RESUMO
Cathode catalyst layers of proton exchange membrane fuel cells (PEMFCs) typically consist of carbon-supported platinum catalysts with varying weight ratios of proton-conducting ionomers. N-Doping of carbon support materials is proposed to enhance the performance and durability of the cathode layer under operating conditions in a PEMFC. However, a detailed understanding of the contributing N-moieties is missing. Here, we report the successful synthesis and fuel cell implementation of Pt electrocatalysts supported on N-doped carbons, with a focus on the analysis of the N-induced effect on catalyst performance and durability. A customized fluidized bed reduction reactor was used to synthesize highly monodisperse Pt nanoparticles deposited on N-doped carbons (N-C), the catalytic oxygen reduction reaction activity and stability of which matched those of state-of-the-art PEMFC catalysts. Operando high-energy X-ray diffraction experiments were conducted using a fourth generation storage ring; the light of extreme brilliance and coherence allows investigating the impact of N-doping on the degradation behavior of the Pt/N-C catalysts. Tests in liquid electrolytes were compared with tests in membrane electrode assemblies in single-cell PEMFCs. Our analysis refines earlier views on the subject of N-doped carbon catalyst supports: it provides evidence that heteroatom doping and thus the incorporation of defects into the carbon backbone do not mitigate the carbon corrosion during high-potential cycling (1-1.5 V) and, however, can promote the cell performance under usual PEMFC operating conditions (0.6-0.9 V).
RESUMO
The reduction of Pt content in the cathode for proton exchange membrane fuel cells is highly desirable to lower their costs. However, lowering the Pt loading of the cathodic electrode leads to high voltage losses. These voltage losses are known to originate from the mass transport resistance of O2 through the platinum-ionomer interface, the location of the Pt particle with respect to the carbon support and the supports' structures. In this study, we present a new Pt catalyst/support design that substantially reduces local oxygen-related mass transport resistance. The use of chemically modified carbon supports with tailored porosity enabled controlled deposition of Pt nanoparticles on the outer and inner surface of the support particles. This resulted in an unprecedented uniform coverage of the ionomer over the high surface-area carbon supports, especially under dry operating conditions. Consequently, the present catalyst design exhibits previously unachieved fuel cell power densities in addition to high stability under voltage cycling. Thanks to the Coulombic interaction between the ionomer and N groups on the carbon support, homogeneous ionomer distribution and reproducibility during ink manufacturing process is ensured.
RESUMO
Anion substitution is an emerging strategy to enhance the photoelectrochemical performance of metal oxide photoelectrodes. In the present work, we investigate the effect of fluorine incorporation on the photoelectrochemical water oxidation performance of BiVO4 and Mo:BiVO4 thin film photoanodes. The BiVO4 and Mo:BiVO4 thin film photoanodes were prepared by a straightforward organometallic solution route involving dip coating and subsequent calcination in air. Fluorine modification was realized by applying a soft and low-cost solid-vapor reaction route involving fluorine-containing polymers and an inert gas atmosphere leading to novel F:BiVO4 and F/Mo:BiVO4 thin film photoanodes with substantially increased photoelectrochemical water oxidation properties. Deposition of the cobalt phosphate (CoPi) water oxidation catalyst allowed further enhancement of the photoelectrochemical performance. While Mo doping mainly improves light-harvesting, charge transport, and charge separation efficiencies, F modification was demonstrated to primarily affect the charge transfer efficiency at the semiconductor-electrolyte interface, thereby leading to a photocurrent increase of 40 and 21% upon fluorination of the BiVO4 and Mo:BiVO4 photoanodes, respectively, and an applied bias photon-to-current efficiency increase of 35 and 5%, respectively. We thereby could demonstrate that cation and anion co-doping in BiVO4 as demonstrated for Mo and F allows combining the photoelectrochemically relevant benefits associated with each type of dopant.
