Open Circuit Voltage Conditions Research Articles

Crosslinked poly(vinylbenzyl chloride) with a macromolecular crosslinker for anion exchange membrane fuel cells

A new material based on crosslinked poly(vinylbenzyl chloride) (PVBC) with a macromolecular crosslinker is synthesized and employed as the membrane for anion exchange membrane fuel cells (AEMFCs). PVBC is used as the hydroxide conducting polymers, while poly(vinyl acetal) (PVAc) containing dimethylamino groups plays the role as macromolecular crosslinker and the supporting matrix simultaneously. Fourier transform infrared (FT-IR) absorption spectra and X-ray photoelectron (XPS) spectra prove successful crosslinking between PVBC and PVAc. The crosslinked membrane shows hydroxide conductivity larger than 0.01 S cm−1 at room temperature, and the swelling by water at elevated temperature is suppressed. The H2/O2 AEMFC using the crosslinked membrane shows a peak power density (Pmax) of 124.7 mW cm−2 at 40 °C, and the decrease of the open circuit voltage (OCV) of the fuel cell is negligible under continuous OCV conditions for 120 h. All the results indicate that the crosslinking with a macromolecular crosslinker may be a promising strategy to fabricate anion exchange membrane for the application in the AEMFCs.

Degradation reduction of polymer electrolyte membranes using CeO2 as a free-radical scavenger in catalyst layer

Ceria nanoparticles were added to the electrodes of proton exchange membrane fuel cells as free-radical scavengers to minimize the degradation of membrane electrode assembly (MEA) components. Accelerated durability tests were performed at low humidity under open circuit voltage (OCV) conditions, and the results were compared with traditional MEAs without CeO2. Gas crossover was monitored during the durability test, and the MEAs were examined by SEM before and after the durability test. The results showed that adding CeO2 as free-radical scavengers to the electrode greatly improves the chemical stability of the membrane. The degradation rate of the MEA with CeO2 in the anode was similar to that of the MEA with CeO2 in the cathode. The fuel cell with CeO2 in the cathode showed better MEA performance that the fuel cell with CeO2 in the anode.

Detection of Radicals by Spin Trapping ESR in a Fuel Cell Operating with a Sulfonated Poly(ether ether ketone) (SPEEK) Membrane

We present experiments in a fuel cell (FC) inserted in the resonator of the electron spin resonance (ESR) spectrometer, which allowed separate monitoring of radical formation at anode and cathode sides. The in situ FC was operated at 300 K under closed- and open-circuit voltage conditions, CCV and OCV, respectively, with a membrane–electrode assembly (MEA) based on sulfonated poly(ether ether ketone) (SPEEK). The formation of radicals was monitored by spin trapping ESR, with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as the spin trap. The magnetic parameters and the relative intensities of DMPO adducts were determined by simulation of the ESR spectra. The major adducts detected at both electrodes were DMPO/OOH and DMPO/H. The generation of the DMPO/OOH adduct at the anode can be explained by reaction of crossover oxygen with hydrogen atoms formed at the Pt catalyst; at the cathode this adduct can be generated by H2 crossover to the cathode, reaction at the Pt catalyst, and reaction with O2. The generation of...

Study of nitrile-containing proton exchange membranes prepared by radiation grafting: Performance and degradation in the polymer electrolyte fuel cell

The fuel cell performance and durability of three kinds of styrene based radiation grafted membranes are investigated and compared in the single cell. The styrene/methacrylonitrile (MAN) co-grafted membrane exhibits the best performance among the tested radiation grafted membranes. The accelerated tests under open circuit voltage (OCV) conditions and post-mortem analysis demonstrate that the nitrile-containing membranes exhibit significantly enhanced durability compared to the pure styrene grafted membrane, which is associated with the reduced gas crossover rates and attributed to the improved gases barrier properties due to the polarity of the nitrile group. To understand the influence of each functional group in the co-monomer units, both styrene/MAN and styrene/acrylonitrile (AN) co-grafted membranes are evaluated in a set of tests at OCV. The degrees of loss of the graft components are subsequently quantitatively analyzed based on FTIR spectra, showing a comparable decomposition rate of grafted styrene units, but more loss of nitrile in case of the styrene/AN co-grafted membrane. The styrene/AN co-grafted membrane, with AN lacking protection at the α-position in contrast to MAN, is found to be susceptible to significant hydrolysis, directly leading to an accelerated degradation in the late stages of the 130 h OCV test and inhomogeneous in-plane degradation.

Effects of the Sm0.2Ce0.8O2-δ Modification of a Ni-Based Anode on the H2S Tolerance for Intermediate Temperature Solid Oxide Fuel Cells

The H2S tolerance and stability of a Sm0.2Ce0.8O2-δ coating on the Ni/YSZ anode pore wall surface was investigated. The sulfur poisoning mechanisms were studied for various H2S concentrations (0-100 ppm) and temperatures (600°C and 700°C) under both open circuit voltage (OCV) and current-loading conditions. For the unmodified Ni/YSZ anode under current-loading conditions, the cell performance decreased significantly with the introduction of H2S, as the sulfur strongly adsorbed at the electrochemical reaction sites (ERSs). For the SDC-modified Ni/YSZ anode under the OCV conditions, the governing mechanism of the H2S poisoning was the formation of Ce2O2S with low oxygen ion conductivities, making the oxygen ion path disconnected. For the SDC-modified anode under current-loading conditions, the mixed ionic and electronic conductivity (MIEC) of the SDC allowed the electrochemical reactions to occur mainly at the SDC/fuel interface (2PB). With these conditions, the adsorbed sulfur was easily removed by the electrochemical oxidation. The decrease in the sulfur poisoning effect resulted in stable cell performances at 500 ppm of H2S over 800 h.

Chemical compatibility, redox behavior, and electrochemical performance of Nd1−xSrxCoO3−δ cathodes based on Ce1.9Gd0.1O1.95 for intermediate-temperature solid oxide fuel cells

The effect of Sr substitution for Nd on Nd1−xSrxCoO3−δ (NSC) (x=0.3, 0.4, 0.5, 0.6, and 0.7) is investigated to evaluate NSC as a cathode material based on Gd0.1Ce0.9O1.95 (GDC) electrolyte for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The NSC powders are prepared by the glycine nitrate method. At a given temperature, the electrical conductivity increases with increasing Sr content up to x=0.5 and then decreases for x>0.5. The redox behavior of NSC (x=0.3, 0.5, and 0.7) cathodes is studied by the coulometric titration at 700°C. In order to investigate the area specific resistances of NSC–GDC cathodes, symmetrical half cells (cathode/electrolyte/cathode) are measured using impedance spectroscopy at various temperatures in air under open circuit voltage (OCV) condition. The electrochemical performance of NSC–GDC cathodes is measured using an NSC–GDC/GDC/Ni-GDC anode supported cell. The maximum power density of NSC–GDC cathodes increases with increasing strontium content up to x=0.5 and then decreases at 700°C. In terms of electrical conductivity and electrochemical performance, Nd1−xSrxCoO3−δ (x=0.5) is more suitable as a cathode material based on GDC electrolyte in IT-SOFC applications.

Systematic evaluation of Co-free LnBaFe2O5+δ (Ln = Lanthanides or Y) oxides towards the application as cathodes for intermediate-temperature solid oxide fuel cells

Co-free oxides with a nominal composition of LnBaFe2O5+δ, where Ln=La, Pr, Nd, Sm, Gd, and Y, were synthesized and phase structure, oxygen content, electronic conductivity, oxygen desorption, thermal expansion, microstructure and electrochemical performance were systematically investigated. Among the series of materials tested, LaBaFe2O5+δ oxide showed the largest electronic conductivity and YBaFe2O5+δ oxide had the smallest thermal expansion coefficient (TEC) of 14.6×10−6K−1 within a temperature range of 200–900°C. All LnBaFe2O5+δ oxides typically possess the TEC values smaller than 20×10−6K−1. The oxygen content, electronic conductivity and TEC values are highly dependent on the cation size of the Ln3+ dopant. The lowest electrode polarization resistance in air under open circuit voltage condition was obtained for SmBaFe2O5+δ electrode and was approximately 0.043, 0.084, 0.196, 0.506 and 1.348Ωcm2 at 800, 750, 700, 650 and 600°C, respectively. The SmBaFe2O5+δ oxide also demonstrated the best performance after a cathodic polarization. A cell with a SmBaFe2O5+δ cathode delivered peak power densities of 1026, 748, 462, 276 and 148mWcm−2 at 800, 750, 700, 650 and 600°C, respectively. The results suggest that certain LnBaFe2O5+δ oxides have sufficient electrochemical performance to be promising candidates for cathodes in intermediate-temperature solid oxide fuel cells.

In situ analysis on the electrical conductivity degradation of NiO doped yttria stabilized zirconia electrolyte by micro-Raman spectroscopy

Phase transformation of 1mol% NiO doped yttria stabilized zirconia (YSZ) under SOFC operation atmospheres was analyzed by in situ Raman spectroscopy with simultaneous measurement of conductivity using a test cell consisting of air, Pt/NiO doped YSZ/Pt, wet Ar balanced 50vol% H2. In situ measurement was carried out under open circuit voltage (OCV) condition at 1173K for more than 20h. Phase transformation from the cubic to the tetragonal phase progressively proceeded from the matting surface of the electrolyte on the anode side with increasing holding time. Simultaneously, the electrical conductivity of the electrolyte also exhibited monotonically decrease from about 0.103Scm−1 to 0.084Scm−1. The phase transformation started from the anode/electrolyte interface. The tetragonal to cubic phase volume ratio evaluated from the Raman spectra, rapidly decreased with increasing depth from the anode/electrolyte interface towards the center of the electrolyte. We have experimentally demonstrated by the in situ measurement that the conductivity degradation of NiO doped YSZ is closely associated to the phase transformation under a reducing condition at a high temperature.

Effect of open circuit voltage on degradation of a short proton exchange membrane fuel cell stack with bilayer membrane configurations

In the present work, membrane electrode assembly degradation of a 4-cell stack with specially designed twin catalyst coated membranes (twin-CCMs) was carried out for 1600h under open circuit voltage (OCV) conditions. Four types of membrane with various thicknesses were employed in the experiment. Each twin-CCM was composed of two membranes of the same type with a catalyst layer coated only on one side. The purpose of this special configuration was to facilitate postmortem analysis of the membrane after degradation due to the detachable membrane structure. By means of several in situ electrochemical measurements, the performance of the individual cells was analysed every 200h during the degradation process. Postmortem analyses such as scanning electron microscopy, atomic force microscopy, and infrared imaging were also conducted to identify the membrane degradation mechanisms after exposure to OCV. The results indicate that membrane degradation, which correlates with electrode degradation, is the most direct reason for the failure of the whole cell/stack during OCV operation, especially for cells with thinner membranes.

In-Depth Profiling of Degradation Processes in a Fuel Cell: 2D Spectral-Spatial FTIR Spectra of Nafion Membranes.

We present in-depth profiling by micro FTIR of cross sections for Nafion 115 membranes in membrane-electrode assemblies (MEAs) degraded during 52 or 180 h at open circuit voltage (OCV) conditions, 90 °C and 30% relative humidity. Analysis of optical images showed highly degraded zones in both MEAs. Corresponding 2D FTIR spectral-spatial maps indicated that C-H and C═O groups are generated during degradation. The highest band intensities for both groups appeared at a depth of 82 μm from the cathode in the MEA degraded for 180 h; the same bands were present but less intense at a depth of 22 μm from the cathode. Degradation at these depths is most likely associated with the location of the Pt band formed from Pt dissolution and migration into the membrane. The two degradation bands, C═O and C-H, appeared at the same depths from the cathode, 82 and 22 μm, suggesting that they are generated by a common mechanism or intermediate. This result is rationalized by a very important first reaction: Abstraction of a fluorine atom from the polymer main chain and side chain by hydrogen atoms, H•. This step is expected to cause main chain and side chain scission and to generate RF-CF2• radicals that can react with H2O2, H2O, and H2 to produce both -COOH and RCF2H groups.

Recoverable Performance Loss Due to Membrane Chemical Degradation in PEM Fuel Cells

Recoverable voltage loss was observed for a PEM fuel cell due to membrane chemical degradation under open circuit voltage (OCV) conditions. The anion analysis of the fuel cell effluent water, collected during both the OCV hold and voltage recovery stages, indicates that sulfate release rate is much higher during recovery than that during the OCV hold. The surge in sulfate anion release occurs simultaneously with the recovery of fuel cell voltage. It was found that the reversible voltage loss, and concurrent sulfate release rate during voltage recovery, is lower for Ce-mitigated NRE211 membrane compared to as-received membrane. The recoverable voltage loss is proposed to be mainly due to the sulfate anion generated by membrane chemical degradation adsorbing onto the platinum catalyst surface.

Recoverable Performance Loss due to Membrane Chemical Degradation in PEM Fuel Cells

Recoverable voltage loss was observed for a PEM fuel cell due to membrane chemical degradation under open circuit voltage (OCV) conditions. The anion analysis of the fuel cell effluent water, collected during both the OCV hold and voltage recovery stages, indicates that sulfate release rate is much higher during recovery than that during the OCV hold. The surge in sulfate anion release occurs simultaneously with the recovery of fuel cell voltage. It was found that the reversible voltage loss, and concurrent sulfate release rate during voltage recovery, is lower for Ce-mitigated NRE211 membrane compared to as-received membrane. The recoverable voltage loss is proposed to be mainly due to the sulfate anion generated by membrane chemical degradation adsorbing onto the platinum catalyst surface.

Characteristics of the Hydrogen Electrode in High Temperature Steam Electrolysis Process

YSZ-electrolyte supported solid oxide electrolyzer cells (SOECs) using LSM-YSZ oxygen electrode but with three types of hydrogen electrode, Ni–SDC, Ni–YSZ and LSCM–YSZ have been fabricated and characterized under different steam contents in the feeding gas at 850°C. Electrochemical impedance spectra results show that cell resistances increase with the increase in steam concentrations under both open circuit voltage and electrolysis conditions, suggesting that electrolysis reaction becomes more difficult in high steam content. Pt reference electrode was applied to evaluate the contributions of the hydrogen electrode and oxygen electrode in the electrolysis process. Electrochemical impedance spectra and over potential of both electrodes were measured under the same testing conditions. Experimental results show that steam contents mainly affect the behavior of the hydrogen electrode but have little influence on the oxygen electrode. Further, contribution from the hydrogen electrode is dominant in the electrolysis process for Ni–based SOECs, but this contribution decreases for LSCM–based SOECs.

Novel Micro-Tubular High Temperature Solid Oxide Electrolysis Cells

NiO-YSZ (yttria-stabilized zirconia) supported micro-tubular LSM ((La0.75Sr0.25)0.95MnO3)-YSZ/YSZ/NiO-YSZ solid oxide electrolysis cells (SOECs) have been prepared by a phase-inversion method. SEM results show that a novel asymmetric microstructure was obtained, with large finger-like pores on inner wall which can be used as gas delivery layer and small finger-like pores adjacent to the electrolyte thin film which can be used as electrochemical functional layer. High temperature steam electrolysis was carried out with H2 as gas carrier and maximum 80 vol. % absolute humidity (AH) steam fed to the micro-tubular cell. The electrolysis cell resistance increased with increase in the fed steam concentration under open circuit voltage conditions. An encouraging H2 generation rate of 3.8 ml min-1 cm-2 was obtained with 60 Vol. % AH steam under an electrolysis voltage of 1.3 V at 850 oC.

Electrochemical Characterization of the Dye-Sensitized Solar Cells

Dye-Sensitized Solar cells (DSSC) has had considerable interest to be a promising solar energy conversion device. In order to improve the performance and long term stability of the DSSC, it is essential to understand the characteristics of DSSC. Various electrochemical experiments were conducted to evaluate DSSC characteristics. Variation in internal interfaces and/or components of DSSC due to exposure time, light intensity and ambient temperature were monitored. Different illumination conditions affected open circuit voltage, short circuit current and fill factor. Change in behavior of DSSC was observed when test conditions were switched from open circuit voltage conditions to potential cycling conditions. Electrochemical impedance spectroscopy (EIS) was selected to scrutinize the interfacial charge transfer resistance variation. Equivalent circuit model for EIS results illustrated that charge transfer resistance at Pt counter electrode was mainly affected by both Voc conditions and potential scanning with same irradiation intensity in the aging study.

Characterization of GaInP/Ge heterostructure solar cells by capacitance measurements at forward bias under illumination

The n-GaInP/n-p Ge heterostructure sloar cells grown by OMVPE were studied by new technique based on the diffusion capacitance measurements under illumination at open circuit voltage conditions. The value of the effective minority carrier lifetime in the p-Ge base τeff= 1.5×10 -6 s was determined from the frequency dependence of the diffusion capacitance, C(f). The numerical simulation model was developed, which is in a good agreement with the experimental C(f), capacitance-voltage and dark current-voltage measurements. The value of the bulk minority lifetime equal to τ b= 5×10 -6 s was obtained by the simulation fit of the experimental data. The simulations have also demonstrated that the diffusion capacitance is very sensitive to the recombination velocity at the back contact, Sbc, for Sbc<10 5

Electrochemical Characterization on SDC/Na2CO3 Nanocomposite Electrolyte for Low Temperature Solid Oxide Fuel Cells

Our previous work has demonstrated that novel core-shell SDC/Na2CO3 nanocomposite electrolyte possesses great potential for the development of low temperature (300-600 degrees C) solid oxide fuel cells. This work further characterizes the nanocomposite SDC/Na2CO3 electrochemical properties and conduction mechanism. The microstructure of the nanocomposite sintered at different temperatures was analyzed through scanning electron microscope (SEM) and X-ray diffraction (XRD). The electrical and electrochemical properties were studied. Significant conductivity enhancement was observed in the H2 atmosphere compared with that of air atmosphere. The ratiocination of proton conduction rather than electronic conduction has been proposed consequently based on the observation of fuel cell performance. The fuel cell performance with peak power density of 375 mW cm(-2) at 550 degrees C has been achieved. A.C. impedance for the fuel cell under open circuit voltage (OCV) conditions illustrates the electrode polarization process is predominant in rate determination.

Characterization of nitrogen gas crossover through the membrane in proton-exchange membrane fuel cells

Gas crossover phenomenon through a membrane is inevitable in a proton-exchange membrane fuel cell (PEMFC). For nitrogen, the concentration at the cathode side is usually higher than that at the anode side, so N 2 permeates to the anode side. Nitrogen gas crossover (NGC) may cause fuel starvation, if N 2 gas accumulates in the hydrogen recirculation loop. Thus, it is important to determine the NGC under various PEMFC operating conditions. In this study, characterization of NGC under both open circuit voltage (OCV) and power generation conditions is investigated using a mass spectrometer. Under OCV conditions with the PEMFC membrane fully hydrated, N 2 concentration in the anode exit stream increases as cell temperature increases. Nitrogen permeability coefficients (NPC) are calculated based on the obtained N 2 concentration data. The results show that NPC exhibits an Arrhenius type relationship. Under OCV conditions, the maximum NPC is 5.14 × 10 −13 mol m −1 s −1 Pa −1 with an N 2 activation energy of 19.83 kJ. Under power generation conditions, the NGC increases with increasing current density, which is the result of elevated membrane temperature and increased water content. When the anode stoichiometric ratio (SR A) is lowered, the N 2 concentration increases under all tested current densities. A low hydrogen flow rate, along with a low SR A at low current density, significantly increases N 2 concentration at the anode outlet.

A diagnosis method for identification of the defected cell(s) in the PEM fuel cells

Incorrect controlling of the pressure, temperature, flow rate and humidity levels of reactant gases can inflict severe and sometimes irreversible damages on the PEM fuel cells. Most important damage is leakage between the two sides of cathode and anode that may lead to physical defects in the stack. Usage of neutral gas method only reveals the overall leakage and does not show the exact location of the defected cell in the stack. This research seeks to determine the exact location of the defective cell using a method that is based on the data received from the voltage–time graphs of the stack under hydrogen sudden stop condition when the stacks are operating in the open circuit voltage condition. This method has been used with respect to two fuel cell stacks with different powers in different working conditions. Also the stack voltage drop due to leakage has been considered theoretically.

Study of Polymer Electrolyte Membrane Degradation under OCV Hold Using Bilayer MEAs

Accelerated degradation tests for polymer electrolyte membrane fuel cells are frequently conducted under open-circuit voltage (OCV) conditions at low relative humidity and high temperature. Comparative experimental studies were carried out to identify the localization of membrane degradation through the thickness direction of the membrane under the OCV hold condition and the effect of platinum band location on the membrane degradation behavior. A bilayer configuration of the membrane electrode assemblies (MEAs) was used for this study. In one test, the fuel cell with the bilayer MEA was exposed to hydrogen and air (regular test); in another test, the bilayer MEA was exposed to 4% hydrogen (balance nitrogen) and pure oxygen to induce the platinum band formation close to the anode side. Hydrogen crossover current, electrochemical areas (ECAs), and polarization curves were evaluated pre- and post-test. After the OCV hold test, the bilayer MEA was separated into two pieces of one-side coated membranes; each piece was characterized by uniaxial mechanical testing, IR spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy (EDS) analysis. The findings suggest highly localized degradation or a defect band formed through the thickness direction of the MEA, and the defect band shifts with the shift of the Pt band.