• Energy Research

Abstract The development of predictive mathematical models for water management in polymer electrolyte membrane fuel cells requires detailed understanding of water distribution and water transport across the Nafion layer. The anisotropic microstructure of Nafion suggests the measurement of water content and mass transport should be along the fuel cell functional direction, i.e. across the membrane. Non-invasive, high resolution, microscopy measurements of this type are very challenging. We report here the calibration of a minimal mathematical model for diffusive water transport in Nafion against data from high-resolution water content maps determined with a new magnetic resonance imaging methodology developed for this purpose. A mock fuel cell was designed to permit well-controlled wetting and drying boundary conditions. With no chemical potential driving force involved, we assume the water transport behavior will be dominated by diffusion. Moreover we show that, in this context, our model is mathematically equivalent to the traditional permeation models based upon saturation dependent pressure gradients via a capillary pressure ansatz. The non-linear equilibrium water distribution across the Nafion membrane measured in this work suggests a bi-modal diffusivity. The model constructed associates distinct transport behaviors to water contents above and below a critical threshold, consistent with a rearrangement of a micro-structural pore network. The experimental observation and the model prediction agree with the primary features of Weber's model of Nafion, which predicts distinct modes of transport for hydration fronts traversing the through-plane direction of the membrane.

To meet the power density, reliability, and cost requirements that will enable a widespread use of fuel cells, many research activities focus on an understanding of the thermodynamics as well as the fluid mechanical and electrochemical processes within a fuel cell. To date, a wide range of experimental diagnostics is imperative not only to help a fundamental understanding of fuel cell dynamics but also to provide benchmark-quality data for modeling research. This two-part paper reviews various tools for diagnosing polymer electrolyte membrane (PEM) fuel cells and stacks, and attempts to incorporate the most recent technical advances in PEM fuel cell diagnosis. In Part I, we review various electrochemical techniques and outline the principle, experimental implementation, and data processing of each technique. Capabilities and weaknesses of these techniques are also discussed. In Part II of the review we will cover physical/chemical methods.

Abstract Chlorine is a major fuel contaminant when by-product hydrogen from the chlor-alkali industry is used as the fuel for proton exchange membrane (PEM) fuel cells. Understanding the effects of chlorine contamination on fuel cell performance and durability is essential to address fuel cell applications for the automotive and stationary markets. This paper reports our findings of chloride contamination effects on PEM fuel cell performance and durability, as our first step in understanding the effects of chlorine contamination. Fuel cell contamination tests were conducted by injecting ppm levels of contaminant into the fuel cell from either the fuel stream or the air stream. In situ and ex situ diagnosis were performed to investigate the contamination mechanisms. The results show that cell voltage during chloride contamination is characterized by an initial sudden drop followed by a plateau, regardless of which side the contaminant is introduced into the fuel cell. The drop in cell performance is predominantly due to increased cathode charge transfer resistance as a result of electrochemical catalyst surface area (ECSA) loss attributable to the blocking of active sites by Cl − and enhanced Pt dissolution.

Abstract A conditioning process is usually needed for a newly fabricated proton exchange membrane (PEM) fuel cell to be activated. Depending on the membrane electrode assemblies, this process can take hours and even days to complete. To provide for accelerated conditioning techniques that can complete the process in a short time, this paper compares various reported methods to condition a PEM single cell. The major objectives are to identify accelerated conditioning approaches that can significantly reduce the conditioning duration for the existing conditioning regime in an operationally easy manner, and to understand the fundamental principles that govern accelerated conditioning. Various effects investigated include temperature, cycling steps, and cycling frequencies. Other techniques, such as short circuiting, hydrogen pumping, and hot water circulation, are also discussed. For each technique, measurements are taken using electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and linear sweep voltammetry (LSV).

Abstract Durability of polymer exchange membrane (PEM) fuel cells under a wide range of operational conditions has been generally identified as one of the top technical gaps that need to be overcome for the acceptance of this fuel cell technology as a commercially viable power source, especially for automotive and portable applications. In this study, a 1200 h lifetime test was conducted with a six-cell PEM fuel cell stack under close to open-circuit conditions. In situ measurements of the hydrogen crossover rate through the membrane, high frequency resistance and electrochemically active surface area of each single cell, in combination with cell polarization curves, were used to investigate the degradation mechanisms. Direct gas mass spectrometry of the cathode exhaust gas indicated the formation of HF, H2O2 and CO2 during the durability testing. The overall cell degradation rate under this accelerated stress testing is approximately 0.128 mV h−1. The cell degradation rate for the first 800 h is much lower than that after 800 h, which may result from the dominance of different degradation mechanisms. For the first period, the degradation of fuel cell performance was mainly attributed to catalyst decay, while the subsequent dramatic degradation is likely caused by membrane failure.

Nanomaterials with a uniform size, large surface area, high adsorption capacity, and excellent dispersion are critical to proton conduction and cell performance when functionalized and incorporated into a proton exchange membrane.

Abstract A group of multi-effect tubular desalination devices which can be driven by solar or waste energy had been designed and constructed. The performances of the devices of single-, two- and three-effect systems were individually tested and analyzed under the conditions of fixed heating power and controlled heating temperature. The research calculated and analyzed the hourly yield and the temperature value of every measured point inside the device. Also the performance ratio of the devices was investigated in a variety of different situations. The productive rate was also tested when the devices were exposed to negative pressure conditions. The experimental curves under different pressure were presented. The experimental results indicate that the performance ratio of the two-effect and the three-effect devices working under environmental pressure reached about 1.4 and 1.7 respectively under conditions of fixed heating power. The yield of the devices under negative pressure was enhanced, in general when the fixed heating power was 300 W, the yield reached about 20.08 kg/(m2·d). These results indicate that the devices exhibit excellent application prospects.

Durability issues have been attracting a great deal of attention in hydrogen/air proton exchange membrane (PEM) fuel cell research. In the present work, membrane electrode assembly (MEA) degradation under open circuit (OC) conditions was carried out for more than 250 h. By means of several on-line electrochemical measurements, the performance of the fuel cell was analysed at different times during the degradation process. The results indicate that structural changes in the PEM and catalyst layers (CLs) are the main reasons for the decline in performance during OC operation. The results also show that degradation due to platinum oxidation or catalyst contamination can be partially recovered by a subsequent potential cycling process, whereas the same cycling process cannot recover the membrane degradation.

Abstract Air-cooled open-cathode low temperature proton exchange membrane fuel cell (AO-LTPEMFC) with low weight, small volume and compact system has become the new potential power source in the unmanned aerial vehicle (UAV). However, the desirable parameters of the cathode channel are a very important factor to influence the cell performance and the compact of the stack, which have the higher requirements put forward to the structural designs of the cathode channel of AO-LTPEMFC. Thus, some single AO-LTPEMFC fabricated with different width:0.9–1.5 mm, depth:1.1–1.5 mm, ratio (width/landing):1:0.7–1:1.3 and bending:0–10° of the cathode channel was investigated to optimize the cell performance and temperature distribution in the 2 mm plate and determine desirable design parameters. The results show that the different design parameters of the cathode channel affect the contact resistance, oxygen mass transfer of cathode and pressure drop in air flow. For AO-LTPEMFC, to keep the best performance, the cathode channel design parameters should be operated at appropriate width, as deep as possible, small ratio and bending. Through the comparison of various designs, combined with practice process, the optimum design size with width (1.1 mm) × depth (1.3 mm) × width/landing (1:0.7) × bending angel (θ) (5°) was obtained based on the 2 mm thickness of the bipolar plate, which could get the maximum performance and improve the compactness of system. Moreover, analysis in this study will provide a new guideline for the development of cathode flow field plate design in application.