Extended-surface thin film platinum electrocatalysts with tunable nanostructured morphologies.

Reducing platinum-group metal (PGM) loadings in fuel cells and electrolyzers is paramount for cost reductions and getting hydrogen to scale to help decarbonize the global economy. Conventional PGM nanoparticle-based ink-cast electrocatalysts lose performance at high current densities owing to mass transport resistances that arise due to the use of ionomer binders. Herein, we report the development of binder-free extended surface thin film platinum electrocatalysts with tunable nanoscale morphology and periodic spacing. The electrocatalysts are prepared by sputtering various loadings of platinum on Al2O3 nanostructures templated from block copolymer (BCP) thin films self-assembled on glassy carbon substrates via sequential infiltration synthesis. Testing for oxygen reduction on a rotating disk electrode setup with ultra-low PGM loadings (5.8 µgPt cm-2) demonstrates electrocatalyst performance that rivals commercial platinum electrocatalysts in terms of mass activity (380 mA mgPt-1 at 0.9 V vs RHE), whilst surpassing commercial catalysts in terms of stability (mass activity loss: 11.45% at after 20,000 potential cycles). Moreover, catalyst performance probed as a function of nanoscale feature size and morphology reveals an inverse correlation between particle size and electroactivity, as well as the superiority of cylindrical morphologies over lamellae, presenting BCP templating as a fabrication pathway towards stable, tunable catalyst geometries

Performance evaluation of different bipolar plate designs of 3D planar anode-supported SOFCs

The performance of Solid Oxide Fuel Cells (SOFCs) is highly sensitive to the fluid dynamics, the interfacial areas, and the residence time of the gases. These parameters are primarily dictated by the geometry of the channels carrying the fuel and the oxidant. However, not many investigations have been made to study the effect of bipolar plate designs on cell performance. We report a detailed comparative study of the performance characteristics of straight and serpentine channel geometries. Simulations of these two channels have been made taking into account fluid flow through the channels and the porous electrodes, multicomponent diffusion, heat transfer, charge transfer reaction kinetics and electrodynamics. Performance of each channel has been compared to in-house experimental data. Extensive parametric analyses have been carried out to evaluate the dependence of cell performance on fuel and air flow rates. Favourable operating ranges of hydrogen and air feeds have been estimated analytically taking into account fuel utilisation, cell temperature, channel pressure drops, and current density. It has been shown that serpentine geometries offer remarkably more uniform distribution of ionic current density, and significantly higher power output and fuel utilisation compared to straight channel geometries. However, these are accompanied by a penalty of pressure drop. This analysis can provide a useful guideline for selecting the channel geometry. (C) 2018 Elsevier Ltd. All rights reserved

Effect of Oxygen Diffusion Constraints on the Performance of Planar Solid Oxide Fuel Cells for Variable Oxygen Concentration

Performance enhancement of solid oxide fuel cell (SOFC) has been a very prominent research problem of the present era. One of the key performance-limiting factors is diffusional constraint encountered in SOFC cathodes during operation with air. While attempts at improving cell performance have been made through the optimization of geometries, flow rates, and operating conditions, very few studies have reported the potential for performance enhancement through the optimization of cathode gas composition. The present study takes an experimental as well as theoretical approach to examine the extent to which SOFC performance can be enhanced through alterations in the oxygen concentration of cathode gas (without varying the oxygen molar flow rate). An SOFC is operated for various cathodic oxygen concentrations, and the experimental protocol is simulated using a parametric computational fluid dynamics study. The results depict remarkable nonlinearities in the dependence of cell performance indicators on cathode gas oxygen concentration, which opens up a new dimension for the optimization of cell efficiency, contingent on the availability of oxygen, and the design and operational constraints unique to specific SOFC development ventures

Counterion condensation or lack of solvation? Understanding the activity of ions in thin film block copolymer electrolytes

© The Royal Society of Chemistry. Polymer electrolytes are found at the heart of numerous electrochemical technologies involved in energy storage, conversion and separations. An important property of these materials is their partitioning behavior of ions in aqueous solutions and its effect on the activity of the ions within the polymer electrolyte. In this work, the fraction of condensed counterions (fc) were quantified in nanostructured block copolymer electrolyte (BCE) thin films with new and established experimental techniques. The transition between the osmotic-controlled regime and condensation-controlled regime in BCEs was identified using environmental GI-SAXS and solution uptake measurements via a quartz crystal microbalance (QCM). Further, the activity coefficients of ions in thin film BCEs were quantified experimentally and these values matched predictions from Manning\u27s theory of counterion condensation if the average distance between fixed charges on the polymer chains were determined accurately. Classical molecular dynamics simulations were also performed to assess counterion condensation and ionic conductivity values. The simulations showed large fc values in the BCE-agreeing with results from GI-SAXS and QCM. Because a holistic approach was adopted, it was uncovered that fc can vary significantly depending on the experimental method and analysis deployed. Interestingly, ionic conductivity measurements of the BCE thin films with aqueous solutions and humidified vapor revealed that solvation is critical for breaking ion pairs, and that the notion of two distinct counterion types, condensed and non-condensed, may not be the most accurate picture despite the utility of Manning\u27s theory for accurately predicting the activity coefficient of ions in polymer electrolytes. This journal i

Understanding the ionic activity and conductivity value differences between random copolymer electrolytes and block copolymer electrolytes of the same chemistry

Herein, a systematic study where the macromolecular architectures of poly(styrene--2-vinyl pyridine) block copolymer electrolytes (BCE) are varied and their activity coefficients and ionic conductivities are compared and rationalized a random copolymer electrolyte (RCE) of the same repeat unit chemistry. By performing quartz crystal microbalance, ion-sorption, and ionic conductivity measurements of the thin film copolymer electrolytes, it is found that the RCE has higher ionic activity coefficients. This observation is ascribed to the fact that the ionic groups in the RCE are more spaced out, reducing the overall chain charge density. However, the ionic conductivity of the BCE is 50% higher and 17% higher after the conductivity is normalized by their ion exchange capacity values on a volumetric basis. This is attributed to the presence of percolated pathways in the BCE. To complement the experimental findings, molecular dynamics (MD) simulations showed that the BCE has larger water cluster sizes, rotational dynamics, and diffusion coefficients, which are contributing factors to the higher ionic conductivity of the BCE variant. The findings herein motivate the design of new polymer electrolyte chemistries that exploit the advantages of both RCEs and BCEs