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.).

Activities of Cu nanoparticles supported on carbon black (VC), single-wall carbon nanotubes (SWNTs), and Ketjen Black (KB) toward CO2 electroreduction to hydrocarbons (CH4, C2H2, C2H4, and C2H6) are evaluated using a sealed rotating disk electrode (RDE) setup coupled to a gas chromatograph (GC). Thin films of supported Cu catalysts are deposited on RDE tips following a procedure well-established in the fuel cell community. Lead (Pb) underpotential deposition (UPD) is used to determine the electrochemical surface area (ECSA) of thin films of 40 wt % Cu/VC, 20 wt % Cu/SWNT, 50 wt % Cu/KB, and commercial 20 wt % Cu/VC catalysts on glassy carbon electrodes. Faradaic efficiencies of four carbon-supported Cu catalysts toward CO2 electroreduction to hydrocarbons are compared to that of electrodeposited smooth Cu films. For all the catalysts studied, the only hydrocarbons detected by GC are CH4 and C2H4. The Cu nanoparticles are found to be more active toward C2H4 generation versus electrodeposited smooth copper films. For the supported Cu nanocatalysts, the ratio of C2H4/CH4 Faradaic efficiencies is believed to be a function of particle size, as higher ratios are observed for smaller Cu nanoparticles. This is likely due to an increase in the fraction of under-coordinated sites, such as corners, edges, and defects, as the nanoparticles become smaller.

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...

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Ultraporous copper/titanium dioxide (Cu/TiO) aerogels supporting <5 nm diameter copper nanoparticles are active for surface plasmon resonance (SPR)-driven photocatalysis. The extended nanoscale Cu‖TiO junctions in Cu/TiO composite aerogels-which arise as a result of photodepositing copper at the surface of the nanoparticulate-bonded TiO aerogel architecture-stabilize Cu against oxidation to an extent that preserves the plasmonic behavior of the nanoparticles, even after exposure to oxidizing conditions. The metallicity of the Cu nanoparticles within the TiO aerogel is verified by aberration-corrected scanning transmission electron microscopy, electron energy-loss spectroscopy, and infrared spectroscopy using CO binding as a probe to distinguish Cu(0) from Cu(i). In contrast, photoreduction of Cu(ii) at a commercial nanoscale anatase TiO powder with primary particle sizes significantly larger than those in the aerogel results in a copper oxide/TiO composite that exhibits none of the plasmonic character of Cu nanoparticles. We attribute the persistence of plasmonic Cu nanoparticles without the use of ligand stabilizers to the arrangement of Cu and TiO within the aerogel architecture where each Cu nanoparticle is in contact with multiple nanoparticles of the reducing oxide. The wavelength dependence of the photoaction spectra for Cu/TiO aerogel films reveals visible-light photocatalytic oxidation activity initiated by an SPR-driven process-as opposed to photo-oxidation initiated by excitation of narrow-bandgap copper oxides.

Lithium-ion batteries are prone to failure at low temperatures and dendrite growth during charging is one suspect. We attempt to understand lithium dendrite growth by observing their number, initiation time and growth rate at ambient and sub-ambient temperatures: −10 • C, 5 • C, and 20 • C using an in-situ optical microscopy cell (Li 0 |Li 0 ). We find that while dendrites initiate quickly at −10 • C, the cells at 5 • C short-circuit most rapidly due in part to a favorable morphology at this temperature. The experimental approach has broad applicability to other electrochemical energy storage technologies where mass transport limitations are present at low temperatures, particularly Li-air, Li-S, and Zn-air batteries. © The Author(s) 2014. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: oa@electrochem.org. [DOI: 10.1149/2.0041502eel] All rights reserved.Manuscript submitted October 29, 2014; revised manuscript received November 24, 2014. Published December 11, 2014 Lithium dendrite formation at the anode during charging of lithium-ion batteries can initiate internal short circuits, a known failure mechanism. Dendrite-induced short circuits at low temperature have been identified as a causal factor in recent lithium-ion battery failures aboard commercial aircraft.1 Low temperature charging affects the kinetic processes occurring within the cell causing lithium ions to reduce and plate metallic lithium (Li 0 ) on the carbon anode rather than favoring ion intercalation into the carbon electrode, according to Equation 1. At best, this Li 0 deposition is manifested as uniform plating leading to a loss of useful capacity of the battery. In the worst case the Li 0 electrodeposits grow uncontrollably to form an internal short to the cathode capable of initiating the thermal runaway reaction. 2Factors which favor unstable Li + reduction to form dendrites rather than ion intercalation at low temperature include: lower solubility of Li + within the liquid electrolyte, greater ion pairing between Li + and the anion (PF 6 − ), and increased viscosity of electrolyte causing slowed ion diffusion.Akolkar recently developed a temperature-dependent diffusionreaction model to predict dendrite growth rate based upon the ratio of the dendrite tip current density to the current density on a flat lithium surface. 4 The model is built upon the principle that dendrite growth is more pronounced at low temperature due to increased mass transport resistance of lithium ions through a viscous electrolyte and reduced charge transfer resistance created by a thinner solid electrolyte interface (SEI) layer. The model predicts −10• C as the critical temperature below whic...

The durability of PEM fuel cell materials is essential to application longevity. This research explores the limits of thermal stability of platinum/Vulcan XC 72 catalysts and a 46 wt % Pt/Vulcan XC 72/Nafion catalyst layer. The thermal stability of Pt/Vulcan XC 72 catalysts and the PEM fuel cell catalyst layer is studied by thermal gravimetric analysis (TGA) in air. The products of decomposition are analyzed with TGA coupled with mass spectrometry (TGA−MS). Low temperature (100−200 °C) carbon combustion in the presence of platinum is confirmed. The high precision and sensitivity of TGA allows differentiation of two oxidative/mass loss regimes for 46% Pt/C. The presence of surface protective groups raises the activation energy for the low-temperature/low-conversion (≤5%) oxidation of 46% Pt/Vulcan XC 72 (197 ± 13 kJ/mol) compared to a higher temperature/higher conversion level (10−30%) process (140 ± 10 kJ/mol). In PEM fuel cell catalyst layers, the thermal decomposition temperature of Nafion is lowered by about 100 °C to 300 °C in the presence of Pt/C catalysts. As a result of the above studies it was found that TGA is convenient for the determination of Pt wt % in catalyst-coated membranes.

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