Electrocatalysts which are efficient relative to their consumption of both energy and platinum (Pt) hold new promise in polymer electrolyte membrane fuel cell applications.
Fuel cells are regarded as an important part of the future energy system, since they provide clean energy with low pollution. However, fuel cells depend on expensive catalyst materials such as platinum (Pt). The project presents a new kind of graphene based catalysts with high catalytic performance and stability and minimal Pt content.
Fuel cells convert chemical energy stored in fuel molecules into electrical energy via electro- chemical reactions. Basic elements of a fuel cell are cathode, anode and electrolyte. Commonly used fuels are dihydrogen, methanol, ethanol and formic acid. While several types of fuel cells exist, the focus of this project is polymer electrolyte membrane fuel cells (PEMFCs) that employ small fuel molecules and dioxygen to produce electrical energy.
PEMFCs offer high specific energy density, but the technology is not widely available, primarily due to the expensive catalyst employed. Highly active catalysts, such as Pt are required, since the mild operating conditions (up to 100 °C) do not contribute significantly to elevating the reaction-driving forces. Pt is a scarce element in the Earths crust, and only a few hundred tons are produced annually, leading to high and increasing prices. Extensive studies have therefore been undertaken to synthesize Pt nanocrystals in order to maximize the catalytic surface area, and thereby the catalytic effect, while minimizing platinum consumption.
In the project, Pt nanoparticles were synthesized. Next, bimetallic Au@Pt nanoparticles were synthesized with atomically thin Pt shells, further increasing the Pt utilization. Graphene was chosen as support due to its unique properties, such as high charge carrier mobility, high conductivity, mechanical strength, and high surface area. A further advantage is the chemical inertness under PEMFC operating conditions.
Chemical anchoring of Pt and Au@Pt nanoparticles was achieved via L-cysteine linker molecules that provide pathways for fast electron transfer during the electro-catalytic reactions. Electrochemical properties of selfassembled L-cysteine monolayers immobilized on single-crystal Au(111) surfaces in ionic liquids were studied and their structures imaged by scanning tunneling microscopy (STM), to investigate the nature of L-cysteine bonds on Au.
Au increases the Pt d-band energy, resulting in stronger bonding to fuel molecules, and higher current densities and power densities in PEMFCs. Furthermore, Au cores protect Pt atomic layers from poisoning. A G-Cys-Au@Pt electrocatalyst in the PEMFC anode setup exhibited superior performance compared to the commercial catalyst used in industry. This was mainly attributed to unique behavior of water while interacting with hydrophopic graphene.
Finally, a direct formic acid, methanol, and ethanol PEMFC station was established. Here, as-synthesized graphene-immobilized Au@Pt nanoparticles exhibit high electrocatalytic performance as anode materials with long stability.
In conclusion, electrochemical power sources pose a promising alternative in a time of depleting fossil fuels. Electrocatalysts which are efficient relative to their consumption of both energy and Pt offer here new perspectives in PEMFC applications.
Illustration:
Scheme of representative FC systems: AFC, PEMFC, DMFC, PAFC, and SOFC. The fuel oxidation occurs at the anode while oxygen is reduced to water at the cathode. Ionic species involved in the reactions of different FC technologies is noted as the electrolyte.