A tutorial is provided for methods to accurately and reproducibly determine the activity of Pt-based electrocatalysts for the oxygen reduction reaction in proton exchange membrane fuel cells and other applications. The impact of various experimental parameters on electrocatalyst activity is demonstrated, and explicit experimental procedures and measurement protocols are given for comparison of electrocatalyst activity to fuel cell standards. (To listen to a podcast about this article, please go to the Analytical Chemistry multimedia page at pubs.acs.org/page/ancham/audio/index.html.).

Hydrous ruthenium oxide (RuO2·xH2O or RuO x H y ) is a mixed proton−electron conductor which could be used in fuel cells and ultracapacitors. Its charge-storage (pseudocapacitance) and electrocatalytic properties vary with water content and are maximized near the composition RuO2·0.5 mol % H2O. We studied the atomic structure of RuO2·xH2O as a function of water content from x = 0.84 to 0.02 using X-ray diffraction and atomic pair density function (PDF). Even though the diffraction patterns of samples containing 0.84 to 0.35 mole of water are suggestive of “amorphous” structures, the PDF analysis clearly shows that up to 0.7 nm, the short-range atomic structure of all of these RuO2·xH2O samples resembles that of the anhydrous rutile RuO2 structure. We conclude that RuO2·xH2O is a composite of anhydrous rutile-like RuO2 nanocrystals dispersed by boundaries of structural water associated with Ru−O. Metallic conduction is supported by the rutile-like nanocrystals, while proton conduction is facilitated by the structural water along the grain boundaries. This structural picture explains the charge-storage and electrocatalytic properties of RuO2·xH2O in terms of competing percolation networks of metallic and protonic conduction pathways, that vary in volume as a function of the water content of the RuO2·xH2O. The control and optimization of electron and proton conducting volumes and pathways will lead to improved performance and guide the design of new materials.

No abstract

The poisoning of the oxygen-reduction reaction ͑ORR͒ by adsorbed sulfur-containing species was quantified for platinum fuel-cell materials using rotating ring disk electrode methodology. Electrodes of Pt on Vulcan carbon ͑Pt/VC͒ were contaminated by submersion in SO 2 -containing solutions. The initial sulfur coverage of the Pt was determined from the total charge consumed as the sulfur was oxidized from S 0 at 0.05 V ͑vs a reversible hydrogen electrode͒ to water-soluble sulfate ͑SO 4 2− ͒ at Ͼ1.3 V. Electrodes were then evaluated for their ORR activity. Significant ͑33%͒ loss in Pt mass activity was measured when approximately 1.2% of the Pt surface had adsorbed the sulfur-containing species. Sulfur coverage of 14% caused a 95% loss in mass activity. When 37% of the Pt surface was covered with sulfur, the reaction pathway of the ORR on the Pt/VC catalyst changed from a 4-electron to 2-electron process reaction for peroxide, a reagent which can aggressively attack Nafion. We conclude that adsorbed sulfur is not removed under typical steady-state operating conditions of a proton exchange membrane fuel cell, so it will affect operation by decreasing mass activity of the catalysts and by enhancing formation of the deleterious H 2 O 2 by-product.For successful operation of commercial proton exchange membrane fuel cells ͑PEMFCs͒, the cathode ͑air͒ catalyst must maintain a high activity for the oxygen reduction reaction ͑ORR͒ over extended periods of time. Activity losses arise from multiple factors, including the corrosion and/or poisoning of the platinum/Vulcan carbon ͑Pt/VC͒ catalysts. Common air-borne poisons are sulfur dioxide ͑SO 2 ͒, nitrogen dioxide ͑NO 2 ͒, and organic contaminants, all of which have deleterious effects on PEMFC performance. [1][2][3][4][5][6] The poisoning studies referenced above were performed on fuelcell membrane electrode assemblies ͑MEAs͒ whereby the current or power densities of the fuel cell were monitored as a function of the concentration of poisons and time. Some of the studies also characterized poisoning of the MEAs by cyclic voltammetry. 3-6 While measuring in situ the gross electrochemical effects that occur when the electrodes are poisoned, these experiments reveal little analytical information about changes in the catalyst kinetics. A further complication is that the catalyst in MEAs is exposed to a mixed phase of gas and water vapor, and the relative concentrations of the poisons in each phase are not known. The effect of SO 2 on Pt in acid electrolyte can be variable, with different sulfur-containing species either enhancing or diminishing Pt electrocatalysis. [7][8][9][10][11] The oxidation state of sulfur changes with potential. At 0.05 V, sulfur is in a zero-valent state ͑S 0 ͒. Sulfur adsorbed on Pt is easily electro-oxidized at high potentials to sulfate ͑SO 4 2− ͒, which desorbs from the Pt surface. [12][13][14][15][16][17][18][19][20][21][22][23] The speciation products of the SO 2 adsorption are still in discussion for the intermediate potentials where fuel cells ope...

Rotating disk electrode (RDE) voltammetry has been touted as a simple means for benchmarking the oxygen reduction reaction (ORR) activity of platinum-based electrocatalysts in proton exchange membrane fuel cells. In practice, the RDE methodology has been highly variable across laboratories, with up to 20% differences in values for the Pt electrochemical surface area and 10x differences in mass activity and area-specific activity reported from the same standard Pt/C electrocatalyst. We confirm that the same ORR activities can be replicated across laboratories when a detailed experimental protocol is followed. From our work and others', we conclude that dominant factors in the RDE experimental protocol include the ink formulation, electrocatalyst film quality and the electrochemical procedures. We make a recommendation for procedures for the reproducible characterization of Pt/C commercial catalysts, present simple metrics for researchers to use for a quick check of their results, and also propose new benchmark values for two Pt/C standards.