Electrochemical impedance analysis of a PEDOT : PSS-based textile energy storage device

A textile-based energy storage device with electroactive PEDOT:PSS (poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate)) polymer functioning as a solid-state polyelectrolyte has been developed. The device was fabricated on textile fabric with two plies of stainless-steel electroconductive yarn as the electrodes. In this study, cyclic voltammetry and electrochemical impedance analysis were used to investigate ionic and electronic activities in the bulk of PEDOT:PSS and at its interfaces with stainless steel yarn electrodes. The complex behavior of ionic and electronic origins was observed in the interfacial region between the conductive polymer and the electrodes. The migration and diffusion of the ions involved were confirmed by the presence of the Warburg element with a phase shift of 45° (n = 0.5). Two different equivalent circuit models were found by simulating the model with the experimental results: (QR)(QR)(QR) for uncharged and (QR)(QR)(Q(RW)) for charged samples. The analyses also showed that the further the distance between electrodes, the lower the capacitance of the cell. The distribution of polymer on the cell surface also played important role to change the capacitance of the device. The results of this work may lead to a better understanding of the mechanism and how to improve the performance of the device

Scanning electrochemical cell microscopy : a versatile technique for nanoscale electrochemistry and functional imaging

Scanning electrochemical cell microscopy (SECCM) is a new pipette-based imaging technique purposely designed to allow simultaneous electrochemical, conductance, and topographical visualization of surfaces and interfaces. SECCM uses a tiny meniscus or droplet, confined between the probe and the surface, for high-resolution functional imaging and nanoscale electrochemical measurements. Here we introduce this technique and provide an overview of its principles, instrumentation, and theory. We discuss the power of SECCM in resolving complex structure-activity problems and provide considerable new information on electrode processes by referring to key example systems, including graphene, graphite, carbon nanotubes, nanoparticles, and conducting diamond. The many longstanding questions that SECCM has been able to answer during its short existence demonstrate its potential to become a major technique in electrochemistry and interfacial science

Hysteresis of Electronic Transport in Graphene Transistors

Graphene field effect transistors commonly comprise graphene flakes lying on SiO2 surfaces. The gate-voltage dependent conductance shows hysteresis depending on the gate sweeping rate/range. It is shown here that the transistors exhibit two different kinds of hysteresis in their electrical characteristics. Charge transfer causes a positive shift in the gate voltage of the minimum conductance, while capacitive gating can cause the negative shift of conductance with respect to gate voltage. The positive hysteretic phenomena decay with an increase of the number of layers in graphene flakes. Self-heating in helium atmosphere significantly removes adsorbates and reduces positive hysteresis. We also observed negative hysteresis in graphene devices at low temperature. It is also found that an ice layer on/under graphene has much stronger dipole moment than a water layer does. Mobile ions in the electrolyte gate and a polarity switch in the ferroelectric gate could also cause negative hysteresis in graphene transistors. These findings improved our understanding of the electrical response of graphene to its surroundings. The unique sensitivity to environment and related phenomena in graphene deserve further studies on nonvolatile memory, electrostatic detection and chemically driven applications.Comment: 13 pages, 6 Figure

Investigation of Microstructural and Carbon Deposition Effects in SOFC Anodes Through Modelling and Experiments

The investigation of the SOFC anode microstructural properties affected by microstructural parameters and degradation is the focus of this research. Imaging and image processing techniques are developed to achieve quantification of the anode microstructural information. The analytical and Computational Fluid Dynamics based modelling of the microstructure including the degradation effects developed in this work will enable the microstructure optimisation for achieving performance enhancements

Simulation study on PEM fuel cell gas diffusion layers using x-ray tomography based Lattice Boltzmann method

The Polymer Electrolyte Membrane (PEM) fuel cell has a great potential in leading the future energy generation due to its advantages of zero emissions, higher power density and efficiency. For a PEM fuel cell, the Membrane-Electrode Assembly (MEA) is the key component which consists of a membrane, two catalyst layers and two gas diffusion layers (GDL). The success of optimum PEM fuel cell power output relies on the mass transport to the electrode especially on the cathode side. The carbon based GDL is one of the most important components in the fuel cell since it has one of the basic roles of providing path ways for reactant gases transport to the catalyst layer as well as excess water removal. A detailed understanding and visualization of the GDL from micro-scale level is limited by traditional numerical tool such as CFD and experimental methods due to the complex geometry of the porous GDL structural. In order to take the actual geometry information of the porous GDL into consideration, the x-ray tomography technique is employed which is able to reconstructed the actual structure of the carbon paper or carbon cloth GDLs to three-dimensional digital binary image which can be read directly by the LB model to carry out the simulation. This research work contributes to develop the combined methodology of x-ray tomography based the three-dimensional single phase Lattice Boltzmann (LB) simulation. This newly developed methodology demonstrates its capacity of simulating the flow characteristics and transport phenomena in the porous media by dealing with collision of the particles at pore-scale. The results reveal the heterogeneous nature of the GDL structures which influence the transportation of the reactants in terms of physical parameters of the GDLs such as porosity, permeability and tortuosity. The compression effects on the carbon cloth GDLs have been investigated. The results show that the c applied compression pressure on the GDLs will have negative effects on average pore size, porosity as well as through-plane permeability. A compression pressure range is suggested by the results which gives optimum in-plane permeability to through-plane permeability. The compression effects on one-dimensional water and oxygen partial pressures in the main flow direction have been studied at low, medium and high current densities. It s been observed that the water and oxygen pressure drop across the GDL increase with increasing the compression pressure. Key Words: PEM fuel cell, GDL, LB simulation, SPSC, SPMC, x-ray tomography, carbon paper, carbon cloth, porosity, permeability, degree of anisotropy, tortuosity, flow transport

Investigation and Propagation of Defects in the Membrane Electrode Assembly of Polymer Electrolyte Membrane Fuel Cells: Quality Control Analysis

Polymer electrolyte membrane fuel cells (PEMFC) have the potential to deliver high power density with a lower weight and volume compared to other fuel cells. However, some of the barriers to the successful commercialization of PEMFCs include problems associated with durability, stability and cost. Fuel cell defects that arise and propagate in the membrane electrode assembly (MEA) components during manufacturing and subsequent operation are the biggest factors limiting their durability and stability, leading to shortened lifetimes, reduced performance or cell failure. Defects in the production line must be minimized if PEMFCs are to become reliable electrochemical energy devices on a commercial scale. A conventional PEMFC electrode consists of layers (CL) of nanoscale Pt catalyst particles mixed with an ionomer on a high surface area carbon support deposited on the polymer electrolyte membrane (PEM) and sandwiched between gas diffusion media (GDM). The defects in these components originate from the raw materials used in the catalyst layers, process conditions during catalyst mixing, coating techniques, drying process, thickness variations in the casting substrate and the temperature and humidity of the processing environment. These defects can lead to reduced performance and can increase fuel cell degradation, specifically in the MEA components. Understanding the MEA component defects that affect fuel cell performance and lifetime is integral to the successful development of an on-line quality control strategy. Previous research studies have been conducted on defects in catalyst-coated membranes (CCMs) and gas diffusion layers (GDLs) with various dimensions that have been introduced artificially at specific locations, which does not satisfactorily mimic the situation with real manufacturing defects. Very few studies on real defects have been reported to date with limited work on localized effects on CL defects such as loss of catalyst, the morphology of defect growth or the effect of defect location within the CCM on the resulting cell performance. This has limited our fundamental and comprehensive understanding of the nature of defects in the beginning-of-life (BOL) state and the manner in which they may or may not propagate during PEMFC operation. The focus of this research is to analyze real catalyst layer defects and membrane pinholes on commercial CCMs that are developed during mass production. Specifically, the objectives of this study are to: (i) develop a non-destructive method to identify and quantify defects in CCM electrodes, (ii) implement a defect analysis framework to age CCMs using open-circuit voltage(OCV)- accelerated stress tests (AST), (iii) characterize the electrochemical performance of CCM/MEAs with varying extent of manufacturing defects (catalyst layer thickness, degree of catalyst non-uniformity) and compare this to a baseline, defect-free CCM/MEA using ASTs as well as in-situ and ex-situ methods and (iv) investigate defects on GDL-microporous layer (MPL) using infrared (IR) imaging and surface conductivity measurements. The first set of quality control experiments were performed on CCMs by using optical microscopy to characterize catalyst layer defects. Defects such as micro/macro cracks, catalyst clusters, missing catalyst layer defects (MCLDs), void/empty areas, CL delamination and pinholes in the CCM were characterized in terms of areal dimension (size, shape, and orientation) prior to electrochemical analysis. The OCV-AST protocol was developed to age defected CCMs in a custom-designed test cell and track defect propagation and behavior during aging. The geometric features of the defects were quantified and their growth measured at regular time intervals from beginning-of-life (BOL) to end-of-life (EOL) until the OCV had dropped by 20% from its initial value (as per the DOE-designed protocol). Overall, two types of degradation were observed: surface degradation caused by catalyst erosion and crack degradation caused by membrane mechanical deformation. Furthermore, the catalyst layer defects formed during the decal transfer process exhibited a higher growth rate at middle-of-life (MOL-1) before stabilizing by EOL. The results of the crack propagation analysis during AST showed that the defected area covered under cracks increased from 2.4% of the total CL area at BOL to 10.5% by EOL with a voltage degradation rate of 2.55mV/hr. This type of analysis should provide manufacturers with baseline information that will allow them to select and reject CCMs, increasing the lifetime of fuel cell stacks. Once the CCM defects were analyzed comprehensively, research was carried out on the MEA stack. MEAs containing defected CCMs (incomplete catalyst layer defects-MCLD), pinhole across sealant and artificial pinholes at inlet/middle/outlet were investigated using a cyclic open-circuit voltage (COCV)-AST. Different RH cycling periods from 80% RH to 20% RH with time delays from 5 mins to 30 mins were applied to the cathode to study the propagation of defects and their effect on overall cell performance. In-situ analysis included the measurement of polarization curves, linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) to measure electrode degradation. Non-destructive ex-situ analysis using IR thermography was conducted every 100 cycles to monitor the evolution of defects in the MEA. The growth of pinholes was studied on the basis on hydrogen crossover curves. Sealing defects were found to have a major impact on performance loss compared to catalyst layer defects. It was also observed that MCLDs degraded within a short period of time and developed pinholes although the extent of this degradation depended on defect thickness. The MCLD defects were unstable and observed to continually grow due to gradual loss of catalyst particles inside the defected areas that accelerated pinhole formation in CCMs. This effect was clearly reflected in the continuous decay of OCV during the fuel cell operation. Therefore, CCMs leaving the production line with missing and /or thin portions of CL are not recommended for MEA fabrication as they ultimately affect the long-term stability of PEMFC. The last set of quality control experiments was conducted on GDL-MPL defects in samples that were being aged by RH cycling in a custom-design test cell. Thermal image analysis using IR thermography was carried out by passing DC current through the GDL sheet mounted on a porous vacuum stage to identify hot and cold spots reflecting defective areas. The morphological features and surface conductivity of MPL cracks were characterized using optical microscopy and four-point probe conductivity measurements. Interestingly, the nature of defects/cracks propagation in the GDL-MPL was found to affect cell performance in the mass transfer region at high currents. Crack propagation in GDL-MPL increased mass transport losses due to water flooding on the cathode, which was clearly observed in the polarization curves. Finally, the overall effects of catalyst layer defects, membrane pinholes and GDL defects on cell performance were compared. MEA sealant defects (pinholes) had such a negative effect on cell performance that EOL was reached after only ~ 50 hours of COCV operation at 80% - 20% RH cycling. Thus, the detection of such a defect in a CCM should be sufficient cause to reject it for use in a commercial stack. We also observed that CCMs with defects that led to 70% reduced thickness of the CL failed faster than those with the same type of defects that had resulted in 30% reduced thickness of the CL, presumably due to less available catalyst for electrochemical reactions. Clearly, CL defects should be given high priority in quality control inspection strategies devised by CCM electrode manufacturers and PEMFC operators

In situ interface engineering for probing the limit of quantum dot photovoltaic devices.

Quantum dot (QD) photovoltaic devices are attractive for their low-cost synthesis, tunable band gap and potentially high power conversion efficiency (PCE). However, the experimentally achieved efficiency to date remains far from ideal. Here, we report an in-situ fabrication and investigation of single TiO2-nanowire/CdSe-QD heterojunction solar cell (QDHSC) using a custom-designed photoelectric transmission electron microscope (TEM) holder. A mobile counter electrode is used to precisely tune the interface area for in situ photoelectrical measurements, which reveals a strong interface area dependent PCE. Theoretical simulations show that the simplified single nanowire solar cell structure can minimize the interface area and associated charge scattering to enable an efficient charge collection. Additionally, the optical antenna effect of nanowire-based QDHSCs can further enhance the absorption and boost the PCE. This study establishes a robust 'nanolab' platform in a TEM for in situ photoelectrical studies and provides valuable insight into the interfacial effects in nanoscale solar cells

In situ scanning electrochemical probe microscopy for energy applications

High resolution electrochemical imaging methods provide opportunities to study localized phenomena on electrode surfaces. Here, we review recent advances in scanning electrochemical microscopy (SECM) to study materials involved in (electrocatalytic) energy-related applications. In particular, we discuss SECM as a powerful screening technique and also advances in novel techniques based on micro- and nanopipets, such as the scanning micropipet contact method and scanning electrochemical cell microscopy and their use in energy-related research