Influence of PTFE coating on gas diffusion backing for unitized regenerative polymer electrolyte fuel cells

Gas diffusion backings (GDBs) with various PTFE loadings for unitized regenerative polymer fuel cells (URFCs) were prepared and the relations between the PTFE loading amount and the URFC performance were examined. As for the GDB of the hydrogen electrode, both the fuel cell and water electrolysis performances were not affected by the amount of PTFE loading on the hydrogen side GDB. However, the URFC performances significantly depended on the PTFE loading amount of the GDB for the oxygen electrode; during the fuel cell and water electrolysis operations, URFC showed higher performances with smaller PTFE loadings but the cell with no PTFE-coated GDB showed a very deteriorated fuel cell performance. Cycle properties of the URFC revealed that the efficiency of the URFC decreased with the increasing cycles when the PTFE loading on oxygen side GDB was too low, however, a stable operation can be achieved with the appropriate PTFE loading on the GDB.     

Thin film electrocatalyst layer for unitized regenerative polymer electrolyte fuel cells

Thin film electrocatalyst layers with various PTFE and Nafion contents for unitized regenerative polymer electrolyte fuel cells (URFCs) were prepared by the paste method and the performance as URFC electrodes was examined. Comparing the terminal voltage versus current density curves of the URFC, it was found that the PTFE content in the electrocatalyst layer affected only the fuel cell performance; the electrode containing 5–7 wt.% PTFE was appropriate for the URFC. The Nafion content in the electrode affected both the fuel cell and water electrolysis performance; the electrode containing 7–9 wt.% Nafion showed good performance. The addition of a small amount of iridium catalyst (about 10 at.%) to the oxygen electrode layer significantly improved the URFC performance. Catalyst loadings can be reduced to <1/3 by using the electrode prepared by the paste method compared to the conventional one without degrading the URFC performance.     

Development and preliminary testing of a unitized regenerative fuel cell based on PEM technology

Results related to the development and testing of a unitized regenerative fuel cell (URFC) based on proton-exchange membrane (PEM) technology are reported. A URFC is an electrochemical device which can operate either as an electrolyser for the production of hydrogen and oxygen (water electrolysis mode) or as a H₂/O₂ fuel cell for the production of electricity and heat (fuel cell mode). The URFC stack described in this paper is made of seven electrochemical cells (256 cм² active area each). The nominal electric power consumption in electrolysis mode is of 1.5 kW and the nominal electric power production in fuel cell mode is 0.5 kW. A mean cell voltage of 1.74 V has been measured during water electrolysis at 0.5 A cm⁻² (85% efficiency based on the thermoneutral voltage of the water splitting reaction) and a mean cell voltage of 0.55 V has been measured during fuel cell operation at the same current density (37% electric efficiency based on the thermoneutral voltage). Preliminary stability tests are satisfactory. Further tests are scheduled to assess the potentialities of the stack on the long term.     

Experimental study on laboratory scale Totalized Hydrogen Energy Utilization System using wind power data

The renewable energy source like wind energy generates electric power with intermittent nature. Hydrogen energy system can help to solve the fluctuation problem of the wind power. Totalized Hydrogen Energy Utilization System (THEUS) consists of a Unitized Reversible Fuel Cell (URFC), a hydrogen storage tank, and other auxiliary components. Wind power is inherently variable; the URFC will be subjected to a dynamic input power profile in water electrolyzer mode operation. This paper describes the THEUS operation and performance at different variations in intermittent wind power. The performance of the THEUS was evaluated in water electrolyzer and fuel cell mode operation. The stack efficiency, system efficiency, and system efficiency including heat output from the URFC were presented at each operation. The total efficiency of the URFC and THEUS were also investigated. The maximum total efficiency of the URFC and THEUS were 53% and 66%, respectively.     

Effect of mode switching on the temperature and heat flux in a unitized regenerative fuel cell

Unitized regenerative fuel cells operate in not only fuel cell but also water electrolyzer mode. Heat management is important for the stable operation of unitized regenerative fuel cells. In this work, temperature and heat flux on the surface of the gas diffusion layer at the hydrogen side of a unitized regenerative fuel cell are experimentally measured using thin film sensors. Four pairs of sensors with good linear relation coefficient are inserted in the unitized regenerative fuel cell. The variation of temperature and heat flux on the gas diffusion layer surface during mode switching is obtained. The effect of mode switching on temperature and heat flux in the unitized regenerative fuel cell is analyzed. Experimental results show that reactant switching significantly affects temperature and heat flux. Reactant switching also causes decreased temperature and variation in heat flux. Despite of the decrease of temperature caused by the low-temperature water, the temperature increases with the operation of the URFC. When the effect of reactant switching is ignored, temperature is further found to increase in fuel cell and water electrolyzer modes, and heat flux remains relatively stable.     

Deposited RuO2–IrO2/Pt electrocatalyst for the regenerative fuel cell

A bifunctional RuO₂–IrO₂/Pt electrocatalyst for the unitized regenerative fuel cell (URFC) was synthesized by colloid deposition and characterized by analytical methods like TEM, XRD, etc. The result reveals that RuO₂–IrO₂ was well dispersed and deposited on the surface of Pt black. With deposited RuO₂–IrO₂/Pt as the catalyst of oxygen electrode, the performance of fuel cell/water electrolysis of unitized regenerative fuel cell (URFC) was studied in detail. URFC with deposited RuO₂–IrO₂/Pt shows better performance than that of URFC with mixed RuO₂–IrO₂/Pt catalyst. Cyclic performance of URFC with deposited RuO₂–IrO₂/Pt is very stable during 10 cyclic tests.     

An experimental investigation of the feasibility of Pb based bipolar plate material for unitized regenerative fuel cells system

The unitized regenerative fuel cell (URFC) system has attracted significant attention and interests because of its round-trip energy conversion with high energy density. However, the identification of low-cost bipolar (BP) plate with higher corrosion resistance is required to produce a sustainable system. In this work, we investigated the possibilities of using cost-effective lead (Pb) metal-based plate as a BP plate material for URFC system. To further enhance the advantageous properties of Pb plate, silver (Ag) was coated onto the Pb plate. Different types of structural and microstructural analyses (XRD, XPS, SEM, EDAX, and mapping) were conducted to characterize the properties of Pb-based plate and the presence of Ag-coated layer on the surface of Pb plate. When compared with the Pb plate, the Ag-coated Pb plate exhibited a higher water contact angle. The obtained water contact angles of Pb and Ag-coated Pb plates were 77.83° and 92.21°, respectively. The obtained water contact angle of the Ag-coated Pb plate is quite acceptable in the URFC system because it offers exceptional advantages during the URFC operation. Furthermore, the interaction between Pb plate and Ag coated layer resulted in an efficient electrochemical performance. Based on these results, we could conclude that the Pb-based BP plate can be possibly considered for the URFC applications.     

Influence of properties of gas diffusion layers on the performance of polymer electrolyte-based unitized reversible fuel cells

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.     

Mass transfer and cell performance of a unitized regenerative fuel cell with nonuniform depth channel in oxygen‐side flow field

In this study, the cell performance of nonuniform depth and conventional straight channel in a unitized regenerative fuel cell (URFC) is compared. Various shapes of oxygen‐side channel cases are also proposed. Several parameters, such as the distribution of reactants and products and current density and powers in fuel cell (FC) and electrolytic cell (EC) modes, are investigated. A steady‐state model of two‐dimensional, two‐phase, nonisothermal, and coupled electrochemical reaction is developed. Five oxygen‐side channel shapes are also designed, in which the depth along the flow direction is narrowed. Result shows that narrowing the average channel depth can promote and guide the reactant transfer to the catalyst layer and avoid the blocking of the production. Thus, in comparison with the conventional channel, the cell performances of nonuniform depth and shallow straight channel cases are improved in both modes. In addition, with the decrease of average channel depth, the temperature uniformity gets better, which is also conductive to the improvement of cell performance. Furthermore, in FC mode at low voltage and EC mode, the cell net power basically increases with the decrease of the average channel depth ratio. And when the average channel depth is the same, the net power of straight channel is always lower than nonuniform depth case. This study introduces the round‐trip energy efficiency as an evaluation indicator of URFC. This efficiency can be increased by improving the cell performance of both modes, especially at high current density.     

Review of the membrane and bipolar plates materials for conventional and unitized regenerative fuel cells

Fuel cell or hydrogen systems offer the potential for clean, reliable and on-site energy generation. This article review current literature with the objective of identifying the latest development in membrane and bipolar plates for the conventional fuel cell and unitized regenerative fuel cell (URFC). The result shows that the choice of both the bipolar plates and the catalysts for URFC electrodes is a delicate task, for bipolar plate the corrosion in the oxygen side will be the major problem and for the electrodes a very good candidate for fuel cell mode will not function well in the electrolyser mode and therefore it is suggested that a compromise should be considered. It is recommended that aluminum, titanium or for best results titanium with a gold-coated layer is a suitable candidate as the bipolar plate and Pt/IrO_X or Pt/Ru is suitable for an oxygen side catalyst in the URFC. For the conventional fuel cell the task is more easer because the corrosion problem is no more effective and thus the main goals for most of the studies was to concentrate on bipolar plate cost reduction, increase electrical conduction and reducing the platinum loading rate for catalyst.     

Performance of Ti-Ag-deposited titanium bipolar plates in simulated unitized regenerative fuel cell (URFC) environment

Titanium with excellent corrosion resistance, good mechanical strength and lightweight is an ideal BPP material for unitized regenerative fuel cell (URFC), but the easy-passivation property accordingly results in poor cell performance. Surface modification is needed to improve the interfacial conductivity. In this study, Ti-Ag film is prepared on TA1 titanium as bipolar plates for URFC by pulsed bias arc ion plating (PBAIP). Interfacial conductivity of Ti-Ag/Ti is improved obviously, presenting an interfacial contact resistance of 4.3 mΩ cm² under 1.4 MPa. The results tested by potentiodynamic, potentiostatic and stepwise potentiostatic measures in simulated URFC environments show that Ti-Ag/Ti has good anticorrosion performance, especially at high potential. The corrosion current density of Ti-Ag/Ti is approximately 10^−5.0 A cm⁻², similar to that of uncoated titanium, at 2.00 V (vs. NHE) in a 0.5 M H₂SO₄ + 5 ppm F⁻ solution at 70 °C with pressured air purging. Ti-Ag/Ti sample also has low surface energy. The contact angle of the sample with water is 102.7°, which is beneficial for water management in URFC. The bipolar plate with cost-effective Ti-Ag film combines the prominent interfacial conductivity with the excellent corrosion resistance at high potential, showing great potential of application in URFC.     

Electrochemical studies of an unsupported PtIr electrocatalyst as a bifunctional oxygen electrode in a unitized regenerative fuel cell

The electrochemical performance of an unsupported PtIr electrocatalyst was evaluated as a bifunctional oxygen electrode in a unitized regenerative fuel cell (URFC). The catalyst was a mixture of unsupported Pt black and Ir black catalysts in varying proportions. The performance of the unsupported PtIr catalyst was studied by using a rotating ring disc electrode (RRDE) and linear sweep voltammetry (LSV). In addition, a unit cell test was performed simultaneously in the electrolyzer and in the fuel cell mode to evaluate the performance and durability of PtIr catalysts. The catalyst composition consisting of 85 wt.% Pt and 15 wt.% Ir showed high oxygen evolution reactivity and comparable electrochemical activity compared to the unsupported Pt black catalyst. The URFC using Pt₈₅Ir₁₅ catalyst showed the highest round-trip efficiency when estimated at different current densities. The cycle performance of URFC with Pt₈₅Ir₁₅ catalyst was stable for 120 h at an applied current density of 0.5 A cm⁻².     

Modelling of temperature distribution in a solid polymer electrolyte fuel cell stack

The production of electricity in a fuel cell system is associated with the production of an equivalent amount of thermal energy, both for large size power plants and for transportation applications. The heat released by the cells must be removed by a cooling system, characterized by its small size and weight, which must be able to assure uniform work conditions and reduce performance losses. Based upon realistic assumptions, a mathematical model has been developed to determine the temperature and current density distribution in a solid polymer electrolyte fuel cell (SPEFC) stack as a function of operating conditions and stack geometry. The model represents a useful tool to identify operating conditions, such as to have an optimal longitudinal and axial temperature profile, so allowing the design of cooling system and bipolar plates. In this paper, the model has been applied to determine the temperature profile of an experimental SPEFC stack. The model is validated by comparing model results with experimental measurements; simulated and experimental results agree satisfactorily.     

Design and set up of a hybrid power system (PV-WT-URFC) for a stand-alone application in Mexico

The Mexican territory has a large potential for renewable energy development, such as geothermal, hydro, biofuels, wind and solar. Thus, a 2.5 kW hybrid power system (solar, wind and hydrogen) was designed and installed to meet the power demand for a stand-alone application at the University of Zacatecas. The hybrid unit integrates three power energy sources –a photovoltaic system (PV), a micro-wind turbine (WT), a prototype of a unitized regenerative fuel cell (URFC) and energy storage devices (batteries)– in addition to their interaction methodology. The main contribution of this work is the URFC integration to a hybrid power system for the production of H₂ (water electrolyzer mode) and energy (fuel cell mode). These three energy technologies were connected in parallel, synchronized to the energy storage system and finally coupled to a power conversion module. To achieve the best performance and energy management, an energy management and control strategy was developed to the properly operation of the power plant. A meteorological station that has wireless sensors for the temperature, the humidity, the solar radiation and the wind speed provides the necessary information (in real time) to the monitor and control software, which computes and executes the short and mid–term decisions about the energy management and the data storage for future analysis.     

High-durability titanium bipolar plate modified by electrochemical deposition of platinum for unitized regenerative fuel cell (URFC)

The electrochemical deposition of platinum on a titanium bipolar plate (Pt/Ti) was studied for applications in a unitized regenerative fuel cell (URFC). Platinum deposition on the titanium plate was carried out in the platinum precursor solution (1.8 g dm⁻³) at constant acidity (pH 1.0) and temperature (90 °C). The pre-treatment of the titanium plate and the applied deposition current density were optimized to obtain uniform deposition of platinum on the titanium plate. New bipolar plates were prepared using the optimized deposition process and were used in a URFC. Electrochemical deposition of platinum on the titanium plate can effectively prohibit the formation of a passive oxide layer and corrosion on the surface of the bipolar plate, leading to lower resistance and better performance. In addition, the stability of URFC performance after the operation of the cell at 2.0 V for 1 h was significantly improved by the platinum deposition on the titanium bipolar plate. This improvement was mainly due to reduced corrosion on the surface of the bipolar plate.     

Numerical and experimental studies on unitized regenerative proton exchange membrane fuel cell

Increasing energy need and running out of fossil-based fuels direct us to renewable energy resources. Although hydrogen is not an energy source by itself, it is an energy carrier with a high specific heat capacity. As it is used as fuel in unitized regenerative PEM fuel cells, water is separated in electrolyzer mode and stored by producing hydrogen when there is no need for energy. In this study, performance tests on the unitized regenerative PEM electrolyzer/fuel cell were carried out and numerical modelling has been performed. The validity of the developed model was confirmed by the results of the experimental study. Before starting the performance tests, the cell's leakproofness tests were carried out, and the appropriate torque force was optimized, reducing the contact resistance that causes performance loss. The material selection of the cell components and corrosion-resistant materials that can operate in both electrolyzer and fuel cell modes were preferred.In this study, 0.019 slpm of hydrogen and 0.0095 slpm of oxygen gas is produced in the electrolyzer mode, while a power density of 0.353 W/cm² is obtained in the fuel cell mode at 80 °C, from a unitized regenerative PEM fuel cell with a 5 cm² active area, whose cell elements are combined with a 3 Nm clamping torque by using 12 bolts.     

Diagnostic analysis of a single-cell Proton Exchange Membrane unitised regenerative fuel cell using numerical simulation

Fabrication and testing of Proton Exchange Membrane (PEM) fuel cells to improve performance is an expensive and time-consuming process. This paper presents a novel procedure for using computer simulation – namely the ANSYS PEM Fuel Cell Module – to identify key performance limiting factors in fuel cell mode of a PEM Unitised Regenerative Fuel cell (URFC) fabricated at RMIT by comparing its performance with a higher performing URFC reported in the literature. The diagnostic analysis is performed in two steps: firstly, changing operating conditions to ensure both cells are compared based on the same conditions; secondly identifying differences in cell properties, specifically catalyst exchange current densities and membrane conductivity. The simulation results show that applying the more optimal operating conditions of the higher performing cell doubled the maximum power of the RMIT cell (from 0.163 W/cm² to 0.327 W/cm²). To overcome the remaining performance deficit in the ohmic polarization region, the value of the protonic conduction coefficient in the modelled RMIT cell had to be increased. Overall the study indicates that computer simulation modelling, in conjunction with carefully focussed experiments, can be a very useful tool in diagnosing fuel-cell performance problems.     

Performance of gold-coated titanium bipolar plates in unitized regenerative fuel cell operation

The corrosion of the carbon-based bipolar plate was studied under unitized regenerative fuel cell (URFC) operation conditions. At overpotentials higher than 2.0 V vs. normal hydrogen electrode (NHE), cell performance in the electrolyzer mode significantly decreases with time due to the increased ohmic resistance of the carbon-based bipolar plates. During fuel cell operation, the unit cell shows an ohmic resistance of approximately 0.15 Ω. After the operation in the electrolyzer mode, the ohmic resistance of the cell increases up to 1.24 Ω. The surface image of the carbon-based bipolar plate after water electrolysis reaction at 2.0 V shows a drastic corrosion at the contact area of the bipolar plate with the electrode. The corrosion of the rib in the flow-field increases the contact resistance between the electrode and the bipolar plate, which leads to the observed decrease in cell performance. A gold coating of 1 μm on the titanium bipolar plates is very effective in preventing titanium oxidation during the URFC operation. The ohmic resistance of the cells that are prepared with bare titanium and gold-deposited titanium bipolar plates is 0.40 Ω and 0.18 Ω, respectively. In fact, the gold coating serves as a barrier layer, which inhibits the formation of the passive layer on the surface of titanium-based bipolar plates. The cycling experiments in the fuel cell and in the electrolyzer mode indicate that the gold-coated titanium bipolar plates exhibit a stable performance.     

Gas diffusion layers for PEM fuel cells: Materials,properties and manufacturing – A review

The complexity in proton exchange membrane fuel cell (PEMFC) stack stems from the fact that numerous physio-chemical processes as well as multi-functional components are involved in its operation. Among the various components a Gas Diffusion Layer (GDL) being an integral component that plays a significant role in determining the performance, durability, and the dynamic characteristics, when air is used as oxidant. In addition, it serves as an armour to safeguard the membrane (Nafion), which is a delicate as well as one of the most expensive components of the PEMFC stack. A comprehensive insight on the GDL can help us to assess the fuel cell stack performance and durability. Apparently, the gas (hydrogen and air/oxygen) being converted to the energy in a PEM fuel cell needs to be diffused uniformly for which surface attributes and porosity must also be well interpreted. This review is a comprehensive assessment made on the fundamental mechanism of the diffusion process along with the various materials involved and evaluating their pros and cons. Eventually, the various manufacturing techniques involved in the GDL fabrication process are also reviewed holistically. It is envisaged that the additive manufacturing process can be a potential option to fabricate a GDL in a cost-effective and simple manufacturing approach.     

A numerical investigation on multi-phase transport phenomena in a proton exchange membrane fuel cell stack

In this study, the simulation of a fuel cell stack is performed by applying a general numerical model with VOF method that has been successfully applied to single PEMFC model to investigate the fluid dynamics, mass transport, flooding phenomenon and the effects of liquid water on the stack performance. The performance of three single cells in series connection in the fuel cell stack is examined according to the presence of liquid water in different single cells. The distributions of fluid flow, species concentration and the current density are presented to illustrate the effects of liquid water on the performance of each single cell. The numerical results locate that the low distributions of species in the flooding cell certainly degrade the performance of this cell. Moreover, it can be seen that the performance of the flooding cell will significantly affect the whole stack performance since the values of average current density must be identical in all single cells.