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Cooling down ceramic fuel cells

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Science  18 Sep 2015:
Vol. 349, Issue 6254, pp. 1290
DOI: 10.1126/science.aad0432

Ceramic fuel cells capable of achieving high power densities are based on oxygen-ion conductors that operate at high temperatures. The development of ceramic fuel cells that exhibit practical power densities at lower temperatures, with proton-conducting electrolytes, has been a long-standing dream, whose realization could lead to large-scale implementation of fuel cells. On page 1321 of this issue Duan et al. (1) report on three important contributions in the development of protonic ceramic fuel cells (PCFCs): the demonstration of the effective use of sintering aids to make difficult-to-prepare electrolytes that exhibit high protonic conductivities; the design of a new cathode material for PCFCs; and the development of a scalable fabrication process for cell production. Impressive performance was obtained with small-scale PCFCs at moderate temperatures. Taken together with other reports of high performance in PCFCs (2), practical ceramic fuel cells could be within reach.

Although the intrinsic conductivities of ceramic proton conductors have long been known to be higher than that of traditional oxygen-ion conductors, it has been difficult to take advantage of this in devices. The conductivities of the most promising proton-conducting ceramic, doped BaZrO3, are usually limited by barriers at grain boundaries and are therefore sensitive to processing conditions. Duan et al. demonstrate that this problem can be solved with simple and inexpensive sintering aids. Improved cathode materials allow lower operating temperatures that avoid the materials-stability and corrosion problems that occur in traditional solid oxide fuel cells (SOFCs) (3). Operation above 700°C requires that the membrane-electrode assembly be made from special alloys that can withstand the extremely corrosive environment. High temperatures accelerate the coarsening of nanostructured electrodes. Also, startup from ambient conditions is simplified when the fuel cell generates power at low temperatures.

PCFCs will likely find their most exciting applications with H2 as the fuel, in devices where proton-exchange membrane fuel cells (PEMFCs) are presently being considered. The electrodes in PEMFCs almost always contain precious metals, whereas none is required by PCFCs. Also, PCFCs can operate at just the right temperatures. With PEMFCs, the electrolytes must be wet, limiting their maximum operating temperature to near the boiling point of water. Under these conditions, the precious-metal electrodes are highly susceptible to poisoning by impurities in the H2, especially CO. Also, rejection of waste heat is difficult when operating near the ambient temperature. Indeed, much effort has gone into searching for proton-conducting electrolytes capable of operation up to 500°C (4).

Fuel cell operation.

The generation of water in the anode compartment of traditional SOFCs results in the dilution of the H2 fuel by H2O, which can cause oxidation of the anode that can in turn cause cracking of the electrolyte. In PCFCs, H2O is formed at the cathode, where it does not appreciably affect the O2 concentration.

ILLUSTRATION: K. SUTLIFF/SCIENCE

In addition to the lower operating temperatures compared to conventional SOFCs, there is no dilution of the H2 by H2O within the anode compartment of PCFCs (see the figure). Avoiding H2 dilution with water improves performance at higher fuel conversions and averts the serious problem that Ni-based anodes can be oxidized to NiO in traditional SOFCs. Oxidation of Ni anodes in SOFCs is potentially catastrophic because it can cause cell fracture due to the expansion that occurs upon NiO formation.

For larger-scale fuel cells operating on natural gas, conventional SOFCs will likely still be preferred. Although H2 can be generated internally from natural gas using the methane steam reforming (MSR) reaction, H2O + CH4 → CO + 3H2, temperatures greater than 500°C are required. Furthermore, system efficiency is increased when the waste heat generated by the fuel cell is consumed in driving the strongly endothermic MSR reaction. This is more easily accomplished in a conventional SOFC operating at higher temperatures (5). The MSR reaction can also be used for cooling to control cell temperature.

The demonstration of high protonic fluxes at just the right temperature also opens the exciting possibility of using PCFCs for producing H2 for reactions. Friebe et al. (6) used a short-circuited PEMFC to separate H2 from a mixture of gases, transporting the H2 from one side of the cell to the other. However, higher PCFC operating temperatures greatly expand the number of possible reactions that can be done in such a membrane. For example, H2 is industrially generated together with CO by the MSR reaction. The temperature range of 350° to 500°C offered by PCFCs is nearly ideal for producing extra H2 from CO by the water–gas shift (WGS) reaction (H2O + CO → H2 + CO2). Removing H2 as it forms theoretically allows the WGS reaction to be carried out to 100% conversion, so that both H2 and CO are utilized.

Much work still remains to take fuel cells from demonstration units to wide-scale, commercial reality. However, advances like those demonstrated by Duan et al. bring us one step closer to having practical devices for real applications by providing high performance at just the right temperatures, and using commercially viable fabrication procedures.

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