were reduced under a hydrogen atmosphere at 350 8C. Copper catalysts supported on metal oxides were prepared by use of a precipitation method. The metal oxide powders were added to a solution of Cu(NO3)2 and were stirred at 80 8C, with 0.1 M KOH added to adjust the pH to 10. The catalyst composition was 5 wt.% Cu and 95 wt.% metal oxide. The precipitates were washed with water, dried, and reduced under hydrogen at 400 8C. For the active layer, a 100-mg portion of carbon black (Gunbai, 92 m2 g 1, Denki Kagaku Kogyo, Tokyo, Japan) and an aliquot of PTFE aq. dispersion (Daikin D-1), containing 30 mg solids were mixed on a PTFE sheet, and then a 100-mg portion of the catalyst-loaded ACF or Cu-loaded metal oxide was added to the material and mixed. After drying, this paste was spread over the surface of a commercial hydrophobic gas-diffusion layer (Tanaka Precious Metals),
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which was cut into a 2.4 /2.4 cm2, and pressed together under a load of 4 tons. This electrode was then heattreated at 350 8C under hydrogen. The counter electrode was formed by pressing together a porous Ni sheet (Nilaco Co., Ltd.) and a piece of one-sided hydrophobic gas diffusion layer (e-Tek). The electrode structure is shown in Fig. 1. Both electrodes were mechanically supported by Ni mesh (Nilaco), which was also the current collector. The electrochemical cell diagram is shown in Fig. 2. Dry CO2 gas was fed into the gas distribution area for the cathode and dry Ar gas was fed into that for the anode during the measurements. The electrolyte solution, which was aq. 0.5 M KHCO3 was allowed to flow down through the separator (filter paper, type No. 3, Advantec Toyo) by gravity by feeding it from the top. A Pt-catalyzed gas diffusion layer (e-Tek), fed with hydrogen gas at 1 atm, was used as a reversible hydrogen electrode (RHE). The reduction products were allowed to accumulate in a Tedlar bag and were analyzed by use of a gas chromatograph (Ohkura GC-202, Porapak-Q column, FID; Hitachi 163, MS-13X column, TCD), and a high performance liquid chromatograph (Tosoh UV8010, Shodex KC811 column, 210 nm UV). The electrolysis was carried out with a DC power supply (Kenwood TMI PAR20-4H), which was connected across the anode and cathode, as shown in Fig. 3. This unit also includes a voltmeter, which was used to measure the cell voltage. The electrical connections for the measurement of the cathode potential are shown. The anode potential was obtained simply by subtracting the cathode potential from the cell voltage. No corrections were made for ohmic resistance, i.e. all potentials and voltages include IR drop .
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Fig. 2. Schematic diagram of the rectangular-type electrolytic cell used in the present work.
3. Results and discussion 3.1. Current /potential behavior of the cathode The current /potential behavior for various types of cathodic electrocatalysts is shown in Fig. 4. The performance was generally superior for the ACF/Ni catalysts compared to the Cu/metal oxide catalysts. We have previously shown that the latter are promising, due
Fig. 1. Schematic diagrams of the structures for the gas-diffusion anode (left) and cathode (right). The former is modified with a gas-diffusion layer for purposes of wet-proofing.
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Fig. 3. Schematic diagram showing the electrical connections to the electrolytic cell. During the polarization measurements, the current is set on the DC power supply, and the cell voltage is read directly from the power supply. The cathode potential is measured separately with respect to the hydrogen-fed reference electrode with a high-impedance voltmeter, and the anode potential is obtained by difference from the cell voltage and the cathode potential. No corrections were made for ohmic resistance.
Fig. 4. Steady-state current /potential behavior (uncorrected for ohmic resistance) for various gas-diffusion cathodes obtained in 0.5 M KHCO3 for GDEs fabricated from (a, b) Ni/ACF (m, m); (c) Cu/ ZrO2 (%); and (d) Cu/ZnO (").
to their high selectivity for CO production (Cu/ZnO) and ethylene production (Cu/ZrO2) [22]. The behavior for the Cu/ZrO2 catalyst was similar to that for ACF/Ni at very low CD at a potential of ca. 0.0 V, as seen in the Tafel plots (Fig. 5), but at more negative potentials, the CD values were significantly lower than those for the better of the two ACF/Ni electrodes but very similar to those for the poorer of the two ACF/Ni electrodes. The CD values for the Cu/ZnO catalyst were very much
Fig. 5. Tafel plots for cathode performance obtained in 0.5 M KHCO3 for GDEs fabricated from (a, b) Ni/ACF (m, m); (c) Cu/ZrO2 (%); and (d) Cu/ZnO ("), based on the data in Fig. 3. A line with a slope of /120 mV per decade is shown for reference.
lower. The values for the onset of current are in the neighborhood of 0.0 V versus RHE. This compares favorably with the theoretical value of (1.281 /1.229) / 0.052 V versus RHE. It was somewhat unexpected to find that the results for the two ACF/Ni electrodes did not reproduce more closely. In our previous work, the reproducibility was much better due to the better control of the electrode fabrication. With this new generation of GDEs, the fabrication procedures have not been optimized to the same degree. This is a typical problem that is encountered with the scale-up of electrode fabrication. It should be noted, however, that the behavior for the better of the ACF/Ni electrodes shown was somewhat better than that for the electrodes examined earlier in half-cell measurements [21], as shown below. In order to make a precise comparison with the previous, half-cell work, it is necessary to convert the RHE reference scale to the SCE scale. Based on the calculated pH of 7.51 for 0.5 M KHCO3 saturated with 1 atm CO2, which was confirmed experimentally (7.41), the standard potential for the RHE would be /0.445 V versus SHE, which would be /0.685 V versus SCE. Similarly, the standard potential for the reversible oxygen electrode (ROE) would be /0.784 V versus SHE, and /0.543 V versus SCE. Making use of the latter value, we can compare the cathodic performance at /1.6 V versus SCE in the previous work, which was ca. 40 mA cm 2. In the present work, the corresponding potential would be /1.6/0.685 / /0.915 V versus RHE, and the corresponding cd was ca. 70 mA cm 2, which is in reasonable agreement.
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For this electrode, the current /potential behavior at very low CD approaches that for a kinetically limited system (Fig. 5, curve a). In the potential region near 0.0 V, the Tafel slope was rather close to 120 mV per decade, which is indicative of a first electron transfer step being rate-determining. A line indicating this slope is also shown in the figure. This behavior is similar to that expected for hydrogen evolution from a nickel electrode in alkaline electrolyte [23]. As shown later, the principal product in this potential region is hydrogen. In the previous work, Tafel slopes of ca. 240 mV per decade were found for the partial current densities (PCD) for hydrogen evolution at relatively negative potentials, i.e. those negative of /1.6 V versus SCE or /0.915 V versus RHE, based on product analysis for hydrogen. This was proposed to be due to a type of mass-transport-limited behavior, due to the porous nature of the electrode, leading to a slope-doubling effect, as described by de Levie [24]. This would result in a slope of ca. 240 mV per decade, based on an intrinsic value of 120 mV per decade. It is not possible to make a direct comparison of the previous work with the present work in this case, because the present performance, since it was not corrected for IR drop, was already resistancelimited in this potential region. In summary, from an examination of the shapes of the current /potential curves for the best-performing Ni/ ACF electrode in Figs. 4 and 5, it appears that the behavior at low CD is consistent with kinetically limited hydrogen evolution. At higher CD, the behavior becomes limited by ohmic drop, as evidenced by the linear behavior. The Tafel plot will be discussed in more detail later.
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Fig. 6. Steady-state current /potential behavior (uncorrected for ohmic resistance) for oxygen-evolving anodes obtained in 0.5 M KHCO3, with cells in which various cathodes were used: (a, b) Ni/ ACF (m, m); (c) Cu/ZrO2 (%); and (d) Cu/ZnO (").
3.2. Current /potential behavior for the anode Fig. 6 shows current /potential curves for the same type of anode (Ni foam) used in cells with various cathode catalysts. Although the behavior should in principle be the same in each case, it varied greatly. The reasons for this variation are not clear at present, but it is possible that soluble products from the Cubased electrodes became incorporated in the anode. The performance was best for the cell in which Cu/ZnO was used as the cathode catalyst, although points could not be obtained for very high CD, and worst for the cell in which Cu/ZrO2 as used. The behavior for the two cells in which ACF/Ni was used did not differ greatly. The onset of current was in the region near the reversible potential for the oxygen electrode, which would be /1.229 V versus RHE. This is an encouraging result. 3.3. Current efficiencies and CD for electrolysis products The potential dependencies of the current efficiencies (CE) corresponding to the electrolytic generation of
Fig. 7. Potential dependence of the CE for H2 evolution (m), CO evolution (m) and CH4 evolution (%) for a GDE fabricated from Ni/ ACF.
various products when performing electrolysis with the best-performing ACF/Ni catalyst, are shown in Fig. 7. These data were obtained with the use of the electrode whose cathodic performance is shown as curve b in Fig. 4. At a potential of ca. /0.6 V, the CEs for CO production and H2 generation became almost equal. This point would correspond to the production of a 1:1 CO/H2 syngas. At more negative potentials, the CE for CO production drops off, reaching essentially zero at ca. /1.2 V versus RHE.
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Fig. 8. Tafel plot for H2 evolution (m), CO evolution (m), CH4 evolution (%) and total CE (k) for a GDE fabricated from Ni/ACF.
Fig. 8 shows the potential dependencies of the PCD. The PCD for CO production becomes essentially constant at potentials negative of ca. /0.9 V, while that for H2 generation continues to climb, finally becoming nearly constant at ca. /2.0 V versus RHE, with about two orders of magnitude greater CD compared to that for CO production (ca. 100 vs. ca. 2.5 mA cm 2). In the potential range intermediate between these extremes, it is possible to control the composition of the CO/H2 mixture that is produced, which is an attractive feature. The composition can be varied to suit the intended use of the syngas, e.g. 1:1 for dimethyl ether production and 1:2 for methanol production [25].
Fig. 9. Steady-state current /potential behavior for both cathode (m), and anode (m) in 0.5 M KHCO3 for a GDE fabricated from Ni/ACF. Simulated curves are shown as lines (see text for details). The cell voltage is also shown (right-hand axis).
and cathode were different: ca. 6.5 V cm2 for the cathode and ca. 18 V cm2 for the anode. These are reasonable values based on the somewhat poor conductivities of KHCO3 electrolyte, which are approximately a factor of three poorer than the corresponding values for KOH [26]. Moreover, within the electrolyte gap (ca. 1.5 mm), a sizable fraction of the electrolyte volume is taken up by the separator. The difference between the values for the anode and cathode is due to the placement of the reference electrode, which is on the same side of the separator as the cathode. Thus, the total
3.4. Simulation of current/potential behavior It is instructive to look at the overall performance of the best-performing cell (corresponding to curve a in Fig. 4) on a single potential axis, showing both anode and cathode curves. Such a plot is shown in Fig. 9 in terms of the total measured CD versus the potential on the RHE scale. The potential differences between the two curves are simply the uncorrected cell voltages, as shown in the figure. Since the CD is proportional to potential in the high CD region (greater than ca. 50 mA cm 2), both electrodes are considered to be limited by ohmic resistance. To illustrate this point, we have inserted theoretical curves that include a Tafel term and an ohmic term, as follows: E=V 0:24 log(I=mA)0:018(I=mA)1:36 E=V 0:12 log(I=mA)0:0065(I=mA) 0:11
(3) (4)
Approximate agreement was obtained. It is necessary to point out that the apparent resistivities for the anode
Fig. 10. Tafel plot for both cathode (m), and anode (m) in 0.5 M KHCO3 for a GDE fabricated from Ni/ACF, based on the data in Fig. 9. Simulated curves are shown as lines (see text for details). The cell voltage is also shown (right-hand axis).
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resistivity was ca. 24 V cm 2, which, at a CD of 10 mA cm 2, would correspond to a voltage loss of 0.24 V. The corresponding Tafel plot is shown in Fig. 10, together with the simple simulations. Again, the agreement is reasonable and shows that there are mass transport limitations in the intermediate CD region for both electrodes. This type of plot is highly useful in evaluating the aspects that are in need of improvement, e.g. the ohmic resistance, the mass transport and the electrocatalysis, and for which electrode. These aspects are also dependent upon the optimum operating potential and CD in terms of the product, in this case a 1:1 CO/H2 mixture, which was obtained at a CD of ca. 10 mA cm 2 for the electrode corresponding to curve b in Fig. 4. For the electrode corresponding to curve a, a CD of ca. 35 mA cm 2 was obtained at the same cathode potential. At 10 mA cm 2, as already pointed out, the ohmic loss would be ca. 0.24 V. At 35 mA cm 2, the value would be 0.84 V, which could be decreased by decreasing the thickness of the separator and increasing the electrolyte concentration. Both the electrocatalysis and mass transport can be substantially improved for both electrodes, however. 3.5. Estimation of energy efficiency The overall energy efficiency that is obtained at this operating point (electrode corresponding to curve b in Fig. 4) was estimated from the cell voltage, 3.054 V, together with the enthalpic voltage of 1.36 V for the generation of the 1:1 CO/H2 mixture, to be ca. 45%. Based on corresponding values typically obtained for water electrolysis under highly optimized conditions, (ca. 85.6%, [16]), this value is in need of significant improvement, although it is reasonable for a nonoptimized system. It is expected that this value could be increased somewhat by focusing on improvements in the electrocatalysis and mass transport. In addition, the ohmic losses can be decreased somewhat. Recent work with porphyrin electrocatalysts in our laboratory, for example, have shown that high selectivities for CO production can also be obtained at less negative potentials, by up to 400 mV [27]. Furthermore, the anode performance can be expected to improve with the use of high-area films of Ni /Fe oxides, for example.
4. Summary and conclusions Based on our earlier work with gas-diffusion electrodes for the cathodic reduction of ambient pressure, ambient temperature CO2, we have constructed a prototype rectangular electrolytic cell with the purpose of producing useful products at both the cathode and anode. With high-area nickel supported on ACF, the cathode performance was superior, reaching a total
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current density of ca. 10 mA cm 2, with ca. 5 mA cm 2 each for CO generation and H2 evolution, at a potential of ca. /0.6 V versus RHE, compared to the reversible potential of /0.052 V versus RHE. At this operating point, the cell voltage, uncorrected for ohmic resistance, was 3.054 V. The polarization losses were concluded to be due to a combination of electrode kinetics, ohmic drop and mass transport within the porous structure. Based on simple simulations of the current /potential behavior, including a Tafel term and an ohmic term, resistivities were estimated for the anode and cathode. The total was ca. 24 V cm 2, which, at a CD of 10 mA cm 2, corresponds to a voltage loss of 0.24 V. The overall energy efficiency obtained at the optimum operating point was estimated for the generation of the 1:1 CO/H2 mixture to be ca. 45%. This value can be improved significantly in the future by focusing on improvements in the electrocatalysis and mass transport, and by decreasing the ohmic resistance.
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Production of syngas plus oxygen from CO2 in a gas-diffusion electrode-based electrolytic cell Toshio Yamamoto a, Donald A. Tryk a,1, Akira Fujishima1 a,*, Hiroshi Ohata b a
Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan b Electric Power Development Co., Ltd., 6-15-1 Ginza, Chuo-ku, Tokyo 104-8165, Japan Received 16 January 2002; received in revised form 8 April 2002
Abstract Most of the research published in electrochemical CO2 reduction has been reported for half-cells, with little consideration of the overall system. However, it is necessary to consider the eventual involvement of full cells. We conducted CO2 reduction and water oxidation in a CO2-reducing full cell with larger geometric surface area (2 /2 cm2) and with a relatively small inter-electrode gap (1 /2 mm) in order to minimize ohmic losses. The result was an ca. 1:1 CO/H2 (v/v) gas ratio at a current density of 10 mA cm 2 and a cell voltage of 3.05 V, producing O2 at the counter electrode. Based on an enthalpic voltage of 1.36 V, this constitutes an overall energy efficiency of 44.6%. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: CO2 reduction; Full cell; Gas diffusion electrode
1. Introduction Recently the buildup of greenhouse gases such as CO2 has become increasingly linked with global warming. There has been much interest in developing ways to convert CO2 into other forms, including fuels, rather than releasing it directly into the atmosphere. Of the various types of chemical approaches that are being considered for the conversion of CO2, the electrochemical approach is attractive. It has the following advantages: (1) the possibility of using water as the proton source [1]; (2) low-temperature operation; (3) intrinsic high efficiency; and (4) the production of pure oxygen gas as a by-product. However, an important consideration is the cost and availability of the required electric power. Therefore, it is important to develop ways to electrochemically reduce CO2 with high efficiency, as well as high selectivity at high current densities (CD). One of the major techniques for achieving high CD for CO2 reduction involves the use of gas diffusion * Corresponding author. Tel.: /81-3-5841-7245; fax: /81-3-38126227 E-mail address: [email protected] (A. Fujishima1). 1 ISE member.
electrodes (GDEs). Furuya and co-workers investigated electrochemical CO2 reduction with GDEs prepared from a variety of electrocatalysts, including supported metals [2 /5] and metal phthalocyanines [6,7]. They achieved high partial CD, i.e. CD in terms of production of specific products of CO2 reduction, up to several hundreds of mA cm 2. Cook et al. [8] and Ikeda et al. [9,10] investigated electrochemical CO2 reduction with GDEs that utilized high area copper supported on carbon black. Hara et al. [11 /14] reported very high partial CD, up to 1 A cm 2, for CO2 electroreduction with GDEs under high CO2 pressure. Most of the research published in electrochemical CO2 reduction has thus far been reported for half-cells, with little consideration of the overall system, including the product at the anode and the total cell voltage and thus energy efficiency. The development of full cells has been neglected, perhaps because it has been assumed (1) that such development must wait until it has been demonstrated that it is justified, or (2) that this step is somewhat trivial. However, in order to accelerate and focus the efforts in the area of CO2 electroreduction, it is necessary to consider the eventual involvement of full cells. One of the early works that addressed this subject that of Russell et al., examined the electrochemical reduction of CO2 to formic acid on various metal
0013-4686/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 2 ) 0 0 2 5 3 - 0
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electrodes [15]. They calculated the energy efficiency as follows: o
DH (percent faradaic efficiency)
(1)
nF(Erev hc ha hIR )
where DH is 269 kJ mol 1 and Erev is 1.43 V for the reaction CO2(gas)/H2O(liquid)0/HCOOH(aq)/1/ 2O2(gas). The term (Erev/hc/ha/hIR) is simply the total cell voltage. These workers obtained a 1 mA cm 2 current density for formic acid production on mercury at a cathodic overpotential hc of 1.15 V. Assuming a value of 0.5 V for the anodic oxygen evolution overpotential ha, a value of 45% was obtained for the overall efficiency, neglecting IR losses. They also estimated a maximum real efficiency, including IR losses, of 33% with an indium electrode in 0.05 M Li2CO3 at a current density of 2.9 mA cm 2. A simpler form of Eq. (1) can be written, based on the commonly used equation used in water electrolysis [16], modified slightly to include the faradaic efficiency: o
DEH (percent faradaic efficiency)
(2)
DEexp
where DEH can be called the enthalpic voltage, and DEexp is the experimental cell voltage. This equation is also analogous to those that are used to calculate efficiencies for fuel cells [17]. Formic acid is just one of the possible products of CO2 reduction. Others include alcohols, alkanes and alkenes. The enthalpy and free energy changes, together with the corresponding enthalpic and reversible cell voltages, for the generation of various cathodic pro-
Table 1 Possible overall cell reactions for electrochemical CO2 reduction, with corresponding enthalpy changes, free energy changes, and enthalpic and reversible cell voltages Reaction
DDfH 8 DEH (kJ mol 1) (V)
CO2 0 CO1/2O2 CO2H2O0 HCOOH1/ 2O2 CO2H2O0 HCHOO2 CO22H2O0 CH3OH1/ 2O2 CO22H2O0 CH43/2O2 H2O0 H21/2O2 CO2H2O0 COH2O2
283 255
1.466 257 1.319 270
1.333 1.400
571 727
1.479 529 1.255 702
1.371 1.212
890 286 569
1.153 818 1.481 237 1.474 494
1.060 1.229 1.281
DDfG 8 DErev (kJ mol 1) (V)
These values were calculated on the basis of thermodynamic data given in the ‘Physical Chemistry’, Sixth Edition, by P.W. Atkins, Oxford University Press, Oxford (1998). The values for H2O, HCOOH, CH3COOH are for the liquid state; others: gas, 1 bar pressure. Abbreviations: DDfH 8, change in standard enthalpy of formation; DDfG 8, change in standard free energy of formation; DEH, enthalpic cell voltage; DErev, reversible cell voltage.
ducts, together with oxygen gas as the anodic product, are given in Table 1. Included is the direct production of a 1:1 CO/H2 synthesis gas or syngas. The electrochemical approach for producing syngas must be carefully compared with conventional ones such as steam reforming (SR), partial oxidation of methane (PMO) and CO2 reforming of methane (CRM). SR and CRM are quite endothermic, but the electrochemical route is even more endothermic, because it uses water rather than methane as a hydrogen source, and therefore the energy cost of producing hydrogen is included explicitly [18,19]. In our previous work, we have been examining the use of various types of electrocatalysts for CO2 reduction in GDEs and have carried out the measurements with small GDEs (effective area, 0.5 cm2) in an H-type cell [20,21]. This type of cell is not optimized for full-cell operation, because it includes high ohmic resistance. The half-cell results, however, have been encouraging enough to motivate us to examine a full cell, particularly with the electrocatalyst materials based on ultrafine nickel particles dispersed in the pores of activated carbon fibers (ACF), which contain slit-shaped pores with widths of ca. 2 nm. Therefore, we have recently scaled up to a 2 /2 cm GDE cathode and an oxygengenerating anode of a similar size, mounted in a rectangular cell with a relatively small inter-electrode gap (1 /2 mm) in order to minimize ohmic losses.
2. Experimental The ACF/Ni catalysts were prepared as previously reported [21]. ACF (type KF-1500) was obtained from Toyobo Co., Ltd. (Osaka, Japan). The fibers were placed in contact with aq. solutions of Ni(NO3)2 (0.5 M) and were stirred at room temperature for ca. 10 h. The fibers were then washed thoroughly with water, and, after air drying, the adsorbed metal ions were reduced under a hydrogen atmosphere at 350 8C. Copper catalysts supported on metal oxides were prepared by use of a precipitation method. The metal oxide powders were added to a solution of Cu(NO3)2 and were stirred at 80 8C, with 0.1 M KOH added to adjust the pH to 10. The catalyst composition was 5 wt.% Cu and 95 wt.% metal oxide. The precipitates were washed with water, dried, and reduced under hydrogen at 400 8C. For the active layer, a 100-mg portion of carbon black (Gunbai, 92 m2 g 1, Denki Kagaku Kogyo, Tokyo, Japan) and an aliquot of PTFE aq. dispersion (Daikin D-1), containing 30 mg solids were mixed on a PTFE sheet, and then a 100-mg portion of the catalyst-loaded ACF or Cu-loaded metal oxide was added to the material and mixed. After drying, this paste was spread over the surface of a commercial hydrophobic gas-diffusion layer (Tanaka Precious Metals),
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which was cut into a 2.4 /2.4 cm2, and pressed together under a load of 4 tons. This electrode was then heattreated at 350 8C under hydrogen. The counter electrode was formed by pressing together a porous Ni sheet (Nilaco Co., Ltd.) and a piece of one-sided hydrophobic gas diffusion layer (e-Tek). The electrode structure is shown in Fig. 1. Both electrodes were mechanically supported by Ni mesh (Nilaco), which was also the current collector. The electrochemical cell diagram is shown in Fig. 2. Dry CO2 gas was fed into the gas distribution area for the cathode and dry Ar gas was fed into that for the anode during the measurements. The electrolyte solution, which was aq. 0.5 M KHCO3 was allowed to flow down through the separator (filter paper, type No. 3, Advantec Toyo) by gravity by feeding it from the top. A Pt-catalyzed gas diffusion layer (e-Tek), fed with hydrogen gas at 1 atm, was used as a reversible hydrogen electrode (RHE). The reduction products were allowed to accumulate in a Tedlar bag and were analyzed by use of a gas chromatograph (Ohkura GC-202, Porapak-Q column, FID; Hitachi 163, MS-13X column, TCD), and a high performance liquid chromatograph (Tosoh UV8010, Shodex KC811 column, 210 nm UV). The electrolysis was carried out with a DC power supply (Kenwood TMI PAR20-4H), which was connected across the anode and cathode, as shown in Fig. 3. This unit also includes a voltmeter, which was used to measure the cell voltage. The electrical connections for the measurement of the cathode potential are shown. The anode potential was obtained simply by subtracting the cathode potential from the cell voltage. No corrections were made for ohmic resistance, i.e. all potentials and voltages include IR drop .
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Fig. 2. Schematic diagram of the rectangular-type electrolytic cell used in the present work.
3. Results and discussion 3.1. Current /potential behavior of the cathode The current /potential behavior for various types of cathodic electrocatalysts is shown in Fig. 4. The performance was generally superior for the ACF/Ni catalysts compared to the Cu/metal oxide catalysts. We have previously shown that the latter are promising, due
Fig. 1. Schematic diagrams of the structures for the gas-diffusion anode (left) and cathode (right). The former is modified with a gas-diffusion layer for purposes of wet-proofing.
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Fig. 3. Schematic diagram showing the electrical connections to the electrolytic cell. During the polarization measurements, the current is set on the DC power supply, and the cell voltage is read directly from the power supply. The cathode potential is measured separately with respect to the hydrogen-fed reference electrode with a high-impedance voltmeter, and the anode potential is obtained by difference from the cell voltage and the cathode potential. No corrections were made for ohmic resistance.
Fig. 4. Steady-state current /potential behavior (uncorrected for ohmic resistance) for various gas-diffusion cathodes obtained in 0.5 M KHCO3 for GDEs fabricated from (a, b) Ni/ACF (m, m); (c) Cu/ ZrO2 (%); and (d) Cu/ZnO (").
to their high selectivity for CO production (Cu/ZnO) and ethylene production (Cu/ZrO2) [22]. The behavior for the Cu/ZrO2 catalyst was similar to that for ACF/Ni at very low CD at a potential of ca. 0.0 V, as seen in the Tafel plots (Fig. 5), but at more negative potentials, the CD values were significantly lower than those for the better of the two ACF/Ni electrodes but very similar to those for the poorer of the two ACF/Ni electrodes. The CD values for the Cu/ZnO catalyst were very much
Fig. 5. Tafel plots for cathode performance obtained in 0.5 M KHCO3 for GDEs fabricated from (a, b) Ni/ACF (m, m); (c) Cu/ZrO2 (%); and (d) Cu/ZnO ("), based on the data in Fig. 3. A line with a slope of /120 mV per decade is shown for reference.
lower. The values for the onset of current are in the neighborhood of 0.0 V versus RHE. This compares favorably with the theoretical value of (1.281 /1.229) / 0.052 V versus RHE. It was somewhat unexpected to find that the results for the two ACF/Ni electrodes did not reproduce more closely. In our previous work, the reproducibility was much better due to the better control of the electrode fabrication. With this new generation of GDEs, the fabrication procedures have not been optimized to the same degree. This is a typical problem that is encountered with the scale-up of electrode fabrication. It should be noted, however, that the behavior for the better of the ACF/Ni electrodes shown was somewhat better than that for the electrodes examined earlier in half-cell measurements [21], as shown below. In order to make a precise comparison with the previous, half-cell work, it is necessary to convert the RHE reference scale to the SCE scale. Based on the calculated pH of 7.51 for 0.5 M KHCO3 saturated with 1 atm CO2, which was confirmed experimentally (7.41), the standard potential for the RHE would be /0.445 V versus SHE, which would be /0.685 V versus SCE. Similarly, the standard potential for the reversible oxygen electrode (ROE) would be /0.784 V versus SHE, and /0.543 V versus SCE. Making use of the latter value, we can compare the cathodic performance at /1.6 V versus SCE in the previous work, which was ca. 40 mA cm 2. In the present work, the corresponding potential would be /1.6/0.685 / /0.915 V versus RHE, and the corresponding cd was ca. 70 mA cm 2, which is in reasonable agreement.
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For this electrode, the current /potential behavior at very low CD approaches that for a kinetically limited system (Fig. 5, curve a). In the potential region near 0.0 V, the Tafel slope was rather close to 120 mV per decade, which is indicative of a first electron transfer step being rate-determining. A line indicating this slope is also shown in the figure. This behavior is similar to that expected for hydrogen evolution from a nickel electrode in alkaline electrolyte [23]. As shown later, the principal product in this potential region is hydrogen. In the previous work, Tafel slopes of ca. 240 mV per decade were found for the partial current densities (PCD) for hydrogen evolution at relatively negative potentials, i.e. those negative of /1.6 V versus SCE or /0.915 V versus RHE, based on product analysis for hydrogen. This was proposed to be due to a type of mass-transport-limited behavior, due to the porous nature of the electrode, leading to a slope-doubling effect, as described by de Levie [24]. This would result in a slope of ca. 240 mV per decade, based on an intrinsic value of 120 mV per decade. It is not possible to make a direct comparison of the previous work with the present work in this case, because the present performance, since it was not corrected for IR drop, was already resistancelimited in this potential region. In summary, from an examination of the shapes of the current /potential curves for the best-performing Ni/ ACF electrode in Figs. 4 and 5, it appears that the behavior at low CD is consistent with kinetically limited hydrogen evolution. At higher CD, the behavior becomes limited by ohmic drop, as evidenced by the linear behavior. The Tafel plot will be discussed in more detail later.
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Fig. 6. Steady-state current /potential behavior (uncorrected for ohmic resistance) for oxygen-evolving anodes obtained in 0.5 M KHCO3, with cells in which various cathodes were used: (a, b) Ni/ ACF (m, m); (c) Cu/ZrO2 (%); and (d) Cu/ZnO (").
3.2. Current /potential behavior for the anode Fig. 6 shows current /potential curves for the same type of anode (Ni foam) used in cells with various cathode catalysts. Although the behavior should in principle be the same in each case, it varied greatly. The reasons for this variation are not clear at present, but it is possible that soluble products from the Cubased electrodes became incorporated in the anode. The performance was best for the cell in which Cu/ZnO was used as the cathode catalyst, although points could not be obtained for very high CD, and worst for the cell in which Cu/ZrO2 as used. The behavior for the two cells in which ACF/Ni was used did not differ greatly. The onset of current was in the region near the reversible potential for the oxygen electrode, which would be /1.229 V versus RHE. This is an encouraging result. 3.3. Current efficiencies and CD for electrolysis products The potential dependencies of the current efficiencies (CE) corresponding to the electrolytic generation of
Fig. 7. Potential dependence of the CE for H2 evolution (m), CO evolution (m) and CH4 evolution (%) for a GDE fabricated from Ni/ ACF.
various products when performing electrolysis with the best-performing ACF/Ni catalyst, are shown in Fig. 7. These data were obtained with the use of the electrode whose cathodic performance is shown as curve b in Fig. 4. At a potential of ca. /0.6 V, the CEs for CO production and H2 generation became almost equal. This point would correspond to the production of a 1:1 CO/H2 syngas. At more negative potentials, the CE for CO production drops off, reaching essentially zero at ca. /1.2 V versus RHE.
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Fig. 8. Tafel plot for H2 evolution (m), CO evolution (m), CH4 evolution (%) and total CE (k) for a GDE fabricated from Ni/ACF.
Fig. 8 shows the potential dependencies of the PCD. The PCD for CO production becomes essentially constant at potentials negative of ca. /0.9 V, while that for H2 generation continues to climb, finally becoming nearly constant at ca. /2.0 V versus RHE, with about two orders of magnitude greater CD compared to that for CO production (ca. 100 vs. ca. 2.5 mA cm 2). In the potential range intermediate between these extremes, it is possible to control the composition of the CO/H2 mixture that is produced, which is an attractive feature. The composition can be varied to suit the intended use of the syngas, e.g. 1:1 for dimethyl ether production and 1:2 for methanol production [25].
Fig. 9. Steady-state current /potential behavior for both cathode (m), and anode (m) in 0.5 M KHCO3 for a GDE fabricated from Ni/ACF. Simulated curves are shown as lines (see text for details). The cell voltage is also shown (right-hand axis).
and cathode were different: ca. 6.5 V cm2 for the cathode and ca. 18 V cm2 for the anode. These are reasonable values based on the somewhat poor conductivities of KHCO3 electrolyte, which are approximately a factor of three poorer than the corresponding values for KOH [26]. Moreover, within the electrolyte gap (ca. 1.5 mm), a sizable fraction of the electrolyte volume is taken up by the separator. The difference between the values for the anode and cathode is due to the placement of the reference electrode, which is on the same side of the separator as the cathode. Thus, the total
3.4. Simulation of current/potential behavior It is instructive to look at the overall performance of the best-performing cell (corresponding to curve a in Fig. 4) on a single potential axis, showing both anode and cathode curves. Such a plot is shown in Fig. 9 in terms of the total measured CD versus the potential on the RHE scale. The potential differences between the two curves are simply the uncorrected cell voltages, as shown in the figure. Since the CD is proportional to potential in the high CD region (greater than ca. 50 mA cm 2), both electrodes are considered to be limited by ohmic resistance. To illustrate this point, we have inserted theoretical curves that include a Tafel term and an ohmic term, as follows: E=V 0:24 log(I=mA)0:018(I=mA)1:36 E=V 0:12 log(I=mA)0:0065(I=mA) 0:11
(3) (4)
Approximate agreement was obtained. It is necessary to point out that the apparent resistivities for the anode
Fig. 10. Tafel plot for both cathode (m), and anode (m) in 0.5 M KHCO3 for a GDE fabricated from Ni/ACF, based on the data in Fig. 9. Simulated curves are shown as lines (see text for details). The cell voltage is also shown (right-hand axis).
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resistivity was ca. 24 V cm 2, which, at a CD of 10 mA cm 2, would correspond to a voltage loss of 0.24 V. The corresponding Tafel plot is shown in Fig. 10, together with the simple simulations. Again, the agreement is reasonable and shows that there are mass transport limitations in the intermediate CD region for both electrodes. This type of plot is highly useful in evaluating the aspects that are in need of improvement, e.g. the ohmic resistance, the mass transport and the electrocatalysis, and for which electrode. These aspects are also dependent upon the optimum operating potential and CD in terms of the product, in this case a 1:1 CO/H2 mixture, which was obtained at a CD of ca. 10 mA cm 2 for the electrode corresponding to curve b in Fig. 4. For the electrode corresponding to curve a, a CD of ca. 35 mA cm 2 was obtained at the same cathode potential. At 10 mA cm 2, as already pointed out, the ohmic loss would be ca. 0.24 V. At 35 mA cm 2, the value would be 0.84 V, which could be decreased by decreasing the thickness of the separator and increasing the electrolyte concentration. Both the electrocatalysis and mass transport can be substantially improved for both electrodes, however. 3.5. Estimation of energy efficiency The overall energy efficiency that is obtained at this operating point (electrode corresponding to curve b in Fig. 4) was estimated from the cell voltage, 3.054 V, together with the enthalpic voltage of 1.36 V for the generation of the 1:1 CO/H2 mixture, to be ca. 45%. Based on corresponding values typically obtained for water electrolysis under highly optimized conditions, (ca. 85.6%, [16]), this value is in need of significant improvement, although it is reasonable for a nonoptimized system. It is expected that this value could be increased somewhat by focusing on improvements in the electrocatalysis and mass transport. In addition, the ohmic losses can be decreased somewhat. Recent work with porphyrin electrocatalysts in our laboratory, for example, have shown that high selectivities for CO production can also be obtained at less negative potentials, by up to 400 mV [27]. Furthermore, the anode performance can be expected to improve with the use of high-area films of Ni /Fe oxides, for example.
4. Summary and conclusions Based on our earlier work with gas-diffusion electrodes for the cathodic reduction of ambient pressure, ambient temperature CO2, we have constructed a prototype rectangular electrolytic cell with the purpose of producing useful products at both the cathode and anode. With high-area nickel supported on ACF, the cathode performance was superior, reaching a total
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current density of ca. 10 mA cm 2, with ca. 5 mA cm 2 each for CO generation and H2 evolution, at a potential of ca. /0.6 V versus RHE, compared to the reversible potential of /0.052 V versus RHE. At this operating point, the cell voltage, uncorrected for ohmic resistance, was 3.054 V. The polarization losses were concluded to be due to a combination of electrode kinetics, ohmic drop and mass transport within the porous structure. Based on simple simulations of the current /potential behavior, including a Tafel term and an ohmic term, resistivities were estimated for the anode and cathode. The total was ca. 24 V cm 2, which, at a CD of 10 mA cm 2, corresponds to a voltage loss of 0.24 V. The overall energy efficiency obtained at the optimum operating point was estimated for the generation of the 1:1 CO/H2 mixture to be ca. 45%. This value can be improved significantly in the future by focusing on improvements in the electrocatalysis and mass transport, and by decreasing the ohmic resistance.
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