Unitized Regenerative Fuel Cell Research Articles

Electrochemical Characterization of Sb-Doped SnO2 as Electrocatalyst Support for Electrochemical Energy Conversion Systems

Electrochemical energy conversion systems, mainly Unitized Regenerative Fuel Cells (URFC) and Solid Polymer Electrolyzer (SPE), are considered to have a promising future due to the generation of hydrogen in a clean and sustainable way. However, the design of oxygen electrodes is the main limiting factor for an efficient performance. In this matter, most of the studies have been focused in the development of efficient electrocatalyst for the oxygen reduction and water electrolysis reactions, and the use of electrocatalysts supports different from conventional carbon-based materials, which is susceptible of corrosion at the URFC and SPE operation conditions. Corrosion resistance supports, with improved surface area and conductivity, would reduce the electrocatalyst loading,the membrane electrode assembly degradation rate, and the global construction cost of the systems. The present work deals with the synthesis and electrochemical characterization of a Sb – doped SnO2 (ATO) for their potential use as electrocatalyst support in the oxygen electrode in electrochemical energy conversion systems. The synthesis of the support was performed by the sol –gel technique with a surface area modification with dodecylamine as surfactant followed by a thermal treatment. Electrochemical stability was evaluated by cyclic voltammetry (CV) with 250, 400 and 500 cycles in a wide range (0 – 2.0 V vs ENH), while the chronoamperometry (CA) was evaluated at high anodic potential (1.8 V vs ENH) in a continuous period of time of 5 h and 8 h. The results revealed that the Sb doped SnO2 support, present good electrical properties and electrochemical stability at a higher potential of 1.8 V vs. NHE in the presence of O2, as compared to other carbon-based and commercial sub-stoichiometric titanium oxide (Ebonex ®) supports.

A Carbon-Free, Precious-Metal-Free, High-Performance O2 Electrode for Regenerative Fuel Cells and Metal-Air Batteries

There have been extensive research efforts focusing on developing technologies for renewable electricity, particularly solar photovoltaics and wind turbines. However, the intermittent and localized nature of wind and solar energy necessitates the development of a cost-effective way to balance energy supply and demand and to deliver electricity from remote places to cities.[1] The short-term option is the electricity grid; however, it can only support intermittent renewable electricity in a stable fashion up to approximately 20 % of grid capacity which means that fossil resources would still account for the remaining 80 % of grid electricity.[2] Clearly, the development of cost-effective energy storage devices is needed to establish a path toward fossil-free energy.The development of high-performance and cost-effective electrodes for oxygen evolution and oxygen reduction is critical for enabling the use of energy storage devices based on O2-H2O chemistries such as metal-air batteries and unitized regenerative fuel cells (URFCs).[3] Herein, we report a precious-metal-free and carbon-free O2 electrode synthesized via electrodeposition of manganese oxide (MnOx) on a stainless steel (SS) substrate followed by high-temperature calcination at 480 oC. The MnOx-SS electrode displays high oxygen reduction and water oxidation activities when tested in an electrochemical cell, comparable to that of a precious-metal based electrode, Pt/C-SS. Accelerated durability testing reveals the excellent stability of the MnOx-SS electrode compared to both the Pt/C-SS electrode and a carbon-based electrode with MnOx and Ni catalysts. This can be rationalized by the carbon-free nature of the MnOx-SS electrode which circumvents carbon corrosion at the high electrochemical potentials during water oxidation and O2 reduction. Integrating the MnOx-SS electrode as the O2 electrode in an anion exchange membrane (AEM) URFC produces round trip efficiencies of 42-45 % at 20 mA.cm-2 over 10 cycles, and exhibits significantly enhanced durability compared to the carbon-based analogue. This work demonstrates the MnOx-SS electrode’s potential for use as a high performance, scalable, precious-metal-free and carbon-free O2 electrode in AEM-URFCs and metal-air batteries.

Strategic optimisation of microgrid by evolving a unitised regenerative fuel cell system operational criterion

In this paper, the problem of microgrid optimisation with storage has been addressed in an unaccounted way rather than confining it to loss minimisation. Unitised regenerative fuel cell (URFC) systems have been studied and employed in microgrids to store energy and feed it back into the system when required. A value function-dependent on line losses, URFC system operational cost and stored energy at the end of the day are defined here. The function is highly complex, nonlinear and multi dimensional in nature. Therefore, heuristic optimisation techniques in combination with load flow analysis are used here to resolve the network and time domain complexity related with the problem. Particle swarm optimisation with the forward/backward sweep algorithm ensures optimal operation of microgrid thereby minimising the operational cost of the microgrid. Results are shown and are found to be consistently improving with evolution of the solution strategy.

A carbon-free, precious-metal-free, high-performance O2 electrode for regenerative fuel cells and metal–air batteries

The development of high-performance and cost-effective electrodes for oxygen evolution and oxygen reduction is critical for enabling the use of energy storage devices based on O2–H2O chemistries such as metal–air batteries and unitized regenerative fuel cells (URFCs). Herein, we report a precious-metal-free and carbon-free O2 electrode synthesized via electrodeposition of manganese oxide (MnOx) on a stainless steel (SS) substrate followed by high-temperature calcination at 480 °C. The MnOx–SS electrode displays high oxygen reduction and water oxidation activities when tested in an electrochemical cell, comparable to that of a precious-metal based electrode, Pt/C–SS. Accelerated durability testing reveals the excellent stability of the MnOx–SS electrode compared to both the Pt/C–SS electrode and a carbon-based electrode with MnOx and Ni catalysts. This can be rationalized by the carbon-free nature of the MnOx–SS electrode which circumvents carbon corrosion at the high electrochemical potentials during water oxidation and O2 reduction. Integrating the MnOx–SS electrode as the O2 electrode into an anion exchange membrane (AEM) URFC produces round-trip efficiencies of 42–45% at 20 mA cm−2 over 10 cycles, and exhibits significantly enhanced durability compared to the carbon-based analogue. This work demonstrates the MnOx–SS electrode's potential for use as a high performance, scalable, precious-metal-free and carbon-free O2 electrode in AEM-URFCs and metal–air batteries.

Optimization of Electrode Structure to Suppress Electrochemical Carbon Corrosion of Gas Diffusion Layer for Unitized Regenerative Fuel Cell

To improve the cyclic performance of a unitized regenerative fuel cell (URFC) employing a carbon-based gas diffusion layer (GDL), a novel IrO2/Pt/IrO2 layered-electrode structure was proposed in which Pt is sandwiched between two IrO2 layers in contact with a membrane and a GDL. It was found that the distribution of Pt and IrO2 catalysts in the electrode has an enormous influence on the cyclic performance of the URFC. The developed three layered electrode showed the same round trip efficiency but much higher cyclic performance compared with the conventional homogeneously mixed electrode based on cyclic performance test. The improved cyclic performance is attributed to the unique layered electrode structure that prevents direct contact between the Pt and the GDL, resulting in the suppression of electrochemical carbon corrosion of the GDL.

Comparison of corrosion behaviors between SS304 and Ti substrate coated with (Ti,Zr)N thin films as Metal bipolar plate for unitized regenerative fuel cell

In this study, a (Ti,Zr)N coating consisting of nano-composite structure of the TiN and ZrN phases is deposited on SS304 and titanium substrates as the metal bipolar plate through the cathodic arc evaporation techniques. This improves the corrosion resistance of SS304 and Ti metal when used in a unitized regenerative fuel cell (URFC). The corrosion behaviors of these two substrates are compared. Scanning electron microscope, energy dispersive spectrometer, and X-ray diffractometer are used to characterize the microstructure, thickness, chemical composition, as well as crystalline and phase states of the deposited thin films. The potentiodynamic polarization of the (Ti,Zr)N coating is tested in a simulated URFC H2 environment, i.e., 0.50M H2SO4 with 3ppm NaF solution purging with H2 gas at 60°C. Experimental results demonstrate that the (Ti,Zr)N coating improves the corrosion resistance of SS304 (about 215 times that of uncoated SS304) better than that of the Ti substrate (about 200 times that of uncoated Ti). The enhanced corrosion resistance is attributed to the nano-composite structure of TiN and ZrN, which has a dense and columnar microstructure that is impermeable to a corrosive medium. The coating of the (Ti,Zr)N layer has better corrosion resistance than the increased sheet resistance for both substrates. Therefore, the (Ti,Zr)N–SS304 and (Ti,Zr)N–Ti samples, specifically the (Ti,Zr)N–SS304 sample, are good candidates as the material for URFC H2 side metal bipolar plate.

Hydrogen Production Using Unitized Regenerative Fuel Cell

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Electrode structure optimization combined with water feeding modes for Bi-Functional Unitized Regenerative Fuel Cells

Bi-functional Unitized Regenerative Fuel Cells (URFCs) based on proton-exchange membrane (PEM) electrolyte are promising in the field of energy storage. Their performance was investigated with catalyst loadings, carbon paper hydrophobicity and water feeding modes in both fuel cell (FC) and water electrolysis (WE) operations. The optimum membrane electrode assembly (MEA) is with 0.20 mg cm−2 Pt (in 50% Pt/C catalyst) for the hydrogen electrode (HE), and a total catalyst loading of 0.80 mg cm−2 for the oxygen electrode (OE). And its performance can reach 2.18 A cm−2 at 1.80 V in WE test and 0.64 A cm−2 at 0.60 V in FC test with air oxidant. Comparison of different carbon papers reveals that suitable hydrophobicity of OE carbon papers is obtained when the PTFE content accounts for 26.95 wt.%. Choice of water feeding modes, which has an influence on URFC performance in WE mode, is proven to have relations with current density and carbon paper hydrophobicity in a sophisticated way.

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.

Plasma-assisted Pt and Pt–Pd nano-particles deposition on carbon carriers for application in PEM electrochemical cells

Plasma-assisted deposition of platinum and platinum-palladium nano-particles at the surface of carbonaceous electronic carriers for application in proton-exchange membrane (PEM) electrochemical cells has been carried out using a conventional DC magnetron sputtering system. Different types of carrier have been used for that purpose: carbon powder (Vulcan XC-72), carbon nanotubes and carbon nano-fibers. The interest of initial chemical pretreatment or metallization of the electronic carrier to improve surface adhesion of catalyst nano-particles has been analyzed. Nanostructured catalytic powders thus obtained have been analyzed and characterized using TGA, SEM, TEM, XRD, XRF and cyclic voltammetry. The electrochemical performances of Pt/C and Pt–Pd/C electrodes have been measured in single-cell PEM fuel cell (PEMFC), water electrolyzer (PEMWE) and unitized regenerative fuel cell (URFC). Results show a high active surface area (up to 44 m2 g−1) and high electrochemical activity for a number of synthesized samples. A qualitative correlation has been established between sputtering parameters, type of carbon carrier and performances as electrocatalyst.

The Compatibility of the Composite Membrane Based on Sulfonated Poly(Ether Ether Ketone) (sPEEK) / Poly(Vinylidenefluoride) (PVdF) / Urethane Acrylate Non-Ionomer (UAN) for a Unitized Regenerative Fuel Cell (URFC)

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Standalone PV-diesel system vs. PV-H2 system: An economic analysis

Hydrogen as a long-term storage medium in photovoltaic systems has been a subject of interest in recent years. Such a system uses an electrolyser – H2 storage – fuel cell combination along with battery as short-term storage to minimize the loss of load probability. Conventionally, the same goal is achieved including a diesel generator (DG) in the photovoltaic (PV) systems. In present work, an economic comparison is carried out between DG based system and various possible configurations of H2 based systems suitable for standalone application in the range of 5 kW. Both the systems are compared with the help of boundary curve obtained from life cycle cost analysis and excess energy available in the PV-DG system. Boundary curve enables in determining cost-effective system for a site, specified by on-site fuel cost including transportation cost and seasonal solar energy difference. It is found, a system with unitized regenerative fuel cell (URFC) and metal hydride storage offers most cost-effective solution. Further, the scope of the PV-H2 system is enhanced if the salvage value of the fuel cell is considered. With steeply rising fossil fuel prices and developments in H2 technology, globally more regions will be cost-effective for PV-H2 systems.

Evaluation of Pt-Ru-Ir as Bifunctional Electrocatalysts for the Oxygen Electrode in a Unitized Regenerative Fuel Cell

Evaluation of Pt-Ru-Ir as Bifunctional Electrocatalysts for the Oxygen Electrode in a Unitized Regenerative Fuel Cell

Preparation of Pt/Irx(IrO2)10 − x bifunctional oxygen catalyst for unitized regenerative fuel cell

Bifunctional Pt/Irx(IrO2)10−x (x<10) catalyst for unitized regenerative fuel cell (URFC) has been prepared by depositing Pt on Irx(IrO2)10−x support which is obtained initially from Adams fusion method. X-ray diffraction and transmission electron microscopy show that ultrafine and narrow distributed Pt/Ir3(IrO2)7 nanocomposites are formed. Electrochemical measurements demonstrate that among the series of catalysts studied, Pt/Ir3(IrO2)7 catalyst possesses the highest electrochemical surface area (24.74m2g−1) and the highest activity towards oxygen reduction reaction (ORR) (21.71mAmg−1 at 0.85V). Meanwhile, considerably high activity towards oxygen evolution reaction (OER) (42.35mAmg−1 at 1.55V) is also observed for Pt/Ir3(IrO2)7 catalyst. Kinetic analyses indicate that ORR on Pt/Ir3(IrO2)7 catalyst follows four-electron mechanism. This work opens a new way to fabricate efficient bifunctional oxygen catalyst for URFC.

Nanosized Pt/IrO2 electrocatalyst prepared by modified polyol method for application as dual function oxygen electrode in unitized regenerative fuel cells

A new generation of highly efficient and non-polluting energy conversion and storage systems is vital to meeting the challenges of global warming and the finite reality of fossil fuels. In this work, nanosized Pt/IrO2 electrocatalysts are synthesized and investigated for the oxygen evolution and reduction reactions in unitized regenerative fuel cells (URFCs). The catalysts are prepared by decorating Pt nanoparticles (2–10 nm) onto the surface of a nanophase IrO2 (7 nm) support using an ultrasonic polyol method. The synthesis procedure allows deposition of metallic Pt nanoparticles on Ir-oxide without causing any occurrence of metallic Ir. The latter is significantly less active for oxygen evolution than the corresponding oxide. This process represents an important progress with respect to the state of the art in this field being the oxygen electrocatalyst generally obtained by mechanical mixing of Pt and IrO2. The nanosized Pt/IrO2 (50:50 wt.%) is sprayed onto a Nafion 115 membrane and used as dual function oxygen electrode, whereas 30 wt.% Pt/C is used as dual function hydrogen electrode in the URFC. Electrochemical activity of the membrane-electrode assembly (MEA) is investigated in a single cell at room temperature and atmospheric pressure both under electrolysis and fuel cell mode to assess the perspectives of the URFC to operate as energy storage device in conjunction with renewable power sources.

Honeycomb-like nanocomposite Ti-Ag-N films prepared by pulsed bias arc ion plating on titanium as bipolar plates for unitized regenerative fuel cells

Honeycomb-like nanocomposite Ti-Ag-N films are prepared by pulsed bias arc ion plating, and for the first time served as surface modification layer on TA1 titanium as bipolar plates for unitized regenerative fuel cells (URFCs). Phase structure and surface morphology of the coated samples are investigated by XRD and SEM. Corrosion resistance of bare Ti plates and Ti-Ag-N coated samples is evaluated in simulated URFC conditions, and the interface contact resistance (ICR) between bipolar plates and carbon papers is measured before and after potentiostatic polarization tests. Ti 2N phase with a preferred orientation of (1 1 0) plane is observed in the Ti-Ag-N film. SEM observation indicates that the film appears honeycomb-like and forms a nanocomposite microstructure with Ag nanoparticles embedded in Ti 2N matrix. Compared to bare titanium plates, the coated sample shows improved corrosion resistance and ultra-low ICR (2 mΩ cm 2 under a compaction pressure of 1.4 MPa). The superior conductivity of the coated sample is attributed to nanocomposite microstructure containing highly conductive Ag nanoparticles, and honeycomb-like rough surface obtained in this study. Nanocomposite Ti-Ag-N coated titanium bipolar plate exhibits prominent interfacial conductivity and excellent corrosion resistance at high potential, being a promising candidate as bipolar plate for applications in URFCs.

The effect of a modified Nafion binder on the performance of a unitized regenerative fuel cell (URFC)

A modified Nafion binder using a different dispersion solvent, dimethylacetamide (DMAc), was applied to an electrode for a unitized regenerative fuel cell (URFC). The properties of the new Nafion binder, such as dissolution rate, proton conductivity, water uptake, and I–V curves, were characterized in the study. The new Nafion binder showed less brittleness, leading to good structural integrity in the electrode and improvement of stability in the URFC performance. SEM images showed that the interfacial adhesion between the membrane and the electrode was improved by using the new Nafion binder. Therefore, the new Nafion binder was suggested as a promising candidate for a binding material in the electrode of the URFC.

Development of supported bifunctional oxygen electrocatalysts and corrosion-resistant gas diffusion layer for unitized regenerative fuel cell applications

A novel bifunctional oxygen electrode (BOE) consisting of titania supported electrocatalysts (Pt/TiO 2 and Ir/TiO 2) and a corrosion-resistant gas diffusion layer (GDL) were developed for application in unitized regenerative fuel cells (URFCs). The corrosion-resistant GDL comprised of a conventional carbon substrate and a protective micro-porous layer (MPL) of iridium–titanium nitride (Ir–TiN). Transmission electron microscopy (TEM) images revealed uniform distribution of Pt and Ir nanoparticles on the TiO 2 support with particle sizes of 4.5 and 2.0 nm, respectively, which was also confirmed by the XRD analysis. Among the various Pt–Ir compositions prepared, Pt 85Ir 15 (with a Pt/Ir weight ratio of 85/15) showed the highest catalyst efficiency towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The URFC testing results showed that the round-trip energy conversion efficiency ( ɛ RT) of supported Pt–Ir/TiO 2 (42%) was significantly higher than that of unsupported Pt–Ir black (30%). The TiO 2 support provided high surface area for the uniform dispersion of the catalyst particles. The URFC performance increase was ascribed to the uniform dispersion and better utilization of noble metal catalysts. Furthermore, the stability of URFC cycle performance was significantly improved by using Ir–TiN as an additional protective MPL mainly due to reduced carbon corrosion of the GDL especially during water electrolysis.

TiC supported Pt–Ir electrocatalyst prepared by a plasma process for the oxygen electrode in unitized regenerative fuel cells

Unitized regenerative fuel cells (URFCs) have become more attractive for some time due to its potentially wide energy storage application such as in fields of space and renewable energy. In this study, TiC supported Pt–Ir electrocatalysts (Pt–Ir/TiC) for oxygen electrode in URFCs were synthesized, respectively, by chemical reduction process and plasma reduction process. Their physical and electrochemical properties are characterized and compared using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), cyclic voltammogram (CV), potentiostatic technique, and electrochemical impedance spectroscopy (EIS). The results from XRD, XPS and TEM demonstrate that the plasma process gives a finer metal crystals and higher metal dispersion on the TiC support. The CV, polarization, potentiostatic and EIS results show that the Pt–Ir/TiC electrocatalyst prepared by the plasma reduction process is obviously more active than that by the chemical reduction process, in agreement with the above metal-dispersion observations. The plasma process is a promising way for the preparation of supported electrocatalysts.

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 cm 2 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 H 2SO 4 + 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.