Carbon nitride materials used for improved efficiency in alternative energy fuel cells
Fuel cells convert chemical energy into electricity through redox reactions at the anode and cathode of an electrochemical cell. The reactions taking place produce no pollution and require only hydrogen as a fuel. The reduction and oxidation generates ions that move between the two sides of the cell, and it is this movement of charge that can be harnessed and used as electricity. Some form of catalysis is required to drive the reactions. Currently, fuel cell systems are dependent on catalysts made of scarce metals (such as platinum), which makes these devices unsustainable and drives up their cost. The catalyst can be as much as 42% of the total device cost. Research is therefore aimed at reducing the use of catalysts made of metals with low abundancies and improving the lifetime and efficiency of these devices.
Catalysts in fuel cells are generally in the form of high-surface-area nanoparticles, which are uniformly deposited on larger support-structure molecules. Supported catalyst structures allow for optimised catalyst activity by maximising the electrochemically active surface area of the nanoparticle catalyst. In this case, the catalyst activity can be thought of as the electricity productivity of the redox reactions. These support structures therefore play a vital role in increasing the device’s efficiency. Most current research addresses the development of sustainable catalysts, but UCL scientists are looking into the nature of the support materials used and their interaction with the catalyst nanoparticles – an equally important component of the catalyst activity.
Carbon black is currently the support material of choice for catalysts in fuel cells. This is due to its ready availability, low cost and minimal environmental impact. The problem with this material is that it oxidizes under fuel-cell operating conditions and therefore its catalytic activity (which is vital for the operation of the fuel cell) is lost relatively quickly. This degradation of the support also restricts the movement of ions through the fuel cell, as the oxidation blocks the channels through which they would ordinarily migrate. Further, platinum is a known catalyst for carbon corrosion and so the catalyst and carbon-black support do not interact well.
The search for catalyst supports with high stability and positive interactions with the catalyst particles is intense. Researchers at UCL believe that graphitic carbon nitride materials may provide the solution. A wide range of alternative catalyst supports have been tested. The known connection between the nitrogen content of the support and stabilized catalyst nanoparticles has pointed most of the research in the direction of carbon materials with a few nitrogen defects introduced (nitrogen-doped carbons). These molecules contain a few percent atomic nitrogen, located throughout the carbon material completely randomly. This randomness means that there is no control over how the introduction of nitrogen will affect the overall properties of the carbon molecule and thus its catalytic activity. UCL have therefore developed a catalyst support made of graphitic carbon nitrides (gCN), which are comprised of a back-bone of carbon-nitrogen bonds and nitrogen sites located at defined positions within carbon graphene layers. The defined nature of the nitrogen locations allows for more control over the interaction of the support and the catalyst.
It has been demonstrated that reactions that take place within a fuel cell can be more efficiently catalyzed with gCN supports than with carbon black supports. Platinum-ruthenium nanoparticle catalysts supported on gCN demonstrated 78-83% higher power density than the same catalyst supported by carbon black. This huge increase in power density that can be obtained from fuel cell confirms the importance of UCL’s research in increasing the efficiency of fuel cell operation.
Additionally, gCNs have excellent resistance, which gives them the ability to endure harsh fuel-cell operating conditions. The improved chemical resistance to corrosion over carbon black can be explained by the comparison of the materials’ band structures, which tell us that the corrosion of the carbon-nitrogen backbone is thermodynamically unfavourable at fuel-cell operating conditions. The more durable and long-lasting gCN catalyst support will allow for more efficient use of the catalyst material (meaning that less will need to be used). This will greatly reduce the cost of the technology, allowing it to become a much more readily available energy-conversion device.
Other research looking to reduce the cost of the device focuses on developing a fuel cell that can operate in alkaline conditions, where the support will be less prone to the corrosion that occurs due to the acidic conditions of current devices. The fuel cell reactions also occur more readily in alkaline media, which would permit the use of non-precious metal catalysts. Unfortunately, the components of the device that are required for this do not have comparable performance with the components that have been developed for acidic conditions.
UCL researchers remain optimistic, however, that advances in the durability and efficiency of the catalyst support will help to bring the fuel cell to a wider audience.
- N. Mansor, T. S. Miller, I. Dedigama, A. B. Jorge, J. Jia, V. Brázdová, C. Mattevi and others, Graphitic carbon nitride as a catalyst support in fuel cells and electrolyzers, Electrochim. Acta 222, p. 44–57, 2016.