WO2005008820A1

WO2005008820A1 - Triode fuel cell and battery and method for conducting exothermic chemical reactions

Info

Publication number: WO2005008820A1
Application number: PCT/GR2003/000032
Authority: WO
Inventors: Constantinos G. Vayenas; Stella Balomenou
Original assignee: University Of Patras
Priority date: 2003-07-23
Filing date: 2003-07-23
Publication date: 2005-01-27

Abstract

Disclosed herein are a device as well as a method for tuning and improving the performance of batteries and of fuel cells for the production of electrical energy in an efficient manner. The device consists of an auxiliary electrical circuit comprising a galvanostat or potentiostat or power supply together with an auxiliary electrochemical cell which utilizes the anode or cathode of the battery or fuel cell as one of its electrodes. The other electrode of the auxiliary electrochemical cell is also in contact with the electrolyte of the battery or fuel cell. The method consists in applying current or potential via the galvanostat or potentiostat or power supply of the auxiliary circuit to the auxiliary electrochemical cell and thus modifying and improving the performance (power output, efficiency) of the battery or fuel cell.

Description

TRIODE FUEL CELL AND BATTERY AND METHOD FOR CONDUCTING EXOTHERMIC CHEMICAL REACTIONS Disclosed herein are a device as well as a method for tuning and improving the performance of batteries and of fuel cells for the production of electrical energy in an efficient manner. The device consists of an auxiliary electrical circuit comprising a galvanostat or potentiostat or power supply together with an auxiliary electrochemical cell which utilizes the anode or cathode of the battery or fuel cell as one of its electrodes. The other electrode of the auxiliary electrochemical cell is also in contact with the electrolyte of the battery or fuel cell. The method consists in applying current or potential via the galvanostat or potentiostat or power supply of the auxiliary circuit to the auxiliary electrochemical cell and thus modifying and improving the performance (power output, efficiency) of the battery or fuel cell.

FUEL CELLS AND BATTERIES FOR CONDUCTING EXOTHERMIC CHEMICAL REACTIONS Chemical reactions which liberate large quantities of heat, i.e. exothermic reactions, can be carried out in batteries (batch operation mode) or fuel cells (continuous operation mode), where a large fraction of the Gibbs energy change of the reaction is converted into electricity rather than heat. This direct conversion of chemical energy into electrical energy is not subject to the Carnot limitations and therefore very high thermodynamic efficiencies can be achieved. Primary and secondary batteries are well known for many decades and are used in numerous commercial applications. Fuel cells are also known since the works of Grove and Schonbein (1837) and had so far limited application, primarily in the space program. In recent years, however, both solid oxide fuel cells (SOFC) and polymer electrolyte membrane fuel cells (PEM) are thoroughly investigated for commercial utilization. Such fuel cells can be used not only for electrical power generation (Hegedus et al, US Patent 4,463,065) but also for the cogeneration of electrical power and chemicals as demonstrated by Vayenas et al (Science 208, 593, 1980 and US Patent 4,272,336) for the case of SOFC fuel cells with Pt electrodes used to convert ammonia to nitric oxide with simultaneous generation of electrical power. One of the main problems for the successful operation of batteries and fuel cells, both PEM and SOFC, is the minimization of the overpotential losses at the anode and cathode which, together with the ohmic losses in the battery or fuel cell, constitute the total cell overpotential, η_TOτ (Figure 1). This is because in actual battery or fuel cell operation only a fraction ε = (Urev-Ti._TOT) U_re of the negative of the reaction Gibbs energy change, ΔG, is converted into electrical energy, the rest being converted into heat. Here ε is the battery or fuel cell thermodynamic efficiency and U_re_v is the reversible reaction potential, which is computable from the Nernst equation. The main approach so far for minimizing anodic and cathodic overpotential is the use of efficient electrocatalysts as anodes and cathodes. In the case of PEM fuel cells where both anodic and cathodic overpotential are quite significant, it has been shown by Ren and Gottesfeld (US Patent 6,458,479 B1) that cofeeding small amounts of oxygen with the fuel at the anode can decrease significantly the anodic overpotential caused by CO poisoning. We disclose here a totally new device and method for significantly decreasing the overpotential of anodes and cathodes of batteries and fuel cells. The main idea is to use the anode or cathode under consideration simultaneously as one of the two electrodes of an auxiliary electrochemical cell using the same or other electrolyte (Figure 2). This auxiliary circuit contains a galvanostat or potentiostat or power supply that can be used to apply current or potential to the auxiliary electrochemical cell during the operation of the battery or fuel cell. In this way we have found that both the power output of the fuel cell and its thermodynamic efficiency, ε, can be controlled and profoundly enhanced (at least by a factor of ten). Moreover we have found that the increase in the power produced by the fuel cell can be comparable with and under certain operating conditions, can exceed the electrical power supplied to the auxiliary electrochemical cell. Under these conditions (e.g. as disclosed in Example 5 below) the thermodynamic efficiency, ε_Sγs, of the overall fuel cell (or battery) plus auxiliary circuit system, exceeds the thermodynamic efficiency, εs_YS , without auxiliary current application, i.e. the thermodynamic efficiency of prior art operation. In general the operation of the auxiliary cell reduces the anodic or cathodic overpotential of the fuel cell and thus causes less heat to be rejected to the environment and more electrical power to be produced by the fuel cell. The overall system thermodynamic efficiency, ε_Sγs, introduced above is defined as the ratio of the total system electrical energy output (i.e. work produced by the fuel cell minus work consumed in the auxiliary circuit) divided by the negative of the Gibbs energy change of the process. Fruhwald (US Patent 4,642,172) had proposed the use of a bias circuit in conjunction with an electrochemical cell serving as a gas detector, including an operational amplifier tied to the anode and reference terminals and a zener diode arrangement for maintaining a predetermined voltage drop between the outputs of the operational amplifier. That invention, however, is different from the present one in that (a) it optimizes the performance of a gas sensor, not of a power producing fuel cell, (b) it includes an operational amplifier and a zener diode which is not the case with the present invention.

DESCRIPTION OF DRAWINGS Figure 1 shows the general type of fuel cell (or battery) potential dependence on fuel cell (or battery) current density and shows the definition of the reversible (or thermodynamic) cell potential, U_rev, of the total overpotential, η_TOτ, and of the fuel cell efficiency ε°. Superscripts here and throughout the text refer to prior art battery or fuel cell operation, i.e. without an auxiliary circuit. Figure 2 shows schematically the principle and mode of operation of the present invention for enhancing the performance of the battery (or fuel cell) anode. Figure 3 shows schematically the principle and mode of operation of the present invention for enhancing the performance of the battery or fuel cell cathode. Figure 4 shows an alternative design for the cell depicted in Figure 2 in which the counter electrode is on the same side of the electrolyte as the anode. Figure 5 shows an alternative design for the cell depicted in Figure 3 in which the counter electrode is on the same side of the electrolyte as the cathode. Figure 6 shows the experimental setup used to obtain the results of Examples 1 to 5. The fuel was continuously fed to the anode and ambient air was supplied to the cathode and counter electrode. Figure 7 refers to example 1 and shows the dependence of fuel cell potential (top), power output (middle) and parameter (ΔP_fc/P_aux)_Re_x defined below (bottom) on fuel cell current density. Figure 8 refers to the curve l=-30 μA of Figure 8 and clarifies the meaning of the curves U_fc (l_fc), U_fC (I_fC), Ua_UX(lfc) and U_an,soE(lfar) discussed in the text. The dotted area corresponds to power consumed in the auxiliary circuit while the shaded areas correspond to power produced by the fuel cell as described in the text. Figure 9 refers to Example 2 and shows the dependence of fuel cell potential, for l_aux=0,

μA and parameter (ΔP_fo/Paux on fuel cell current density. Figure 10 refers to the data of Figure 9 and shows the corresponding values of overall system (fuel cell plus auxiliary circuit) efficiency, εsγs_> in comparison with ε°_YS , i.e. overall system efficiency with zero auxiliary current. Figure 11 provides the same information as Figure 9, but now for Example 3. Figure 12 refers to Example 4 and shows the dependence of fuel cell potential and efficiency ε (top), power output (middle) and parameter (ΔP_fc/Pa_uxj_Re_x on fuel cell current density. Figure 13 refers to Example 5 and shows the dependence of fuel cell potential and efficiency ε (top), power output (middle) and parameter (ΔP_fς/Pa_uxj_Re_x on fuel cell current density. Figure 14 refers to Example 5 and shows the dependence of overall system efficiency, ε_Sγs. and prior art (i.e. without auxiliary circuit) overall system efficiency

(εs_YS ) for the data of Figure 13 referring to _uχ—50 μA. It can be seen that for operating points A, B and C, ε_Sγs is higher thanεs_YS . Note that the horizontal displacement at each point corresponds to the (negative) auxiliary current l_aux

DESCRIPTION OF TRIODE FUEL CELL OR BATTERY AND ITS OPERATION

As shown in Figure 2 the novel fuel cell (or battery) system contains three electrodes: a: The fuel cell anode, which also serves simultaneously as the working electrode (W) of the auxiliary electrochemical cell, b: The fuel cell cathode c: The counter electrode of the auxiliary electrochemical cell.

A reference electrode can also be in contact with the electrolyte, similarly with the above three electrodes. In this design (Figure 2) the auxiliary circuit is used to control and tune the overpotential of the anode. An alternative but equivalent design for tuning the overpotential of the cathode is shown in Figure 3. Two alternative designs for tuning the overpotential of the anode and cathode are shown in Figures 4 and 5. Combinations of the above designs for simultaneous tuning of anode and cathode overpotential can also be realized. The said three electrodes are all in electrolytic contact (Figure 2) and form two electrical circuits: A. The fuel cell circuit which, in addition to the anode and cathode, comprises a variable resistance, RE_X, or other device for dissipating the electrical power, P_fc, produced by the fuel cell. B. The auxiliary circuit which, in addition to the working electrode (same as the anode or cathode of the fuel cell) and the counter electrode, comprises a galvanostat or potentiostat or power supply, as well as an ammeter for measuring the current l_aux of the auxiliary circuit. When

i.e. when the auxiliary circuit is open, then the fuel cell (or battery) operates in the normal mode of prior art, i.e., without an auxiliary circuit. It produces a power, denoted hereafter by P°, which is the product of the fuel cell potential, U_fC, and of the fuel cell current, I_fC . Both U_fC and I_fC vary as the external resistive load of the fuel cell circuit, R_ex, is varied, e.g. as in Figure 1. The triode fuel cell operation consists of imposing via the galvanostat or potentiostat or power supply a current _ux≠O in the auxiliary system. We have termed the new fuel cell device described here a triode fuel cell due to the fact that its central element are three electrodes (anode=working, cathode and counter electrode) and thus its resemblance to the vacuum tube triodes where the grid potential is used to control the flux of electrons to the cathode. It is evident that the main difference of the triode fuel cell disclosed here from the vacuum tube triode is the presence of the electrolyte, liquid or solid, and the occurance of a chemical reaction which produces electric energy and heat. Denoting by lt_ar the net Faradaic fuel consuming current, it is noted that in the designs of Figures 4 and 5 it is l_far=l_fc, while in the designs of Figures 2 and 3 it is from Kirchhoff s first Law:

The key reason of the success of the new device to affect overpotentials and enhance fuel cell performance is that the auxiliary circuit forces the anode (and/or cathode) to operate under different and controlled (via l_aUχ) potentials in the standard oxygen electrode (SOE) or standard hydrogen electrode (SHE) scale (Tsiplakides and

Vayenas, 2001).

EXAMPLE 1 The solid oxide fuel cell (SOFC) depicted schematically in Figure 6 was used for H₂ oxidation at the anode with the cathode exposed to ambient air for the supply of O₂. The experiments were carried out at temperatures 400° to 700°C but in this example we present only results obtained at T=400°C. The results at higher operating temperatures are presented and discussed in Example 2. The solid electrolyte was 8 mol% Y₂O₃- stabilized-ZrO₂ (YSZ) in the form of a tube closed at one end. The tube OD was 19 mm and its ID 16 mm, so that the wall, but also bottom, thickness was 1.5 mm. The anode was a porous Pt film, of superficial area 2 cm² and thickness 3 μm deposited on the inside of the YSZ tube bottom, using Engelhard Pt paste followed by calcination at 850°C. The anode also served as the working electrode of the auxiliary circuit. The cathode (C) was a similarly deposited Pt film of ring shape (Fig. 6) deposited at the periphery of the outside surface of the bottom wall of the YSZ tube. It had a surface area of 1 cm². The counter electrode was a circular dot Pt electrode of area 0.33 cm² deposited around the center of the same outer surface. Figure 7 was obtained by varying the value of the external resistance, R_ex, of the fuel cell circuit at various imposed currents, l_aUχ, in the auxiliary circuit. The case corresponds to normal, classical, fuel cell operation. The top part of the Figure (7a) shows the dependency of U_fc on I_fc, the middle part (7b) shows the corresponding dependence of the power output, P_fc, of the fuel cell while the bottom part (Fig. 7c) shows the corresponding dependency on l_fc of the parameter ΔP_fc/P_aux where ΔP_fc is the change (gain) in the power output of the fuel cell due to the operation of the auxiliary circuit and P_aux is the power consumed in the auxiliary circuit. These parameters are computed from the raw U_fc, l_fc, U_aux and l_aux data from the equations: ΔP_fc = P_fC -P_fc = U_fcI_fc -U?_c -I?_c (2)

As shown in Figure 7b application of l_aUχ=-50μA to the auxiliary circuit causes an eightfold enhancement in the maximum power output of the fuel cell. However, as shown in Figure 7c, in this particular set of experiments the ratio ΔP_fc/P_au has a maximum value of 0.28. In determining the ratio ΔP_fc P_aux in Figure 7c and subsequent figures, we have kept constant the value of the external resistive load R_ex. Figure 8 shows in more detail the data of Figure 7 which correspond to l_aux=0 and l_aux ⁼-30 μA. The open-circle curve shows the fuel cell performance for l_au_x=0, i.e. is shows U_fC (I_fC). The dotted triangle curves show the corresponding U_fc(l_fc) curves obtained for

μA. Note that the distance DG on the I axis corresponds to l_far(=l_fc- _ux)- The open-triangle cells shows the corresponding dependence of l_far on the anode potential, U_an,so_E S. the standard oxygen electrode. As an example Figure 8 also shows in detail the change of fuel cell operating point from point B'

to point B (l_auχ=-30 μA) and the point H corresponding to B' on the U_{an S}o_E Ctar) curve. In this experiment the fuel cell power output increases by a factor of six (area (ABCD) vs area (A'B'C'D)) but the power consumed in the auxiliary system (area KFEB) is larger by a factor of ten, so that as already shown in Figure 7c (ΔP_fc/PauxjRex is near 0.1. One therefore can conclude from Figure 8, that, although the thermodynamic efficiency of the fuel cell, ε, is enhanced relative to the ε° value corresponding to normal operation

the overall thermodynamic efficiency of the system, ε_Sγs⁼W_Toτ/(-ΔG), is decreased.

EXAMPLE 2 The same solid oxide fuel cell (SOFC) and experimental apparatus of Example 1 was used at temperatures 400° to 700°C. Figure 9 shows the fuel cell performance for l_aux=0 (Fig. 9a), l_aUχ=-50 μA (Fig. 9b) and also the measured value of the parameter ΔP_fo/P_aux (Fig. 9c). As shown in Figure 9c this parameter takes values up to 0.5 for T=700°C. The corresponding values of total thermodynamic system efficiency are shown in Figure 10 as a function of the Faradaic current It_ar- It can be seen that at700°C the new device and mode of operation leads to ε_Sγs values approaching the initial ε value denoted by ε _YS .

EXAMPLE 3 The same SOFC and experimental apparatus of Examples 1 and 2 is used but now the fuel is ethane (C₂H₆) instead of H₂. As shown in Figure 11 the behaviour is qualitatively similar as in the case of H₂ fuel. The maximum fuel cell power output P_fc is enhanced (by a factor of 6) but the ratio ΔP_f0 P_aux is up to 0.4.

EXAMPLE 4 The same type of SOFC of examples 1 , 2 and 3 is used for the electrochemical combustion of CH₄ at 710°C. Here the anode and cathode are more active and fuel cell produces current densities up to 19 mA/cm² and power densities up to 4 mW/cm² (Figure 12). Application of l_aux=-8 mA causes a 50% increase in fuel cell efficiency at current densities below 5 mA, a 70% increase in power output (7 mW/cm²) while the (ΔP_fc/P_aUχ)_Rex value is up to 0.2.

EXAMPLE 5 The same type of SOFC of examples 1 ,2,3 and 4 is used again for the combustion of CH₄ at 700°C. The system has been sintered at 930°C (vs 850°C in the previous examples) so that the current density is lower than in Example 4, i.e. up to 1.6 mA/cm² (Fig. 13). Application of negative currents leads again to significant improvement of the fuel cell performance (Fig. 13 top) and nearly three-fold enhancement in power output (Fig. 13, middle). For l_aux=-50 μA and =-150 μA, the parameter (ΔP_fc/P_aux)_Rex takes values exceeding unity (Figure 13 bottom), i.e. 1.28 and 1.01 respectively. Under these conditions the enhancement in the power output of the fuel cell exceeds the power consumed by the auxiliary power supply. As a result of this, under these operating conditions the overall system thermodynamic efficiency ε_Sγs exceeds the initial system efficiency, ε°_YS , for the corresponding operating points A, B and C as shown in Figure 14. Further optimization is clearly possible. References Cited:

US Patent 4,272,336 6/1981 Vayenas et al

US Patent 4,463,065 7/1984 Hegedus et al

US Patent 6,458,479B1 10/2002 Ren et al

US Patent 4,642,172 2/1987 Fruhwald

Other publications:

"Cogeneration of Electric Energy and Nitric Oxide" Science, Vol. 208, May 9, 1980 pp. 593-594, C.G. Vayenas and R.D. Farr "Electrode work function and absolute potential scale in solid state electrochemistry", J. Electrochemical Soc, Vol. 148(5), 2001 pp. E189-E202, D. Tsiplakides and C.G. Vayenas

Claims

1. A fuel cell equipped with an auxiliary electrical circuit comprising a galvanostat, potentiostat or power supply and an auxiliary electrochemical cell where one of its two electrodes is the anode or cathode of the fuel cell.

2. A method for improving the performance of fuel cells where electrical power is supplied via the galvanostat, potentiostat or power supply to the auxiliary electrical circuit of the fuel cell as in claim 1.

3. A battery equipped with an auxiliary electrical circuit comprising a galvanostat, potentiostat or power supply and an auxiliary electrochemical cell where one of its two electrodes is the anode or cathode of the battery.

4. A method for improving the performance of batteries where electrical power is supplied via the galvanostat, potentiostat or power supply to the auxiliary electrical circuit of the battery as in claim 3.

5. A fuel cell as in claim 1 where the electrolyte is a O^2" or proton conductor.

6. A fuel cell as in claim 1 where two auxiliary electrical circuits are used, one comprising the anode and the other the cathode of the fuel cell.

7. A battery as in claim 3 where two auxiliary electrical circuits are used, one comprising the anode and the other the cathode of the battery.