No abstract

Facile sol–gel synthesis of Mo:BiVO4 thin films with optimized morphology results in reduced surface recombination and enhanced hole transfer efficiency.

Nitrogen-enriched porous carbons have been discussed as supports for Pt nanoparticle catalysts deployed at cathode layers of polymer electrolyte membrane fuel cells (PEMFC). Here, we present an analysis of the chemical process of carbon surface modification using ammonolysis of preoxidized carbon blacks, and correlate their chemical structure with their catalytic activity and stability using in situ analytical techniques. Upon ammonolysis, the support materials were characterized with respect to their elemental composition, the physical surface area, and the surface zeta potential. The nature of the introduced N-functionalities was assessed by X-ray photoelectron spectroscopy. At lower ammonolysis temperatures, pyrrolic-N were invariably the most abundant surface species while at elevated treatment temperatures pyridinic-N prevailed. The corrosion stability under electrochemical conditions was assessed by in situ high-temperature differential electrochemical mass spectroscopy in a single gas diffusion layer electrode; this test revealed exceptional improvements in corrosion resistance for a specific type of nitrogen modification. Finally, Pt nanoparticles were deposited on the modified supports. In situ X-ray scattering techniques (X-ray diffraction and small-angle X-ray scattering) revealed the time evolution of the active Pt phase during accelerated electrochemical stress tests in electrode potential ranges where the catalytic oxygen reduction reaction proceeds. Data suggest that abundance of pyrrolic nitrogen moieties lower carbon corrosion and lead to superior catalyst stability compared to state-of-the-art Pt catalysts. Our study suggests with specific materials science strategies how chemically tailored carbon supports improve the performance of electrode layers in PEMFC devices.

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.

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 Ndoped 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).