Abstract
Rotating disk electrode experiments were used to demonstrate the influence of dissolved Ru species on the oxygen reduction activity of a Pt/C electrocatalyst. Dissolved Ru in micromolar levels was found to deposit instantly onto Pt, thereby blocking the electrode surface for ORR at low overpotentials. Ru contamination can decrease oxygen reduction kinetics by eightfold or increase the overpotential by ca. 160 mV. This facet of fuel cell durability needs special attention from the perspective of appropriate materials choice, i.e., preventing the leaching of Ru from PtRu anodes and its crossover to the cathode across the membrane electrolyte.
Abstract
Noble metal chalcogens are the preferred choice of electrocatalyst materials over pure metals and metal alloys in many industrial processes involving operation in highly corrosive environment. The depolarised electrolysis of hydrochloric acid represents one of such processes; in this case, rhodium sulfide is incorporated into gas diffusion electrode structure for use as oxygen-consuming cathode. An increased dispersion of the rhodium/sulfurcompound is an evident goal to obtain highly active catalyst systemswhilemaintaining similar activity. In studying the effect ofRhxSy loading on carbon on oxygen reduction reaction activity, it is paramount to understand and optimize the structure sensitivity of the reaction.Doing so will not only aid in determining the optimal metal loading but more importantly will control the commercial viability of the electrocatalyst. In the present work, the RhxSy loading onVulcanXC72-R was studied in terms of morphological characteristics andORR activity. Physicochemical characterization suggests that the preparation methodology of such chalcogens plays a fundamental role in terms of chemical structure. ORR kinetics was addressed using a series of rotating disk electrode experiments in 1 MHCl electrolyte, in which the optimal RhxSy loading was found to be at 15 wt.%, a value two times lower with respect to the commercially available 30 wt.% material. Any higher dispersion results in no significant increase in the overall electrocatalytic performance. Based on the increased Rh utilization and enhanced activity found for the low-loading RhxSy/C samples, we report a significant development in terms of materials and material employment for the oxygen depolarized electrolysis of HCl.
Site Specific vs. Specific Absorption of Anions on Pt and Pt Based Alloys
Abstract
X-ray absorption spectroscopy (XAS) is utilized in situ to gain new insights into the electronic and chemical interactions of anions specifically adsorbed on Pt/C. A novel difference methodology was utilized, along with full-multiple scattering calculations using the FEFF8 code, to interpret the X-ray absorption near edge structure (XANES). Significant direct contact (“specific”) anion adsorption occurs in 1 M H2SO4 and 6 M TFMSA, while it does not in 1 M HClO4 and 1 M TFMSA. This specific anion adsorption significantly hinders O(H) chemisorption, particularly formation of subsurface O, causes the Pt nanoparticle to become more round, and weakens the Pt−Pt bonding at the surface. The specific anion adsorption becomes site-specific only after lateral interactions from other chemisorbed species such as OH on the surface force the anions to adsorb into specific sites. Alloying has a profound effect on the strength of the anion adsorption and whether site-specific or just specific adsorption occurs.
Chalcogenide Electrocatalysis for Oxygen Depolarized Aqueous Hydrochloric Acid Electrolysis
J. M. Ziegelbauer, A. F. Gulla, C. O’Laoire, C. Urgeghe, R. J. Allen and S. Mukerjee Electrochim. Acta,52, 6282 (2007)
Abstract
Several Rh- and Ru-based carbon-supported chalcogenide electrocatalysts were evaluated as oxygen-depolarized cathodes for HCl electrolysis applications. The roles of both crystallinity and morphology of the electrocatalysts were explored by investigating several synthetic processes for materials, specifically patented E-TEK methods and the non-aqueous method. The activity of the electrocatalysts for ORR was evaluated via RDE studies in 0.5 M HCl, and compared to state of the art Pt/C and Rh/C systems. RhxSy/C, CoxRuySz/C, and RuxSy/C materials synthesized from the E-TEK methods exhibited appreciable stability and activity for ORR under these conditions. The amorphous non-aqueous moieties, while exhibiting little depolarization due to the presence of high concentrations of Cl− in the RDE studies, were unsuitable for operation in a true ODC HCl electrolyzer cell because of irreversible dissolution resulting from the high concentration (5 M) of HCl. In contrast, the Ru-based materials from the E-TEK methods were unaffected by the depolarizing conditions of an uncontrolled shutdown. These Ru-based electrocatalysts, being on the order of seven times less expensive than the state of the art RhxSy material, may prove to be of economic benefit to the HCl electrolysis industry.
CO Coverage/Oxidation Correlated with PtRu Electrocatalyst Particle Morphology in 0.3 M Methanol by in situ XAS
Abstract
In situ X-ray absorption spectroscopy (XAS) measurements, including both X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) at the Pt L3 and Ru K edges, were carried out on three different carbon-supported PtRu electrocatalysts in an electrochemical cell in 1 M HClO4 with 0.3 M methanol. The CO and OH adsorbate coverage on Pt and Ru were determined as a function of the applied potential via the novel delta XANES technique, and the particle morphology was determined from the EXAFS and a modeling technique. Both the bifunctional and direct CO oxidation mechanisms, the latter enhanced by electronic ligand effects, were evident for all three electrocatalysts; however, the dominant mechanism depended critically on the particle size and morphology. Both the Ru island size and overall cluster size had a very large effect on the CO oxidation mechanism and activation of water, with the bifunctional mechanism dominating for more monodispersed Ru islands, and the direct surface ligand effect dominating in the presence of larger Ru islands.
5 M) of HCl. In contrast, the Ru-based materials from the E-TEK methods were unaffected by the depolarizing conditions of an uncontrolled shutdown. These Ru-based electrocatalysts, being on the order of seven times less expensive than the state of the art RhxSy material, may prove to be of economic benefit to the HCl electrolysis industry.