Polymer Electrolyte Membrane Fuel Cell Stack Research Articles

About the Necessity to Consider Membrane Electrolyte Degradation Statistically

Polymer Electrolyte Membrane Fuel Cells (PEMFCs) are subjected to aging under pure mechanical, pure chemical and the combination of both stressors. Progression of degradation leads to membrane failures. In this study, seven different accelerated stress test (AST) protocols are applied to degrade two types of commercially available membrane electrode assemblies (MEAs) to demonstrate the necessity to evaluate lifetime of this system with statistical methods. In total, data from 56 samples is reported. Membrane lifetime is derived from hydrogen crossover and open circuit voltage (OCV) which is tracked over the course of degradation. The characteristic membrane lifetime distribution of each AST protocol is described via a cumulative Weibull distribution function (CDF). As a result, the scatter of lifetime distribution correlates with the lifetime itself and thus conclude that less repeats are required for ASTs creating short lifetimes compared to those causing long lifetimes. As the latter is required especially for membrane lifetime prediction, these conclusions are relevant for anybody designing the lifetime of PEMFC stacks.

Dynamic transport characteristics and performance response of commercial-size polymer electrolyte membrane fuel cell stack: An experimental study

The stability and durability of polymer electrolyte membrane fuel-cell stacks are closely related to their dynamic performance responses and internal transport characteristics, particularly for commercial-size stacks in automotive applications. In this study, the dynamic behaviors of a 10-cell stack (active area: 310 cm2) are investigated under various loading/unloading processes and cathode stoichiometric ratios; additionally, the joint influence of these two factors is analyzed. Voltage hysteresis is observed, and the dominant roles of dynamic oxygen, membrane water, and liquid transport are elucidated. The results indicate that the electro-osmotic effect dominates and results in anode-membrane dehydration as the loading process proceeds (>1.5 A cm−2). A larger airflow velocity enables faster downstream oxygen delivery, causing the oxygen concentration to reduce initially and then increase gradually with time after loading, thus voltage undershoots appear. Voltage hysteresis is enabled during the continuous loading/unloading/loading process owing to the difference in accumulated liquid water drainage ability after step loading and unloading. Although the synergistic loads and cathode stoichiometric ratio variations enable less cell-performance deterioration than that on contrary strategy, the dynamic characteristics are much more unstable and more likely to induce cell-stack failure and durability problems. This study provides insights into the dynamic transport characteristics in a commercial-size fuel cell stack and presents a guidance for control strategy development.

Data Driven Model for a Fuel Cell stack development in a complex Multi-source Hybrid Renewable Energy System

Fuel cells based on polymer electrolyte membrane are considered as the most hopeful clean power technology. The operating principles of polymer electrolyte membrane fuel cells (PEMFC) system involve electrochemistry, thermodynamics and hydrodynamics theory for which it is difficult to establish a mathematical model. In this paper a nonlinear data driven model of a PEMFC stack is developed using Neural Networks (NNs). The model presented is a black-box model, based on a set of measurable exogenous inputs and is able to predict the output voltage and cathode temperature of a high power module working at the CNR- ITAE. A 5 kW PEM fuel cell stack is employed to experimentally investigate the dynamic behaviour and to reveal the most influential factors.

Experimental study of the steady-state and dynamic characteristics of 1 kW water-cooled PEMFC

The Polymer Electrolyte Membrane Fuel Cell (PEMFC) has been widely developed in the shipping sector, prompting the need to accurately understand its characteristics under various operating conditions and load variations. This study presents an experimental investigation into the steady-state and dynamic characteristics of a 1 kW water-cooled PEMFC stack, aiming to optimize reactant consumption and to assess the PEMFC voltage response under varying load conditions. In the steady-state tests, experiments were conducted at three different stoichiometries of hydrogen and air to evaluate the effect of stoichiometry on PEMFC efficiency. The results showed that the electrical efficiency improved by 10% at lower hydrogen stoichiometry (1.2) compared to the manufacturer's recommended values (1.6). In the dynamic tests, the behaviors of undershoot voltage and open circuit voltage (OCV) were examined using load-step and load-ramp conditions. Results showed that the undershoot voltage could be reduced either by narrowing the load step size and ramp rate or by implementing a reactant supply strategy, which increasing the gas flow rates before increasing the current. A consistent 0.55 V recovery was observed at the OCV after 10 s, regardless of the step sizes or ramp rates of load, indicating that the PEMFC was in good state of health after dynamic load conditions. OCV can serve as an effective diagnostic tool for assessing PEMFC health.

Improving Water Management and Reactant Distribution in PEM Fuel Cells Via Flow Fields with Biomimetic Auxiliary Channels

The bipolar plate, a component of the polymer electrolyte membrane (PEM) fuel cell is responsible for reactant delivery, product removal, and mechanical stability of PEM fuel cell stacks. Bipolar plates also account for 70-90% of the weight and volume, and 18-28% of the production cost of stacks (1). Improving the function of the flow fields embedded in the bipolar plates can significantly impact the energy density of PEM fuel cells by improving liquid water management and reactant distribution. Previous works show that water preferentially accumulates under the lands of flow fields in the gas diffusion layer (GDL) for various GDL materials, creating a heterogeneous distribution of water and reactants, thereby increasing cathode mass transport overpotential(2–4). Biomimetic channel architectures have been shown to enhance preferential water flow; however, these designs have not been tailored to control water accumulation for improved reactant distribution (5,6). Targeting areas of known water accumulation such as the under-land regions of GDLs, could have a profound impact on reducing mass transport losses and improving reactant homogeneity, thereby improving the power density and efficiency of the PEM fuel cell.In this work, biomimetic auxiliary channels were laser-cut into the lands of a parallel PEM fuel cell flow field to enhance the liquid water removal and reactant distribution in the under-land region of the GDL. Constant-current electrochemical testing and electrochemical impedance spectroscopy revealed a 29% increase in peak power density resulting from a 54% decrease in oxygen mass transport overpotential using the biomimetic flow field (BFF) compared to a baseline trapezoidal flow field (TFF). Operando synchrotron X-ray radiography revealed reduced GDL water accumulation when using the BFF compared to the TFF. Water accumulation under the BFF flow fields was more homogenous especially near the catalyst layer (CL) – GDL interface, indicating enhanced reactant distribution near the CL reaction sites. Therefore, the reduced mass transport overpotential and corresponding power density increase were attributed to enhanced liquid water removal and reactant distribution due to the biomimetic auxiliary channels. These results demonstrate that significant performance enhancements can be realized by embedding alternate pathways for air and water in the lands of flow field components. Ultimately, this work can be used to further optimize the design of bipolar plates for more efficient stacks and accelerate the commercialization of PEM fuel cells. Y. Wang, D. F. Ruiz Diaz, K. S. Chen, Z. Wang, and X. C. Adroher, Materials Today, 32, 178–203 (2020).N. Ge et al., Electrochim Acta, 328, 135001 (2019).D. Muirhead et al., Int J Hydrogen Energy, 42, 29472–29483 (2017).S. Chevalier et al., J Electrochem Soc, 164, F107–F114 (2017)N. Guo, M. C. Leu, and U. O. Koylu, Int J Hydrogen Energy, 39, 21185–21195 (2014).S. Feng et al., Science, 373, 1344–1348 (2021).

Evaluation of Pt-Co Nano-Catalyzed Membranes for Polymer Electrolyte Membrane Fuel Cell Applications

The membrane electrode assembly (MEA) encompassing the polymer electrolyte membrane (PEM) and catalyst layers are the key components in Polymer Electrolyte Membrane Fuel Cells (PEMFCs). The cost of the PEMFC stacks has been limiting its commercialization due to the inflated price of conventional platinum (Pt)-based catalysts. As a consequence, the authors of this paper focus on developing novel bi-metallic (Pt-Co) nano-alloy-catalyzed MEAs using the non-equilibrium impregnation–reduction (NEIR) approach with an aim to reduce the Pt content, and hence, the cost. Herein, the MEAs are fabricated on a Nafion® membrane with a 0.4 mgPtcm−2 Pt:Co electrocatalyst loading at three atomic ratios, viz., 90:10, 70:30, and 50:50. The High Resolution-Scanning Electron Microscopic (HR-SEM) characterization of the MEAs show a favorable surface morphology with a uniform distribution of Pt-Co alloy particles with an average size of about 15–25 µm. Under standard fuel cell test conditions, an MEA with a 50:50 atomic ratio of Pt:Co exhibited a peak power density of 0.879 Wcm−2 for H2/O2 and 0.727 Wcm−2 for H2/air systems. The X-ray diffractometry (XRD), SEM, EDX, Cyclic Voltammetry (CV), impedance, and polarization studies validate that Pt:Co can be a potential affordable alternative to high-cost Pt. Additionally, a high degree of stability in the fuel cell performance was also demonstrated with Pt50:Co50.

Interaction phenomena of electrically coupled cells under local reactant starvation in automotive PEMFC stacks

The local current density distribution of polymer electrolyte membrane fuel cells (PEMFCs) can be distorted by various error states. Differences in current density distributions (CDDs) of adjacent cells in a stack are equilibrized by in-plane currents within the sandwiched bipolar plates. Degradation stressors such as detrimental differences in local cell voltage and current density maxima can thus be generated. A novel method was therefore developed to intentionally manipulate CDD profiles by integrating local artificial starvation into only one fuel cell in an assembly. This technique is applied to automotive-sized PEMFCs single cells as well as in 20 cell short-stack to analyze such voltage and current redistribution phenomena. A drastic distortion of local cell voltage is only observed for stacks, which is explained by a supplementary simulation. The local voltage distribution of an electrically coupled fuel cell is therefore calculated by combining CDD measurements with a spatially resolved polarization curve model. The capabilities and limits of a multipoint cell voltage monitoring measurement device are discussed on this basis. The inspected correlation between these two independent online measurement techniques allows to localize such error states with considerable accuracy during operation of automotive sized PEMFC stacks.

Adapted Galvanostatic Charge Method for cell-individual in-situ characterization of PEM fuel cell stacks and systems

Increasingly challenging lifetime requirements for automotive Polymer-Electrolyte-Membrane (PEM) fuel cells call for onboard characterization methods to enable state-of-health (SOH) adaptive operation strategies. In this work, an adapted Galvanostatic Charge Method (GCM) is presented, that allows for cell-individual electrochemical characterization without nitrogen flush, making it system- and potentially onboard-compatible. Key innovation is to utilize the system’s standard shutdown procedure, i.e. oxygen depletion with closed air valves, to create an inert atmosphere in the cathode volume in which the characterization takes place. The electrochemical processes triggered during the characterization with GCM are now embedded in a closed-volume-problem instead of a steady-flow-environment. This causes the hydrogen concentration difference between anode and cathode to be dynamic instead of constant during the characterization. In this work, a 0D-simulation model of the closed-volume-problem and complementary measurements on a system testbench are presented. Additionally, a novel model-based evaluation method is introduced which extracts the SOH-indicators electrochemical active surface area (ECSA), double layer capacity, hydrogen permeability of the membrane and short circuit resistance from the measurements by fitting simulated to measured voltage curves. The simulation model is thoroughly analyzed and a systematic error, evoked by the fluidic coupling of the individual cells, is found to influence the results obtained for membrane permeability. The influence of different initial conditions (among them initial hydrogen concentration) on the evaluation results is experimentally investigated and found to have no significant influence. For further studies, improvement potential was found in modeling, measurement procedure and evaluation method.

Effect of the high oxygen excess ratio design on the performance of air-cooling polymer electrolyte membrane fuel cells for unmanned aerial vehicles

Oxygen excess ratio (OER) is a key parameter that affects output power and operating current density of air-cooling polymer electrolyte membrane fuel cells (PEMFCs). In this work, the design method and performance model of air-cooling PEMFCs for UAVs with high OER are proposed. The high OER is realized by using high-speed airflow behind propeller to feed PEMFC cathode. The effect of high OER on the PEMFC performance is investigated by measuring OER and the output power of PEMFC. The test results indicate that the high OER design can further release the performance potential of the PEMFC constrained by heat dissipation while discarding the parasitic power caused by cooling fans. Compared with current PEMFC with low OER by using cooling fans, the temperature distribution variation of PEMFC stack is reduced by 29.8% and the voltage consistency of the single cell at high current density is doubled. The PEMFC with the high OER design exhibits the “current jump” effect, the continuous operating current of PEMFC is increased by 25.0%. The PEMFC net power is increased from 1070 W to 1320 W, and the power density is also increased by 67.8%. Such results provide good reference values for the current PEMFC-powered propulsion system.

Design and Analysis of Bipolar Plate of Polymer Electrolyte Membrane Fuel Cell Assembly used for Automotive Applications

A polymer electrolyte membrane fuel cell (PEMFC) is defined as a type of fuel cell used to generate voltage and current. A fuel cell produces very small amount of electrical energy about 0.7 volts. So, it is essential to stack the fuel cells in bipolar plate series connection for the production of the large amount of electrical energy to fulfil the requirement. However, it is required to stack them with uniform pressure distribution in order to minimize the chance of BPP, MEA and GDL damage, fuel leakage and contact resistance. The mechanical properties and geometrical attributes of PEMFC stack components were collected with the help of many journal papers and books for the sake of their design and simulation work. In this study, the finite element analysis (FEA) were employed to simulate the bipolar plates meant for the assessment of the uniform stress dissemination.

Multi-objective optimization of the cathode catalyst layer micro-composition of polymer electrolyte membrane fuel cells using a multi-scale, two-phase fuel cell model and data-driven surrogates

Polymer electrolyte membrane fuel cells (PEMFCs) are considered a promising alternative to internal combustion engines in the automotive sector. Their commercialization is mainly hindered due to the cost and effectiveness of using platinum (Pt) in them. The cathode catalyst layer (CL) is considered a core component in PEMFCs, and its composition often considerably affects the cell performance (Vcell) also PEMFC fabrication and production Cstack costs. In this study, a data-driven multi-objective optimization analysis is conducted to effectively evaluate the effects of various cathode CL compositions on Vcell and Cstack. Four essential cathode CL parameters, i.e., platinum loading (LPt), weight ratio of ionomer to carbon (wtI/C), weight ratio of Pt to carbon (wtPt/c), and porosity of cathode CL (εcCL), are considered as the design variables. The simulation results of a three-dimensional, multi-scale, two-phase comprehensive PEMFC model are used to train and test two famous surrogates: multi-layer perceptron (MLP) and response surface analysis (RSA). Their accuracies are verified using root mean square error and adjusted R2. MLP which outperforms RSA in terms of prediction capability is then linked to a multi-objective non-dominated sorting genetic algorithm II. Compared to a typical PEMFC stack, the results of the optimal study show that the single-cell voltage, Vcell is improved by 28 mV for the same stack price and the stack cost evaluated through the U.S department of energy cost model is reduced by $5.86/kW for the same stack performance.

Experimental study on the effect of flow channel parameters on the durability of PEMFC stack and analysis of hydrogen crossover mechanism

Polymer electrolyte membrane fuel cell (PEMFC) is a leading technology with a bright future in the field of transportation. The bipolar plate structure has a profound impact on the mass and heat transfer of PEMFC. In this study, the durability tests of two stacks with different channel depths are carried out for 1000 h respectively on the 1 kW fuel cell stack test platform. We detect the overall performance change of the stack and characterize the membrane electrode assembly at the end of the test. Based on the first principle calculation, the mechanism of membrane degradation and hydrogen crossover is analyzed, and some durability experimental results are explained. The results show that the durability of the PEMFC with a shallow flow channel is worse, the voltage drops significantly, and the hydrogen crossover increases significantly after 500 h. The first principle calculation results reveal that the chemical structure change of the membrane is caused by the free radical attack. Hydrogen crossover promotes ∙H to attack carbon-fluorine bonds. This paper reveals the mechanism of hydrogen crossover and its influence on the catalyst layer, which is helpful to improve the durability of PEMFC.

Adaptive optimization strategy of air supply for automotive polymer electrolyte membrane fuel cell in life cycle

• Adaptive optimization matching method of the PEMFC system is presented. • The adaptive model of degradation stack can be universally applied in life cycle. • Validation is completed on experimental data of commercialized fuel cell engine. • Effects of air compressor parameters on the system efficiency are investigated. • System efficiency could be improved above 5% by adaptive optimization method. In this study, an adaptive optimization matching method of the air supply is developed to maintain the high-efficiency operation of the automotive polymer electrolyte membrane fuel cell (PEMFC) system in the life cycle. A 1-D non-isothermal model of the PEMFC stack with 150 kW designed power and a centrifugal air compressor model are developed, considering the fuel cell performance degradation. The genetic algorithm (GA) is used to optimize the overall system efficiency under various output powers to achieve adaptive matching. The 1-D stack model is validated with the experimental test results at two states (before and after 800 h degradation), considering the effect of degradation on the matching strategies. Through the optimization method, the centrifugal air compressor is adaptively matched with the stack of the proposed two states to develop the compressor matching strategies under various stack conditions individually. It is found that the efficiency of the system with this optimized method is 3.8% higher than that of the system without an optimized method under the full system power range. In addition, the new matching strategy between the air compressor and the stack after degradation is exploited by the adaptive optimization method. With the help of this method, the efficiencies of the system and the stack are 5.7% and 2.9% higher than that of the matching strategy without adaptive updating. It is shown that this adaptive optimization method not only improves the output efficiency of the stack but also reduces the additional parasitic power consumed by the compressor.

A study into Proton Exchange Membrane Fuel Cell power and voltage prediction using Artificial Neural Network

Polymer Electrolyte Membrane fuel cell (PEMFC) uses hydrogen as fuel to generate electricity and by-product water at relatively low operating temperatures, which is environmentally friendly. Since PEMFC performance characteristics are inherently nonlinear and related, predicting the best performance for the different operating conditions is essential to improve the system’s efficiency. Thus, modeling using artificial neural networks (ANN) to predict its performance can significantly improve the capabilities of handling multi-variable nonlinear performance of the PEMFC. This paper predicts the electrical performance of a PEMFC stack under various operating conditions. The four input terms for the 5 W PEMFC include anode and cathode pressures and flow rates. The model performances are based on ANN using two different learning algorithms to estimate the stack voltage and power. The models have shown consistently to be comparable to the experimental data. All models with at least five hidden neurons have coefficients of determination of 0.95 or higher. Meanwhile, the PEMFC voltage and power models have mean squared errors of less than 1 × 10−3 V and 1 × 10−3 W, respectively. Therefore, the model results demonstrate the potential use of ANN into the implementation of such models to predict the steady state behavior of the PEMFC system (not limited to polarization curves) for different operating conditions and help in the optimization process for achieving the best performance of the system.

Analysis of Electrode Degradation By High-Potential Environment-Using Electrochemical Impedance Spectroscopy in Polymer Electrolyte Membrane Fuel Cells

A Polymer electrolyte membrane fuel cell (PEMFC) is one of the most important applications for decarbonization of energy using hydrogen as an energy source and is considered a promising future power source due to its high efficiency, low operating temperature, fast startup, and low noxious emissions. In particular, improvement in durability of PEMFC is one of the major issues to spur the commercialization of the PEMFC system. Therefore, to improve the durability of PEMFC systems, diagnostic strategies are required to identify faulty conditions of PEMFC systems such as fuel starvation, dehydration, flooding, load cycling, and startup-shutdown (SU/SD). The problematic form of the membrane electrode assembly (MEA) that causes a decrease in the performance of PEMFCs in steady-state and dynamic operating conditions includes Pt dissolution, ionomer degradation, carbon corrosion [1] , and mechanical problems of the membrane in some circumstances. Among them, the undesired corrosion reaction, namely, the electrochemical carbon oxidation reaction, results in a drastic decrease in electrochemical double layer capacitance (EDLC), generation of oxygen functional groups in carbon supports, and severe electrochemical surface area (ECSA) fading [2, 3], followed by deteriorated PEMFC performance. Typically, the high potential inducing carbon corrosion reactions would occur during SU/SD and fuel starvation conditions. In detail, as for fuel starvation, a temporary shortage of H2 supply to one or several cells in a PEMFC stack causes the cell voltage reversal, accompanied by a potential strike over 1.0 V vs. RHE (Figure 1) with significant electrode degradation [4 , 5]. Also, the generation of hydrogen/air boundary at the anode due to the gas crossover through the membrane results in a high potential (~1.4 V vs. RHE), accelerating the decomposition of carbon support [6]. Therefore, in this work, to investigate the effects of high potential inducing electrode degradation on PEMFC performance, we conducted accelerated stress tests (AST), and in particular, we studied the correlation between electrode degradation behaviors as well as structural properties of porous carbon such as ionic resistance, EDLC with PEMFC performance in the systemic point of view. Specially, we could see the contrasting degradation behaviors and various performance decay rates using electrochemical impedance spectroscopy (EIS) analysis. EIS can be used to isolate the contribution of many processes to performance loss, allowing investigation of the effect of carbon corrosion-induced catalyst layer changes on fuel cell performance [ 7]. The measured EIS data is evaluated through the parametric fitting of the transmission line model (TLM) to the EIS spectrum. In addition, relaxation time distribution (DRT) analysis for direct analysis of internal resistance factors by reinforcing the TLM-based impedance analysis results was applied as a diagnostic method to evaluate electrode degradation. During the ASTs, electrochemical measurements (i-V, CV, LSV, and EIS) were performed to estimate the degree of PEMFC degradation, we also conducted ex-situ surface analysis of MEA using SEM, TEM, and XPS to elucidate the specific degradation components.

Investigation of Pt/Fe-N-C Hybrids Towards ORR in Acidic Environment

Polymer electrolyte membrane fuel cells (PEMFCs) are recognized as one of the renewable energy sources in portable, automobile, and stationary applications. Currently, both low temperature (LT) and high temperature (HT) PEMFCs use platinum group metal (PGM) based catalysts for oxygen reduction reaction (ORR) containing usually Pt nanoparticles on carbon black. To reduce the total cost of PEMFC stack, worldwide researchers have considerable attention to find an inexpensive alternative catalyst. Recently, metal-nitrogen-carbon (M-N-C) compounds such as Fe-N-Cs are the most promising PGM-free catalysts for ORR. However, they suffer from insufficient volumetric activity and electrochemical stability in PEMFCs. [1,2] Xiao et al. have proven an improved electrochemical stability of Pt-Fe-N-C electrocatalysts consisting of atomically dispersed Pt and Fe atoms or Pt-Fe alloy nanoparticles in comparison with Fe-N-C only. [3] Mechler et al. have reported that the addition of 1-2 wt.% Pt in hybrid Pt/Fe-N-C catalyst performs well with an increased stability in LT-PEMFCs. [4] Liao et al. have recently shown better ORR activity of Pt/Fe-N-C than Pt/C. [5] With the overriding goal of reducing LT- and HT-PEMFC production costs, Pt/Fe-N-C activity, selectivity and stability with systematically reducing the Pt-content has not been investigated yet. In this study, Pt/Fe-N-C hybrids are synthesized using PMF-011904 from Pajarito Powder (USA) as catalyst support and wet-chemically precipitated Pt nanoparticles with targeted Pt-contents of 40, 5, 1 and 0 wt.%. First, ICP mass spectrometry is used for Pt quantification along all catalysts, and transmission electron microscopy is carried out to investigate morphology and Pt nanoparticle diameter and distribution on the Fe-N-C catalyst. Second, ORR activity and selectivity is investigated by rotating ring disc electrode (RRDE) experiments in 0.1 mol L-1 HClO4, and electrochemical surface area of Pt is calculated by hydrogen underpotential deposition and CO stripping voltammetry. Last, accelerated stress testing (AST) is performed with 5,000 triangle potential cycles between 0.6–1.5 VRHE to evaluate the catalyst stability. Figure 1 shows the initial ORR curves during the RRDE experiments. The mass activities of 40, 5 and 1 wt.% Pt/Fe-N-C determined at 0.9 VRHE are 222.5 ± 41.2 A gPt -1, 170.8 ± 80.4 A gPt -1 and 49.0 ± 8.6 A gPt -1, respectively. Thus, the mass activity depends on Pt-content in the hybrid catalyst strongly. In addition, the other electrochemical characterization results are also going to be discussed in terms of catalyst selectivity, stability and the correlation with morphological aspects during the presentation. Figure 1 ORR curves in O2-saturated 0.1 mol L-1 HClO4 electrolyte with rotation speed of 1,600 rpm and scan rate of 5 mV s-1 (averaged results of three separate electrodes per each catalyst).[1] Y. He, S. Liu, C. Priest, Q.Shi , G. Wu, Chem. Soc. Rev. 2020, 11, 3484–3524.[2] T. Reshetenko, A. Serov, M. Odgaard, G. Randolf, L. Osmieri, A. Kulikovsky, Electrochem. Commun. 2020, 118, 106795.[3] F. Xiao, G.-L. Xu, C.-J. Sun, I. Hwang, M. Xu, H.-W. Wu, Z. Wei, X. Pan, K. Amine, M. Shao, Nano Energy 2020, 77, 10592.[4] A. K. Mechler, N. R. Sahraie, V. Armel, A. Zitolo, M. T. Sougrati, J. N. Schwämmlein, D. J. Jones, F. Jaouen, J. Electrochem. Soc. 2018, 165, F1084.[5] W. Liao, S. Zhou, Z. Wang, F. Liu, H. Pan, T. Xie, Q. Wang, ChemCatChem 2021, 13, 23, 4925-4930. Figure 1

Al2O3–H2O nanofluids for cooling PEM fuel cells: A critical assessment

Polymer electrolyte membrane (PEM) fuel cells are promising eco-friendly and sustainable power generation technology. Thermal management of these cells is one of the main challenges facing their commercialization. Alumina nanofluids, in addition to others, have been proposed to overcome excess heat generated within the PEM fuel cell stacks. However, most the previous studies investigated the hydrothermal performance of the nanofluids as a function of Reynolds number (Re). It is evident that all parameters must be kept constants except one to examine its effect on the performance. To this end, choosing Re as a parameter (independent variable) is problematic as it is a function not only of nanofluid's velocity, but also its density and viscosity. In this study, a well-validated CFD model has been built to simulate the hydrothermal performance of alumina nanofluids of different volumetric concentrations (0%, 0.05%, 0.1% and 0.2%) as functions of Re (170 < Re < 530), flow rate (20 ml/min ≤ Vf ≤ 50 ml/min), and pumping power (0.025 mW < Pp < 0.3 mW). This study shows a maximum reduction of 7.9% in the thermal effectiveness and a maximum increase of 6% in the pumping power when using the nanofluids instead of water. This deterioration in the hydrothermal performance is due to the reduction in the effective specific heat capacity and increase in viscosity of the nanofluids. Also, it is concluded that it is incorrect to use Re as an independent variable in analysing the hydrothermal performance of nanofluids because of the dependency of Re on some of the thermophysical properties of the nanofluids, which are functions of the concentration. Adopting such analysing procedure leads to false conclusions.

Power equalizer for a series fuel cell architecture based on load tracking control

This paper analyses the safe operation of a multi-stack Polymer Electrolyte Membrane Fuel Cell (PEMFC) under normal conditions and partial faults using an innovative power-sharing technique based on Load-Following (LF) control. LF control references are designed based on the power flow balance to track the dynamic load profile. These references control the power flow of DC-DC boost converters connected to PEMFC stacks, performing power sharing and output voltage restoration as a final result. The behavior of multi-stack FC architecture is presented in conditions of partial drying and flooding of PEMFCs. If the transitory faults are short, the controllability of the boost converters is ensured by LF control and multi-stack FC architecture continues to safety operate in degraded-operation mode.

Experimental analysis of the performance of the air supply system in a 120 kW polymer electrolyte membrane fuel cell system

For analyzing the performance of 120 kW polymer electrolyte membrane fuel cell (PEMFC) system and its air supply system, an air system test bench was built, then applied on a 120 kW PEMFC system test bench composed of air supply subsystem, hydrogen supply subsystem, stack, cooling subsystem and electronic control subsystem. The strategy composed of feedforward table and Piecewise proportional integral (PI) feedback control strategy is employed to regulate the flow rate and pressure of air supply system. Firstly, the air compressor map and the mapping relationship between the speed of air compressor, opening of back-pressure valve and stack current are obtained by carrying out experiments on the PEMFC air system bench. Then, the max output performance, steady-state performance, the startup performance, the dynamic response abilities of PEMFC system are tested, respectively. During the experiments, performances under different test conditions were analyzed by comparing parameters such as voltage inconsistency, average voltage, minimum voltage, voltage range, net power of the PEMFC system, and stack power. The test results show that the air supply system can provide qualified flow rate and pressure for the PEMFC stack. The peak power of the stack is 120 kW and net power of the system is 97 kW when the current is 538 A. The response time from rated net power to idle net power is 12 s and from idle net power to rated net power is 23 s. The overshoot of average voltage and minimum voltage in the process of increasing load is both 0.01 V, which are 0.015 V and 0.02 V lower than that when the load is decreased, respectively. The dynamic response speed and stability of the PEMFC system in the process of decreasing the load are better than those in the process of increasing the load.

EDITORIAL

The editorial of Thermal Engineering of this issue continues the discussion on scientific research needs in vital areas in which thermal engineering has important participation. The main goal is to motivate the readers, within their specialties, to identify possible subjects for their future research. It is estimated that the existing amount of fossil fuels will last for many years. However, there is a need to look for alternative sources of energy in order to preserve the environment. De Angelis et. al., in his article, Energy Research Outlook. What to Look for in 2018 (ACS Energy Lett. 3(2018) 261-263), argues that generation and storage of technical and economically viable renewable forms of energies are the main obstacles to be overcome. In his article, De Angelis also lists some technological areas that need more research effort in the energy field: energy materials, electrochemical energy conversion and energy storage, solar cells, solar fuels, LED and display devices and, the last but not least, theory and computational modeling. Possible answers for those questions can be given from what it is called constructal theory. Constructal theory states that geometry (flow architecture) is generated by seeking the global performance subjected to global restrictions. According to the constructal law, the optimization of the flow architecture begins in a small scale (elementary level), in which, even though small, the system still keeps its identity (e.g., a brook in a river basin, a single polymer electrolyte membrane fuel cell (PEMFC) in a PEMFC stack, a cell in a multicellular organisms). The irreversibility caused by the flow resistance is minimized for a maximum global performance at the level of the complete system. Any physical system is a combination of several flow systems (e.g., electric, chemical, fluid and heat). Therefore, it can be seen that the optimization of the architecture of flow systems is as common in engineering as is in nature, where the most fit organisms (optimum configuration) survive selection (global restrictions). The mission of Thermal Engineering is to document the scientific progress in areas related to thermal engineering (e.g., energy, oil and renewable fuels). We are confident that we will continue to receive articles’ submissions that contribute to the progress of science.