Uses of Gold in Fuel Cell Systems
Fuel cells are already accepted as reliable power sources for space and military applications and are being developed for a wide spectrum of applications, including vehicles. Although widely demonstrated and technically proven, their cost needs to be reduced to make them truly competitive in mass markets. Carbon supported gold alloy catalysts are being investigated as replacements for expensive platinum within the fuel cell stack, while other gold catalysts have a role in producing pure hydrogen from fuels such as natural gas, propane or biogas.
Fuel cells continuously convert chemical energy to electrical energy. The majority combine hydrogen with atmospheric oxygen, their operation being the converse of the electrolysis of water. This electrochemical process avoids the limitations of the Carnot heat cycle, and fuel cell systems can utilize a wide range of fuels much more efficiently than conventional generators, as well as being quiet and pollution free. Many fuel cells includes a series of bipolar plates typically made of a conductive material like stainless steel. To achieve good corrosion resistance and reduce contact resistance between the plate and diffusion media in the cell a thin layer of conductive gold can be used.
Gold in Fuel Cell Catalysis
In alkaline electrolytes gold is an effective oxygen reduction catalyst (being used in Space Shuttle Orbiter fuel cells), and is also capable of catalysing the direct oxidation of sodium borohydride with high utilisation. Until quite recently, a reputation for low activity in acid electrolytes has led to gold has playing almost no role in the development of acidic, proton-exchange membrane (PEM) fuel cells. The need to reduce the cost of the platinum needed to catalyze low temperature fuel cells has led to carbon supported PdAu alloy anode catalysts being examined at Lawrence Berkeley National Laboratory. Normally, carbon monoxide acts as a powerful poison for platinum catalysts, but these alloys have demonstrated high CO tolerance in acid electrolytes. Under cyclic voltammetry conditions, carbon supported Pd80Au20 alloys appear to be some 3-5 times more active for CO/H2 oxidation than standard Pt50Ru50 carbon supported catalysts in sulphuric acid electrolyte (0.5M H2SO4 in 250 ppm CO/H2 mixture). It has been postulated that the gold stabilises palladium so that CO is not strongly adsorbed.
Electrocatalysts with reduced platinum loading and enhanced activity have been prepared at Brookhaven National Laboratory. Carbon supported Pt/Au/Ni catalysts with a core-shell of AuNi alloy nanoparticles (i.e.1-2 monolayers of Au around a Ni core) exhibited a Pt mass-specific activity about 20 times that of platinum/carbon catalyst in acidic electrolyte. Surface segregation of Au was verified using x-ray powder diffraction technique.
Carbon supported Pt/Au catalysts have been shown to be considerably more stable than Pt/C supported catalysts by Adzic et al, while work by Zhong et al has shown that the Au/Pt system exhibits significantly different electrocatalytic properties to either Au or Pt alone. The reasons for this have been discussed by Bond. Work is continuing to optimise these catalysts for alkaline and acid fuel cells, while new techniques for preparing high surface area gold alloy catalysts should provide an exciting competitor for platinum catalysts for acidic as well as alkaline electrolytes.
Leading Japanese manufacturer Hitachi Maxell recently announced development of a new gold-based catalyst that is used in oxygen reduction reaction at cathode of polymer electrolyte fuel cells (PEFC). The new catalyst is made of gold-platinum (AuPt) nanoparticles, 2 to 3 nanaometer in size, and the new AuPt catalyst generates approximately 4.8 times higher oxygen reduction current per unit area than that of a commercial platinum catalyst. This success represents a large step closer to the practical use of fuel cells for applications requiring large current, such as power sources for automobiles and homes.
Gold in Hydrogen Generation and Purification
Expensive catalysts are used to generate pure hydrogen for fuel cell systems, and gold is likely to make a significant impact in reducing costs. Major efforts are in progress to develop compact reactors for stationary fuel cell systems and vehicles. Most of these consist of a hydrocarbon fuel reforming stage, followed by a water gas shift reaction to yield hydrogen rich gas mixtures. These invariably contain a small proportion of carbon monoxide, and steps must be taken to remove this, since it acts as a fuel cell catalyst poison.
In the water gas shift (WGS) reactor, carbon monoxide is reacted with steam to form more hydrogen:
CO + H2O ¨ CO2 + H2
The water gas shift reactor catalysts need to achieve high conversion at comparatively low temperatures, while lowering the reaction temperature reduces the proportion of CO present in the equilibrium. Supported gold catalysts have demonstrated comparable activity to commercially available CuO/ZnO/Al2O3 materials, with activity reported at temperatures as low as 120‹C. The gold-based catalysts also require no special activation steps, and are resistant to start up and shut down cycles. In particular, Au/CeO2-ZrO2 catalysts have demonstrated high activity and durability.
Low temperature polymer electrolyte membrane fuel cells can only be operated on reformate mixtures after removal of CO from the fuel gas stream. Using a preferential oxidation (PROX) reactor, oxygen or air (equivalent to 0.5-1% oxygen) is injected into the fuel stream upstream of the reactor to preferentially oxidize CO to CO2, even in the presence of hydrogen.
2CO + O2 ¨ 2CO2
Gold supported on metal oxides such as Fe2O3, Co3O4, and TiO2 will catalyse CO oxidation at temperatures as low as -70‹C in the presence of hydrogen, Au/a-Fe2O3 catalysts exhibiting higher activity at lower temperatures than commercially available Pt/ƒÁ-Al2O3 PROX catalysts. Luengnaruemitchai et al found that Au/CeO2 catalysts were very effective.
Overall, fuel cells have reached an exciting stage of development and are rapidly making inroads into several markets, and gold catalysts are making a significant contribution to their success by providing technically superior materials at reduced cost.
Source: World Gold Council
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