Continuous Gas Concentration Cells as Thermally Regenerative

2 Continuous Gas Concentration Cells as Thermally Regenerative, Galvanic Cells JOHN C. ANGUS

Downloaded by CORNELL UNIV on May 17, 2017 | http://pubs.acs.org Publication Date: January 1, 1967 | doi: 10.1021/ba-1967-0064.ch002

Case Institute of Technology, Cleveland, Ohio

The use of a gas concentration cell as a means of converting thermal to electrical energy is described. This cell, which is in the early stages of development, shows promise because of the absence of any chemical regeneration and separation steps. Initial results on cells using I vapor and aPbI electrolyte are given, as well as estimated characteristics of more advanced cells using alkali metal vapors and alkali metal halide electrolytes. 2

2

'T'hermal energy may be converted into electrical energy by means of a continuous gas concentration cell (hereafter denoted CGCC). This device has received very little attention but offers some unique advantages compared with other schemes. Principle of Operation The electrochemical nature of the C G C C is identical to that of the well-known gas concentration cell. In the CGCC, however, the pressure difference across the isothermal electrolyte is maintained by using the change in vapor pressure with temperature of the electrochemically active, gas phase species (working fluid). As in all gas concentration cells, work is done by electrochemical expansion of the working fluid through the electrolyte. After the expansion, the working fluid is condensed in a cold reservoir and, if desired, can be recycled to the high temperature-high pressure side of the cell by means of a pump. The system is thermodynamically very similar to a conventional power cycle in which a working fluid is vaporized at a high temperature and pressure, expanded through a turbine, condensed at a low temperature and pressure, and then pumped back to the high pressure side. In the 11

Crouthamel and Recht; Regenerative EMF Cells Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

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REGENEBATTVB B M P CELLS

CGCC, expansion is through the electrolyte rather than through the turbine.


Nomenclature The abbreviations used in this paper are: J? « cell potential; F m Faraday's constant; / « fugacity of working fluid at high pressure elec trode;/! » fugacity of workingfluidat low pressure electrode, taken as a constant; AH — enthalpy of gaseous workingfluidminus enthalpy of con densed working fluid; η — electron number of electrode reaction; Ρ = vapor pressure of working fluid at high pressure electrode; P i « vapor pressure of working fluid at low pressure electrode, taken as a constant; R = universal gas constant; Τ « vaporization temperature; and T condensation temperature, taken aô a constant. x

Operating Characteristics The theoretical operating voltage is given by the Nernst equation:

The fugacities of the working fluid may be approximated by the partial pressures which may be estimated from the Clausius-Clapeyron equation: d(lnP) AH dT ™ RT*

m w

Differentiating Equation 1 and using Equations 1 and 2, one has: dΕ Ε dΤ " Τ

AH nFT

/«χ w

If AH is constant with T, Equation 2 may be integrated:

>•(£)--¥[*-*] Combining Equations 1, 3, and 4, we have the simplified expression: dE dT--^F\Tj

AH(l\ ( 5 )

Using Equation 2, this becomes: dE _ RTl d(lnP)" nFTx

m w

Numerous, small-scale laboratory versions of the CGCC have been constructed and operated (see Figure 1). Two different, working fluid/elec trolyte combinations were used—one with It as the workingfluidand Pbli as the electrolyte, and another with Hg as the working fluid and an


2.

ANGUS

Concentration Cells

13

ELECTROOESI

THS SECTION A TUBE FURNACE


PYREX-

\3 -AUXILIARY HEATER FOR ΗΒΗ PRESSURE RESERVOIR

LOW PRESSURERESERVOIR

1

>e

Figure Î. Early Pyrex cell HgjClt/HgCU electrolyte. Both N i and Pt wire electrodes were used with the I2 cell; W and Pt wire electrodes were used with the Hg cell. Typical results for the I2/PM2 cell are shown in Table I. The voltage agrees with Equation 1 ; however, the agreement was not always this good. For example, when the Pbl* electrolyte was held just above its melting point (402°C), the voltage was approximately double the predicted value. This may be caused by a change in the electron number for the electrode reaction. The predicted voltages were calculated with π = 2 in accordance with the simple electrode reaction I + 2e — 2I~. Regeneration of the cells was accomplished by cooling the original hot end and vaporizing the I* from the original cold end. The cell voltage reversed as expected. It was not possible to reduce the internal resistance to acceptable t

Table I. Voltage Characteristics of I /Pbl CGCC w i t h N i Electrodes 2

Vaporization temperature, T , °C. Condensation temperature, T\ °C. Electrolyte temperature, °C. Voltage (low pressure side is negative) Predicted voltage from Equation 1 2

t

177 24 538 0.22 0.207

2

193 24 538 0.28 0.22


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REGENERATIVE EMP CELLS

levels using the small glass cells. For this reason, several more advanced types of cells have been constructed. The most successful configuration is shown in Figure 2. The contacts to the electrodes are made secure by screwing the inner, graphite cylinder in until the entire electrode assembly is in compression. The porous electrodes and thin electrolyte cavity are completely filled with electrolyte in another apparatus. I was used as the working fluid and P b l as the electrolyte. The electrodes were porous N i with a relative density of 40% and a mean pore diameter of 22 μ. In the initial running of this cell, 6.2 ma. were obtained through an external load of 24.5 ohms. The open-circuit potential of the cell was 0.17 volt, which corresponds to an internal cell resistance of 2.9 ohms. The cell resistance increased markedly with continued running of the cell. Inspection of the electrode-electrolyte assembly indicated the pressure difference had forced most of the P b l through the porous N i electrodes. Corrosion of the N i electrodes by the I may have been responsible for part of the internal resistance increase. In these runs, the full vapor pressure of I was not used; instead, only enough I was placed in the cell to produce a total pressure of one atmosphere. Work is under way on similar cells, with N a as the working fluid and NaCl as the electrolyte. 2


2

2

2

2

2

THREADED BORON NITRIOE SLEEVE

QUARTZ INSULATOR

BORON NITRIOE SEPARATOR RING POROUS METAL ELECTRODES ELECTROLYTE CAVITY

CONDENSED WORKING FLUIO

Figure 2. Latest cell configuration


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ANGUS

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Concentration Cells


Comparison with Existing Methods The principal advantage of the CGCC is the absence of any chemical regeneration step. Because the working fluid undergoes no chemical change, no regeneration and separation steps are necessary. The ultimate values of power/mass and power/volume for the CGCC would therefore appear to be quite favorable. The CGCC appears similar to the thermocell—which is simply a thermocouple in which one leg of the couple is an electrolyte (1, 4, £)· In the CGCC, however, the temperature gradient does not appear across the electrolyte, thus minimizing one major source of irreversibility (heat leak). There will, however, still be an unavoidable heat loss down the current-carrying leads which go from the electrolyte to the load. This irreversibility cannot be eliminated because the thermal conductivity of these current conductors cannot be reduced below the value given by the Wiedemann-Franz Law. This type of irreversibility (or its equivalent) is inherent in all regenerative galvanic cell schemes. Because there are no irreversibilities within the CGCC when no cur rent is drawn, the open-circuit potential can be computed using the methods of classical equilibrium thermodynamics. These methods cannot be rigor ously applied to the thermocell because even under open-circuit conditions there is an irreversible heat flux through the cell (1). Nevertheless, the potentials of an I / A g l / I thermocell have been found to agree with the results of approximate equilibrium thermodynamic calculations (5). The potentials are also close to those of the I / P b I / l 2 CGCC. The principal difficulty associated with the CGCC is the necessity of maintaining the integrity of a liquid electrolyte subjected to a pressure gradient. Another difficulty for some systems is the low, open-circuit potential. In Table II, the computed operating characteristics for several working fluid/electrolyte combinations are summarized. The N a / N a C l and K / K C 1 systems have open-circuit potentials of 0.88 and 0.71 volt, respectively, which are of the same order as conventional regenerative galvanic cells. The pressure difference for these systems is not excessive and is of the order that may be contained with surface tension effects. 2

2

2

2

Table II. Characteristics of the CGCC with Several Working Fluid/Electrolyte Systems Working Fluid/Electrolyte h/Fbh Hg/HgîCh Na/NaCl K/KC1

a

ρ Ohm-Cm. 2.1 2.0 .273 .44

Vaporization Condensation Open-Circuit Temp. Temp. Voltage (T, °C.) (Γι, °C.) 410 305 827 827

119 100 327 327

0.3 .22 .88 .71

b

Ρ mm.

Pi mm.

31,900 246.8 433 1408

93.4 .27 .0394 .635

b

β

Values of resistivity are taken from Smithells (S) and are given for the vaporization temperature, Γ. Vapor pressures and fugacities for calculation of the open-circuit voltage were taken from Hultgren {#). 6


REGENERATIVE EMF CELLS

16


The pressures at the low pressure electrode are not so small that intolerable gas phase concentration polarization will occur. Values of current density and power density referred to unit elec trolyte cross sectional area have been calculated for matched load condi tions. They are .394 amp./sq. cm. and .13 watt/sq. cm. for the Na/NaCl cell, and ,202 amp./sq. cm. and .0537 watt/sq. cm. for the K/KC1 system. For these calculations, a voltage efficiency of 0.75 was assumed. The elec trolyte was taken to be 1 cm. thick and to have a resistance three times the value calculated from the resistivity of the pure electrolyte. These predicted characteristics compare favorably with other, more complex, regenerative, galvanic cell systems.

Summary A new, thermally regenerative, galvanic cell based on the principle of continuous gas concentration cell is described. Results obtained with cells using I* as the workingfluidand Pbli as electrolyte are given. The estimated operating characteristics of cells using Na/NaCl and K/KC1 appear favorable in comparison with existing devices.

Acknowledgments The author is indebted to the Valley Co. and the National Science Foundation for supporting this work and to Ε. E. Hucke for many stimu lating discussions. C. C. Liu, M . Rothstein, R. Campbell, R. GUckman, J. Lopez, E. Home, W. Munroe, and J. Meyer all contributed to various aspects of the experimental program.

Literature Cited (1) Agar, J. N., "Thermogalvanic Cells," Chapter 2, Vol. 3, "Advances in Electro chemistry and Electrochemical Engineering," P. Delahay, ed., Interscience, New York, 1963. (2) Hultgren, R., Orr, R. L., Anderson, P. D., Kelley, Κ. K., "Selected Values of Thermodynamic Properties of Metals and Alloys," John Wiley, New York, 1963. (3) Smithells, C. J., "Metals Reference Book, Volume II," 3rd ed., Butterworths, Washington, 1962. (4) Weininger, J. L., "Thermocell," U. S. Patent 2,890,259 (June 9, 1957). (5) Weininger, J. L., Electrochem. Soc. 111(7), 769 (1964). RECEIVED November 10, 1965.


Continuous Gas Concentration Cells as Thermally Regenerative

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