Energy & Fuels 2008, 22, 2771–2774
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Energy Storage using Aqueous Hydrogen Peroxide Robert S. Disselkamp* Institute for Interfacial Catalysis, Pacific Northwest National Laboratory, Richland, Washington 99352 ReceiVed January 22, 2008. ReVised Manuscript ReceiVed March 21, 2008
As alternative energy sources, such as solar, geothermal, wind, and wave, become viable in the future and cost-competitive with and environmentally favorable to conventional carbon-based energy sources, it will be increasingly important to develop low-cost energy-storage systems. These energy-storage systems may be either carbon-based or non-carbon-based but must possess a high energy-storage density, high operating efficiency, low cost, and ease of use, such as operation from the extensive electrical grid that exists throughout developed countries. Here, we present a novel energy-storage concept employing aqueous hydrogen peroxide (H2O2). The electrochemical synthesis of H2O2 from aerated water using a solid superacid cathodic electrode is proposed on the basis of a prior literature study and serves as the mechanism of stored energy. The subsequent generation of H2 plus O2 (again via electrolysis on a cathodic superacid electrode) for use in a polymer electrolyte membrane (PEM) fuel cell is described and thus comprises energy release. For a 50 wt % aqueous H2O2 solution using a tungstated zirconia electrocatalyst with an energy-storage density of >0.49 MJ/kg solution and a full-cycle (synthesis to electrical energy output) operating efficiency of >35% has been estimated to be feasible based on prior literature work.
1. Introduction A long-range global energy production and use goal is to make a transition from carbon-based energy sources to an environmentally clean (e.g., noncarbon) energy economy. This difficult task may entail both an energy production aspect, such as electricity production from solar, wind, and wave sources, and energy storage in carbon or noncarbon materials on a large scale. The energy-storage component is required as a “buffer” because peak energy demand, say on the electrical grid, is not always in-phase with energy production.1 Furthermore, because vehicles consume 28% of the U.S. energy expenditure,2 the development of novel, low-cost, transportable energy storage materials is critical. Traditional batteries, because of their high cost, cannot store energy on the large scale required as part of an energy-buffering system. Here, we examine a concept of energy storage based on prior experimental work, which involves the production and use of aqueous solutions of hydrogen peroxide (H2O2). Hydrogen peroxide is an ideal energy-storage medium for many reasons, including (1) being a stable liquid when not exposed to metals that can decompose it to water and oxygen or organic compounds that pose a combustion threat, (2) remaining fairly nontoxic but corrosive up to ∼20 M, (3) providing portability in plastic containers, and (4) containing a high energy density. It is necessary to point out though that concentrated H2O2 is an Environmental Protection Agency (EPA)-regulated compound and deemed hazardous at >52 wt %, which should suggest an upper concentration limit for practical application. Nevertheless, this limitation should not preclude its application here. * To whom correspondence should be addressed: Pacific Northwest National Laboratory, 3335 Q Avenue; Post Office Box 999, Richland, WA 99352. Telephone: 509-371-3107. Fax: 509-371-3501. E-mail:
[email protected]. (1) Bair, E. J. Connecting the Dots to Future Electric Power; Authorhouse: Bloomington, IN, 2007. (2) Energy Information Administration. Energy flow trend. U.S. Department of Energy, Annual Energy Review, 2006 (http://www.eia.doe.gov/ emeu/aer/overview.html).
Scheme 1. Gibb’s Free-Energy Diagram Illustrating the Electrolysis of a Standard Solution of H2O2 into H2 Plus O2, Followed by H2 Plus O2 Conversion into H2O Plus (1/2)O2a
a Assuming 100% efficiency for the process, the reaction is exothermic by -117 kJ/mol H2O2.
The purpose of this paper is to assimilate prior experimental studies that support the use of aqueous H2O2 as an energystorage medium and briefly outline additional work needed to test this concept. Interestingly, H2O2 can act as both a fuel (i.e., H2) and oxidant (i.e., O2) under varying reaction pathways.3 The general concept begins with the membrane-type electrochemical production of H2O2 from the reaction H2O + (1/2)O2 f H2O2 in a sulfuric acid electrolyte4,5 or on a solid superacid(3) Schumb, W. C.; Satterfield, C. N.; Wentworth, R. L. Hydrogen Peroxide; ACS Monograph Series, Reinhold Publishing Corporation: New York, 1955. (4) Gopal, R. Electrochemical synthesis of hydrogen peroxide. U.S. Patent 6,712,949, Electrosynthesis Company, Lancaster, NY, March 30, 2004.
10.1021/ef800050t CCC: $40.75 2008 American Chemical Society Published on Web 06/11/2008
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containing cathode material at neutral pH.6 Next, the energy of hydrogen peroxide can be released by first forming H2 plus O2, via H2O2 electrolysis in concentrated sulfuric acid solution, followed by polymer electrolyte membrane (PEM) fuel cell use of the H2 (cathode) and O2 (anode) gases that creates electrical energy and water, thus completing the cycle. We will discuss below an improvement to this approach by using electrodes containing solid superacid materials, thus eliminating the need for the aqueous sulfuric acid electrolyte. 2. System Operation, H2O2 Use, and Electrochemical Efficiencies Scheme 1 illustrates that, at standard state solution composition conditions, 117 kJ/mol is liberated in these latter two processes above if operating at 100% efficiency. For the high H2O2 concentrations considered here, the change from standard state conditions of less than 7.5 kJ/mol, which does not affect our results significantly, can be expected. In this way, H2O2 is an energy-storage compound and is used in a system similar to a battery, although the reactions do not occur spontaneously, using electrical energy that, in principle, can be supplied from a renewable source. It differs from a battery though in that the initial release of energy contained in H2O2 is not spontaneous but can be self-sustaining by feedback of a portion of the electrical energy from the PEM fuel cell (energy generation) into the electrolysis (energy consumption) process. Energy generation exceeds consumption by a factor of ∼2:1 at 100% efficiency. Each of the above steps will now be discussed further, and a direction for future research will be planned for process implementation and testing. A primary concern with any electrochemical energy-storage scheme is the overall system efficiency, which here is simply the product of H2O2 formation, H2O2 electrolysis to H2/O2, and H2/O2 PEM fuel cell use efficiencies. Each of these efficiencies will be discussed now. For H2O2 production, indirect evidence supporting the concept here (i.e., for slightly alkaline solutions) can be obtained from ∼90% system efficiencies as reported by Spalek and co-workers.7,8 They demonstrated that the twoelectron reduction of dioxygen to H2O2 in a slightly alkaline (e.g., 35 wt % KHCO3) buffer solution employing a carbon/ polytetrafluoroethylene (PTFE) hydrophobic cathode and nickel anode exhibited a current efficiency of ∼90% and that 2 M (7 wt %) H2O2 was achieved. These studies illustrated that the carbon electrode (i.e., carbon black) exhibited stability at 0.8-0.5 V (RHE) over hundreds of hours of polarization in the 10-60 mA/cm2 current density range. An added benefit from an economic perspective is that the manufacture of the electrode was straightforward from inexpensive materials. Another study examined a sulfuric acid (1 N H2SO4) solution that measured an efficiency of ∼95% at 100 mA/cm,2 yielding a H2O2 solution concentration of 2.2 M, as reported for a membrane-type electrode assembly.4 The membrane in that study consisted of a carbon/PTFE/nafion composite cathode catalyst. Of greatest interest here, however, was a recent study by Yamanaka and Murayama6 that synthesized H2O2 by dioxygen reduction to 8 wt % concentration on a nafion/carbon cathode in water at neutral pH. They observed a current efficiency of 26% at a cathode potential of -0.3 V Ag|AgCl. This study has demonstrated for the first time that highly alkaline or acidic aqueous solutions are not necessary for substantial H2O2 generation. The judicious selection of acidity of the (5) Campos-Martin, J. M.; Blanco-Brieva, G.; Fierro, J. L. G. Angew. Chem., Int. Ed. 2006, 45, 6962.
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cathode material (e.g., using superacid impregnation) can be sufficient for efficient chemistry to occur, even in low ionic strength solutions. A second potentiometric efficiency to characterize is that associated with the conversion of H2O2 to H2 plus O2 during electrolysis. The early literature describes observations of H2 evolution at the cathode and O2 evolution at the anode for electrolysis of H2O2 in concentrated acid solutions.3,9–11 With decreasing potential and current density, the hydrogen evolved at the cathode became less and less with combined H2/O2 evolution, such that eventually at low current density only O2 was evolved at the cathode.3,9 Weiss has studied this effect and shown that various mineral acids promote H2 evolution. In one representative but nonoptimized experiment measuring efficiency, a mercury amalgam cathode/Pt anode electrode assembly operating at 0.35 M H2O2/62 mM H2SO4 and 40 mA/ cm2 yielded 90% H2 (with 10% O2) cathode gases with a current efficiency of 50%.9 This current efficiency, when interpreted further, says that 2 faradays of charge converts 1 mol of H2O2 into 90% H2 at the cathode. The H2O2 conversion into H2 plus O2 may entail either a one-electron or two-electron process. For either, an improvement to the 50% current efficiency is possible. A final efficiency to consider is the PEM fuel cell efficiency, which for commercially available devices operates with again a ∼50% efficiency.12 A key to the success of the integrated approach above is that, whereas prior work demonstrated that H2O2 synthesis and electrolysis could be performed in sulfuric acid solutions, it is much preferable to employ solid superacid electrode materials to perform these conversions, thus eliminating hazards arising from strong acid storage on a large scale. Unfortunately, despite the discovery of superacids having occurred 80 years ago13 and that of solid superacids in the early 1970s, only recently has research begun to explore their use in electrochemisty applied to cathode-mediated processes. One way to optimize electrochemical H2 generation in near neutral pH H2O2 solutions is to construct and employ the most highly acidic graphitic/solid superacid composite electrode for both the two-electron dioxygen reduction and H2O2 conversion processes. The solid superacid of choice here must exhibit excellent water stability, which eliminates many potential candidates, such as sulfated zirchonia. Most promising then are commercially available tungstated zirchonia (WOx-ZrO2, pKa < -14.6) or Fe-doped and WOx-doped ZrO214–16 or less preferable, a nafion/silica (SAC13 BASF) catalyst (pKa ) -6.0).6,17 The former materials are promising because they exhibit high Bronsted acidity (6) Yamanaka, I.; Murayama, T. Angew. Chem., Int. Ed. 2008, 47, 1900. (7) Spalek, O.; Balej, J. Collect. Czech. Chem. Commun. 1981, 46, 2052. (8) Spalek, O.; Balej, J.; Balogh, K. Collect. Czech. Chem. Commun. 1977, 42, 952. (9) Weiss, J. The catalytic decomposition of hydrogen peroxide by different metals. Trans. Faraday Soc. 1935, 31, 1547. (10) Berthelot, M. J. Chem. Soc. 1882, 42, 1157. (11) Tanatar, S. J. Chem. Soc. 1903, 84, 202. (12) Garcia, C. P.; Chang, B.-J.; Johnson, D. W.; Bents, D. J.; Scullin, V. J.; Jakupca, I. J. Round trip energy efficiency of NASA Glenn regenerative fuel cell system. NASA Publishing NASA/TM-2006-214054, Jan 2006. (13) Hall, N. F.; Conant, J. B. J. Am. Chem. Soc. 1927, 49, 3062. (14) Santiesteban, J. G.; Vartuli, J. C.; Han, S.; Bastain, R. D.; Chang, C. D. J. Catal. 1997, 168, 431. (15) Boyse, R. A.; Ko, E. I. J. Catal. 1997, 171, 191. (16) Boyse, R. A.; Ko, E. I. Appl. Catal., A 1999, 177, L131. (17) Passos, R. R.; Paganin, V. A.; Ticianelli, E. A. Electrochim. Acta 2006, 51, 5239.
Energy Storage using Aqueous H2O2
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Table 1. Approximate Electricity f H2O2 f Electricity Efficiencies and Specific Energy-Storage Capacity for Current and Projected Aqueous H2O2 Solutions (See the Text) property pKa H2O + (1/2)O2 f H2O2 electrochemical efficiency H2O2 f H2 + O2 electrolysis efficiency H2 + O2 f electricity (PEM fuel cell) efficiency overall efficiency of electricity f H2O2 f electricity (from above) aqueous H2O2 properties (wt %, specific gravity, M) total energy stored in aqueous H2O2 (see Scheme 1) total usable stored energy in aqueous H2O2 solution
parameter -6.0a (nafion) 26% 50% 50 × 161%c 10% 8 wt %, 1.04, 2.5 M (117 kJ/mol H2O2) 0.27 MJ/kg 0.027 MJ/kg solution
< -14.6b (WOx-ZrO2) >86% >50% >50 × 161%c >35% >41 wt %, >1.18, >14.2 M (117 kJ/mol H2O2) >1.41 MJ/kg solution) >0.49 MJ/kg solution
a Corresponds to the nafion membrane cathode (see refs6 and 19). b Corresponds to proposed tungstated zirconia cathode material (see ref 20). c The factor of 161% arises because pure oxygen is used in the fuel cell here instead of air (e.g., 20% O2) at 1 atm (see ref 24).
Scheme 2. Pictorial Representation of Energy Storage and Release from an Aqueous H2O2 Solutiona
a The left side illustrates H2O2 production, whereas the right side shows H2O2 release into electrical energy. Precedence for H2O2 production is work by Yamanaka and Murayama,6 yielding a 26% efficient process generating a 8.0 wt % H2O2 solution (see the text). Conversely, precedence for H2O2 energy release is obtained from work by Weiss,9 yielding a ∼50% efficient process and 90% H2/10% O2 cathodic gas composition (see the text). Half-reactions are shown, both resulting from cathodes employing solid superacid materials that may increase these efficiencies and yields.
because they possess an acid strength comparable to ∼100% H2SO4 (pH < -14.5).18 Table 1 is a summary of the efficiencies discussed above and those based on ref 6, using in part an estimated or scaled efficiency comparing the pKa of nafion19 to tungstated zirconia.20 The scaling factor was computed as the change in pKa divided by the change in percent additional H2O2 formation rate obtained with enhanced acidity. In this table, it is seen that the upper portion of Table 1 gives the efficiencies of H2O2 production/ use, whereas the bottom portion of the table describes solution properties, such as wt %, density, and energy-storage capacity. It is seen that the overall electricity f H2O2 f electricity efficiency, given as a product of the above efficiencies, is 10% using present day (nafion membrane) results6 but could reach >35% for use of best available solid superacids, such as tungstated zirchonia.20 This overall efficiency can be thought of as arising from ohmic and overpotential losses, however, is not theoretically determined but obtained either directly from measurements or from the scale factor. Thus, they can be expected to more accurately represent true system performance. (18) Yang, H.; Lu, R.; Zhao, J.; Yang, X.; Shen, L.; Wang, Z. Mater. Chem. Phys. 2003, 80, 68. (19) Pathapati, P. R.; Xue, X.; Tang, J. Renewable Energy 2005, 30, 1.
Furthermore, for tungstated zirconia, the usable stored energy based on this efficiency is 0.49 MJ/kg solution for a 41 wt % aqueous H2O2 solution. This can be compared to gasoline that possesses 45 MJ/kg stored energy, which when multiplied by the operating efficiency of an internal combustion engine of ∼20% yields 9 MJ/kg usable energy (mechanical or electrical). Therefore, the H2O2-specific energy storage amount, by mass, is a factor of ∼18 lower capacity than that offered by a petroleum source. However, because fuel cells weigh substantially less than the mechanical working assembly of an internal combustion engine, H2O2 may still find use in vehicular applications, with all other factors being equal. In addition to vehicular energy storage applications, aqueous H2O2 may be ideally suited to stationary energy-storage systems, where stored energy per mass concerns are less crucial. When the tungstated zirconia stored specific energy (with estimated efficiencies) listed as the final entry of Table 1 of 0.49 MJ/kg is compared to that of a lithium ion battery of 1.04 MJ/kg,21 it is seen that the concept presented here compares favorably to battery energy storage. This suggests that its application could have merit, even at these non-optimized efficiencies. Scheme 2 illlustrates a working electrochemical system for H2O2 “production” (left side) and H2O2 energy “release” (right
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side). The half-cell reactions are listed,22 and it is seen that the entire process is an energy-storage device, with an efficiency as estimated above of ∼35%. The aqueous solution in its simplest form is aerated aqueous H2O2. It may prove to be advantageous from an efficiency perspective, however, to have the H2O2 solution contain an inorganic salt (such as peroxy borate, peroxy carbonate, peroxy sulfate, phosphates, etc.), which further research can determine. Other potential energy uses for H2O2 in addition to the possibility discussed here exist, such as energy use in a direct NaBH4/H2O2 fuel cell.23 It is seen then that, if successful, the low cost and nontoxic nature of the system components here offer an advantage over many (20) Arata, K. AdVanced Catalysis; Academic Press, Inc.: New York, 1990; Vol. 37, p 202. (21) Achiha, T.; Shibata, S.; Nakajima, T.; Ohzama, Y.; Tressaud, A.; Durand, E. J. Power Sources 2007, 171, 932. (22) Bard, A. J.; Parsons, R.; Jordan, J. Standard Potentials in Aqueous Solution; International Union of Pure and Applied Chemistry (IUPAC)Physical and Analytical Chemistry Divisions, Marcel Dekker, Inc.: New York, 1985.
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traditional (e.g., metal-based) battery storage systems. Research into the energy-storage concept presented here is now necessary to increase the H2O2 solution concentration and efficiencies to test the unique capabilities offered by this promising system. Acknowledgment. The author thanks Dr. James E. Amonette for valuable discussions and support during the formative stages of this project. This project was performed at the Institute for Interfacial Catalysis (ICC) at Pacific Northwest National Laboratory (PNNL). The work was carried out in the Environmental Molecular Sciences Laboratory (EMSL) at PNNL, a National Scientific User facility supported by the U.S. Department of Energy Office of Biological and Environmental Research. PNNL is operated by Battelle Memorial Institute for the U.S. Department of Energy. EF800050T (23) Miley, G. H.; Luo, N.; Mather, J.; Burton, R.; Hawkins, G.; Gu, L.; Byrd, E.; Gimlin, R.; Shrestha, P. J.; Benavides, G.; Laystrom, J.; Carroll, D. J. Power Sources 2007, 165, 509. (24) Rho, Y. W.; Velev, O. A.; Srinivasan, S. J. Electrochem. Soc. 1994, 141 (8), 2084.
