The broad area of the research activity in the group is numerical study of chemical reactions in condensed phases using the density functional theory based molecular dynamics (DFTMD) method ("Car-Parrinello"). This method combines electronic structure calculation of the Density Functional Theory (DFT) variety and Molecular Dynamics (MD). Reactive species and their liquid or solid environment are treated at strictly the same level of theory. This is crucial for the application to reactions at electrochemical interfaces controlled by transitions between localized molecular states in the electrolyte and the delocalized (conducting) states of the electrode. We have developed a DFTMD method for the computation of the free energy of oxidation and acid dissociation reactions based on reversible insertion of electrons and protons. Transfers of electron and proton are treated in a consistent half reaction scheme. This makes it possible to couple oxidation/reduction and (de)protonation. Proton coupled electron transfer is an important redox potential levelling mechanism in energy conversion and storage which has become one of our main topics of research. This is also how the standard hydrogen electrode (SHE) works. Applying this method we can therefore refer the computed redox energies to the SHE and compare directly to experiment. Using the solvation free energy of the proton as reference also enables us to compute key quantities in interfacial electrochemistry such as the potential of polarisable electrodes, acidities of metal oxide surfaces and structure and capacitance of electrical double layers. Systems currently under investigation include the interface between aqueous electrolytic solutions and reducible metal oxides such as TiO2 (see figure), SnO2 and MnO2 . These and other metal (hydr)oxides are used in heterogeneous electrocatalytic oxidation and reduction (for example in environmental cleanup) and as electrodes in batteries and fuel cells. The DFTMD code we use is CP2K (cp2k.berlios.de).
Figure: DFTMD model system of a TiO2 rutile (110) water interface at the point of zero net proton charge (PZC) consisting of 48 TiO2 units (Ti ions in yellow) and 71 H2O molecules in a periodically replicated orthorhombic MD cell (size 11.9 × 13.2 × 24.2 Å). The system has been reduced by, inserting one electron and one proton. The excess electron visualized by the green spin density contours occupies a semilocalized orbital in the TiO2 slab. The proton is attached to the highlighted water molecule. The free energy of the double insertion gives the electron affinity of TiO2 (effectively the conduction band minimum) in contact with water relative to SHE after adding the gas phase formation energy of H+ and a correction for zero point energy of the proton. The value we find using the PBE functional is -0.6 V versus SHE compared to the experimental value of -0.35 V.