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.     

Investigation on electrochemical behavior and surface conductivity of titanium carbide modified Ti bipolar plate of PEMFC

The titanium carbide (TiC) modified layer is prepared by plasma surface modification technology on the surface of Ti plate (TA1) to meet the performance requirements of bipolar plate of PEMFC. The microstructure characterization confirms that a compact and defectless TiC modified layer is formed on the surface of Ti bipolar plate. The corrosion current density of TiC modified plate in simulated PEMFC environment is reduced by approximately an order of magnitude and the self-corrosion potential is significantly improved compared with bare Ti plate. The interfacial contact resistance (ICR) of TiC modified plate (7.5 mΩ cm², under loading pressure of 140 N) is evidently lower than bare Ti plate (98.1 mΩ cm²). Even after potentiostatic polarization, the ICR of TiC modified plate still remains at satisfactory values. Furthermore, the contact angle of TiC modified plate reaches a higher value of 112°, which is beneficial to the water discharge of PEMFC.     

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.     

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.     

Fabrication of multi-filler thermoset-based composite bipolar plates for PEMFCs applications: Molding defects and properties characterizations

The bipolar plate (BP) material should possess contradictory properties. They should also manufacture from low-cost methods and materials. In the current investigation, thermoset-based composite materials reinforced with conductive fillers, and the compression molding process is implemented. In addition to fabricating the bipolar plates (BPs) with and without the flowing channels, alleviating the defects during the molding process is performed. The channels are perfectly formed on the plates with the designed depth of 0.65 mm and 0.5 mm of width. In the meanwhile, we alleviate different molding defects, which spoil the surface appearance and part properties. Regarding the physical properties, the water contact angle is measured to be around 85°. The through-plane electrical conductivity of molded plates showed high values up to 38 S/cm, and the interfacial contact resistance measured to be 18–24 mΩ cm². The mean value of the flexural strength of the produced samples was equal to 47 MPa, which is almost twice the DOE target (>25 MPa).     

Ag-coated polyurethane fibers membranes absorbed with quinary fatty acid eutectics solid-liquid phase change materials for storage and retrieval of thermal energy

The novel quinary fatty acid eutectic (CA-LA-MA-PA-SA) of capric acid, lauric acid, myristic acid, palmitic acid and stearic acid was successfully prepared with the mass ratio of 61.09/24.61/8.13/4.01/2.16. Thereafter, the innovative Ag-coated polyurethane (PU) fibers membranes with different concentrations of Ag, which were selected as a supporting material to adsorb the CA-LA-MA-PA-SA eutectics, were successfully fabricated through electrospinning followed by magnetron sputter. The energy dispersive X-ray confirmed that Ag nanoclusters were successfully deposited on the surface of PU fibers as a result of sputter coating. The observations of atomic force microscope indicated that the surface roughness of the PU fibers significantly increased with increase in coating time. The scanning electron microscope images demonstrated that the CA-LA-MA-PA-SA eutectics were uniformly distributed into the three-dimensional porous structures of uncoated and Ag-coated PU fibers membranes. Furthermore, the differential scanning calorimeter curves suggested that the CA-LA-MA-PA-SA/PU/Ag composites phase change materials (PCMs) possessed melting enthalpies about 110 kJ/kg and melting temperature around 17 °C. The absorption ratios of the CA-LA-MA-PA-SA eutectic in composite PCMs was approximately at 73.74%–83.18%. The investigation on thermal performance indicated that we achieved higher melting and freezing rates of the CA-LA-MA-PA-SA/PU/Ag composites PCMs by increasing coating time. In addition to this, after depositing Ag nanoparticles the melting and freezing times of composites PCMs were shortened to about 21%–65%.     

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.     

Frost formation on a bionic super‐hydrophobic surface under natural convection conditions

A bionic super‐hydrophobic surface has a multiple micro‐nano‐binary structure (MNBS) similar to the lotus leaf surface microstructure. This kind of surface has a contact angle of water greater than 150^° and a roll angle smaller than 5^°. In this paper, the frost deposition phenomena on a bionic super‐hydrophobic surface were observed. The surface has many micro bumps and its contact angle is 162^°. The formation of water droplets, the droplet freezing process, the formation of initial frost crystals and the frost layer structure on a cold bionic super‐hydrophobic surface under natural convection conditions were closely observed. The frost layer structure formed on the super‐hydrophobic surface shows remarkable differences to that on a plain copper surface: the structure is weaker, looser, thin, and easily removed and most importantly, it is of a very special pattern, a pattern similar to a chrysanthemum, a frost layer structure that has not been reported before to the best of the present authors knowledge. The experimental results also show that a super‐hydrophobic surface has a strong ability to restrain frost growth. The frost deposition on this bionic surface was delayed 55 minutes when compared with a plain copper surface under the conditions of a cold plate temperature of ?10.1^°C, air temperature of 18.4^°C, and relative humidity of 40%. A theoretical analysis was also presented to explain the observed phenomena. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(7): 412–420, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20216     

Effect of Ni content on the electrical and corrosion properties of CrNiN coating in simulated proton exchange membrane fuel cell

In this paper, CrNiN coatings with various Ni content are deposited on 304ss bipolar plates by closed field unbalanced magnetron sputter ion plating from CrNi alloy targets. Simulative working environment of proton exchange membrane fuel cell (PEMFC) is applied to test the electrical and corrosion properties of uncoated 304ss and CrNiN-coated samples. The influence of Ni content on microstructure, phase structure, contact angle with water and electrochemical performance is investigated. Results show that all the coated samples significantly enhanced the corrosion resistance of the 304ss, and the CrN-coated 304ss sample without Ni has the best corrosion resistance of 153.8 and ?141.9 mV in the simulated anodic and cathodic environments, respectively. Electrochemical impedance spectroscopy (EIS) studies reveal that the resistance of CrN coating is higher than that of other coated samples and 304ss in the cathodic environment. Furthermore, Interfacial contact resistance (ICR) studies revealed that CrN coating has a superior ICR of 11 mΩ cm² at a compaction force of 160 N cm^?2. In addition, the contact angle of the CrNiN coatings with water is approximately 114°, which is beneficial for water management in PEMFC. Analysis result indicates that the enhanced performance of the coated 304ss bipolar plates is related to the high film density determined by closed field unbalanced magnetron sputter ion plating, and the synergistic function of the CrNiN layered structure.     

Self-healing of super hydrophobic and hierarchical surfaces for gas diffusion layer

Surface wettability of gas diffusion media (GDM) is one of the key issues related to the water management in fuel cells. In this study, a facile coating approach of combining carbon black and polydimethylsiloxane (PDMS) is developed to fabricate the gas diffusion layer (GDL) with super hydrophobic and hierarchical surfaces. Due to the Wenzel and Cassie's effect, the fabricated GDL shows the average contact angle as high as 158° and the roll angle less than 5°. Its super durability could be identified by the constant potential oxidation with the oxidization peak current approaching to 0.1 mA cm⁻², an order of magnitude smaller than that of conventional GDL coated with polytetrafluoroethylene (PTFE) and carbon black (10/90 wt/wt). Furthermore, these hierarchical hydrophobic surfaces exhibit a recovery of hydrophobicity from 107° to 133° by heat treatment. The mechanism of the exceptional self-healing capability is investigated by microscopic and spectroscopic analysis. It is indicated that ring siloxanes with lower surface tension formed on GDL surface during heat treatment process. This paper provides a fundamental research on the hierarchical superhydrophobic surfaces of GDL and a promising solution to develop long-live super hydrophobic GDL.     

Test of conductor and semiconductor electrocatalysts in high voltage alkaline electrolyzer as production media for green hydrogen

Despite the restricted success of conductor and semiconductor electrodes in solving hydrogen production problems, they provide a promising alternative to expensive conventional electrodes in water electrolysis investigations. Titanium dioxide (TiO₂) and silver (Ag) are widely used as photocatalysts in water splitting systems for hydrogen generation. Though TiO₂ is an inactive chemical semiconductor with poor conductivity, it has not been entirely investigated as an electrocatalyst yet. Two criteria were used to achieve this target: supplying high voltage to overcome the TiO₂ large band gap and immersing it in an alkaline solution to activate its inert surface. For comparison study, Ag noble metal nanoparticles coating was employed as a competitive electrocatalyst. In this regard, the application of Ag and TiO₂ coated on Ti electrodes in a hydrogen production system operated under high voltage was reported. The nanoparticles were synthesized using cost-effective and simple methods based on UV-deposition for Ag nanoparticles and the chemical precipitation method for TiO₂ nanoparticles. Then the synthesized nanoparticles were deposited on the Ti electrodes by simple immersion. The synthesized nanoparticles and coated electrodes were tested by XRD, SEM, and EDS to study their morphology, structure, particle size, and surface composition. Based on these results, TiO₂ nano-powder and coated electrodes exhibited homogenous spheres with a mixture of rutile and anatase phases, the majority being the anatase phase. The Ag-coated Ti substrate possessed a smaller crystallite size compared to TiO₂ coated substrate. To evaluate the performance of Ag/Ti and TiO₂/Ti electrodes toward hydrogen production, H₂ flow rates were measured in a 3.6 M KOH electrolytic solution at 6 V. Hydrogen flow rates obtained for pure Ti, Ag, and TiO₂ electrodes at a steady state were 21, 35, and 37 SCCM (standard cm³/min), respectively. Also, it was found that energy consumption was reduced when the electrodes were coated with nanoparticles. Furthermore, the electrolyzer's performance was assessed by calculating the hydrogen production efficiency and the voltage efficiency. The results showed that using TiO₂ electrodes gave the best hydrogen production and voltage efficiencies of 27% and 23%, respectively. This study brings new insights about Ag and TiO₂ coated electrodes in alkaline water electrolysis at high voltage regarding nanoparticle performance, hydrogen production, system performance, and energy consumption. In addition, minimizing the fabrication and operation costs of hydrogen production is the major enabler for the broad commercialization of water electrolysis devices.     

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.     

Analysis of a Finned Heat Exchanger Working in an Adsorption Refrigeration System Using Zeolite and Methanol

A new refrigeration system that uses a specially designed finned plate heat exchanger and works with zeolite and methanol is proposed. The integration of heat transfer and adsorption via a finned surface coated with zeolite CBV 901 and the use of a connected, twin active bed system to enable heat recuperation are novel features. The thermophysical properties of zeolite and methanol were first studied with the intention of designing a high performance heat exchanger (generator) for the adsorption refrigeration system. Here, the major problem is related to poor conductivity at the interface between the heat exchanger and the zeolite. The adsorbent must be heated (desorption phase) and then cooled (adsorption phase) back to ambient temperature in order to complete a thermodynamic cycle. To manufacture a sufficiently small system, there must be high rates of heat transfer in and out of the adsorbent. Therefore, the surface of the heat exchanger is finned in order to increase the heat transfer area (the fins are coated with 2 mm layer of specially prepared zeolite paste). The following characteristics were estimated from initial calculation: heating temperature, 120°C; outside tube temperature, 119.6°C; middle fin temperature, 117°C; and coated layer of zeolite paste temperature, 115.3°C. The mathematical code developed to calculate the effects of operating conditions and the Coefficient of Performance (COP) was presented at HPC 2001 in Paris. It is based on the Dubinin-Astakhov equation and thermodynamic analyses. The results obtained shows that 0.535 is the COP for a single bed and 0.925 for a double bed.     

黄登水电站水轮机转轮上冠泵板对顶盖取水的影响

针对不同上冠斜置泵板结构方案影响黄登水电站机组顶盖取水的问题,基于计算流体动力学(CFD)和工程流体力学的基本理论,以商用CFD分析软件为平台,从顶盖取水口压力、上冠轴向水推力和泵板能耗的角度,分析了辐射径向布置泵板及泵板逆转轮旋转方向斜置30°、45°三种方案7种工况下的上冠流道内部流动特性。结果表明,斜置45°的泵板结构能获得最大的取水压力且产生的上冠轴向水推力最小,斜置30°的泵板结构能耗最低,在顶盖取水系统设计中应根据水轮机运行工况范围特点选择不同的泵板结构。     

Carbon-based films coated 316L stainless steel as bipolar plate for proton exchange membrane fuel cells

Carbon-based films on 316L stainless steel were prepared as bipolar plates for proton exchange membrane fuel cells (PEMFCs) by pulsed bias arc ion plating. Three kinds of films were formed including the pure C film, the C–Cr composite film and the C–Cr–N composite film. Interfacial conductivity of the bipolar plate with C–Cr film was the highest, which showed great potential of application. Corrosion tests in simulated PEMFC environments revealed that the C–Cr film coated sample always showed better anticorrosive performance than 316L stainless steel either in reducing or oxidizing environments. The C–Cr film coated bipolar plate sample also had high surface energy. The contact angle of the C–Cr film coated sample with water was 92°, which is beneficial for water management in a fuel cell.     

Experimental investigations of frost release by hydrophilic surfaces

Frost formation occurs when water vapor in the surrounding air comes into contact with cold surfaces through heat and mass transfer. It is usually an undesirable phenomenon in most refrigeration and cryogenic systems. A few studies have shown that changing the surface energy, such as increasing the surface hydrophilicity or hydrophobicity, has significant effects on frost growth. In this paper, a kind of hydrophilic polymer paint is formulated to counteract frost deposition on cold surfaces. The coated surface can retard frost formation up to three hours under low plate temperatures (− 15.3°C) and high air humidity (72%). To test the antifrosting performance of the hydrophilic paint under more practical conditions, it is applied to a fin-and-tube heat exchanger and a domestic refrigerator at a coating thickness of 30 μm. Comparisons of frost deposition, pressure drops, and outlet temperatures are made between uncoated and coated heat exchangers. Under conditions of high air temperature (2.2°C) and relative high air humidity (90%), the paint prolongs the defrosting interval from 80 to 137 min. Experimental observations also show that the coated hydrophilic fins are free of frost deposition during the entire course of the test and that the coating has no significant additional thermal resistance.     

Ag–polytetrafluoroethylene composite coating on stainless steel as bipolar plate of proton exchange membrane fuel cell

Forming a coating on metals by surface treatment is a good way to get high performance bipolar plate of proton exchange membrane fuel cell (PEMFC). In our research, Ag–polytetrafluoroethylene (PTFE) composite film was electrodeposited with silver-gilt solution of nicotinic acid by a bi-pulse electroplating power supply on 316 L stainless steel bipolar plate of PEMFC. Surface topography, contact angle, interfacial conductivity and corrosion resistance of the bipolar plate samples were investigated. Results showed that the defects on the Ag–PTFE composite coating are greatly reduced compared with those on the pure Ag coating fabricated under the same condition; and the contact angle of the Ag–PTFE composite coating with water is 114°, which is much bigger than that of the pure Ag coating (73°). In addition, the interfacial contact resistance of the composite coating stays as low as the pure Ag coating; and the bipolar plate sample with composite coating shows a close corrosion resistance to the pure Ag coating sample in potentiodynamic and potentiostatic tests. Coated 316 L stainless steel plate with Ag–PTFE composite coating exhibits well hydrophobic characteristic, less defects, high interfacial conductivity and good corrosion resistance, which shows a great potential of the application in PEMFC.     

Wetting on a plate with three‐dimensional random roughness and heterogeneity

A theoretical study was conducted to investigate the wetting behavior of liquid meniscus on a vertical plate with three‐dimensional random characteristics of heterogeneity and roughness. The thermodynamic stable condition was derived by considering the minimum of system free energy. The local stable condition leads to a result similar to that obtained for a plate with two‐dimensional characteristics, that is, the system has many meta‐stable states. For the stable condition of the whole system, a relation was derived between the macroscopically observed contact angle and the surface characteristics. The product of cosine of the contact angle and liquid surface tension is equal to the energy difference for the liquid to wet the plate by apparent unit area. If the liquid wets the solid surface reversibly, there is only one contact angle observed macroscopically. This fact suggests that the contact angle hysteresis is caused by the irreversible motion when the liquid advances or recedes on the solid surface. The well‐known Cassie's and Wenzel's contact angles are explained as those corresponding to a thermodynamically stable condition when the liquid wets the solid reversibly. © 2001 Scripta Technica, Heat Trans Asian Res, 30(5): 371–382, 2001     

In-situ hydrogen wettability characterisation for underground hydrogen storage

Hydrogen storage in subsurface aquifers or depleted gas reservoirs represents a viable long-term energy storage solution. There is currently a scarcity of subsurface petrophysical data for the hydrogen system. In this work, we determine the wettability and Interfacial Tension (IFT) of the hydrogen-brine-quartz system using captive bubble, pendant drop and in-situ 3D micro-Computed Tomography (CT) methods. Effective contact angles ranged between 29° and 39° for pressures 6.89–20.68 MPa and salinities from distilled water to 5000 ppm NaCl brine. In-situ methods, novel to hydrogen investigations, confirmed the water-wet system with the mean of the macroscopic and apparent contact angle distributions being 39.77° and 59.75° respectively. IFT decreased with increasing pressure in distilled water from 72.45 mN/m at 6.89 MPa to 69.43 mN/m at 20.68 MPa. No correlation was found between IFT and salinity for the 1000 ppm and 5000 ppm brines. Novel insights into hydrogen wetting in multiphase environments allow accurate predictions of relative permeability and capillary pressure curves for large scale simulations.     

Electrochemical synthesize and characterization of ZnO/ZnS nanostructures for hydrogen production

In present study, zinc oxide/zinc sulfide (ZnO/ZnS) nanostructures were fabricated by anodization of Zn. The morphological, structural and compositional properties of ZnO/ZnS were studied by field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD). From FESEM results, well-defined and smooth spherical-like ZnO/ZnS nanostructures were obtained with tube-like ZnO nanostructures underneath the compact layer. XRD patterns revealed that achieved materials offer high crystallinity hexagonal ZnO as dominant with hexagonal close-packed polycrystalline Zn. Moreover, measure of water contact angle was realized to learn about wetting property of the electrodes. Electrochemical impedance spectroscopy (EIS) was recorded to examine electrocatalytic performance of electrodes against hydrogen evolution reaction (HER) in 1 M KOH solution. The values of energy consumption and energy efficiency were calculated as 571.9 kJ mol⁻¹ and 49.5% at current density of 50 mA cm⁻² for the HER on 40-ZnO/ZnS/Zn electrode at 25°C.