Reversible and irreversible degradation in fuel cells during Open Circuit Voltage durability testing

In this study reversible and irreversible voltage loss in a polymer electrolyte membrane fuel cell undergoing an open circuit voltage (OCV) durability test was studied. OCV durability testing is thought to promote chemical degradation of the electrolyte membrane material via radical attack and degradation of the catalyst layer. The results for degradation under constant relative humidity showed that voltage degradation rates measured in the first 20–50 h after polarization curve measurement consisted of a reversible, or transient, and irreversible component. A steady voltage decay rate became evident after 50 h of operation. Comparison to the voltage decay rates obtained from polarization curves showed that the steady voltage decay rate was representative of irreversible voltage loss due to irreversible changes in materials as shown by crossover and active surface area measurements. This study highlights the necessity of understanding the difference between reversible and irreversible voltage decay rates since the reversible decay rates were found to be much higher than irreversible decay rates.     

Optimization of power allocation for wind-hydrogen system multi-stack PEM water electrolyzer considering degradation conditions

Hydrogen production from wind power has become one of the most important technologies for the large-scale comprehensive development and utilization of wind power, but the randomness of wind power has a large negative impact on the stability and cost of such wind-hydrogen hybrid energy systems. In this work, we initially analyze the relationship between electrolyzer efficiency and degradation with a three-dimensional multi-physics field model of PEMWE single-cell. Optimization of a power allocation strategy for wind-hydrogen system with a multi-stack PEM water electrolyzer (PEMWE) is proposed by considering degradation conditions. The multi-stack PEMWE power allocation strategy consists of the control module and execution module. In the control module, the degradation of PEMWE is quantified using the voltage degradation rate under different operating conditions. By setting the turning power point and external power supply and calculating the power allocation order online to reduce the degradation of PEMWE. In the execution module, the extended duty cycle interleaved buck converter (EDCIBC) based on fuzzy PID control is used to power each PEMWE single-stack. Case studies are carried out via computer simulation based on the configuration and experimental data for a specific wind farm located in Cixi, Zhejiang, China. Our results show that the energy efficiency of the wind-hydrogen system is 61.65% in a one-year operation, the voltage degradation of the PEMWE single-stack is 7.5 V, and the maximum efficiency is 6.29% lower than that when it is not aged. The EDCIBC output current ripple is as low as 0.053%, which rapidly and accurately follows the generated power allocation signal.     

Numerical analysis of the electrochemical dissolution of iridium catalyst and evaluation of its effect on the performance of polymer electrolyte membrane water electrolyzers

A physicochemical model of a water electrolyzer with a polymer electrolyte membrane (PEM) was developed, taking into account the electrochemical dissolution of an anodic iridium catalyst. The dependencies of the rates of iridium loss and electrolysis voltage increase upon the current density were calculated in order to analyze the effect of the iridium dissolution on degradation of the electrolysis cell (EC) performance. As an estimated characteristic of the techno-economic costs of the electrolysis process, the amount of iridium loss from the anode catalyst layer (as a result of electrochemical dissolution) in the course of the generation of 1 kg of hydrogen was calculated. Data were analyzed and a number of regularities of the iridium dissolution and its influence on the rate of degradation of the EC performance were found. In particular, the most efficient ECs in terms of electrolysis voltage (energy consumption for gas production) are, simultaneously, the most unstable (prone to performance degradation) in relation to the iridium dissolution process. An aim of current requirements for water electrolyzers includes reducing the specific consumption of iridium required for hydrogen generation.     

Investigation of the recoverable degradation of PEM fuel cell operated under drive cycle and different humidities

Recoverable degradation of a proton exchange membrane fuel cell (PEMFC) under different relative humidities (RHs) after a whole night rest was investigated. A single cell was operated under drive cycle to simulate the working conditions of fuel cell vehicle. It was found that the cell performance decreased after 5 h operation and recovered mostly after one night rest at higher humidities, i.e. 100%, 75% and 50% RH for both cathode and anode sides; while continuous decrease took place at lower humidity, 35%RH. Polarization curve, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were conducted before and after every 5 h drive cycle for investigating the mechanism of the recoverable degradation. It was found that water content, current density and thermal management might be the main contributions to the performance degradation, by impacting the membrane conductivity, internal resistance, electrode kinetics, and catalyst utilization. A good understanding of voltage recovery phenomenon after several hours rest and its effect on durability will be helpful in improving the reliability and durability of PEMFC.     

Reactive power market management considering voltage control area reserve and system security

This paper presents a new algorithm to optimize reactive power procurement through commercial transactions considering system voltage security. The proposed algorithm minimizes reactive power provision and transmission loss costs in addition to maximizing system voltage security margin through a multiobjective function. In order to maintain the voltage profile of power system during sever contingencies or due to load uncertainty, all voltage control areas (VCA) of the system are detected and then optimal reactive power reserve is provided for each VCA during the market settlement. A four-stage multiobjective mathematical programming method is proposed to settle the reactive power market. The proposed algorithm has been applied on IEEE-RTS test system. The simulation results show the effectiveness of the proposed algorithm for reactive power market management.     

The voltage characteristics of proton exchange membrane fuel cell (PEMFC) under steady and transient states

In this paper, voltage sensors were developed to explore the voltage distribution characteristics inside the fuel cell under both steady and transient states. The effects of air stoichiometry and current density on the voltage distribution under steady state were discussed, and the dynamic voltage response due to the load change under transient state was also investigated. Results showed that under transient state, the fuel cell would experience a temporary voltage fluctuation due to the air starvation. Thus could probably lead to the degradation of materials, such as the catalyst, membrane, etc. To lessen the degree of air starvation, a method of pre-supplying certain amount of air before loading was adopted. The relationship between the voltage response at the loading transient and the amount of pre-supplied air was also studied, and a minimum value of the pre-supplied air was obtained. The experimental results of this paper could be applied to the optimization of vehicular fuel cell system.     

Natural degradation and stimulated recovery of a proton exchange membrane fuel cell

In this paper, the stimulated recovery of a proton exchange membrane (PEM) fuel cells after natural degradation has been investigated. The performance degradation of a 63-cell PEM fuel cell stack over a storage interval of 40,000 h at temperature 24 °C and relative humidity 65% was analyzed by static and dynamical tests. The average cell voltage degradation rate was 309 μV h⁻¹, averaged over a range of currents. The performance was then partially recovered by application of a high frequency pulsing procedure after which the effective average degradation rate (from the commencement of storage to after the recovery) was approximately 170 μV h⁻¹. This indicates the existence of both recoverable and irrecoverable degradations in the fuel cell. Furthermore, the equivalent circuit model and membrane resistance were used to investigate the degradation mechanisms, suggesting that the natural degradation of the fuel cell is mainly caused by the increase of the resistance, which is most likely caused by membrane dehydration.     

Photovoltaic system testing techniques and results

Over the past three years, the New Mexico Solar Energy Institute (NMSEI) has tested and collected data on eight intermediate-size flat-plate photovoltaic systems. These data are now included in a valuable database for determining component reliability and system degradation trends. The specific test techniques used by NMSEI and the reliability of photovoltaic modules revealed by this testing are described and discussed. These methods are: I-V curve plotting, operating voltage and current measurements, and shading tests. These use of I-V curve data are important in determining array peak power rating and quickly locating large system faults. Operating voltage and current measurements are used in determining the location of module level faults. Bypass diode current measurements in conjunction with intentional module shading are used for isolating module faults in intermediate-size systems and systems with inaccessible module wiring. Of the 64000 modules tested, 362 modules (0.6%) were not contributing power     

Voltage degradation of high-temperature PEM fuel cells operating at 200 °C under constant load and start-stop conditions

High temperature proton exchange membrane fuel cells (HT-PEM) offer significant advantages over conventional low temperature fuel cells (LT-PEM), including improved fuel impurity tolerance and increased electrode kinetics. These advantages enable use of reformate fuels with potentially lower costs and simplified handling versus ultra-pure hydrogen fuel required for LT-PEM. Although HT-PEM fuel cell operation has been demonstrated at temperatures above 120 °C, relatively few studies have focused on operation at 200 °C or higher where fuel impurity tolerance is maximized, but at the cost of accelerated performance degradation. To help address this research gap, the present study investigated the voltage degradation of HT-PEM fuel cells operating at 200 °C and 0.4 A/cm² under continuous load conditions, and at 200 °C and 0.6 A/cm² during start-stop cycling. Results based on triplicate measurements show an average constant load degradation rate of 102 μV/h, as compared to literature values of 10 μV/h or less at lower temperature and current density. The start-stop experiments showed relatively high degradation rates per cycle up to 50 cycles, with decreasing average degradation rates over 80 and 100 cycles.     

基于电压时间序列的电力系统暂态电压稳定分析

基于实时获取的广域测量信息,提出一种利用机端电压幅值偏差序列的最大Lyapunov指数指标(LLE)与机端电压幅值序列相结合的暂态电压稳定判别方法.基于无需系统模型的LLE计算方法,建立LLE与机端电压幅值偏差序列的数学关系,根据最大Lyapunov指数曲线运动的轨迹特征,以及机端电压幅值变化率-电压幅值偏差(Vmag...     

Effects of operating conditions on durability of polymer electrolyte membrane fuel cell Pt cathode catalyst layer

In this study, we investigated the effects of humidity and oxygen reduction on the degradation of the catalyst of a polymer electrolyte membrane fuel cell (PEMFC) in a voltage cycling test. To elucidate the effect of humidity on the voltage cycling corrosion of a carbon-supported Pt catalyst with 3 nm Pt particles, voltage cycling tests based on 10,000 cycles were conducted using 100% relative humidity (RH) hydrogen as anode gas and nitrogen of varying humidities as cathode gas. The degradation rate of an electrochemical surface area (ECSA) was almost 50% under 189% RH nitrogen atmosphere and the Pt average particle diameter after 10,000 cycles under these conditions was about 2.3 times that of a particle of fresh catalyst because of the agglomeration of Pt particles.The oxygen reduction reaction (ORR) that facilitated Pt catalyst agglomeration when oxygen was employed as the cathode gas also demonstrated that Pt agglomeration was prominent in higher concentrations of oxygen. The ECSA degradation figure in 100% RH oxygen was similar to that in 189% RH nitrogen. It was concluded that liquid water, which was dropped under a supersaturated condition or generated by ORR, accelerated Pt agglomeration. In this paper, we suggest that the Pt agglomeration degradation occurs in a flooding area in a cell plane.     

Terminal voltage build-up and control of a DFIG based stand-alone wind energy conversion system

The paper presents the voltage build-up process and the terminal voltage control of a doubly-fed induction generator (DFIG) driven by a pitch controlled wind turbine for the supply of autonomous system without any auxiliary source. A control strategy for the complete system including voltage build-up phase is developed with a view to provide as well as possible the required power for load. Indirect stator flux-oriented vector control is proposed to keep the stator voltage constant by means of a back-to-back converter connected to the rotor side, while the management system is supported by the pitch angle and the load shedding controllers. A novel scheme for voltage build-up is presented, which requires no additional hardware support, and physical interpretation of how self-excitation can occur from residual magnetism in the machine core is examined. A reliable start-up process is accomplished by using an appropriate voltage reference ramp which enables minimizing energy loss during the starting. The proposed system is modeled and simulated using Matlab/Simulink software program to examine the dynamic characteristics of the system with proposed control strategy. Dynamic simulation results for different transient conditions demonstrate the effectiveness of the proposed control strategy.     

Effects of temperature uncertainty on the performance of a degrading PEM fuel cell model

The performance of a fuel cell is subject to uncertainties on its operational and material parameters. Among operational parameters, temperature is one of the most influential factors. This work focuses on this parameter. A statistical analysis is developed on the output voltage of proton exchange membrane fuel cell models. The first model does not include any degradation, whereas the second one introduces a degradation rate on the cell active area. To complete the simulation work, a full factorial design is carried out and a statistical sensitivity analysis (ANOVA) is used to compute the effects and contributions of important parameters of the model on the output voltage.     

Experimental study on dynamic response characteristics and performance degradation mechanism of hydrogen-oxygen PEMFC during loading

The instantaneous voltage overshoot caused by current loading is one of the important factors for the performance degradation of hydrogen-oxygen proton exchange membrane fuel cells (PEMFCs). In this study, the dynamic response characteristic parameters, including first-stage delay (FTD), second-stage delay (STD), and voltage undershoot (VU), were studied quantitatively to analyze the voltage changes during current loading. The effects of loading range and operating parameters including temperature, stoichiometric ratio, and relative humidity on dynamic response characteristics were experimentally analyzed. The results show that the FTD and the STD were shortened by a smaller loading amplitude. The FTD under the loading range of 200 mA/cm²-600 mA/cm² was 0.5 s shorter than that under 200 mA/cm²-1000 mA/cm², and the STD was shortened by 7.5 s. The STD was reduced by 30.9% with the operating temperature increased from 55 °C to 75 °C. What's more, the FTD and VU reached minimum values with the relative humidity of the anode and cathode controlled at 50% and 70%, respectively. In addition, it was found that after the current loading experiment, the performance decreased by 2.5%, the charge transduction resistance increased by 8.94%, and the electrochemical active surface area decreased by 11.68%. The findings reported are expected to provide guidance for optimizing the working conditions of hydrogen-oxygen PEMFC, to reduce the performance degradation caused by current loading and thus improve its working life.     

Experimental analysis on the degradation behavior of overdischarged lithium-ion battery combined with the effect of high-temperature environment

A set of experiments are performed in the present work to investigate the degradation behavior of lithium-ion battery during overdischarge cycling, as well as the influence of a high-temperature environment on the degradation. Among, different discharge cut-off voltages (1.0, 0.5, and 0.2 V) are included. During the overdischarge process, batteries experience a stage where a violent electro-thermal behavior is exhibited, involving sharp decreases in the voltage and current, and a fierce increase in the surface temperature; moreover, several parameters such as the discharge capacity, energy density, and internal resistances are all increased after overdischarge. Besides, a poor rate capacity and serious capacity degradation can also be seen during the overdischarge cycling, which is further reflected by the evolution of battery surface temperature, charge/discharge voltage, and internal resistances. What is more, it is found that battery electro-thermal parameters, eg, temperature rise, degradation rate, and internal resistances, increase exponentially as overdischarge deepens. Finally, a high-temperature environment is verified to deteriorate the degradation of overdischarged battery.     

One thousand-hour long term characteristics of a propane-fueled solid oxide fuel cell hot zone

One thousand-hour continuous test of a propane-fueled portable solid oxide fuel cell (SOFC) based hot zone has been successfully performed in order to assess the degradation characteristics of its performance. Comparing the different operating modes, the degradation rate based on constant current mode was three times lower than that based on constant voltage mode. The stack power output initially increased 3.7% during the first 34 h probably due to electrode activation processes improving cell performance under polarization during the early stage of operation, and then gradually decreased. It has been clearly illustrated that operating condition of constant current is more beneficial to the long term performance test. Further, based on thermodynamics analysis, the electromotive force of nickel oxidation is 13.2 V for the stack voltage at the stack temperature of 740 °C. From the initial current-power curve data, it can be derived that if the hot zone durability test was performed at constant current of 9 A from the beginning, the stack degradation rate would be 15% per 1000 h. The 1000-h durability test and analysis can better understand how to run longer term stability on the hot zone and guide the optimization of hot zone operating conditions.     

Voltage pulsing for performance recovery of yttria-stabilised zirconia membranes in oxygen/sulfur dioxide separation

The regeneration of yttria-stabilized zirconia (YSZ) membranes exposed to high concentration sulfur dioxide in oxygen at 850 °C using DC voltage pulses was investigated by in-situ impedance spectroscopy. The membranes consisted of a dense YSZ layer as the solid electrolyte coated with two platinum layers as electrodes. On operation in the presence of SO₂, the serial resistance and polarization resistance of the Pt/YSZ cell increased. This is most likely due to the formation of sulfide at the interface area of the electrode and electrolyte combined with sulfur adsorption on electrode surface. DC Voltage pulses were found to have an effect on the charge transfer and mass transfer properties of the Pt/YSZ cell, assisting the removal of sulfur on the cathode surface and leading to performance recovery of the cell. In these experiments, the greatest rate of membrane performance recovery is achieved with a cathodic DC bias of 10 V, applied for 0.08 s. Higher or longer voltage pulses may cause the rate controlling step for the oxygen reduction reaction to shift to oxygen supplied in feed from oxygen surface exchange and diffusion processes. A relatively steady membrane performance was achieved during 20 h SO₂ exposure tests. It is concluded that DC voltage pulses show promise as a method for reducing the performance degradation effects of poisoning due to sulfur containing gases in the fields of fuel cells and in the sulfur family of thermochemical cycles.     

Experimental study of the potential degradation due to the polarization curve of a high temperature proton exchange membrane fuel cell

The polarization curve is the most common characterization used to describe the steady-state of a fuel cell. The methods proposed in the literature to realize polarization curves for High Temperature Proton Exchange Membrane Fuel Cells (HT-PEMFC) are mainly focused on ensuring stable operating conditions. However, if degradations are caused during its realization, the interpretation of the experimental results may be biased. In order to understand and highlight these different phenomena, an experimental campaign is carried out on Advent PBI (formerly BASF Celtec®-P 1100W) membrane-electrode assemblies (MEA). Four different methodologies for the realization of polarization curves including impedance spectroscopies are proposed. Each one is cycled 30 times in order to compare their impact on fuel cell degradation. The use of a CO₂ sensor confirms that carbon corrosion is the main degradation caused by the passage to high voltage. Moreover, the analysis shows that this degradation can be intensified according to the conditions preceding the switch to these voltages. However, the realization of a current truncated polarization curve in order to avoid this degradation can be at the origin of the generation of reversible losses which can distort the interpretation of the results. The atmospheric pressure variation also generates a bias as the polarization curves are not necessarily realized at the same value because the gas outlet pressures are not regulated. A methodology of voltage readjustment according to the operating conditions is thus applied in order to compensate for this effect. Finally, the cycling of one of the polarization curve realization methods shows an interest for improvingthe break-in period.     

Short-term performance degradation prediction of a commercial vehicle fuel cell system based on CNN and LSTM hybrid neural network

Short-term performance degradation prediction is significant for fuel cell system control and health management. This paper presents a hybrid deep learning method by combining the convolutional neural network (CNN) and long short-term memory (LSTM) network to predict the short-term degradation of a 110 kW fuel cell system used for the commercial vehicle. First, the complete ensemble empirical mode decomposition (CEEMD) is applied to decompose the nonlinear and non-stationary voltage sequence extracted by the sliding window into modality sequences with different characteristic time scales. Then, these modality sequences are input into the corresponding CNN-LSTM for voltage prediction. Experimental results show that the proposed CNN-LSTM can reduce the root mean square error (RMSE) by 13.55% and 34.40%, respectively, compared to the single CNN and LSTM because it combines the spatial feature extraction ability of CNN and the powerful prediction ability of LSTM. Furthermore, the CEEMD–CNN–LSTM can reduce RMSE by 36.92% compared to CNN-LSTM since the impact of exogenous factors on the recoverable decay and intrinsic decay of the fuel cell can be separated easily for better model learning. The CEEMD–CNN–LSTM is also compared with other recently published deep learning models based on the same data set, and the results show that the prediction framework in this paper has higher accuracy.     

Experimental investigation on statistical characteristics of cell voltage distribution for a PEMFC stack under dynamic driving cycle

The proton exchange membrane fuel cell (PEMFC) stack consists of individual cells in series. Its operating life is subjected to performance of the weakest cell because of the short-board effect, thus voltage uniformity during dynamic long-running process is significant to its durability. In this work, based on a 1044 h aging experiment on a 6.55 kW PEMFC stack under dynamic driving cycle, voltage uniformity is analyzed. In a single cycle, voltage uniformity becomes worse with the increase of loading current and there are some local maxima of voltage coefficient variation (C_v) at the moment of loading or unloading step. C_v value at higher current is greater and increases faster with cycles. At the end of experiment, C_v at 135 A is more than 6%. Besides, skewness (S_k) is used to evaluate the skew direction and degree of cell voltage data in a cycle. In most cycles, S_k values at 34.22 A are above 0 and S_k values at 59.70 A and 135 A are below 0. After the Box-Cox transformation, which is used to improve symmetry of data and reduce S_k, the cell voltage data have passed the verifications of normal fitting, probability-probability plot and quantile-quantile plot. Therefore, it is found that cell voltage data tend to obey skewed normal distribution, which is of positive significance for improving voltage uniformity and durability of PEMFC stack.