A rapid transition from fossil to more sustainable energy sources is mandatory in the next years. Fuel cells (FCs) represent a promising and flexible technology to convert the chemical energy stored in fuels into electricity with a high efficiency, applicable both in stationary and mobile applications.
Hydrogen is often discussed as the energy carrier of choice for use in FCs, and hydrogen based FCs (H-FCs) represent the most efficient and clean FCs, since the exhaust is solely water. However, there are some serious hurdles for the application of H-FCs: hydrogen is not a natural resource and it is normally produced by hydrocarbon steam reforming or by water electrolysis. The latter represents a significant waste of primary energy, only sustainable if electricity from renewable sources is available. Moreover, issues regarding hydrogen storage and distribution need to be solved, especially in the transportation sector. Hence, there is a need for the implementation of alternative energy carriers into FCs.
Bioethanol (EOH) is an attractive alternative energy carrier, especially when it is not produced at the expense of food production, e.g. by fermentation of lignocellulose, which uses crop or wood as precursors. EOH can be produced by converting biomass into sugars, which are then fermented. EOH is considered an alternative to petroleum and diesel and its popularity is emerging as a fuel for cars, particularly well established in Brazil. It has a number of advantages over conventional fuels: it comes from a renewable and not from a finite resource, such as crops. These crops are specifically grown for energy use and include corn, maize and wheat crops, waste straw, willow and popular trees, sawdust, reed canary grass, cord grasses, Jerusalem artichoke, myscanthus and sorghum plants. Therefore, development of efficient FCs exploiting a direct conversion of the chemical energy stored in EOH into electricity would have a tremendous impact on FC technology and commercialization.
Within the rich list of different types of FCs, the Direct Ethanol Fuel Cell (DEFC) represents a viable route to achieve such a goal because:
However, existing catalysts convert EOH into acetic acid and acetaldehyde, a process that releases just a couple of electrons per EOH molecule, hence generates low currents. Breaking down EOH molecules further to produce CO2 would release far more electrons (a total of 12 per molecule) and generate higher currents. For a complete EOH oxidation to CO2 at the anode, the carbon-carbon bond needs to be broken, which requires both higher operating temperatures (in the 150- 200°C range, often referenced as intermediate-T) and the use of appropriate catalysts.
For an efficient design of a DEFC a number of questions still have to be addressed, mainly because the technological solutions so far optimized for standard hydrogen polymer electrolyte membrane (PEM) FCs (which usually operate at low temperature 60 – 120°C) are not directly transferable to DEFCs.
To this end the contribution from the methods and tools of the emerging field of Nanoscience&Nanotechnology (N&N) can play a relevant role. When the catalyst is dispersed into nanodimensional objects (i.e. nanoparticles, NPs), the catalyst becomes much more efficient due to the higher intrinsic specific surface area, and due to the new active chemisorption sites peculiar for the NPs themselves. The big challenge is the stabilization of the catalysts, which have a low intrinsic stability. In this context, the interaction with the catalyst support plays a major role, both for the stabilization of the catalyst and for the innovative chemical properties induced by the catalyst/support interaction. It is commonly recognized that also the catalyst support plays a pivotal role in both activity and durability of the catalyst itself.
Besides being stable (chemically and electrochemically in the operating potential and temperature ranges and in the used electrolyte) and conductive, the support should be specifically tailored to avoid degradation of the active catalyst due to events such as metal dissolution, sintering, as well as agglomeration induced by temperature and electrochemical potential. In general, a well-matched physicochemical and electronic metal-support interaction is beneficial to improve both the activity and durability of the catalyst. Reaching such goals when operating at intermediate-T becomes even more difficult using standard carbon-based supports. Thus alternative support materials are required in order to increase the electrode lifetime for intermediate-T DEFCs.
The best methodological route to face such challenges is to adopt a rational design of the catalyst/support assembly aiming at efficient electrodes, mainly based on an approach where the outcome of rigorous studies on model electrodes are capitalized to implement efficient real electrodes to be properly tested under realistic working conditions. This is actually what DECORE plans to do.
Another pivotal point for a widespread FC commercialization relies on reducing (or even avoiding) the noble metal loading in the catalyst without compromising FC performance. State-of-the-art catalysts for both anode- and cathode-side are based on noble metals, especially platinum. Among the components in a PEM-FC, Pt-based electrodes contribute to ca. 55% of the total costs. As an example, in the automotive field, a FC stack needs the power of about 100 kW. A state-of-the-art Pt/C electrode has a Pt specific power density of about 0.5 gPt kW-1 (grams of Pt per kW), and thus 50 g Pt are needed for the required FC stack. Precious metals like Pt are on the one hand expensive, but even more critical is the fact that the Pt resources are limited and they are confined outside of EU in countries politically not extremely stable. The annual production of Pt is currently estimated to some 220t with a clear trend for further increase. Short supply coupled with an expected increasing demand, is likely to cause prices to rise in the future. Considering these facts, it is highly strategic for EU to investigate alternative catalyst materials where the precious metal is reduced or completely avoided