- Email: [email protected]
Enhanced performance of CO poisoned proton exchange membrane fuel cells via triode operation
Electrochimica Acta 56 (2011) 6966–6975
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Enhanced performance of CO poisoned proton exchange membrane fuel cells via triode operation F.M. Sapountzi a,∗ , S.C. Divane a , M.N. Tsampas a , C.G. Vayenas a,b a b
Department of Chemical Engineering, University of Patras, Caratheodory 1 St, GR-26504 Patras, Greece Academy of Athens, Panepistimiou 28 Ave., GR-10679 Athens, Greece
a r t i c l e
i n f o
Article history: Received 23 February 2011 Received in revised form 3 June 2011 Accepted 4 June 2011 Available online 12 June 2011 Keywords: PEM fuel cell CO poisoning Triode operation Self-sustained potential and current oscillations Power output enhancement
a b s t r a c t The effect of triode operation on the performance of CO poisoned PEM fuel cells was investigated. In this mode of operation a third, auxiliary, electrode is introduced in addition to the anode and the cathode. Application of electrolytic current in the auxiliary circuit, comprising the cathode and the auxiliary electrode was found to significantly enhance the time-averaged power output of a state-of-the-art PEMFC unit operating with a 70 ppm CO in H2 atmospheric pressure mixture. Both normal and triode operation were found to lead to self-sustained current and potential oscillations in the fuel cell circuit over wide ranges of external resistive load. The time averaged increase in power output was found to be typically a factor of three higher than the power output in conventional fuel cell operation and up to a factor of 1.32 larger than the power sacrificed in the auxiliary circuit. The mechanism of the enhanced anodic electrocatalysis was investigated via the use of two reference electrodes and the results are discussed together with a possible design for application of the triode concept in stacks. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction One of the main problems associated with the practical utilization of PEMFC units is that of CO poisoning of the Pt-based anode [1–4]. This is because hydrogen-rich reformates of light hydrocarbons or liquid alcohols inevitably contain significant levels of carbon monoxide that poisons the anode and degrades fuel cell performance [3–6]. Carbon monoxide competes with hydrogen for active sites on the catalyst at normal anode operating potentials. With even relatively low levels of gaseous CO, the catalyst surface is blocked by CO due to stronger adsorption. As an example for 1% CO/H2 mixture the CO coverage on the platinum surface is estimated to be near 0.98 at 25 ◦ C [7]. This results in poor overall fuel cell performance. The CO adsorbed on the catalytic sites can be removed by raising the anode potential to about 0.7 V versus RHE [8]. At this potential, hydroxyl species formed on the platinum surface oxidize CO. For lower potentials, CO adsorption is strong and keeps the surface essentially completely blocked. For higher potentials the surface remains partially cleaned. For intermediate potentials selfsustained oscillatory behavior is frequently observed [3,8]. This high (∼0.7 eV) overpotential for CO oxidation is prohibitive for practical PEMFC applications.
∗ Corresponding author. Tel.: +30 2610 997576; fax: +30 2610 997269. E-mail address: [email protected] (F.M. Sapountzi). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.06.012
Several approaches have been proposed to mitigate the CO poisoning of PEMFC anodes, i.e.: (i) advanced reformer design [8–16], (ii) use of more CO-tolerant anodes [17–22], (iii) oxidant bleeding into the fuel feed stream (air, oxygen or hydrogen peroxide) [23–26], (iv) use of a bilayer anode structures [27–30], (v) higher operating temperature [31–34], (vi) use of membranes for CO separation [35–37] and (vii) pulsing the cell voltage between specific values during cell operation, thus forcing the anode potential to operate for a short time at a potential positive enough to electrochemically oxidize CO [2,38]. The objective of this work is to investigate an alternative approach for enhancing the PEMFC performance under CO poisoning conditions by using the recently described triode fuel cell design and operation [39–41]. In addition to the anode and the cathode, a triode fuel cell contains a third, auxiliary, electrode and thus a second, auxiliary, circuit is formed which is run in the electrolytic mode (Fig. 1). This permits fuel cell operation under previously inaccessible anode–cathode potential differences [39–41] and Fig. 2. The triode fuel cell concept has been explored and tested already in solid oxide fuel cells (SOFC) with polarizable Pt electrodes [39], in a PEMFC with a Pt–Ru anode fed with 100 ppm CO in H2 [40] and in a PEMFC with a Pt anode fed with a methanol reformate mixture with very high (760 ppm) CO concentration at the Pt–Ru anode [40]. Both studies have shown that the power output of the fuel cell can be very significantly increased via application of auxiliary potential or current. Furthermore, this increase, Pfc , was found to be comparable with or higher than the power, Paux , sac-
F.M. Sapountzi et al. / Electrochimica Acta 56 (2011) 6966–6975
6967
Fig. 1. Schematic of the triode fuel cell concept, showing (a) the fuel cell and auxiliary circuits, (b) the cell geometry and the reactions taking place at the electrodes (c), the exact geometry of the triode PEM fuel cell and the electrical circuits and (d) the geometry of the membrane electrode assembly. The fuel cell cathode acts simultaneously as an electrode of the auxiliary circuit. P/G: potentiostat–galvanostat.
rificed in the auxiliary circuit. The latter leads to enhanced overall thermodynamic efficiency [39,40]. Recently, Cloutier and Wilkinson have pioneered the use of the triode design in PEM electrolyzers [42]. In the present study, in order to investigate the mechanism of CO mitigation and power output enhancement, we have used, in addition to the anode, the cathode and the auxiliary electrode, two reference electrodes as shown in Fig. 1d. This allows for direct measurement of the anode and cathode overpotentials both in normal and in triode PEM fuel cell operation.
2. Operating principle In a triode fuel cell the three electrodes are all in electrolytic contact and form two electrical circuits (Fig. 1a): (i) the fuel cell circuit, which comprises the anode, the cathode, and a variable resistance, Rext , for dissipating the electrical power, Pfc , produced and (ii) the auxiliary circuit, which comprises the auxiliary electrode, the cathode of the fuel cell and a potentiostat–galvanostat.
Fig. 2. Ifc –Vfc curves of the fuel cell circuit obtained with the conventional operation mode (Iaux = 0) and with the triode operation mode (Vaux = −1.9 V) by varying the external resistive load, Rext . Anode feed: 70 ppm CO, 14 kPa H2 in He balance.
When the applied auxiliary current, Iaux , is zero then the fuel cell operates in the conventional mode. It produces a power, denoted o , which is the product of the fuel cell potential, V o , and of the by Pfc fc o . Both V o and I o vary as the external resistive fuel cell current, Ifc fc fc load of the fuel cell circuit, Rext , is varied.
6968
F.M. Sapountzi et al. / Electrochimica Acta 56 (2011) 6966–6975
The triode fuel cell operation consists of imposing an electrolytic current Iaux < 0 or potential Vaux < 0 on the auxiliary circuit. This affects the potentials and thus the overpotentials of both the anode and the cathode. In the present case in this way protons migrate from the fuel cell cathode to the auxiliary electrode (Fig. 1a and b) and a fraction of them migrates to the anode and back to the cathode (Fig. 1a and b). Fig. 1c and d shows the exact geometry of the PEMFC triode unit used in the present investigation which also includes two reference electrodes as shown in Fig. 1d. These reference electrodes allow for direct measurement of the anodic and cathodic overpotential. Denoting by Ifar the net Faradaic fuel-consuming current, one notes that in view of Fig. 1 and Kirchhoff’s first law it is: Ifar = Ifc + Iaux
(1)
where Iaux < 0 and the net Faradaic current Ifar is that corresponding via Faraday’s law to the net consumption of fuel at the anode or the net consumption of O2 at the cathode. Two parameters have been introduced to quantify the results of triode operation [39]. The first is the power enhancement ratio, P , defined as [39]: P =
Pfc o Pfc
(2)
which quantifies the increase in power output in the fuel cell circuit. The second parameter is the power gain ratio, P , defined as [39]: P =
o Pfc − Pfc
Paux
(3)
This parameter is the ratio of the increase in power output of the fuel cell circuit, divided by the power sacrificed in the auxiliary circuit, Paux . The latter is the product of the auxiliary cell potential, Vaux , and of the auxiliary cell current, Iaux . When P > 1 then the thermodynamic efficiency of the triode unit is higher than that corresponding to conventional fuel cell operation. One reason for the ability of the triode fuel cell operation to affect overpotentials and to enhance fuel cell performance is that the auxiliary circuit can force the fuel cell anode or cathode to operate under controlled (via Iaux ) and previously inaccessible, for a PEM fuel cell, potentials in the standard hydrogen electrode or in the absolute potential scale [43,44]. A major reason, however in the present case of PEM fuel cells is the supply of protons to the anode via the auxiliary electrode which is found to cause significant reduction of the anodic overpotential. It is clear from Fig. 1 that all three electrodes operate in a corrosion-type mode with part of their surface used for oxidation and part of their surface used for reduction. Despite this rather intense mode of operation no performance deterioration was observed during operation for several weeks provided the negative of the auxiliary potential does not exceed 1.9 V. 3. Experimental A state-of-the-art PEM fuel cell [40] was used (NuVant, Fig. 1c) with state-of-the-art Pt and Pt–Ru electrodes deposited by NuVant on E-TEK carbon cloth and equipped with a third auxiliary electrode as shown in Fig. 1d. The loading of the Pt electrode was 0.5 mg Pt/cm2 (unsupported Pt black) and the loading of the Pt (30%) Ru (15%), supported on Vulcan XC-72 carbon, was 0.5 mg/cm2 , thus 1.8 mg/cm2 metal basis. The superficial surface area of the cathode electrode (Pt) was 5.29 cm2 , of the anode electrode (PtRu) was 3.85 cm2 and of the auxiliary electrode (PtRu) was 0.49 cm2 (Fig. 1). The cathode was a square (Fig. 1) and the auxiliary electrode was a smaller square
located in the center of the hollow square anode (Fig. 1). The membrane was Nafion 117 with nominal thickness 185 m. Two reference Pt electrodes were also introduced in order to measure anodic and cathodic overpotentials in conjunction with the current interruption technique which was used to separate the IR component. The location of the Pt reference electrodes is shown in Fig. 1d. The proper performance of both reference electrodes was confirmed by monitoring the potential difference between them during fuel cell operation and noting that it changes less than 10 mV when currents as high as 1 A flow between the anode and cathode both in normal and in triode operation. The membrane electrode assembly (MEA) was prepared by hot pressing in a model C Carver hot press at 120 ◦ C and under pressure of 1 metric ton for 3 min. Two cells were used in order to check reproducibility and both gave practically the same results. Preliminary investigation showed that the negative of the applied auxiliary potential, Vaux , i.e. −Vaux should not exceed 1.9 V as this leads to CO2 formation at the cathode via oxidation of the carbon support. The concentration of CO and CO2 in the anode and cathode feed and effluent was monitored used a Fuji Electric’s Infrared Analyzer ZRJ-4UNOR 6N IR CO/CO2 analyzer. The fuel cell circuit included a decade resistance Box (Time Electronics Ltd 1051) in order to vary the external load. The current and the potential were measured by two digital multimeters (Metex ME 21) (Fig. 1c). Constant potentials or currents in the auxiliary circuit were applied using an AMEL 553 Potentiostat–Galvanostat. The gas feeds to the cathode and anode compartments (the latter includes also the auxiliary electrode, Fig. 1) were continuously humidified using thermostated gas saturators. The cell temperature was typically set at the same temperature (25 ◦ C) with the gas saturators. The anode compartment gas feed was Messer Griesheim certified gas mixture of 150 ppm CO/30 kPa H2 in He balance, which could be further diluted with Air Liquide He. The cathode feed was humidified Air Liquide synthetic air. 4. Results and discussion 4.1. General overview As shown in Fig. 2 the use of the triode operation mode via application of an auxiliary potential, Vaux , of −1.9 V leads to very significant enhancement of the CO poisoned PEMFC performance. Thus in normal operation and external resistive loads, Rext , below 3 , the cell potential drops below 0.22 V. For higher Rext values, self-sustained oscillations are obtained between an active and a severely poisoned state. Triode operation extends this oscillatory region to Rext values as low as 0.5 and leads to a 300% increase in the time averaged power output of the cell. In order to better rationalize these results it is useful to first present the results obtained in the conventional fuel cell operation mode without and with CO added to the anode feed gas and then to proceed with the detailed presentation of the triode operation results. 4.2. Conventional operation mode, H2 feed The top panel in Fig. 3 shows the Ifc –Vfc curve obtained in the conventional fuel cell mode by using a 30 kPa H2 in He gas mixture. We use the subscript “fc” to denote that I and V refer to the fuel cell circuit. The Ifc –Vfc curve has been obtained by varying the external resistance, Rext , of the decade box from zero to 200 . As the resistive load is decreased (and thus the fuel cell current, Ifc , is increased), the operating voltage of the cell drops below the (o) (o) open-circuit voltage, Vfc , and the difference = Vfc − Vfc is the total cell overpotential [45]. The types of overpotential which
F.M. Sapountzi et al. / Electrochimica Acta 56 (2011) 6966–6975
6969
Fig. 3. Effect of the cell current or current density on the cell potential and on the ohmic, anodic and cathodic overpotential for conventional operation mode (Iaux = 0) under 30 kPa H2 /He.
limit the cell performance are (a) the anodic and cathodic activation overpotential caused by slow electrocatalysis at the anode and cathode respectively, (b) the ohmic overpotential associated with the ohmic resistance of the electrolyte and the electrodes and (c) the concentration overpotential associated with slow mass transfer of the reactants to the catalyst–electrode–gas three phase boundaries. The latter leads to a pronounced sharp decrease in cell potential as the limiting current is approached [45]. For the case of Fig. 3, it is clear that no limiting current is reached and thus the concentration overpotential is negligible. Consequently the cell overpotential is the sum of three components, i.e. of the ohmic overpotential (Ifc Rfc ), the anodic overpotential (a ) and the cathodic overpotential (c ), with [45]: = Ifc Rfc + a + c
(4)
Fig. 3 shows the contribution of each type of overpotential to the total cell losses. The anodic and cathodic potential losses were determined by means of the two Pt reference electrodes, which allowed both for monitoring the potentials Ea and Ec of the anode and cathode in the SHE scale and also for the separation of the anodic and cathodic overpotential from the ohmic losses via the current interruption technique. The procedure is shown in Fig. 4a
Fig. 4. Transient response of the Vfc (a) and Vc (b) signals, upon changing the external resistive load, Rext , from 0 to 200 during current interruption measurements. The arrows indicate the estimated values of the ohmic losses and of the net cell (a) and cathodic (b) overpotential.
and b for the determination of and Ifc Rfc (Fig. 4a) and c (Fig. 4b) respectively from the total change Vfc and Vc induced by current interruption. In this way, a total cell resistance, Rfc , of 0.37 was estimated by following the transient response of the Vfc signal (Fig. 4a). The same value of Rfc = 0.37 was also obtained from the slope of the linear region of the Ifc –Vfc curve of Fig. 3. The ohmic overpotential of the cell equals the product Ifc Rfc . As shown in Fig. 4b, the transient potential deviation Vc also includes part of the ohmic losses, and thus the Ifc Ra and Ifc Rc components should be extracted in order to obtain the net a and c values. From the thus calculated values of Ra = 0.17 and Rc = 0.18 (Fig. 4b), it is obvious that the two ohmic components contribute equally to the total cell ohmic resistance, Rfc , of 0.37 , which confirms the proper location of the two reference electrodes
6970
F.M. Sapountzi et al. / Electrochimica Acta 56 (2011) 6966–6975
Fig. 5. Comparison of the cell overpotential as obtained from the Ifc –Vfc curve and as determined via addition of the ohmic overpotential, Ifc Rfc and of the anodic and cathodic activation overpotential measurements by means of reference electrodes in conjunction with current interruption measurements. Anode feed: 30 kPa H2 .
which are roughly equipotential with the symmetry plane of the Nafion electrolyte membrane [45]. As already shown in Fig. 3, a is negligible during fuel cell operation in hydrogen, and the cell overpotential is mainly due to the cathodic overpotential, c , and to the ohmic losses. Fig. 5 provides a confirmation for the accuracy of the measurements. It shows that the sum of the above determined values of Ifc Ifc , (o) a and c indeed equals the cell overpotential, = Vfc − Vfc , with an accuracy better than 3%. 4.3. Conventional fuel cell operation, CO poisoning Fig. 6a shows Ifc –Vfc curves obtained in the conventional fuel cell mode in 30 kPa hydrogen (squares) and in a 14 kPa H2 stream containing 70 ppm CO, denoted hereafter 70 ppm CO/14 kPa H2 mixture (cycles). The curves have been obtained by varying the external resistance, Rext , of the decade box from zero to 200 . As expected, one observes that the presence of CO causes a significant decrease in power output and creates three distinct regions [6]. In the high potential-low current region (Rext > 20 ) and in the low potential-high current region (Rext > 3 ) stable steady-state behavior is observed, while in the intermediate region (3 ≤ Rext ≤ 20) self-sustained potential and current oscillations are obtained with a period of the order of 11–18 s. These oscillations are shown in some detail in Fig. 6b. Since the cell is run at constant Rext , it follows that these oscillations correspond to straight line segments in the Ifc –Vfc plane, as shown in Fig. 6a. The contribution of each of the overpotential components for the CO-poisoned cell was estimated, as discussed above for the case of H2 operation, via potential measurements using the two reference electrodes and the current interruption technique. Fig. 7a shows the dependence of the Vfc , Ifc Rfc , a and c on the cell current Ifc during normal fuel cell operation under 70 ppm CO/14 kPa H2 . One can observe that in presence of CO in the hydrogen supply, the overall losses in the cell performance are mainly caused by the high a values which result from the poisoning of the anodic active sites due to strong CO adsorption. Focusing on the oscillatory region at the intermediate current densities of the Ifc –Vfc curve, one can observe that it is accompanied by a pronounced oscillatory behavior of the anodic overpotential. Thus, it is obvious that the
Fig. 6. (a) Ifc –Vfc curves of the fuel cell circuit obtained with the conventional operation mode (Iaux = 0) by varying the external resistive load, Rext under 30 kPa H2 and under 70 ppm CO/14 kPa H2 gas mixtures. The arrows indicate the values of the current and potential oscillation limits for constant Rext . (b). Effect of Rext for Iaux = 0 on the fuel cell current Ifc and potential Vfc , showing the region of oscillations.
oscillations in the cell current and potential are due primarily to periodic cleaning and blocking of the anode surface from CO. The cathodic overpotential, c , also exhibits some smaller amplitude oscillation but its value remains, as expected, at the same levels (∼0.2 V) obtained in absence of CO at the anode. As shown in Fig. 7b, the data of Fig. 7a fit equation (4) with an accuracy better than 15% even in the oscillatory region. 4.4. Triode operation Fig. 8 shows the effect of constant electrolytic potential application in the auxiliary circuit, Vaux = −1.9 V, on the auxiliary circuit current (top panel) and on the fuel cell current, Ifc , and potential, Ufc (bottom two panels). The initial conditions (Vfc = 114 mV, Ifc = 220 mA) have been chosen in the low Rext region (Rext = 0.5 ). One observes that imposition of a constant electrolytic potential (−1.9 V) and current (−21 mA) in the auxiliary circuit causes the induction of self-sustained high amplitude oscillations in the fuel cell current (from 220 to 520 mA) and potential (from 120 to 294 mV). It is remarkable that the imposition of such a small auxiliary electrolytic current (Iaux = −21 mA) causes such a pronounced increase in current (up to 300 mA) and potential in the fuel cell
F.M. Sapountzi et al. / Electrochimica Acta 56 (2011) 6966–6975
6971
Fig. 8. Effect of a constant auxiliary potential application, Vaux = −1.9 V on the auxiliary current Iaux and on the cell potential Ufc and cell current Ifc , under fixed external resistive load (Rext = 0.5 ). Anode feed: 70 ppm CO/14 kPa H2 .
Fig. 7. (a) Effect of the cell current or current density on the cell potential and on the ohmic, anodic and cathodic overpotential for the conventional operation mode (Iaux = 0) under 70 ppm CO/14 kPa H2 . (b) Comparison of the cell overpotential as obtained from the Ifc –Vfc curve and as obtained by addition of the ohmic and activation overpotentials.
Fig. 9. Time variation of fuel cell power output, Pfc , sacrificed power in the auxiliary cell, Paux , power enhancement ratio, P , and power gain ratio, P and corresponding time-averaged values for triode operation during imposition of a constant auxiliary potential, Vaux = −1.9 V, under a fixed external resistive load (Rext = 0.5 ). Anode feed: 70 ppm CO/14 kPa H2 .
6972
F.M. Sapountzi et al. / Electrochimica Acta 56 (2011) 6966–6975
Fig. 11. Effect of the cell current on the cell potential and on the ohmic, anodic and cathodic overpotentials. Open symbols refer to the conventional operation mode (Iaux = 0), filled symbols refer to triode operation mode (Vaux = −1.9 V).
Fig. 10. Effect of fixed auxiliary potential application (Vaux = −1.9 V) on the limits of fuel cell current–potential oscillations (a) and on the current–power output oscillations (b). The dark and light dashed areas show the region of self-sustained oscillations for normal and triode operation respectively.
circuit. The induction of this oscillatory behavior is reversible, as oscillations disappear when the cell operation returns to the conventional mode (Iaux = 0, Fig. 8). Fig. 9 is based on Fig. 8. It presents the time variation of the fuel cell power output, Pfc , and compares it with the power sacrificed in the auxiliary cell, Paux . The figure also shows the time variation of o ), and of the power gain the power enhancement ratio, P (= Pfc /Pfc ratio, P (= Pfc /Paux ) and the corresponding time-averaged values of these two quantities. These are computed on the basis of Eqs. (2) and (3) from: ¯ P =
1 T
T
dt
(5) 4.5. Investigation of the triode operation mechanism
0
where T is the oscillation period and:
T
¯P=
o ) dt (P − Pfc 0 fc
T 0
Paux dt
One observes that the fuel cell power output, Pfc , is always higher o. in the triode operation mode than in the conventional mode, Pfc The power enhancement ratio, P , varies from 1 to 6, and the timeaveraged enhancement ¯ P is 3.1. The power gain ratio, P , varies from 0 to 3.4, showing that there are small time intervals where P < 1 and thus the power gain in the fuel cell circuit is smaller than the power consumed in the auxiliary circuit. However the beneficial effect of the triode mode (1 < P < 3.4) is more pronounced, thus the time-averaged power gain is approximately 1.32. Thus there is a net gain in power and thus on overall thermodynamic efficiency. Fig. 10a shows the Ifc –Vfc curves obtained during normal (Iaux = 0) and triode (Vaux = −1.9 V) operation, in all Rext regions, while Fig. 10b shows the significant advantages of the triode operation in terms of power output. As shown in these figures, the triode operation enhances very significantly the region of selfsustained oscillations and thus leads to very significant increase in the time-averaged fuel cell performance, particularly in the high current–high power output region.
(6)
In order to enhance our understanding of the mechanism of triode operation, the response was examined of the three overpotential components (Ifc Rfc , a and c ) upon application of constant electrolytic potential in the auxiliary circuit, Vaux = −1.9 V
F.M. Sapountzi et al. / Electrochimica Acta 56 (2011) 6966–6975
6973
Fig. 12. Schematic of a possible design of a triode PEMFC stack, showing the overall geometry and proton flow (a), the separator plate geometry (b), and the processes taking place in a single cell (c).
(Fig. 11). The filled and open symbols in Fig. 11 correspond to triode and conventional operation respectively. As already noted, one factor for the ability of the triode fuel cell operation to affect overpotentials and enhance fuel cell performance is that the auxiliary circuit forces the fuel cell anode or cathode to operate under controlled (via Iaux ) and previously inac-
cessible, for fuel cells, potentials in the standard hydrogen electrode scale. Thus as shown in Fig. 11 the fuel cells potential difference Ufc under open-circuit conditions increases from 950 mV for normal fuel cell operation (Iaux = 0) to 1420 mV for Vaux = −1.9 V. As a result, nominally negative values for the cell overpotential
6974
F.M. Sapountzi et al. / Electrochimica Acta 56 (2011) 6966–6975 (o)
( = Vfc − Vfc ) are obtained, in the very low current density region in relation to normal open-circuit operation. This is because, during the triode operation there is an extra supply of protons to the anode from the auxiliary electrode, creating a previously inaccessible proton electrochemical potential and hydrogen chemical potential at the anode, which is reflected by a small nominal negative anodic overpotential of −20 mV and a concomitant positive anode potential value of Ea = 20 mV in the SHE scale, also confirmed by the use of a reference electrode. This increased proton electrochemical potential and hydrogen chemical potential, however small, can cause an increased coverage of H at the anode and therefore it can decrease the coverage of CO. The CO desorption from the anode during triode operation at low cell current densities was also confirmed by continuous gas analysis of the fuel cell effluent. At larger current densities, where a stable poisoned steady state is observed when Iaux = 0, the triode operation leads to activation of an oscillatory behavior between a CO free and a CO poisoned anode surface state, as already mentioned. One observes that the application of electrolytic current or potential affects both the anodic and the cathodic overpotential (Fig. 11) and leads to enhanced performance. This enhanced performance is mainly related to the significant decrease of the anodic overpotential, a , observed during triode operation, as shown in Fig. 11, although there is a concomitant decrease in cathodic overpotential. Thus, the advantageous effect of triode operation is based primarily on the enhanced cleaning of the poisoned anodic active sites via the supply of protons to the anode, and the concomitant minimization of the anodic potential losses. This is corroborated by the fact that an increase from 24 to 30% in the CO conversion was observed during triode operation. It thus follows that the CO mitigation mechanism at large cell current densities is due to enhanced CO electrooxidation. The reaction which is enhanced is CO + H2 O → CO2 + 2H+ + 2e−
(7)
It would appear that supplying protons which are a product of (7) could cause a decrease in the rate. However, if the supply of protons decreases, via competitive adsorption, the sites available for CO adsorption then this can lead to significant enhancement in catalytic rate. This appears to be the case here. Therefore, to some extent, proton supply is similar to a temperature increase. They both lead to decreased CO coverage. 4.6. Stack considerations A crucial factor for the practical viability of triode PEMFCs will be the ability to design efficient stacks where each cell is a triode cell with a minimum of electrical connections. Such a design is shown in Fig. 12. The only difference from a common PEMFC stack is in the design of the separator (bipolar) plates which in the present case include an insulating section (Fig. 12). The fuel cell units are electrically connected in the usual mode, i.e. using conducting carbon separator plates. The resulting terminal stack voltage Vfc,s equals nVfc where Vfc is the potential of each unit cell, and n is the number of unit cells. The auxiliary cells are formed using the bipolar electrolysis concept [46] between the separator plates and the auxiliary electrodes (Fig. 12) and a total potential, Vaux,s , equal to nVaux is applied between the two end auxiliary sections of the separator plates. 5. Conclusions Triode operation of CO poisoned PEM fuel cells leads to very significant, threefold, increase in power output of the fuel cell circuit. This increase in power output is up to a factor of 1.32 larger than the power sacrificed in the auxiliary circuit and is mainly due to a
significant decrease in anodic overpotential caused by the supply of protons to the anode via the auxiliary electrode. This proton supply increases the electrochemical potential of protons and chemical potential of hydrogen at the anode, thus decreasing the coverage of CO and enhancing its electrooxidation rate. To some extent the triode operation of fuel cells is similar to electrochemical promotion of catalysis (EPOC or NEMCA effect [44,47–49]). In both cases a potential or current is applied between a working electrode and an auxiliary electrode in order to enhance performance. In the case of EPOC some power is sacrificed to enhance the rate of production of a chemical. In the case of a triode fuel cell some power is sacrificed in the auxiliary circuit in order to enhance the overall production of electrical power. In both cases (EPOC or triode operation) significant enhancement in chemicals or power production is possible only when the working electrode is at least partly polarizable and thus the ionic conduction in the electrolyte is not rate limiting, i.e. under high Wagner number conditions [39,40,50]. In the case of CO poisoned PEMFC units, the triode operation leads to CO poisoning mitigation due to enhanced CO electrooxidation via the supply of protons to the anode. This was confirmed by also measuring the electrode potentials individually by means of two reference electrodes. It appears that the triode design is worth a more detailed investigation with different anode compositions and CO poisoning levels. Application in suitably modified PEMFC stacks also appears feasible.
References [1] S. Ye, in: J. Zhang (Ed.), CO-tolerant Catalysts in PEM FC Electrocatalysts and Catalyst Layers, Fundamentals and Applications, Springer-Verlag, London, 2008. [2] A.H. Thomason, T.R. Lalk, A.J. Appleby, J. Power Sources 135 (2004) 204. [3] C.G. Farell, C.L. Gardner, M. Ternan, J. Power Sources 171 (2007) 282. [4] X. Cheng, Z. Shi, N. Glass, L. Zhang, J. Zhang, D. Song, Z.-S. Liu, H. Wang, J. Shen, J. Power Sources 165 (2007) 739. [5] J.J. Baschuk, X. Li, Int. J. Energy Res. 25 (2001) 695. [6] A. Katsaounis, S. Balomenou, D. Tsiplakides, S. Brosda, S. Neophytides, C.G. Vayenas, Appl. Catal. B: Environ. 56 (2005) 251. [7] D.C. Papageorgopoulos, F.A. de Bruijn, JECS 149 (2003) A140. [8] W.A. Adams, J. Blair, K.R. Bullock, C.L. Gardner, J. Power Sources 145 (2005) 55. [9] A. de Lucas-Consuegra, A. Princivalle, A. Caravaca, F. Dorado, C. Guizard, J.L. Valverde, P. Vernoux, Appl. Catal. B 94 (3–4) (2010) 281. [10] F.M. Sapountzi, M.N. Tsampas, C.G. Vayenas, Catal. Today 127 (2007) 295. [11] F.M. Sapountzi, M.N. Tsampas, C.G. Vayenas, Catal. Today 146 (2009) 319. [12] T. Dorio, K. Yasuda, Z. Siroma, N. Fujiwara, Y. Miyazaki, J. Electrochem. Soc. 150 (2003) A1225. [13] J.H. Wee, K.Y. Lee, J. Power Sources 157 (2005) 128. [14] S.T. Kuk, A. Wieckowski, J. Power Sources 141 (2005) 1. [15] J.S. Spendelow, C.Q. Lu, P.J.A. Kenis, A. Wieckowski, J. Electroanal. Chem. 568 (2004) 215. [16] W.F. Lin, T. Iwasita, W. Vielstich, J. Phys. Chem. B 103 (1999) 3250. [17] A. Pozio, L. Giorgi, E. Antolini, E. Passalacqua, Electrochim. Acta 46 (2000) 555. [18] G. Papakonstantinou, F. Paloukis, A. Siokou, S.G. Neophytides, J. Electrochem. Soc. 154 (2003) B989. [19] S. Mukerjee, J. McBreen, J. Electrochem. Soc. 146 (1999) 600. [20] M.T.M. Koper, Surf. Sci. 548 (2004) 1. ´ P.N. Ross, J. Phys. Chem. 99 (1995) 8945. [21] H.A. Gasteiger, N.M. Markovic, ´ Electrochem. Commun. 5 [22] M. Arenz, V. Stamenkovic, P.N. Ross, N.M. Markovic, (2003) 809. [23] S. Gottesfeld, J. Pafford, J. Electrochem. Soc. 135 (1988) 2651. [24] F. Sapountzi, M.N. Tsampas, C.G. Vayenas, Top. Catal. 44 (2007) 461. [25] R.J. Bellows, E. Marucchi-Soos, R.P. Reynolds, Electrochem. Solid State Lett. 1 (1998) 69. [26] G.Q. Lu, P. Waszczuk, A. Wieckowski, J. Electroanal. Chem. 532 (2002) 49. [27] T.R. Ralph, M.P. Hogarth, Platinum Met. Rev. 46 (3) (2002) 117. [28] H. Yu, Z. Hou, B. Yi, Z. Lin, J. Power Sources 105 (2002) 52. [29] C.-H. Wan, Q.-H. Zhuang, Electrochim. Acta 52 (2007) 4111. [30] A. Haug, R.E. White, J.W. Weidner, W. Huang, S. Shi, N. Rana, J. Electrochem. Soc. 149 (2002) A868. [31] N. Gourdoupi, K. Papadimitriou, S. Neophytides, J.K. Kallitsis, Fuel Cells 8 (3–4) (2008) 200. [32] J.K. Kallitsis, M. Geormezi, S.G. Neophytides, Polym. Int. 58 (11) (2009) 1226. [33] M. Geormezi, V. Deimede, N. Gourdoupi, N. Triantafyllopoulos, S. Neophytides, J.K. Kallitsis, Macromolecules 41 (23) (2008) 9051. [34] B. Smitha, S. Sridhar, A.A. Khan, J. Membr. Sci. 259 (1–2) (2005) 10. [35] B.C. Benicewicz, Advances in Materials for PEM Fuel Cell Systems, Polymer Division, American Chemical Society, Asilomar, CA, 2003, p. 23. [36] E. Kikuchi, Catal. Today 56 (2000) 97.
F.M. Sapountzi et al. / Electrochimica Acta 56 (2011) 6966–6975 [37] F.J. Varela-Gandia, A. Berengue-Murcia, D. Lozano-Castelló, D. Cazorla-Amorós, J. Membr. Sci. 351 (1–2) (2010) 123. [38] L.P.L. Carrette, K.A. Friedrich, M. Huber, U. Stimming, Phys. Chem. 3 (2001) 320. [39] S.P. Balomenou, C.G. Vayenas, J. Electrochem. Soc. 151 (11) (2004) A1874. [40] S.P. Balomenou, F. Sapountzi, D. Presvytes, M. Tsampas, C.G. Vayenas, Solid State Ionics 177 (2006) 2023. [41] S. Balomenou, C.G. Vayenas, WO 2005/008820 (2005). [42] C.R. Cloutier, D.P. Wilkinson, ECS Trans. 25 (2010) 47. [43] D. Tsiplakides, C.G. Vayenas, J. Electrochem. Soc. 148 (2001) E189. [44] C.G. Vayenas, S. Bebelis, C. Pliangos, S. Brosda, D. Tsiplakides, Electrochemical Activation of Catalysis: Promotion, Electrochemical Promotion and Metal–Support Interactions, Kluwer Academic/Plenum Publishers, New York, 2001.
6975
[45] J.O.M. Bockris, A.K.M. Reddy, M. Gamboa-Aldeco, Modern Electrochemistry, Kluwer Academic/Plenum Publishers, New York, 2000. [46] W. Hanni, A. Perret, Ch. Comninellis, Electrolytic cell with bipolar electrode including diamond, US Patent No. 6306270 (2001). [47] A. Jaccoud, C. Falgairette, G. Fóti, Ch. Comninellis, Electrochim. Acta 52 (2007) 7927. [48] X. Li, F. Gaillard, P. Vernoux, Top. Catal. 44 (2007) 391. [49] C. Jiménez-Borja, F. Dorado, A. de Lucas-Consuegra, J.M. Garcia-Vargas, J.L. Valverde, Catal. Today 146 (2009) 326. [50] G. Foti, I. Bolzonella, C. Comninellis, in: C.G. Vayenas, B.E. Conway, R.E. White (Eds.), Modern Aspects of Electrochemistry, No. 36, Kluwer Academic/Plenum Publishers, New York, 2003, p. 191.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Enhanced performance of CO poisoned proton exchange membrane fuel cells via triode operation F.M. Sapountzi a,∗ , S.C. Divane a , M.N. Tsampas a , C.G. Vayenas a,b a b
Department of Chemical Engineering, University of Patras, Caratheodory 1 St, GR-26504 Patras, Greece Academy of Athens, Panepistimiou 28 Ave., GR-10679 Athens, Greece
a r t i c l e
i n f o
Article history: Received 23 February 2011 Received in revised form 3 June 2011 Accepted 4 June 2011 Available online 12 June 2011 Keywords: PEM fuel cell CO poisoning Triode operation Self-sustained potential and current oscillations Power output enhancement
a b s t r a c t The effect of triode operation on the performance of CO poisoned PEM fuel cells was investigated. In this mode of operation a third, auxiliary, electrode is introduced in addition to the anode and the cathode. Application of electrolytic current in the auxiliary circuit, comprising the cathode and the auxiliary electrode was found to significantly enhance the time-averaged power output of a state-of-the-art PEMFC unit operating with a 70 ppm CO in H2 atmospheric pressure mixture. Both normal and triode operation were found to lead to self-sustained current and potential oscillations in the fuel cell circuit over wide ranges of external resistive load. The time averaged increase in power output was found to be typically a factor of three higher than the power output in conventional fuel cell operation and up to a factor of 1.32 larger than the power sacrificed in the auxiliary circuit. The mechanism of the enhanced anodic electrocatalysis was investigated via the use of two reference electrodes and the results are discussed together with a possible design for application of the triode concept in stacks. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction One of the main problems associated with the practical utilization of PEMFC units is that of CO poisoning of the Pt-based anode [1–4]. This is because hydrogen-rich reformates of light hydrocarbons or liquid alcohols inevitably contain significant levels of carbon monoxide that poisons the anode and degrades fuel cell performance [3–6]. Carbon monoxide competes with hydrogen for active sites on the catalyst at normal anode operating potentials. With even relatively low levels of gaseous CO, the catalyst surface is blocked by CO due to stronger adsorption. As an example for 1% CO/H2 mixture the CO coverage on the platinum surface is estimated to be near 0.98 at 25 ◦ C [7]. This results in poor overall fuel cell performance. The CO adsorbed on the catalytic sites can be removed by raising the anode potential to about 0.7 V versus RHE [8]. At this potential, hydroxyl species formed on the platinum surface oxidize CO. For lower potentials, CO adsorption is strong and keeps the surface essentially completely blocked. For higher potentials the surface remains partially cleaned. For intermediate potentials selfsustained oscillatory behavior is frequently observed [3,8]. This high (∼0.7 eV) overpotential for CO oxidation is prohibitive for practical PEMFC applications.
∗ Corresponding author. Tel.: +30 2610 997576; fax: +30 2610 997269. E-mail address: [email protected] (F.M. Sapountzi). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.06.012
Several approaches have been proposed to mitigate the CO poisoning of PEMFC anodes, i.e.: (i) advanced reformer design [8–16], (ii) use of more CO-tolerant anodes [17–22], (iii) oxidant bleeding into the fuel feed stream (air, oxygen or hydrogen peroxide) [23–26], (iv) use of a bilayer anode structures [27–30], (v) higher operating temperature [31–34], (vi) use of membranes for CO separation [35–37] and (vii) pulsing the cell voltage between specific values during cell operation, thus forcing the anode potential to operate for a short time at a potential positive enough to electrochemically oxidize CO [2,38]. The objective of this work is to investigate an alternative approach for enhancing the PEMFC performance under CO poisoning conditions by using the recently described triode fuel cell design and operation [39–41]. In addition to the anode and the cathode, a triode fuel cell contains a third, auxiliary, electrode and thus a second, auxiliary, circuit is formed which is run in the electrolytic mode (Fig. 1). This permits fuel cell operation under previously inaccessible anode–cathode potential differences [39–41] and Fig. 2. The triode fuel cell concept has been explored and tested already in solid oxide fuel cells (SOFC) with polarizable Pt electrodes [39], in a PEMFC with a Pt–Ru anode fed with 100 ppm CO in H2 [40] and in a PEMFC with a Pt anode fed with a methanol reformate mixture with very high (760 ppm) CO concentration at the Pt–Ru anode [40]. Both studies have shown that the power output of the fuel cell can be very significantly increased via application of auxiliary potential or current. Furthermore, this increase, Pfc , was found to be comparable with or higher than the power, Paux , sac-
F.M. Sapountzi et al. / Electrochimica Acta 56 (2011) 6966–6975
6967
Fig. 1. Schematic of the triode fuel cell concept, showing (a) the fuel cell and auxiliary circuits, (b) the cell geometry and the reactions taking place at the electrodes (c), the exact geometry of the triode PEM fuel cell and the electrical circuits and (d) the geometry of the membrane electrode assembly. The fuel cell cathode acts simultaneously as an electrode of the auxiliary circuit. P/G: potentiostat–galvanostat.
rificed in the auxiliary circuit. The latter leads to enhanced overall thermodynamic efficiency [39,40]. Recently, Cloutier and Wilkinson have pioneered the use of the triode design in PEM electrolyzers [42]. In the present study, in order to investigate the mechanism of CO mitigation and power output enhancement, we have used, in addition to the anode, the cathode and the auxiliary electrode, two reference electrodes as shown in Fig. 1d. This allows for direct measurement of the anode and cathode overpotentials both in normal and in triode PEM fuel cell operation.
2. Operating principle In a triode fuel cell the three electrodes are all in electrolytic contact and form two electrical circuits (Fig. 1a): (i) the fuel cell circuit, which comprises the anode, the cathode, and a variable resistance, Rext , for dissipating the electrical power, Pfc , produced and (ii) the auxiliary circuit, which comprises the auxiliary electrode, the cathode of the fuel cell and a potentiostat–galvanostat.
Fig. 2. Ifc –Vfc curves of the fuel cell circuit obtained with the conventional operation mode (Iaux = 0) and with the triode operation mode (Vaux = −1.9 V) by varying the external resistive load, Rext . Anode feed: 70 ppm CO, 14 kPa H2 in He balance.
When the applied auxiliary current, Iaux , is zero then the fuel cell operates in the conventional mode. It produces a power, denoted o , which is the product of the fuel cell potential, V o , and of the by Pfc fc o . Both V o and I o vary as the external resistive fuel cell current, Ifc fc fc load of the fuel cell circuit, Rext , is varied.
6968
F.M. Sapountzi et al. / Electrochimica Acta 56 (2011) 6966–6975
The triode fuel cell operation consists of imposing an electrolytic current Iaux < 0 or potential Vaux < 0 on the auxiliary circuit. This affects the potentials and thus the overpotentials of both the anode and the cathode. In the present case in this way protons migrate from the fuel cell cathode to the auxiliary electrode (Fig. 1a and b) and a fraction of them migrates to the anode and back to the cathode (Fig. 1a and b). Fig. 1c and d shows the exact geometry of the PEMFC triode unit used in the present investigation which also includes two reference electrodes as shown in Fig. 1d. These reference electrodes allow for direct measurement of the anodic and cathodic overpotential. Denoting by Ifar the net Faradaic fuel-consuming current, one notes that in view of Fig. 1 and Kirchhoff’s first law it is: Ifar = Ifc + Iaux
(1)
where Iaux < 0 and the net Faradaic current Ifar is that corresponding via Faraday’s law to the net consumption of fuel at the anode or the net consumption of O2 at the cathode. Two parameters have been introduced to quantify the results of triode operation [39]. The first is the power enhancement ratio, P , defined as [39]: P =
Pfc o Pfc
(2)
which quantifies the increase in power output in the fuel cell circuit. The second parameter is the power gain ratio, P , defined as [39]: P =
o Pfc − Pfc
Paux
(3)
This parameter is the ratio of the increase in power output of the fuel cell circuit, divided by the power sacrificed in the auxiliary circuit, Paux . The latter is the product of the auxiliary cell potential, Vaux , and of the auxiliary cell current, Iaux . When P > 1 then the thermodynamic efficiency of the triode unit is higher than that corresponding to conventional fuel cell operation. One reason for the ability of the triode fuel cell operation to affect overpotentials and to enhance fuel cell performance is that the auxiliary circuit can force the fuel cell anode or cathode to operate under controlled (via Iaux ) and previously inaccessible, for a PEM fuel cell, potentials in the standard hydrogen electrode or in the absolute potential scale [43,44]. A major reason, however in the present case of PEM fuel cells is the supply of protons to the anode via the auxiliary electrode which is found to cause significant reduction of the anodic overpotential. It is clear from Fig. 1 that all three electrodes operate in a corrosion-type mode with part of their surface used for oxidation and part of their surface used for reduction. Despite this rather intense mode of operation no performance deterioration was observed during operation for several weeks provided the negative of the auxiliary potential does not exceed 1.9 V. 3. Experimental A state-of-the-art PEM fuel cell [40] was used (NuVant, Fig. 1c) with state-of-the-art Pt and Pt–Ru electrodes deposited by NuVant on E-TEK carbon cloth and equipped with a third auxiliary electrode as shown in Fig. 1d. The loading of the Pt electrode was 0.5 mg Pt/cm2 (unsupported Pt black) and the loading of the Pt (30%) Ru (15%), supported on Vulcan XC-72 carbon, was 0.5 mg/cm2 , thus 1.8 mg/cm2 metal basis. The superficial surface area of the cathode electrode (Pt) was 5.29 cm2 , of the anode electrode (PtRu) was 3.85 cm2 and of the auxiliary electrode (PtRu) was 0.49 cm2 (Fig. 1). The cathode was a square (Fig. 1) and the auxiliary electrode was a smaller square
located in the center of the hollow square anode (Fig. 1). The membrane was Nafion 117 with nominal thickness 185 m. Two reference Pt electrodes were also introduced in order to measure anodic and cathodic overpotentials in conjunction with the current interruption technique which was used to separate the IR component. The location of the Pt reference electrodes is shown in Fig. 1d. The proper performance of both reference electrodes was confirmed by monitoring the potential difference between them during fuel cell operation and noting that it changes less than 10 mV when currents as high as 1 A flow between the anode and cathode both in normal and in triode operation. The membrane electrode assembly (MEA) was prepared by hot pressing in a model C Carver hot press at 120 ◦ C and under pressure of 1 metric ton for 3 min. Two cells were used in order to check reproducibility and both gave practically the same results. Preliminary investigation showed that the negative of the applied auxiliary potential, Vaux , i.e. −Vaux should not exceed 1.9 V as this leads to CO2 formation at the cathode via oxidation of the carbon support. The concentration of CO and CO2 in the anode and cathode feed and effluent was monitored used a Fuji Electric’s Infrared Analyzer ZRJ-4UNOR 6N IR CO/CO2 analyzer. The fuel cell circuit included a decade resistance Box (Time Electronics Ltd 1051) in order to vary the external load. The current and the potential were measured by two digital multimeters (Metex ME 21) (Fig. 1c). Constant potentials or currents in the auxiliary circuit were applied using an AMEL 553 Potentiostat–Galvanostat. The gas feeds to the cathode and anode compartments (the latter includes also the auxiliary electrode, Fig. 1) were continuously humidified using thermostated gas saturators. The cell temperature was typically set at the same temperature (25 ◦ C) with the gas saturators. The anode compartment gas feed was Messer Griesheim certified gas mixture of 150 ppm CO/30 kPa H2 in He balance, which could be further diluted with Air Liquide He. The cathode feed was humidified Air Liquide synthetic air. 4. Results and discussion 4.1. General overview As shown in Fig. 2 the use of the triode operation mode via application of an auxiliary potential, Vaux , of −1.9 V leads to very significant enhancement of the CO poisoned PEMFC performance. Thus in normal operation and external resistive loads, Rext , below 3 , the cell potential drops below 0.22 V. For higher Rext values, self-sustained oscillations are obtained between an active and a severely poisoned state. Triode operation extends this oscillatory region to Rext values as low as 0.5 and leads to a 300% increase in the time averaged power output of the cell. In order to better rationalize these results it is useful to first present the results obtained in the conventional fuel cell operation mode without and with CO added to the anode feed gas and then to proceed with the detailed presentation of the triode operation results. 4.2. Conventional operation mode, H2 feed The top panel in Fig. 3 shows the Ifc –Vfc curve obtained in the conventional fuel cell mode by using a 30 kPa H2 in He gas mixture. We use the subscript “fc” to denote that I and V refer to the fuel cell circuit. The Ifc –Vfc curve has been obtained by varying the external resistance, Rext , of the decade box from zero to 200 . As the resistive load is decreased (and thus the fuel cell current, Ifc , is increased), the operating voltage of the cell drops below the (o) (o) open-circuit voltage, Vfc , and the difference = Vfc − Vfc is the total cell overpotential [45]. The types of overpotential which
F.M. Sapountzi et al. / Electrochimica Acta 56 (2011) 6966–6975
6969
Fig. 3. Effect of the cell current or current density on the cell potential and on the ohmic, anodic and cathodic overpotential for conventional operation mode (Iaux = 0) under 30 kPa H2 /He.
limit the cell performance are (a) the anodic and cathodic activation overpotential caused by slow electrocatalysis at the anode and cathode respectively, (b) the ohmic overpotential associated with the ohmic resistance of the electrolyte and the electrodes and (c) the concentration overpotential associated with slow mass transfer of the reactants to the catalyst–electrode–gas three phase boundaries. The latter leads to a pronounced sharp decrease in cell potential as the limiting current is approached [45]. For the case of Fig. 3, it is clear that no limiting current is reached and thus the concentration overpotential is negligible. Consequently the cell overpotential is the sum of three components, i.e. of the ohmic overpotential (Ifc Rfc ), the anodic overpotential (a ) and the cathodic overpotential (c ), with [45]: = Ifc Rfc + a + c
(4)
Fig. 3 shows the contribution of each type of overpotential to the total cell losses. The anodic and cathodic potential losses were determined by means of the two Pt reference electrodes, which allowed both for monitoring the potentials Ea and Ec of the anode and cathode in the SHE scale and also for the separation of the anodic and cathodic overpotential from the ohmic losses via the current interruption technique. The procedure is shown in Fig. 4a
Fig. 4. Transient response of the Vfc (a) and Vc (b) signals, upon changing the external resistive load, Rext , from 0 to 200 during current interruption measurements. The arrows indicate the estimated values of the ohmic losses and of the net cell (a) and cathodic (b) overpotential.
and b for the determination of and Ifc Rfc (Fig. 4a) and c (Fig. 4b) respectively from the total change Vfc and Vc induced by current interruption. In this way, a total cell resistance, Rfc , of 0.37 was estimated by following the transient response of the Vfc signal (Fig. 4a). The same value of Rfc = 0.37 was also obtained from the slope of the linear region of the Ifc –Vfc curve of Fig. 3. The ohmic overpotential of the cell equals the product Ifc Rfc . As shown in Fig. 4b, the transient potential deviation Vc also includes part of the ohmic losses, and thus the Ifc Ra and Ifc Rc components should be extracted in order to obtain the net a and c values. From the thus calculated values of Ra = 0.17 and Rc = 0.18 (Fig. 4b), it is obvious that the two ohmic components contribute equally to the total cell ohmic resistance, Rfc , of 0.37 , which confirms the proper location of the two reference electrodes
6970
F.M. Sapountzi et al. / Electrochimica Acta 56 (2011) 6966–6975
Fig. 5. Comparison of the cell overpotential as obtained from the Ifc –Vfc curve and as determined via addition of the ohmic overpotential, Ifc Rfc and of the anodic and cathodic activation overpotential measurements by means of reference electrodes in conjunction with current interruption measurements. Anode feed: 30 kPa H2 .
which are roughly equipotential with the symmetry plane of the Nafion electrolyte membrane [45]. As already shown in Fig. 3, a is negligible during fuel cell operation in hydrogen, and the cell overpotential is mainly due to the cathodic overpotential, c , and to the ohmic losses. Fig. 5 provides a confirmation for the accuracy of the measurements. It shows that the sum of the above determined values of Ifc Ifc , (o) a and c indeed equals the cell overpotential, = Vfc − Vfc , with an accuracy better than 3%. 4.3. Conventional fuel cell operation, CO poisoning Fig. 6a shows Ifc –Vfc curves obtained in the conventional fuel cell mode in 30 kPa hydrogen (squares) and in a 14 kPa H2 stream containing 70 ppm CO, denoted hereafter 70 ppm CO/14 kPa H2 mixture (cycles). The curves have been obtained by varying the external resistance, Rext , of the decade box from zero to 200 . As expected, one observes that the presence of CO causes a significant decrease in power output and creates three distinct regions [6]. In the high potential-low current region (Rext > 20 ) and in the low potential-high current region (Rext > 3 ) stable steady-state behavior is observed, while in the intermediate region (3 ≤ Rext ≤ 20) self-sustained potential and current oscillations are obtained with a period of the order of 11–18 s. These oscillations are shown in some detail in Fig. 6b. Since the cell is run at constant Rext , it follows that these oscillations correspond to straight line segments in the Ifc –Vfc plane, as shown in Fig. 6a. The contribution of each of the overpotential components for the CO-poisoned cell was estimated, as discussed above for the case of H2 operation, via potential measurements using the two reference electrodes and the current interruption technique. Fig. 7a shows the dependence of the Vfc , Ifc Rfc , a and c on the cell current Ifc during normal fuel cell operation under 70 ppm CO/14 kPa H2 . One can observe that in presence of CO in the hydrogen supply, the overall losses in the cell performance are mainly caused by the high a values which result from the poisoning of the anodic active sites due to strong CO adsorption. Focusing on the oscillatory region at the intermediate current densities of the Ifc –Vfc curve, one can observe that it is accompanied by a pronounced oscillatory behavior of the anodic overpotential. Thus, it is obvious that the
Fig. 6. (a) Ifc –Vfc curves of the fuel cell circuit obtained with the conventional operation mode (Iaux = 0) by varying the external resistive load, Rext under 30 kPa H2 and under 70 ppm CO/14 kPa H2 gas mixtures. The arrows indicate the values of the current and potential oscillation limits for constant Rext . (b). Effect of Rext for Iaux = 0 on the fuel cell current Ifc and potential Vfc , showing the region of oscillations.
oscillations in the cell current and potential are due primarily to periodic cleaning and blocking of the anode surface from CO. The cathodic overpotential, c , also exhibits some smaller amplitude oscillation but its value remains, as expected, at the same levels (∼0.2 V) obtained in absence of CO at the anode. As shown in Fig. 7b, the data of Fig. 7a fit equation (4) with an accuracy better than 15% even in the oscillatory region. 4.4. Triode operation Fig. 8 shows the effect of constant electrolytic potential application in the auxiliary circuit, Vaux = −1.9 V, on the auxiliary circuit current (top panel) and on the fuel cell current, Ifc , and potential, Ufc (bottom two panels). The initial conditions (Vfc = 114 mV, Ifc = 220 mA) have been chosen in the low Rext region (Rext = 0.5 ). One observes that imposition of a constant electrolytic potential (−1.9 V) and current (−21 mA) in the auxiliary circuit causes the induction of self-sustained high amplitude oscillations in the fuel cell current (from 220 to 520 mA) and potential (from 120 to 294 mV). It is remarkable that the imposition of such a small auxiliary electrolytic current (Iaux = −21 mA) causes such a pronounced increase in current (up to 300 mA) and potential in the fuel cell
F.M. Sapountzi et al. / Electrochimica Acta 56 (2011) 6966–6975
6971
Fig. 8. Effect of a constant auxiliary potential application, Vaux = −1.9 V on the auxiliary current Iaux and on the cell potential Ufc and cell current Ifc , under fixed external resistive load (Rext = 0.5 ). Anode feed: 70 ppm CO/14 kPa H2 .
Fig. 7. (a) Effect of the cell current or current density on the cell potential and on the ohmic, anodic and cathodic overpotential for the conventional operation mode (Iaux = 0) under 70 ppm CO/14 kPa H2 . (b) Comparison of the cell overpotential as obtained from the Ifc –Vfc curve and as obtained by addition of the ohmic and activation overpotentials.
Fig. 9. Time variation of fuel cell power output, Pfc , sacrificed power in the auxiliary cell, Paux , power enhancement ratio, P , and power gain ratio, P and corresponding time-averaged values for triode operation during imposition of a constant auxiliary potential, Vaux = −1.9 V, under a fixed external resistive load (Rext = 0.5 ). Anode feed: 70 ppm CO/14 kPa H2 .
6972
F.M. Sapountzi et al. / Electrochimica Acta 56 (2011) 6966–6975
Fig. 11. Effect of the cell current on the cell potential and on the ohmic, anodic and cathodic overpotentials. Open symbols refer to the conventional operation mode (Iaux = 0), filled symbols refer to triode operation mode (Vaux = −1.9 V).
Fig. 10. Effect of fixed auxiliary potential application (Vaux = −1.9 V) on the limits of fuel cell current–potential oscillations (a) and on the current–power output oscillations (b). The dark and light dashed areas show the region of self-sustained oscillations for normal and triode operation respectively.
circuit. The induction of this oscillatory behavior is reversible, as oscillations disappear when the cell operation returns to the conventional mode (Iaux = 0, Fig. 8). Fig. 9 is based on Fig. 8. It presents the time variation of the fuel cell power output, Pfc , and compares it with the power sacrificed in the auxiliary cell, Paux . The figure also shows the time variation of o ), and of the power gain the power enhancement ratio, P (= Pfc /Pfc ratio, P (= Pfc /Paux ) and the corresponding time-averaged values of these two quantities. These are computed on the basis of Eqs. (2) and (3) from: ¯ P =
1 T
T
dt
(5) 4.5. Investigation of the triode operation mechanism
0
where T is the oscillation period and:
T
¯P=
o ) dt (P − Pfc 0 fc
T 0
Paux dt
One observes that the fuel cell power output, Pfc , is always higher o. in the triode operation mode than in the conventional mode, Pfc The power enhancement ratio, P , varies from 1 to 6, and the timeaveraged enhancement ¯ P is 3.1. The power gain ratio, P , varies from 0 to 3.4, showing that there are small time intervals where P < 1 and thus the power gain in the fuel cell circuit is smaller than the power consumed in the auxiliary circuit. However the beneficial effect of the triode mode (1 < P < 3.4) is more pronounced, thus the time-averaged power gain is approximately 1.32. Thus there is a net gain in power and thus on overall thermodynamic efficiency. Fig. 10a shows the Ifc –Vfc curves obtained during normal (Iaux = 0) and triode (Vaux = −1.9 V) operation, in all Rext regions, while Fig. 10b shows the significant advantages of the triode operation in terms of power output. As shown in these figures, the triode operation enhances very significantly the region of selfsustained oscillations and thus leads to very significant increase in the time-averaged fuel cell performance, particularly in the high current–high power output region.
(6)
In order to enhance our understanding of the mechanism of triode operation, the response was examined of the three overpotential components (Ifc Rfc , a and c ) upon application of constant electrolytic potential in the auxiliary circuit, Vaux = −1.9 V
F.M. Sapountzi et al. / Electrochimica Acta 56 (2011) 6966–6975
6973
Fig. 12. Schematic of a possible design of a triode PEMFC stack, showing the overall geometry and proton flow (a), the separator plate geometry (b), and the processes taking place in a single cell (c).
(Fig. 11). The filled and open symbols in Fig. 11 correspond to triode and conventional operation respectively. As already noted, one factor for the ability of the triode fuel cell operation to affect overpotentials and enhance fuel cell performance is that the auxiliary circuit forces the fuel cell anode or cathode to operate under controlled (via Iaux ) and previously inac-
cessible, for fuel cells, potentials in the standard hydrogen electrode scale. Thus as shown in Fig. 11 the fuel cells potential difference Ufc under open-circuit conditions increases from 950 mV for normal fuel cell operation (Iaux = 0) to 1420 mV for Vaux = −1.9 V. As a result, nominally negative values for the cell overpotential
6974
F.M. Sapountzi et al. / Electrochimica Acta 56 (2011) 6966–6975 (o)
( = Vfc − Vfc ) are obtained, in the very low current density region in relation to normal open-circuit operation. This is because, during the triode operation there is an extra supply of protons to the anode from the auxiliary electrode, creating a previously inaccessible proton electrochemical potential and hydrogen chemical potential at the anode, which is reflected by a small nominal negative anodic overpotential of −20 mV and a concomitant positive anode potential value of Ea = 20 mV in the SHE scale, also confirmed by the use of a reference electrode. This increased proton electrochemical potential and hydrogen chemical potential, however small, can cause an increased coverage of H at the anode and therefore it can decrease the coverage of CO. The CO desorption from the anode during triode operation at low cell current densities was also confirmed by continuous gas analysis of the fuel cell effluent. At larger current densities, where a stable poisoned steady state is observed when Iaux = 0, the triode operation leads to activation of an oscillatory behavior between a CO free and a CO poisoned anode surface state, as already mentioned. One observes that the application of electrolytic current or potential affects both the anodic and the cathodic overpotential (Fig. 11) and leads to enhanced performance. This enhanced performance is mainly related to the significant decrease of the anodic overpotential, a , observed during triode operation, as shown in Fig. 11, although there is a concomitant decrease in cathodic overpotential. Thus, the advantageous effect of triode operation is based primarily on the enhanced cleaning of the poisoned anodic active sites via the supply of protons to the anode, and the concomitant minimization of the anodic potential losses. This is corroborated by the fact that an increase from 24 to 30% in the CO conversion was observed during triode operation. It thus follows that the CO mitigation mechanism at large cell current densities is due to enhanced CO electrooxidation. The reaction which is enhanced is CO + H2 O → CO2 + 2H+ + 2e−
(7)
It would appear that supplying protons which are a product of (7) could cause a decrease in the rate. However, if the supply of protons decreases, via competitive adsorption, the sites available for CO adsorption then this can lead to significant enhancement in catalytic rate. This appears to be the case here. Therefore, to some extent, proton supply is similar to a temperature increase. They both lead to decreased CO coverage. 4.6. Stack considerations A crucial factor for the practical viability of triode PEMFCs will be the ability to design efficient stacks where each cell is a triode cell with a minimum of electrical connections. Such a design is shown in Fig. 12. The only difference from a common PEMFC stack is in the design of the separator (bipolar) plates which in the present case include an insulating section (Fig. 12). The fuel cell units are electrically connected in the usual mode, i.e. using conducting carbon separator plates. The resulting terminal stack voltage Vfc,s equals nVfc where Vfc is the potential of each unit cell, and n is the number of unit cells. The auxiliary cells are formed using the bipolar electrolysis concept [46] between the separator plates and the auxiliary electrodes (Fig. 12) and a total potential, Vaux,s , equal to nVaux is applied between the two end auxiliary sections of the separator plates. 5. Conclusions Triode operation of CO poisoned PEM fuel cells leads to very significant, threefold, increase in power output of the fuel cell circuit. This increase in power output is up to a factor of 1.32 larger than the power sacrificed in the auxiliary circuit and is mainly due to a
significant decrease in anodic overpotential caused by the supply of protons to the anode via the auxiliary electrode. This proton supply increases the electrochemical potential of protons and chemical potential of hydrogen at the anode, thus decreasing the coverage of CO and enhancing its electrooxidation rate. To some extent the triode operation of fuel cells is similar to electrochemical promotion of catalysis (EPOC or NEMCA effect [44,47–49]). In both cases a potential or current is applied between a working electrode and an auxiliary electrode in order to enhance performance. In the case of EPOC some power is sacrificed to enhance the rate of production of a chemical. In the case of a triode fuel cell some power is sacrificed in the auxiliary circuit in order to enhance the overall production of electrical power. In both cases (EPOC or triode operation) significant enhancement in chemicals or power production is possible only when the working electrode is at least partly polarizable and thus the ionic conduction in the electrolyte is not rate limiting, i.e. under high Wagner number conditions [39,40,50]. In the case of CO poisoned PEMFC units, the triode operation leads to CO poisoning mitigation due to enhanced CO electrooxidation via the supply of protons to the anode. This was confirmed by also measuring the electrode potentials individually by means of two reference electrodes. It appears that the triode design is worth a more detailed investigation with different anode compositions and CO poisoning levels. Application in suitably modified PEMFC stacks also appears feasible.
References [1] S. Ye, in: J. Zhang (Ed.), CO-tolerant Catalysts in PEM FC Electrocatalysts and Catalyst Layers, Fundamentals and Applications, Springer-Verlag, London, 2008. [2] A.H. Thomason, T.R. Lalk, A.J. Appleby, J. Power Sources 135 (2004) 204. [3] C.G. Farell, C.L. Gardner, M. Ternan, J. Power Sources 171 (2007) 282. [4] X. Cheng, Z. Shi, N. Glass, L. Zhang, J. Zhang, D. Song, Z.-S. Liu, H. Wang, J. Shen, J. Power Sources 165 (2007) 739. [5] J.J. Baschuk, X. Li, Int. J. Energy Res. 25 (2001) 695. [6] A. Katsaounis, S. Balomenou, D. Tsiplakides, S. Brosda, S. Neophytides, C.G. Vayenas, Appl. Catal. B: Environ. 56 (2005) 251. [7] D.C. Papageorgopoulos, F.A. de Bruijn, JECS 149 (2003) A140. [8] W.A. Adams, J. Blair, K.R. Bullock, C.L. Gardner, J. Power Sources 145 (2005) 55. [9] A. de Lucas-Consuegra, A. Princivalle, A. Caravaca, F. Dorado, C. Guizard, J.L. Valverde, P. Vernoux, Appl. Catal. B 94 (3–4) (2010) 281. [10] F.M. Sapountzi, M.N. Tsampas, C.G. Vayenas, Catal. Today 127 (2007) 295. [11] F.M. Sapountzi, M.N. Tsampas, C.G. Vayenas, Catal. Today 146 (2009) 319. [12] T. Dorio, K. Yasuda, Z. Siroma, N. Fujiwara, Y. Miyazaki, J. Electrochem. Soc. 150 (2003) A1225. [13] J.H. Wee, K.Y. Lee, J. Power Sources 157 (2005) 128. [14] S.T. Kuk, A. Wieckowski, J. Power Sources 141 (2005) 1. [15] J.S. Spendelow, C.Q. Lu, P.J.A. Kenis, A. Wieckowski, J. Electroanal. Chem. 568 (2004) 215. [16] W.F. Lin, T. Iwasita, W. Vielstich, J. Phys. Chem. B 103 (1999) 3250. [17] A. Pozio, L. Giorgi, E. Antolini, E. Passalacqua, Electrochim. Acta 46 (2000) 555. [18] G. Papakonstantinou, F. Paloukis, A. Siokou, S.G. Neophytides, J. Electrochem. Soc. 154 (2003) B989. [19] S. Mukerjee, J. McBreen, J. Electrochem. Soc. 146 (1999) 600. [20] M.T.M. Koper, Surf. Sci. 548 (2004) 1. ´ P.N. Ross, J. Phys. Chem. 99 (1995) 8945. [21] H.A. Gasteiger, N.M. Markovic, ´ Electrochem. Commun. 5 [22] M. Arenz, V. Stamenkovic, P.N. Ross, N.M. Markovic, (2003) 809. [23] S. Gottesfeld, J. Pafford, J. Electrochem. Soc. 135 (1988) 2651. [24] F. Sapountzi, M.N. Tsampas, C.G. Vayenas, Top. Catal. 44 (2007) 461. [25] R.J. Bellows, E. Marucchi-Soos, R.P. Reynolds, Electrochem. Solid State Lett. 1 (1998) 69. [26] G.Q. Lu, P. Waszczuk, A. Wieckowski, J. Electroanal. Chem. 532 (2002) 49. [27] T.R. Ralph, M.P. Hogarth, Platinum Met. Rev. 46 (3) (2002) 117. [28] H. Yu, Z. Hou, B. Yi, Z. Lin, J. Power Sources 105 (2002) 52. [29] C.-H. Wan, Q.-H. Zhuang, Electrochim. Acta 52 (2007) 4111. [30] A. Haug, R.E. White, J.W. Weidner, W. Huang, S. Shi, N. Rana, J. Electrochem. Soc. 149 (2002) A868. [31] N. Gourdoupi, K. Papadimitriou, S. Neophytides, J.K. Kallitsis, Fuel Cells 8 (3–4) (2008) 200. [32] J.K. Kallitsis, M. Geormezi, S.G. Neophytides, Polym. Int. 58 (11) (2009) 1226. [33] M. Geormezi, V. Deimede, N. Gourdoupi, N. Triantafyllopoulos, S. Neophytides, J.K. Kallitsis, Macromolecules 41 (23) (2008) 9051. [34] B. Smitha, S. Sridhar, A.A. Khan, J. Membr. Sci. 259 (1–2) (2005) 10. [35] B.C. Benicewicz, Advances in Materials for PEM Fuel Cell Systems, Polymer Division, American Chemical Society, Asilomar, CA, 2003, p. 23. [36] E. Kikuchi, Catal. Today 56 (2000) 97.
F.M. Sapountzi et al. / Electrochimica Acta 56 (2011) 6966–6975 [37] F.J. Varela-Gandia, A. Berengue-Murcia, D. Lozano-Castelló, D. Cazorla-Amorós, J. Membr. Sci. 351 (1–2) (2010) 123. [38] L.P.L. Carrette, K.A. Friedrich, M. Huber, U. Stimming, Phys. Chem. 3 (2001) 320. [39] S.P. Balomenou, C.G. Vayenas, J. Electrochem. Soc. 151 (11) (2004) A1874. [40] S.P. Balomenou, F. Sapountzi, D. Presvytes, M. Tsampas, C.G. Vayenas, Solid State Ionics 177 (2006) 2023. [41] S. Balomenou, C.G. Vayenas, WO 2005/008820 (2005). [42] C.R. Cloutier, D.P. Wilkinson, ECS Trans. 25 (2010) 47. [43] D. Tsiplakides, C.G. Vayenas, J. Electrochem. Soc. 148 (2001) E189. [44] C.G. Vayenas, S. Bebelis, C. Pliangos, S. Brosda, D. Tsiplakides, Electrochemical Activation of Catalysis: Promotion, Electrochemical Promotion and Metal–Support Interactions, Kluwer Academic/Plenum Publishers, New York, 2001.
6975
[45] J.O.M. Bockris, A.K.M. Reddy, M. Gamboa-Aldeco, Modern Electrochemistry, Kluwer Academic/Plenum Publishers, New York, 2000. [46] W. Hanni, A. Perret, Ch. Comninellis, Electrolytic cell with bipolar electrode including diamond, US Patent No. 6306270 (2001). [47] A. Jaccoud, C. Falgairette, G. Fóti, Ch. Comninellis, Electrochim. Acta 52 (2007) 7927. [48] X. Li, F. Gaillard, P. Vernoux, Top. Catal. 44 (2007) 391. [49] C. Jiménez-Borja, F. Dorado, A. de Lucas-Consuegra, J.M. Garcia-Vargas, J.L. Valverde, Catal. Today 146 (2009) 326. [50] G. Foti, I. Bolzonella, C. Comninellis, in: C.G. Vayenas, B.E. Conway, R.E. White (Eds.), Modern Aspects of Electrochemistry, No. 36, Kluwer Academic/Plenum Publishers, New York, 2003, p. 191.