Platinum-Based Nanowire Networks with Enhanced Oxygen-Reduction Activity

Pt-Al and Pt-Y-Al thin-film electrodes on yttria-stabilized zirconia electrolytes were prepared by dealloying of cosputtered Pt-Al or Pt-Y-Al films. The selective dissolution of Al from the Pt-alloy compound causes the formation of a highly porous nanowire network with a mean branch thickness below 25 nm and a pore intercept length below 35 nm. The oxygen-reduction capability of the resulting electrodes is analyzed in a micro-solid-oxide fuel-cell setup at elevated temperatures (598–873 K). Here, we demonstrate that these nanoporous thin films excel “state-of-the-art” fuel-cell electrodes in terms of catalytic activity and thermal stability. The nanoporous Pt electrodes exhibit exchange-current densities that are up to 13 times higher than conventional Pt electrodes, measured at 648 K. It is shown that the enhanced intrinsic electrocatalytic activity of these Pt electrodes is achieved by the addition of yttrium as a ternary constituent, which allows for an engineering of the material’s band structure.

Figure 1

Nanowire networks. (a) Three-dimensional (3D) reconstructed FIB thin-film tomography of a ${\mathrm{Pt}}_{0.70}{\mathrm{Al}}_{0.30}$ nanowire network on a YSZ single crystal. (b) Typical scanning electron microscopy top-view image of a nanoporous ${\mathrm{Pt}}_{0.70}{\mathrm{Al}}_{0.30}$ nanowire network. The nanowires are interconnected and are characterized by a mean length of 30 nm and a mean diameter of 12 nm. (c),(d) FIB-polished cross-sections of ${\mathrm{Pt}}_{0.70}{\mathrm{Al}}_{0.30}$ and ${\mathrm{Pt}}_{0.60}{\mathrm{Y}}_{0.26}{\mathrm{Al}}_{0.14}$ nanowire networks on a YSZ single crystal before and after annealing at 873 K for 24 h. (e) Porosity before and after annealing of a ${\mathrm{Pt}}_{0.60}{\mathrm{Y}}_{0.26}{\mathrm{Al}}_{0.14}$ film plotted as a function of the film height perpendicular to the film-substrate interface and compared to the porosity of ${\mathrm{Pt}}_{0.72}{\mathrm{Al}}_{0.28}$ films from Ref. [29].

Figure 2

Fuel-cell setup and device performance. (a) Schematic illustration of the experimental fuel-cell setup. (b) Current-voltage characteristics of the ${\mathrm{Pt}}_{0.60}{\mathrm{Y}}_{0.26}{\mathrm{Al}}_{0.14}$ fuel cells fitted by Eq. (2) (lines) at five different temperatures. (c) Measured peak power density $\rho$ as a function of the operating temperature compared to data from the literature [33, 36]. The orange dotted lines with symbols show an extrapolation of the power density (Pt-Y-Al electrodes) for two different electrolyte thicknesses ($100\mu \mathrm{m}$, $5\mu \mathrm{m}$). The error bars represent the standard deviation between different cells.

Figure 3

FIB-polished cross-sections of a symmetric fuel cell with ${\mathrm{Pt}}_{0.70}{\mathrm{Al}}_{0.30}$ thin films as electrodes deposited on a $5\mu \mathrm{m}$ YSZ foil with a mean grain size of 250 nm. The detail of the picture shows the electrode’s nanostructure.

Figure 4

Exchange-current density and thermal activation. The dotted and dashed lines correspond to Arrhenius fits. Literature data for Pt thin-film electrodes is shown in gray [33]. (a) Exchange-current density $i^{*}$ determined by fitting the Tafel equation [Eq. (1)] to the cells’ $I\text{−}V$ curves. (b) The surface-area-normalized current density $j^{*}$ is plotted as a function of the temperature. The error bars indicate the error accompanied by the fitting procedure. (c) Exchange-current density $i^{*}$ for the ORR and HOR reactions obtained from fits using the Butler-Volmer model [Eq. (2)]. (d) The surface-area-normalized current density $j^{*}$ for the ORR and HOR reactions are plotted as a function of the temperature.