Understanding the transport of water in a polymer electrolyte membrane, for example, Nafion, is important for optimizing and predicting fuel cell performance. It had been shown that the interfacial ...resistance could be responsible for a significant portion of the net water transport. In this study, water vapor sorption experiments were done on thin ionomer films (0.03–3 μm thick) by supporting them on a quartz-crystal microbalance (QCM). The high mass resolution of the QCM enables determination of the membrane water content (λ) of the thin films. A slight depression of the water content was observed in very thin films. Meanwhile, study on thin ionomer films minimizes the contribution of internal water diffusion in the film and allows for simplification of the analyses. Interfacial mass-transfer and mechanical relaxation coefficients were determined as a function of water content and temperature. Empirical equations for the mass-transfer coefficient governing absorption and desorption, each as a function of temperature and ionomer water content, were proposed.
Substantial progress has been made in reducing proton-exchange membrane fuel cell (PEMFC) cathode platinum loadings from 0.4–0.8 mgPt/cm2 to about 0.1 mgPt/cm2. However, at this level of cathode Pt ...loading, large performance loss is observed at high-current density (>1 A/cm2), preventing a reduction in the overall stack cost. This next developmental step is being limited by the presence of a resistance term exhibited at these lower Pt loadings and apparently due to a phenomenon at or near the catalyst surface. This issue can be addressed through the design of catalysts with high and stable Pt dispersion as well as through development and implementation of ionomers designed to interact with Pt in a way that does not constrain oxygen reduction reaction rates. Extrapolating from progress made in past decades, we are optimistic that the concerted efforts of materials and electrode designers can resolve this issue, thus enabling a large step toward fuel cell vehicles that are affordable for the mass market.
The success of PEM fuel cells lies in the design of the cathode where C-supported Pt catalyst is covered by a thin layer of ionomer, allowing easy access of oxygen and protons. The high current ...densities (1-2 A/cm2) are achieved by the optimization of the thickness of the ionomer between two opposing trends: thinner ionomer allows high oxygen through-plane diffusion, while the in-plane conduction of the protons requires reasonable thickness. Anisotropy is likely to develop in such thin films. The purpose of this work was to investigate the effect of a thinner ionomer layer on the diffusion and conductivity and to establish how thin the ionomer layer can be. Oxygen permeability (DH) and proton conductivity in ultra-thin Nafion films (100-1000nm) were respectively characterized on SiO2 substrate via limiting current measurement and electrochemical impedance spectroscopy for a range of temperature and relative humidity conditions. The transport behavior of these films is comparable to the electrode membrane layer of PEM fuel cells. Values of oxygen permeability were shown to increase with decreasing film thickness. The oxygen diffusion through the gas boundary layer becomes significant only at ultra-thin film thickness. The proton conductivity decreases with decreasing thickness and was disproportional to bulk Nafion behavior. Post-test SEM imaging shows buckling of the thin layer on the silicon wafer between the Pt sites, partially explaining the opposing trends in diffusivity and conductivity as film thickness decreases. Anisotropy exists as the thickness becomes comparable to the size of the aqueous regions of the Nafion ionomer. Measurements of thinner layers (<100nm) were experimentally limited, yet the trends indicate that the thinness of the ionomer in the cathode is limited by its ability to allow proton conductance.
Different-sized CdSe quantum dots have been assembled on TiO2 films composed of particle and nanotube morphologies using a bifunctional linker molecule. Upon band-gap excitation, CdSe quantum dots ...inject electrons into TiO2 nanoparticles and nanotubes, thus enabling the generation of photocurrent in a photoelectrochemical solar cell. The results presented in this study highlight two major findings: (i) ability to tune the photoelectrochemical response and photoconversion efficiency via size control of CdSe quantum dots and (ii) improvement in the photoconversion efficiency by facilitating the charge transport through TiO2 nanotube architecture. The maximum IPCE (photon-to-charge carrier generation efficiency) obtained with 3 nm diameter CdSe nanoparticles was 35% for particulate TiO2 and 45% for tubular TiO2 morphology. The maximum IPCE observed at the excitonic band increases with decreasing particle size, whereas the shift in the conduction band to more negative potentials increases the driving force and favors fast electron injection. The maximum power-conversion efficiency ≤1% obtained with CdSe−TiO2 nanotube film highlights the usefulness of tubular morphology in facilitating charge transport in nanostructure-based solar cells. Ways to further improve power-conversion efficiency and maximize light-harvesting capability through the construction of a rainbow solar cell are discussed.
The use of single wall carbon nanotubes (SWCNTs) as conduits for transporting electrons in a photoelectrochemical solar cell and electronic devices requires better understanding of their ...electron-accepting properties. When in contact with photoirradiated TiO2 nanoparticles, SWCNTs accept and store electrons. The Fermi level equilibration with photoirradiated TiO2 particles indicates storage of up to 1 electron per 32 carbon atoms in the SWCNT. The stored electrons are readily discharged on demand upon addition of electron acceptors such as thiazine and oxazine dyes (reduction potential less negative than that of the SWCNT conduction band) to the TiO2–SWCNT suspension. The stepwise electron transfer from photoirradiated TiO2 nanoparticles → SWCNT → redox couple has enabled us to probe the electron equilibration process and determine the apparent Fermi level of the TiO2–SWCNT system. A positive shift in apparent Fermi level (20–30 mV) indicates the ability of SWCNTs to undergo charge equilibration with photoirradiated TiO2 particles. The dependence of discharge capacity on the reduction potential of the dye redox couple is compared for TiO2 and TiO2–SWCNT systems under equilibration conditions.
We demonstrate the unprecedented proton exchange membrane fuel cell (PEMFC) performance durability of a family of dealloyed Pt-Ni nanoparticle catalysts for the oxygen reduction reaction (ORR), ...exceeding scientific and technological state-of-art activity and stability targets. We provide atomic-scale insight into key factors controlling the stability of the cathode catalyst by studying the influence of particle size, the dealloying protocol and post-acid-treatment annealing on nanoporosity and passivation of the alloy nanoparticles. Scanning transmission electron microscopy coupled to energy dispersive spectroscopy data revealed the compositional variations of Ni in the particle surface and core, which were combined with an analysis of the particle morphology evolution during PEMFC voltage cycling; together, this enabled the elucidation of alloy structure and compositions conducive to long-term PEMFC device stability. We found that smaller size, less-oxidative acid treatment and annealing significantly reduced Ni leaching and nanoporosity formation while encouraged surface passivation, all resulting in improved stability and higher catalytic ORR activity. This study demonstrates a successful example of how a translation of basic catalysis research into a real-life device technology may be done.
Single wall carbon nanotube (SWCNT) architecture when employed as conducting scaffolds in a TiO2 semiconductor based photoelectrochemical cell can boost the photoconversion efficiency by a factor of ...2. Titanium dioxide nanoparticles were dispersed on SWCNT films to improve photoinduced charge separation and transport of carriers to the collecting electrode surface. The shift of ∼100 mV in apparent Fermi level of the SWCNT−TiO2 system as compared to the unsupported TiO2 system indicates the Fermi level equilibration between the two systems. The interplay between the TiO2 and SWCNT of attaining charge equilibration is an important factor for improving photoelectrochemical performance of nanostructured semiconductor based solar cells. The feasibility of employing a SWCNT−TiO2 composite to drive the water photoelectrolysis reaction has also been explored.
Development of electrocatalysts with higher activity and stability is one of the highest priorities in enabling cost-competitive hydrogen-air fuel cells. Although the rotating disk electrode (RDE) ...technique is widely used to study new catalyst materials, it has been often shown to be an unreliable predictor of catalyst performance in actual fuel cell operation. Fabrication of membrane-electrode assemblies (MEA) for evaluation which are more representative of actual fuel cells generally requires relatively large amounts (>1 g) of catalyst material which are often not readily available in early stages of development. In this study, we present two MEA preparation techniques using as little as 30 mg of catalyst material, providing methods to conduct more meaningful MEA-based tests using research-level catalysts amounts.
One of the major obstacles to the commercialization of proton exchange membrane fuel cells (PEMFCs) is the usage of scarce platinum in the cathode for the oxygen reduction reaction (ORR). Although ...progress has been made in reducing Pt usage by alloying with transition metals M (M = Co, Ni, Cu, etc.), practical applications of Pt-M/C catalysts are impeded by their insufficient durability under the highly corrosive conditions at a PEMFC cathode. Herein, we reconcile the durability difficulty by demonstrating that the high mass activity of the dealloyed PtNi3/C catalyst with low nanoporosity further increases after 30k voltage cycles in PEMFCs. A novel method has been developed to implement an in situ X-ray absorption spectroscopy study of these PEMFC-cycled catalysts under operating conditions to understand the unusual activity trend. We reveal that the ORR activity of PtNi3/C catalysts with varied nanoporosities exhibits a Sabatier volcano curve as a function of the strain governed by Ni content, and the volcano is skewed toward the Pt–O weak binding leg owing to the asymmetric site-blocking effect. The Ni dissolution during PEMFC operation, which was previously believed to be detrimental, becomes beneficial for the solid PtNi3/C catalysts located on the Pt–O weak binding leg because it leads to the activity ascending toward the apex, and meanwhile the activity remains high throughout the long-term operation owing to the minimal site-blocking effect. More generally, the fundamental insights into the universal asymmetric volcano curve of redox catalysis will potentially guide the rational design of a broad variety of catalytic materials.
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