Use of carbon dioxide in energy storageRichard Williams, Richard S. Crandall, and Allen Bloom Citation: Applied Physics Letters 33, 381 (1978); doi: 10.1063/1.90403 View online: http://dx.doi.org/10.1063/1.90403 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/33/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Carbon capture and storage: More energy or less carbon? J. Renewable Sustainable Energy 2, 031006 (2010); 10.1063/1.3446897 Energy storage of the atomic carbon laser in an electrical discharge J. Appl. Phys. 57, 2637 (1985); 10.1063/1.335504 Energy storage by electrochemical reduction of carbon dioxide Phys. Teach. 17, 246 (1979); 10.1119/1.2340200 The Free Energy of Steam and of Carbon Dioxide J. Chem. Phys. 1, 308 (1933); 10.1063/1.1749293 Carbon dioxide to methanol using geothermal power Phys. Today

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Use of carbon dioxide in energy storage Richard Williams, Richard S. Crandall, and Allen Bloom

RCA Laboratories, Princeton, New Jersey 08540 (Received 11 May 1978; accepted for publication 23 June 1978)

We have investigated an energy-storage cycle in which CO2 is electrochemically reduced to fonnic acid, HCOOH. The product can be used in several ways. By means of a catalyst, it can be converted to hydrgen for use as a fuel or raw material. We have obtained data on the efficiency of the process and analyzed the energetics.

PACS numbers: 84.60.Dn. 82.45.+z, 89.30.+f

Development of new energy sources such as photo­voltaic cells and wind generators has emphasized the need for new methods of energy storage. This is be­cause the power generation is intermittent. Fossil fuels contain the solar energy stored in earlier geologic times and most of our energy practices are built around them. Recently, much thought has been given to the possibility of replacing fossil fuels by hydrogen. 1,2 A likely source for hydrogen will be the electrolysis of water. Some method of hydrogen storage will be needed that can combine some of the convenience and high energy denSity of our current practices for the storage of liquid and solid fuels. Earlier ideas on hydrogen storage are reviewed in Refs, 1 and 2.

In this letter we describe a novel method for hydrogen storage that may have more general applications for energy storage. It involves electrochemical reduction of CG.! in water to form organic compounds. Several re­duction products are possible. 3,4 They may be either solids or liquids. The simplest is formic aCid, 5-7

HCOOH, which can be made by electrochemical reduc­tion of CO:!, then easily stored as a stable product, followed either by catalytic conversion to hydrogen or by direct use as a fuel. The following reaction sequence illustrates the idea:

(1)

where AJfl = + 64. 4 kcal/mol and AFo = + 65. 9 kcal/ mol. The electrochemical reaction consists of separate cathode and anode reactions, as follows: cathode, CO:! + 2ll' + 2e - HCOOH; anode, H20- to:! + 2ll' + 2e,

HCOOH ~ H2 (g) + CG.! (g), (2)

where AJfl=+3.9 kcal/mol and AFo=- 9,2 kcal/mol;

H20(l) - H2(g) + to:!(g), (3)

where AJfl = 68. 3 kcal/mol and AFo = 56, 7 kcal/mol. The standard enthalpy and free energy changes, AIfl and AFo, are given for each reaction. 8 Reaction (1) is the overall reduction of Co:! gas to formiC acid in aqueous solution. The formic acid could be used as a fuel, reversing reaction (1), either by ordinary com­bustion or in a fuel cell. 9 The maximum yield in com­bustion is given by AIfl; the maximum yield in a fuel cell by AFo, In practice, the energy yield would be reduced by overvoltage and Side reactions in the elec­trochemical reduction and by resistance losses.

Reaction (2) is the catalytic decomposition of formic acid to give hydrogen and CG.!. There has been exten­sive study of the details of the reaction mechanism in

the vapor phase at elevated temperatures, 10 Since the free-energy change is negative, the reaction should go spontaneously at room temperature; but, in the absence of a catalyst, formic acid is stable for years. We have found that palladium, supported on fine carbon parti­cles,11 catalyzes a vigorous decomposition of an aqueous solution of formiC acid at room temperature. Analysis of the product gases by mass spectrometry shows that the decomposition is taking place by reaction (2).

By combining reactions (1) and (2) in series we obtain reaction (3). This is equivalent to the production of hydrogen by electrolysis of water. However, by stopping between reactions (1) and (2), we can store the hydrogen indefinitely in a chemically bound form, finally re­generating it with the catalyst as needed. The enthalpy and free energy for reaction (3) are, of course, equal to the sums of the corresponding quantities for reactions (1) and (2). For ordinary combustion of hydrogen the enthalpy is the important quantity and the smaller value in step (1) is augmented by absorption of heat from the ambient in step (2). For fuel cells, free energy is the important quantity. If the hydrogen is to be used in a fuel cell the loss of free energy in step (2) is probably not recoverable. However, this loss could be aVOided if the formic aCid, itself, were used in a fuel cell after step (1).

The energy storage denSity achieved by this method is high. Experiments have showns that concentrations of formic acid as high as 4 mol/liter can be generated by electrolysis. Since each molecule of formic acid contains one recoverable molecule of hydrogen, this is equivalent to storage of gas at a pressure of 100 atm or 1500 psi, a pressure typical for the tank storage of gases. Liquid hydrogen has a concentration of 35 mol/liter, compared with 21 mol/liter for pure concen­trated formic acid. The smaller concentration of the acid could be more than compensated for by the advant­ages of needing no cryogenic eqUipment, presenting no unusual safety hazardS, and having no losses in the storage cycle such as those associated with the liquefac­tion of hydrogen.

We have done experiments to demonstrate that re­action (2) takes place under the deSired conditions, to measure its rate, and to obtain quantitative information on reaction (1), using a new cathode material. Figure 1 shows a schematic diagram of the essential com­ponents of our electrolysis cell, together with current­voltage and yield data. The cathode is in an aqueous

381 Appl. Phys. Lett. 33(5), 1 September 1978 0003-6951/78/3305-0381 $00.50 © 1978 American Institute of Physics 381

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0.6 0.7 0.8 0.9 1.0 1.1 1.2 OVERPOTENTIAL (VOLTS)

FIG. 1. The solid curve is the electrolysis current versus the overpotential 7) at a CO2 concentration of 4x 10-2 mol/liter. The supporting electrolyte was 0.2 mol/liter tetramethyl­ammonium chloride in 0.2 mol/liter NaHCOa• The voltage scan rate was 2.5 mV /sec. pH = 6. 9 and the solution was continuous­ly stirred. The circles indicate the corresponding Faradaic yields Y F and the squares indicate the energy yields Y E' The inset shows a schematic diagram of the apparatus. A stream of CO2 gas passes through a diffuser and then, in the form of fine bubbles, passes over the cathode C of amalgamated nickel. Voltage is measured or controlled by means of the saturated calomel electrode Ca whose tip is close to the cathode. The anode A was of platinum. The ion exchange membrane M separates the cathode and anode compartments. It is permea­ble selectively to cations and prevents formic acid, produced at the cathode, from being subsequently destroyed at the anode. The ion-exchange membrane was not used for the current-voltage measurements. Current is drawn for only a short time and there is no significant buildup of formic acid or other change in electrolyte composition.

solution of NaHCOa (0.2 mol/liter) and tetramethyl ammonium chloride (0.2 mol/liter). The CO2 is sup­plied either by bubbling the gas directly over the cathode or by circulating a solution of the cathode electrolyte saturated with the gas. The anode is a sheet of platinum, Cathode and anode compartments are separated by an ion-exchange membrane that is per­meable to cations but not to anions. This prevents formic acid molecules, formed at the cathode, from migrating, as formate ions, to the anode, where they could be destroyed by an electrode reaction that is the inverse of the one that created them,

The essential experimental quantities are the over­voltage 1), the Faradaic yield YF , and the energy yield YE , Since reaction (1) is endothermic, a voltage must be applied to provide the energy of reaction. The ther­mal equilibrium potential difference EO is related to the free-energy difference AFo by EO = + AFo/2F= 1, 43 eV, where F is the Faraday. The overvoltage is the differ­ence between the applied voltage and Eo, both referred to a saturated calomel reference electrode, The Fara­daic yield is the fraction of the current passing through the cell that goes into reaction (1), as opposed to pos­Sible side reactions. The energy yield is the fraction of recoverable energy contained in the product of electrolysis and is given by the expression YE = EOYF(EO

382 Appl. Phys. Lett., Vol. 33, No.5, 1 September 1978

+ 1)r1• We use the expression here to analyze losses at the cathode and note that, ultimately, one must also include possible losses at the anode, At the current densities we have used they are not importanL

The concentration of formic acid was determined by a titration method12 with potassium permanganate. Cur­rent-voltage data were obtained with a potentiostatic circuit, using saturated calomel electrode as a voltage reference, Amalgamated nickel cathodes were made by applying mercury to the surface of electropolished nickel sheets under concentrated hydrochloric acid in the presence of a small amount of powdered magne­sium, 13 We tried several other materials for the cath­ode, including lead, cadmium, carbon, platinum, and amalgamated copper, 6 The amalgmated nickel was superior to all of these for Faradaic yield of formic acid and for stability in use.

The current-voltage data are shown in Fig. 1, They are similar to data reported earlier for this re­action,5-7 except that our currents are higher at lower overvoltages (1) < O. 9 V). These currents are probably due to side reactions that compete with reaction (1), giving reduced Faradaic yield at low 1). There is a middle range of voltages where YF>O, 9, At higher voltages YF falls off, probably due to competition of reaction (1) with the generation of hydrogen, Values of YE are also shown in Fig. 1.

Figure 2 shows rate data for the evolution of hydrogen from formic acid solutions of various concentrations in the presence of the palladium catalyst. The gas evolved in the reaction is an equimolar mixture of H2 and CO:!. The gas was collected over water in a volu­metric buret. To avoid errors due to the substantial solubility of CO:! in water, we passed the gas through a column of potassium hydroxide pellets before collecting it. This absorbs the CO:! and passes the hydrogen which can then be collected and measured without significant error since its solubility in water is very small. The data shown are for a catalyst having 1 % palladium, supported on carbon particles. The curves show the rate of generation of H2 by 1 g of catalyst in 50 ml of formic acid. Concentration of the acid in mol/liter is indicated by the numbers at the upper end of each curve. The reaction rate is approximately proportional to the concentration of formic acid. For an acid of a given concentration a catalyst having 5% palladium causes

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FIG. 2. Rate data for the catalytic conversion of formic acid to H2 and CO2, As described in the text, only the hydrogen was collected and measured. The numbers at the upper end of each curve indicate the concentration of formic acid in mol/liter. In each case 1 g of catalyst was used in 50 ml of solution. The catalyst contained 1% palladium supported on carbon particles (Alfa Products No. 89113).

Williams, Crandall, and Bloom 382

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the reaction to go approximately five times as fast as the catalyst having 1 % palladium.

It might be desirable for the hydrogen generated by the catalyst to be fed into a system having a pressure higher than atmospheric. The free energy of reaction (2) is sufficient to drive the reaction even when the product gases are at high pressures, The equilibrium constant K~ for the reaction is related to the partial pressures of the product gases, PH2 and Pco2, the con­centration of the formic acid CHCOOH, and the free energy by

(PH2)(PC02) ~- t:.FO) K~ -exp---

CHCOOH - RT'

From this equation it can be seen that, if desired, the hydrogen could be produced at pressures on the order of 103 atm by this reaction,

We have proposed a new method for energy storage, It is a way to store hydrogen in bound form as molecules of formic acid. The energy density of storage compares well with that of existing methods for hydrogen storage. ¥k find an energy efficiency for the process as high as 60%, encouraging at this early stage of development, It compares well with the energy efficiencies reported for hydrogen production by electrolYSiS of water. 14

The cycle of reactions we have described represents an energy storage method of exceptional versatility and generality. It will be important for the storage of elec­trical energy wherever there are periodic surpluses. The original energy is used to generate an organic chemical product with high-energy content which can be readily transported or stored until needed, This mate­rial can then be converted back to energy using a fuel cell, catalytically converted to hydrogen, or used as a chemical raw material,

383 Appl. Phys. Lett., Vol. 33, No.5, 1 September 1978

We are indebted to B, E. Tompkins and C. W, Magee for help with the experiments, and B. F. Williams for helpful encouragement during the course of the work.

lJ. O'M. Bockris, Energy, the Solar Hydrogen Alternative (Wiley, New York, 1975), pp. 207-222.

2S. Srinivasan and R. H. Wiswall, Proceedings of the Sym­posium on Energy Storage, edited by J. B. Berkowitz and H. P. Silverman (E lectrochemical Society, Princeton, N. J. , 1976), pp. 82-108.

3V. Kaiser and E. Heitz, Ber. Bunsenges. Phys. Chem. 77, 818 (1973).

4D. A. Tyssee, J. H. Wagenknecht, M. M. Baizer, and J. L. Chruma, Tetrahedron Lett. 47, 4809 (1972).

5J. Ryu, T.N. Anderson, and H. Eyring, J. Phys. Chem. 76, 3278 (1972).

6K. S. Udupa, G. S. Subramanian, and H. V. K. Udupa, Electrochem. Acta 16, 1593 (1971).

7p. G. Russell, N. Kovac, S. Srinivasan, and M. Steinberg, J. Electrochem. Soc. 124, 1329 (1977). This article contains an extensive bibliography of earlier work on the CO2 reduction.

BW. M. Latimer, The Oxidation States of the Elements and their Potentials in Aqueous Solutions, 2nd ed. (Prentice-Hall, Englewood Cliffs, N. J., 1956).

9R. JaSinski, J. Huff, S. Tomter, and L. Swette, Ber. Bunsenges. 68, 400 (1964).

lOp. Mars, J.J.F. Scholten, and P. Zwietering, Adv. Catal. 14, 35 (1963).

11The catalyst in palladium on activated carbon. It is a standard hydrogenation catalyst and is supplied by the Alfa Ventron Co., Danvers, Mass.

12F. P. Treadwell and W. T. Hall, Analytical Chemistry, 8th ed. (Wiley, New York, 1935), p. 569.

13M.N. Gavze, U.S.S.R. Patent 185067, Vol. 2, 1966 (unpublished).

14S. S. Penner and L. Icerman, Energy (Addison-Wesley, Reading, Mass., 1975), p. 143.

Williams, Crandall, and 8100m 383

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