Lightweight Pressure Vessels And Unitized Regenerative Fuel Cells

F.Mitlitsky,B.Myers,andA.H.Weisberg . Lawrence Livermore National Laboratory 7000 East AVE, P.O. Box 808.L-045 Livermore, CA 94551-0808

High specific energy (>400 Wh/kg) energy storage systems have been designed using lightweight pressure vessels in conjunction with unitized regenerative fuel cells (URFCs). URFCs produce power and electrolytically regenerate their reactants using a single stack of reversible cells. Although a rechargeable energy storage system with such high specific energy has not yet been fabricated, we have made progress towards this goal. A primary fuel cell (FC) test rig with a single cell (0.05 ft2 active area) has been modified and operated reversibly as a URFC. This URFC uses bifunctional electrodes (oxidation and reduction electrodes reverse roles when switching from charge to discharge, as with a rechargeable battery) and cathode feed electrolysis (water is fed from the oxygen side of the cell). Lightweight pressure vessels with state-of-the-art performance factors (burst pressure * internal volume / tank weight = Pb V / W) have been designed and fabricated.'1' These vessels provide a lightweight means of storing reactant gases required for fuel cells (FCs) or URFCs. The vessels use lightweight bladder liners that act as inflatable mandrels for composite overwrap and provide the permeation barrier for gas storage. The bladders are fabricated using materials that are compatible with humidified gases which may be created by the electrolysis of water and are compatible with elevated temperatures that occur during fast fills.

Lightweight vessels have been designed and fabricated to react purely pressure loads or hybridized pressure and structural loads. Use of these hybridized vessels can result in lower system mass for various vehicles, such as high altitude long endurance (HALE) solar rechargeable aircraft (SRA).'2' We have designed, fabricated, and load tested to failure (in bending) a series of prototype hybridized vessels that can withstand the structural loads expected in a HALE SRA, in addition to storing the reactant gases required by a URFC energy storage system.

URFC systems with lightweight pressure vessels were designed for zero emission vehicles (ZEVs). Such systems are shown to be cost competitive with primary FC powered vehicles that . operate on hydrogen/air with capacitors or batteries for power peaking and regenerative braking. URFCs are capable of regenerative braking via electrolysis and power peaking using low volume/low pressure accumulated oxygen for supercharging the power stack.'3' URFC ZEVs effectively carry their infrastructure on-board, enabling electrical recharge at home, work, or the highest power electric vehicle charging stations under consideration (by virtue of the large active area of cells that are sized for power production). URFC ZEVs can be safely and rapidly (< 5 minutes) refueled from high pressure hydrogen sources, when available, to achieve driving ranges in excess of 350 miles. URFC ZEVs can be refueled using home electrolysis units, but procurement of such units becomes an option, rather than a requirement, as is the case of other hydrogen powered vehicles prior to the existence of a widespread hydrogen infrastructure.

A single cell cycle life test for a URFC showed that reversible operation of cell membrane and catalyst is feasible without significant degradation,'4' thus refuting comments to the contrary made at the 1994 Fuel Cell Seminar. This test was performed in the early 1970s at ambient temperature using a membrane that is similar to DuPont's Nafion 120. The catalyst (E-5™) is a proprietary General Electric mixture of Pt, Pt-group metals, and their oxides. This test was a proof-of-principle energy storage system for a long life (7-10 yr) geosynchronous satellite, that was required not to use mechanical pumps (for reliability). The cell used a wicking cloth (typically quartz or Dacron) to feed water to the cell in zero-gravity. Upon disassembly of the cell, the initially hydrophilic wicks had become hydrophobic which degrades wicking and may well account for most of the limited cell degradation (<40 mV) shown in figure 1. It should be noted that other substitutes for wicks exist for zero-gravity operation, and wicks are clearly not required for terrestrial applications. Since this early data is sparse and masked by the unnecessary wicking cloth, we plan to perform a series of lifetime tests to show that high cycle life URFCs are feasible.

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