In the quest for alternative sources of energy, one active area of exploration is fuel cells. These devices use the same net oxidation reaction as burning, but by spatially separating the reactants, fuel cells convert chemical energy directly into electrical energy instead of generating heat as an intermediary (see the review by Sivan Kartha and Patrick Grimes in (Physics Today, November 1994, page 54)So fuel cells have the potential to be a clean and efficient means of electrical energy production.
A critical part of a fuel cell is the electrolyte, which conducts ions between the cell’s electrodes but is impervious to electrons. Electrons must instead flow around the electrolyte and can power external loads. There are many different approaches to fuel cell designs, relying on different electrolytes and the flow of different ions between the electrodes.
For many reasons, solid electrolytes are preferable to liquid ones, but the temperature ranges of operation for such electrolytes are either very high (above 600°C) or rather low (below 90°C). The high-temperature electrolytes are not currently practical in mobile applications such as automobiles—one of the major potential markets for fuel cells—due to start-up and other limitations. Low-temperature fuel cells, on the other hand, are not as flexible in the fuel they can use, and much of the energy they release is needed just to maintain operation. Better performance could be achieved from fuel cells operating at intermediate temperatures. Recently, a team at Caltech led by Sossina Haile has demonstrated that a well known electrolyte, based on a solid acid, may open up opportunities in the intermediate range when used in a fuel cell. 1
Water, water everywhere
The basic design of a fuel cell is illustrated in figure 1. Two electrodes—the anode and cathode—are separated by an electrolyte. A fuel such as molecular hydrogen or methanol is spontaneously oxidized by a catalyst at the anode; molecular oxygen is reduced at the cathode. The two half-reactions are completed by the flow of ions through the electrolyte and electrons through external wiring. Fuel cells thus resemble batteries, but with an external supply of reactants.
Figure 1. Typical fuel cell. Incoming fuel, here hydrogen, is ionized at the anode. Hydrogen ions pass through the electrolyte to the cathode, where they combine with oxygen and with electrons that have traveled through an external load to form water or steam.
Figure 1. Typical fuel cell. Incoming fuel, here hydrogen, is ionized at the anode. Hydrogen ions pass through the electrolyte to the cathode, where they combine with oxygen and with electrons that have traveled through an external load to form water or steam.
The solid-acid fuel cells explored by Haile and colleagues might prove to be a viable intermediate-temperature alternative to the primary fuel-cell technology being pursued for automotive and other mobile applications: the hydrogen–oxygen powered proton exchange membrane (PEM; also known by other names including polymer electrolyte membrane). There, the electrolyte is a membrane composed of a sulfonated fluorocarbon-based polymer that is heavily hydrated. Protons released in the reaction at the anode hitch a ride on water molecules through the membrane to the cathode, where they combine with oxygen to form water.
PEM fuel cells have high current and power densities, but the crucial need for water in the membrane presents several limitations. If the hydration is lost, so is the proton conductivity, and thus the cell must be operated below 100°C, the boiling point of water. (Pressurizing the membrane can raise the operating temperature to 120–130°C, still short of the desired operating range of 150–200°C.) At these low temperatures, expensive platinum catalysts are required for the electrodes. Moreover, the fuel stream must be very pure: Carbon monoxide can “poison” the Pt catalyst at concentrations of only about 50 parts per million. At higher temperatures, CO doesn’t bind to Pt as tightly and catalyst efficiency is increased—the biggest motivation behind the drive for intermediate temperatures.
The choice of fuels for PEM cells is essentially limited to molecular hydrogen. Although methanol can be used as a fuel at low temperatures, and is cheaper to produce than hydrogen, it can permeate almost anything water can, including the PEM. Methanol diffusing through the fuel-cell membrane acts like a chemical short circuit: Instead of generating useful electrical energy by reacting at the anode, methanol reaching the cathode is simply burned, and energy is lost as heat. This methanol crossover problem severely limits the utility of methanol as a fuel for PEM fuel cells.
Water and heat management are also concerns with PEM fuel cells. Several water molecules accompany each proton transported through the membrane from the anode to the cathode, and so a significant fraction of the energy from the fuel must go into recirculating the water back to the anode. Also, the heat generated within the fuel cell must be dissipated, which is harder to do at lower temperatures.
Operating at higher temperatures and reducing the hydration problems are key areas of research in the fuel-cell community. There are other electrolytes besides PEMs, such as alkaline materials and phosphoric acid, that are anhydrous and can operate at intermediate temperatures. But they have their own limitations, including carbon-dioxide intolerance, low efficiency, or corrosive liquids. Haile’s group has investigated a different class of electrolytic materials: solid acids.
A solid-state solution
Solid acids are compounds partway between normal acids, such as H2SO4 or H3PO4, and normal salts, such as K2SO4. With only some of the normal acid’s hydrogen atoms replaced, solid acids still act as proton donors. Some of the solid acids, including the CsHSO4 studied by Haile, have been known for 20 years to undergo a so-called superprotonic phase transition. At low temperatures, the tetrahedral oxyanion groups (such as SO4 2– or PO4 3–) in these compounds are frozen in place, but above a transition temperature, these groups undergo rotational diffusion. Above this superprotonic transition (140°C for CsHSO4), the proton conductivity through the solid increases by several orders of magnitude as protons are passed from one rotating oxyanion group to another.
The Caltech solid-acid fuel cell is shown in figure 2. A layer of solid acid was sandwiched between two catalyst layers formed from the solid acid mixed with Pt, carbon, and a volatile organic material that later was evaporated to create a porous catalyst layer with high surface area. These layers, in turn, were surrounded by graphite current-collecting electrodes. The final structure was about 1.5 mm thick. When heated to 160°C and with hydrogen supplied to the anode and oxygen to the cathode, the assembly developed an open-circuit voltage of 1.11 V, close to the theoretical expectation for a hydrogen–oxygen fuel cell and higher than what is typically obtained from PEMs.
Figure 2. Solid-acid fuel cell developed at Caltech. The black layers are graphite electrodes, which sandwich a layer of CsHSO4, a solid-state acid. Thin electrocatalyst layers are placed between the electrodes and the electrolyte. This fuel cell operates between 150 and 160°C.
Figure 2. Solid-acid fuel cell developed at Caltech. The black layers are graphite electrodes, which sandwich a layer of CsHSO4, a solid-state acid. Thin electrocatalyst layers are placed between the electrodes and the electrolyte. This fuel cell operates between 150 and 160°C.
The solid-acid fuel-cell prototypes studied by Haile and colleagues had a maximum current density of 44 mA/cm2, more than an order of magnitude lower than that of PEM fuel cells. A significant limitation on the current density was the thickness of the solid-acid electrolyte and the resulting ohmic losses from proton conduction across the material. Engineering improvements in making the electrolyte-catalyst-electrode assembly could yield substantial improvements, Haile told us.
Solid acids are soluble in water, a natural by-product of both hydrogen- and methanol-fueled reactions. Indeed, this solubility may have been a significant reason why solid acids were not explored earlier as potential fuel-cell electrolytes. But with the fuel cell operating at 150–160°C, the reaction produces steam, not water, and the electrolyte remains intact. The experimenters found the fuel cell to be stable in the presence of steam over periods of days.
A perhaps greater concern is the possibility that performance will be lost gradually through the reduction of the sulfur in the CsHSO4 electrolyte. The compound can react with molecular hydrogen to form hydrogen sulfide, which poisons the Pt catalyst. Other solid acids, with different oxyanion groups, may not have this problem and thus may be better candidates for fuel-cell electrolytes.
Additional advantages may be realizable from solid-acid electrolytes. At the higher operating temperatures, not only is there a greater tolerance to CO, but the waste heat generated by the cell is more readily disposed of, and might even be used productively to heat water or air. Furthermore, notes Haile, the electrolyte is impermeable to methanol, raising the possibility of directly using methanol vapor as a fuel. But much work remains to be done, including improving the current density, determining the long-term stability, and protecting the soluble electrolyte from water during warm-up and cool-down periods and accidental flooding.
Other efforts
In the meantime, many other efforts are under way worldwide to develop hydrogen–oxygen fuel cells that operate at intermediate temperatures. Several researchers are exploring modifications to the polymer membranes in PEM fuel cells that will allow their use at higher temperatures. Another major focus of activity is achieving a better understanding of proton transport mechanisms through polymer membranes and other materials. Robert Savinell, Morton Litt, and coworkers (Case Western Reserve University) and Jean Claude Lassègues and colleagues (University of Bordeaux) have investigated nonaqueous fuel cells based on phosphoric acid in a polymer matrix. 2 And Klaus-Dieter Kreuer (Max Planck Institute for Solid-State Physics in Stuttgart) and others have explored the use of materials based on so-called heterocyclic compounds such as imidazole (C3H4N2) for proton transport and have developed stable, highly proton-conducting oxides. 3
