Computational Material Science for Energy Conversion
Dr. Clotilde Cucinotta, CRANN, School of Physics, Trinity College Dublin
The quest for environmentally sustainable energy sources, together with the dramatic increase in energy demand calls for the further development of electrochemical devices such as Fuel Cells (FCs). FCs are electrochemical devices in which a hydrogen rich fuel reacts with an oxidizing agent - normally O2. Fuel and oxygen are split at the opposite sides of an electrolyte, through which, an ion diffuses to combine with an ion with opposing charge, at the opposite side. This produces electrons, i.e. electric power (see fig. 1, which shows how a solid oxide fuel cell (SOFC works). Like a combustion engine, FCs use a chemical fuel to generate power, but as ina battery, the chemical energy is converted directly to electricity by means of an electrochemical process without a combustion step. As a consequence, FCs operate at a much higher efficiency and generate fewer pollutants than combustion methods. Among the various types of FCs, SOFCs are of great interest because of their ability to utilize, besides hydrogen, a wide variety of commonly available fuels - such as carbon monoxide, hydrocarbons, alcohols and ammonia - at high efficiency. Thus SOFCs do not rely on the development of an hydrogen infrastructure and represent a mid-term solution to lower both the energy demand and the CO2 emissions. SOFCs find a natural application as combined heat and power devices for residential applications. They are convenient because they work with the present energy distribution system, and they have the potential to drive the transition towards a hydrogen based economy. The FCs industry has grown worldwide by more than 50% annually over the past four years. However, the gap separating the actual state of the art energy conversion technology from that required for widespread commercialization is still large. Limited life span, performance and costs represent the major limitations of the actual technology.
Figure 1: Operation of a Solid Oxide Fuel Cell (SOFC): At the cathode triple phase boundary (TPB) the oxygen is reduced with charge transfer from the cathode to the electrolyte. The O2- ions diffuse through the electrolyte towards the anode. At the anodic chamber TPB oxidation of the fuel and charge transfer from electrolyte to anode occur.
The first important goal of this research has already been fulfilled and delivered an atomistic understanding of thermodynamics and kinetics of the hydrogen oxidation in commercial SOFCs. Commercial SOFCs offer excellent kinetic efficiency (up to 80%), because of their high operational temperature (600-1000°C). In these cells, a ceramic thin film electrolyte separates the anodic and cathodic compartments, where fuel and oxygen are respectively supplied. The most used electrolyte is Yttria (Y2O3) stabilized Zirconia (ZrO2)1-x(Y2O3)(YSZ), selected for its high ionic conductivity at the operational temperature. The anode is multifunctional, it is believed to serve both as a catalyst for the hydrogen oxidation and as an electron collector. Fuel oxidation occurs at the triple phase boundary (TPB) region, i.e. at the interface between the anode, the electrolyte and the gas phase. Here O2-, provided through the ionic conductor, combines with H2 producing water and electrons. In spite of decades of experimental investigations and kinetic modelling of electrode reactions , the detailed mechanism of hydrogen electro-oxidation at the SOFCs anode (H2 + O2- → H2O + 2e-) is still a matter of debate. It is unclear, for instance, whether the formation of H2O takes place on the metal (Ni) or at the electrolyte (YSZ) surface.
For the development of strategies for optimizing cell technology as well as for identifying next generation materials it is crucial to achieve a microscopic understanding of the electrochemical phenomena that occur at the TPB. A compelling microscopic understanding of these processes is also of paramount importance to gain insights into the direct oxidation of hydrocarbon fuels at SOFCs anodes, as this has been demonstrated in recent years. Computer simulations provide essential tools for the investigation of equilibrium and non-equilibrium properties in condensed matter, and can provide information that cannot be obtained by experiments.
However, simulating such systems and processes in a realistic way presents a challenging task, both because of the number of degrees of freedom and the complexity of the quantum problems involved. This requires not only great accuracy in the description of the potential energy of the systems but also the application / development of novel theoretical methodologies. These difficulties are the reasons behind the delay in advance of theoretical material science in this area. However, with today's powerful computers these challenges can now be confronted. Also thanks to the facilities at the Trinity Centre for High Performance Computing (TCHPC) it has been possible to address hydrogen oxidation at the anodic TPB of SOFCs at a reasonable level of approximation. To this end, a realistic model for the TPB is essential. The structure and transport properties of the Ni/YSZ interface depend on composition and can affect the performance of the cell. The electrode dimension and chemical activity must allow charge transfer from O2- to Ni, as it is this that gives rise to the potential drop at the double layer, during SOFCs operation. Our model for the Ni/YSZ interface is shown in figure 2. This realistic model allowed study of the different processes which lead to water desorption from the TPB and their dependence on the local defective structure of the YSZ surface moiety.
Figure 2: Model of the Ni/TPB interface. The Ni/YSZ interfacial nanostructure is modelled adsorbing a 46 atom Ni cluster on top of a c-YSZ(111) surface. The cluster has the shape of a truncated tetrahedron, obtained cutting off a Ni slab only 111 terminated surfaces. The YSZ surface cell lateral dimensions are that of an ideal hexagonal c-ZrOi2(111):5x5 surface. Substituting in this ideal ZiO2(111) surface 12 Zr with Y atoms introduces 6 O vacancies and models a YSZ with a Yttria concentration of x=8.7%.
The main possible oxidation pathways leading to hydrogen oxidation at the TPB of SOFCs are H+, OH- or O2- spillover. In H+ spillover (HNi &rarr HYSZ+eNi) Ni adsorbed H atoms hop either to a hydroxyl ion or an oxygen ion on the YSZ surface and here combine with oxygen to produce water. In O2- spillover (O2-YSZ &rarr ONi + 2eNi) oxygen ions hop from the YSZ surface to the Ni surface, undergoing two charge-transfer reactions that take place before, after or during the hop. In OH- spillover the OH- formed on YSZ surface after H+ spillover jumps back on to the Ni anode. Following O2- and OH- spillover, water formation occurs on the Ni surface. These elementary steps have been compared by means of ab-initio Density Functional Theory based statical and dynamical methods - as implemented in the CP2K suite of codes  - in terms of thermodynamic stability and activation energies along the selected reaction pathways.
Our research proved that the most favourable oxidation pathway involves hydrogen spillover from Ni to YSZ surface, which represents the rate limiting step of the process. We also found out that water formation and desorption take place on the YSZ surface and is mediated by the diffusion of an hydroxyl group on a Zr site close to the Ni/YSZ interface. The process continues with the formation of water by the direct transfer from Ni cluster of a second H atom and its subsequent release . Understanding the basic principles of operation of novel materials and devices for energy conversion opens new horizons within theoretical material science. It allows one to design new strategies - going beyond a mere trial and error procedure - for improving actual technology, developing new materials and establishing basic relationships in the electrochemistry of interfaces and electrolytes. Much is still to be done in this field, but substantial progress will be made possible by the common effort of the scientific community, also thanks to the increasing availability of computational power.
- M. Vogler, A. Bieberle-Hü tter, L. Gzuckler, J. Warnatz, W. G. Bessler, J. Electrochem. Soc., 2009, 156, B663
- CP2K: http://cp2k.berlios.de
- C. Cucinotta, M. Bernasconi and M. Parrinello, submitted