• Energy Research

Most of the countries have increased the production of renewable energy to reduce pollution and their dependency on oil and natural gas. In case of Japan, solar power is increased rapidly, especially after the Fukushima Nuclear Accident. The load leveling and fluctuation absorption are the main bottlenecks to the integration of solar PV power into the future electricity system. Hydrogen is considered as an energy carrier in a future energy system based on renewable resources. Totalized Hydrogen Energy Utilization System (THEUS) consists of a unitized reversible fuel cell and a hydrogen storage tank. The main objective of this paper is to evaluate the THEUS operation and performance at different variations in solar photovoltaic (PV) power during a sunny day and a partly cloudy day and to characterize its dynamic response. Energy efficiency of the THEUS was evaluated in water electrolyzer and fuel cell mode operation.

A Regenerative Fuel Cell system, driven by renewable energy sources, has the potential to overcome the intermittent nature of renewable energy and become a reliable and feasible solution for small power stationary systems, producing electricity without pollutants. The present work describes a new system model for a metal hydride hydrogen storage bed (based on an AB5-type material) integrated into a Regenerative Fuel Cell system. The model has been validated against experimental data obtained from a Savannah River National Laboratory metal hydride bed at different operating conditions and has been integrated into a Regenerative Fuel Cell system using TRNSYS® to simulate the behavior of the overall system for selected scenarios. Results show the technical feasibility of the Regenerative Fuel Cell concept with short term energy storage (i.e. hydrogen storage) and suggest useful solutions to make the system adaptable to long term storage scenarios as well.

Abstract The relationship between through-plan polytetrafluoroethylene (PTFE) distribution on a gas diffusion layer (GDL) and unitized reversible fuel cell (URFC) performance was investigated. Titanium (Ti) - felt was used for the oxygen-side GDL and treated with 10wt.% PTFE dispersion to enhance hydrophobicity. The dependence of PTFE distribution on the PTFE drying conditions was examined using scanning electron microscopy (SEM)-based energy dispersive X-ray spectroscopy (EDS) imaging. The EDS image maps revealed that the PTFE distribution strongly depended on the drying condition of the PTFE; drying under atmospheric pressure yielded a highly non-uniform PTFE distribution in the through-plane direction, whereas drying under vacuum pressure yielded a relatively uniform PTFE distribution. The cell performance of URFCs was then evaluated based on measured current–voltage characteristics during both electrolysis and fuel cell operation modes. Results verified that compared with non-uniform PTFE distribution, a uniform distribution in the Ti-felt GDL improved the fuel cell performance under the fully wet condition (relative humidity = 100%). By applying the Ti-felt GDL with uniform PTFE distribution in the through-plane direction, the current density at the cell voltage of 0.6 V was increased by a factor of about 1.9 compared with the Ti-felt with non-uniform PTFE distribution.

Abstract Hydrogen (H 2 ) cross-over through membrane electrolyte is a critical safety issue in proton exchange membrane (PEM) electrolysis. Permeated H 2 tends to be consumed by oxidation or recombination at the anode. In this study, the contribution of oxidation/recombination to the reduction of H 2 content in the anode compartment was quantitatively evaluated by measuring the H 2 content during electrolysis operation of a unitized reversible fuel cell stack in which the anode catalyst layer (CL) contained platinum (Pt). The results of fitting calculation indicated that over 70% of permeated H 2 flux through the membrane was consumed at the anode by oxidation or recombination when the cathode pressure was under 10 bars. Therefore, promoting H 2 consumption due to the addition of Pt in either the CL and/or current collector is critical for safe PEM electrolysis. Furthermore, optimization of the electrode structure is also important not only to increase the Faraday (current) efficiency but also to reduce the H 2 content in the anode.

Abstract This experimental study identifies the effect of through-plane polytetrafluoroethylene (PTFE) distribution in gas diffusion backing (GDB) on the performance of proton exchange membrane fuel cells (PEMFC). PTFE-drying under vacuum pressure created a relatively uniform PTFE distribution in GDB compared to drying under atmospheric pressure. Carbon paper samples with different PTFE distributions due to the difference in drying conditions were prepared and used for the cathode gas diffusion layer (GDL) of PEMFCs. Also investigated is the effect of MPL application on the performance for those samples. The current density (i) – voltage (V) characteristics of these PEMFCs measured under high relative humidity conditions clearly showed that, with or without MPL, the cell using the GDL with PTFE dried under vacuum condition showed better performance than that dried under atmospheric condition. It is suggested that this improved performance is caused by the efficient transport of liquid water through the GDB due to the uniform distribution of PTFE.

Abstract In-plane permeability of gas diffusion backing (GDB) of proton exchange membrane fuel cells (PEMFCs) was investigated experimentally. Toray-paper and SGL-paper were selected as GDB test samples. Several Toray-papers were treated in-house with polytetrafluoroethylene (PTFE) using the immersion technique, dried either under atmospheric or vacuum pressure, and then sintered. The dependence of PTFE distribution in the through-plane direction on the PTFE drying conditions was examined using scanning electron microscopy (SEM)-based energy dispersive X-ray spectroscopy (EDS) imaging. The EDS image maps revealed that the PTFE distribution strongly depended on the drying condition, and PTFE drying under vacuum pressure yielded a relatively uniform PTFE distribution. The measured in-plane permeability suggests that the homogeneous distribution of PTFE achieved by the vacuum drying produces a porosity-leveling effect. In addition, the relationship between the in-plane permeability and porosity of the Toray-paper samples followed the Kozeny–Carman relation, whereas due to non-fibrous solids such as binder, that of the SGL-paper samples did not.

Abstract The intrinsic effect of properties of a self-supporting microporous layer (MPL) on the performance of proton exchange membrane fuel cells (PEMFCs) is identified. First, a self-supporting MPL is fabricated and applied to a gas diffusion layer (GDL) of a PEMFC, when the GDL is either an integrated sample composed of a gas diffusion backing (GDB, i.e., carbon paper) combined with MPL or a sample with only MPL. Cell performance tests reveal that, the same as the MPL fabricated by the coating method, the self-supporting MPL on the GDB improves the cell performance at high current density. Furthermore, the GDL composed only of the MPL (i.e., GDB-free GDL) shows better performance than does the integrated GDB/MPL GDL. These results along with literature data strongly suggest that the low thermal conductivity of MPL induces a high temperature throughout the GDL, and thus vapor diffusion is dominant in the transport of product water through the MPL.

Abstract A comprehensive overview of the properties of Nafion membranes under electrolysis conditions is presented here to help evaluate and design high-pressure operating proton exchange membrane (PEM) electrolyzers. First, the properties of Nafion membranes are reviewed and summarized based on literature data for PEM electrolysis conditions. Second, the solubility and diffusivity of H2 and O2 in liquid water and the PEM are reviewed based on literature data. Finally, the relationship between current density and the impurity of gases is reviewed and discussed.

Abstract The effect of pore structural properties (porosity and pore diameter) of current collectors in proton exchange membrane (PEM) electrolyzers on electrolysis performance was experimentally evaluated by using various titanium (Ti)-felt substrates with different porosities and pore diameters (measured by capillary flow porometry) as the anode current collectors. The current–potential (j–ΔV) characteristics were measured, and overpotential analysis was performed based on the j–ΔV characteristics and the cell resistance (Rcell) data. The results showed that (1) the effect of the decrease in water supply on the membrane resistance due to produced gas bubbles is limited when the mean pore diameter of the anode current collector is less than 50 μm, but might appear at the concentration overpotential, and (2) enhancing the uniform and sufficient contact between the current collector and the electrode reduces not only the contact resistance but also the activation overpotential.

Polymer electrolyte-based unitized reversible fuel cells (URFCs) combine the functionality of a fuel cell and an electrolyzer in a single device. In a URFC, titanium (Ti)-felt is used as a gas diffusion layer (GDL) of the oxygen electrode, whereas typical carbon paper is used as a GDL of the hydrogen electrode. Different samples of Ti-felt with different structural properties (porosity and fiber diameter) and PTFE content were prepared for use as GDLs of the oxygen electrode, and the relation between the properties of the GDL and the fuel cell performance was examined for both fuel cell and electrolysis operation modes. Experimental results showed that the cell with a Ti-felt GDL of 80 μm fiber diameter had the highest round-trip efficiency due to excellent fuel cell operation under relatively high-humidity conditions despite degradation in performance in the electrolysis mode.