Effect of catalyst distribution and structural properties of anode porous transport electrodes on the performance of anion exchange membrane water electrolysis

Anion exchange membrane (AEM) water electrolyzers are expected to be novel devices for hydrogen (H₂) production that achieve high performance at low capital cost. The effect of catalyst distribution in anode porous transport electrodes (PTEs) on the performance of AEM water electrolysis is experimentally examined. Based on the analysis of the correlation between the PTE structure and the electrolysis performance, it was revealed that the surface catalyst coverage is related to the activation overpotential, and that the location and compactness of the catalyst layer (CL) affects the concentration overpotential. This suggests that the water diffusion through the membrane is related to the concentration overpotential, and that denser CLs can promote water diffusion and thus mitigate the concentration overpotential. Based on the electrolysis data with PTEs of different thickness, it was also revealed that decreasing the thickness of the anode PTE enables good performance with low catalyst loading.     

Direct three-dimensional reconstruction of a nanoporous catalyst layer for a polymer electrolyte fuel cell

The direct three-dimensional reconstruction of a polymer electrolyte fuel cell cathode catalyst layer from focused ion beam/scanning electron microscope (FIB/SEM) images is presented. The carbon and pore distribution is shown and quantitatively analysed. A new catalyst layer sample (Fumapem-F950/HiSpec13100) is sliced with FIB and a series of SEM images is taken. The images are registered, segmented and a three-dimensional stack is reconstructed. The three-dimensional carbon and pore distribution is shown. Based on the reconstruction the pore size distribution is evaluated. The total porosity and the unconnected pores space is analysed. The fully segmented 2D images are provided as supplemental material to this paper for future analysis and modeling work.     

Controllable synthesis of three-dimensional nitrogen-doped graphene as a high performance electrocatalyst for oxygen reduction reaction

Three-dimensional nitrogen-doped graphene (3D-NG@SiO₂) is prepared by pyrolyzing poly (o-phenylenediamine) (POPD) with high nitrogen content. POPD is prepared via an in situ chemical oxidation polymerization of o-phenylenediamine (OPD) in acetic acid with silica colloid as templates. The optimum parameter is OPD:SiO₂ = 1:2, pyrolysis @ 900 °C. SEM and TEM images show the wrinkled and porous graphene structures. Raman spectra indicate that 3D-NG@SiO₂ consists of 4–6 layers graphene. N₂ adsorption–desorption isotherms reveal that the pore size distributions mainly centralize at 5–10 nm. XRD illustrates the amorphous structure. XPS analysis shows that the nitrogen content is 3.6% and nitrogen mainly exists in the form of pyridinic nitrogen and pyrrolic nitrogen. The oxygen reduction reaction (ORR) performance investigated using a rotating disk electrode shows that the initial potential of 3D-NG@SiO₂ is 0.08 V (vs. Hg/HgO). The electron transfer number is 3.92 @ ?0.3 V (vs. Hg/HgO), indicating that 3D-NG@SiO₂ mainly occurs via a four-electron process. The oxygen reduction current density decreases by 21% after 60 h in the chronoamperometry test. The CVs manifests a current density loss of 0.16 mA cm^?2 after scanning for 5000 cycles. The high activity and durability indicate the promising potential of 3D-NG@SiO₂ as ORR catalysts.     

Effect of water distribution in gas diffusion layer on proton exchange membrane fuel cell performance

Water management of proton exchange membrane fuel cells remains a prominent issue in research concerning fuel cells. In this study, the gas diffusion layer (GDL) of a fuel cell is partially treated with a hydrophobic agent, and the effect of GDL hydrophobicity on the water distribution in the fuel cell is examined. First, the effect of the position of the cathode GDL hydrophobic area relative to the channel on the fuel cell performance is investigated. Then, the water distribution in the fuel cell cathode GDL is observed using X-ray imaging. The experimental results indicate that when the hybrid GDL's hydrophobic area lies on the channel, water tends to accumulate under the rib, and the water content in the channel is low; this improves the fuel cell performance. When the hydrophobic area is under the rib, the water distribution is more uniform, but the performance deteriorates.     

A three-dimensional performance analysis of all-glass vacuum tubes with coaxial fluid conduit

The thermal performance of a solar system composed of parallel, all-glass (double skin) vacuum tubes has been investigated by using a three-dimensional analytical model. Each vacuum tube is equipped with a coaxial fluid conduit for water to flow through and collect the sun's thermal energy. The space between the exterior of the fluid conduit and the glass tube is filled with antifreeze solution to facilitate heat transfer from the solar heated absorber surface to water and to prevent the functional problems due to freezing in frigid weather conditions. Different from one-dimensional analytical models, the three-dimensional model considered in the present analysis enables the prediction of spatial variation of water temperatures as it flows through the coaxial conduit. This is quite useful in extracting major variables for the operation of the solar system using all-glass vacuum tubes as considered in the present investigation.     

Fabrication and lithium storage performance of three-dimensional porous NiO as anode for lithium-ion battery

Three-dimensional porous NiO is prepared on Ni foam by a thermal treatment method at various temperatures. The morphology and structure of porous NiO are characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The electrochemical properties of three-dimensional porous NiO anode are evaluated by galvanostatic discharge-charge cycling, cyclic voltammery, and impedance spectral measurements on cells with lithium as the counter and reference electrodes. Results show that porous NiO delivers a stable capacity of 520 ± 20 mAh g⁻¹ with no noticeable capacity fading up to 30 cycles when cycled in the voltage range 0.05-3.0 V at rate of 0.2 C. The porous NiO exhibits higher reversible capacity, better cycleability, as well as higher rate capability in comparison to NiO foil. The observed cyclic voltammograms and impedance spectra are analyzed and interpret a redox reaction of NiO-Ni⁰ with formation and decomposition of Li₂O. The excellent electrochemical performance of porous NiO can be attributed to its large surface area, which lowers the current density of NiO reaction interface, and then an alternative anode is provided for lithium-ion batteries.     

Three-dimensional modeling of pressure effect on operating characteristics and performance of solid oxide fuel cell

A three-dimensional (3-D) model for planar, anode-supported, solid oxide fuel cell (SOFC) is developed to investigate the effect of operating pressure on cell characteristics. The results show that the elevated operating pressure can improve cell performance by increasing open circuit voltage and reducing activation overpotential, and enhance the electrochemical reaction in the vicinity of electrolyte. Besides, the high pressure can also change the distributions of species and internal reforming reactions. Compared to the case using syngas as fuel, the operating pressure has more significant effects on temperature gradient along flow direction when partly pre-reformed gas is supplied. In addition, efficient control of cell temperature could be achieved by decreasing fuel utilization in the case of partly pre-reformed gas, but this is achieved at the expense of cell efficiency, especially under high pressure condition. Another way to reduce the temperature gradient is to adopt higher air ratio. Moreover, when partly pre-reformed gas is used, the counter-flow configuration has a better performance due to the higher overall temperature.     

Investigation of concentration measurement for hydrogen leakage with a new calibration visual approach

Hydrogen leakage concentration rapid measurement is the key issue for hydrogen application as hydrogen leakage is easy to cause hydrogen safety issues such as hydrogen explosions. Non-invasive visual measurement method such as the schlieren measurement technique is the prospective solution. However, the specific relationship between the hydrogen leakage concentration and schlieren image gray remains unclear, which leads that the schlieren technique procedure is developed for visualization and acquiring qualitative information only. This paper aims to decouple the hydrogen leakage concentration from the complicated schlieren image information, and find the mapping relationship between the hydrogen leakage concentration and schlieren image gray, hence realizing a quantitative hydrogen leakage concentration analysis. Therefore, a hydrogen leakage visualization experimental bench is established to simulate and measure hydrogen leakage by a series of experiments under different leakage concentrations. The mapping relationship between the hydrogen leakage concentration and schlieren image gray is obtained by the experiments using the schlieren technique. Then, a new calibration schlieren technique with the function of visually measuring hydrogen leakage concentration is developed and verified under 80% hydrogen leakage concentration. Results of the trials demonstrated the ability of the proposed technique successfully measure concentration distributions with satisfactory accuracy.     

The conception of in-plate adverse-flow flow field for a proton exchange membrane fuel cell

To improve species concentration and current density distribution uniformity of a proton exchange membrane (PEM) fuel cell, an in-plate adverse-flow (IPAF) flow field is developed. Its utility is conceptually examined through three-dimensional numerical simulation comparison between three typical fuel and air flow combinations out of those it can support. Under isothermal condition and constant velocity reactant feeding mode, as the simulation results indicate, there is no significant cell performance improvement by the new flow filed unless in mass transport limited region, while the species concentration and current density distribution uniformities are substantially improved. As data analysis supports, there are two mechanisms in the new flow field that are responsible for the distribution uniformity improvement: the along-channel offset effect and the across-rib transport effect, and their respective pure contributions to the improvement are well discerned.     

A three-dimensional model for transient performance of a solid oxide fuel cell

This paper presents an analysis of transient behavior of an anode-supported solid oxide fuel cell (SOFC) using a model, which has recently been built for steady state operation. The model is three dimensional (3D), which takes into account heat and mass transport, chemical and electrochemical reactions taking place simultaneously in the cell. The electrochemical processes are assumed to take place in a layer of finite thickness at electrode–electrolyte interfaces. A repeating unit of a planar anode-supported SOFC with co-flow configuration is investigated. Step changes of working voltage and fuel composition are applied to the cell. Results for the dynamic profiles of the temperature, the current density and the activation overpotential distributions in the cell are presented and discussed.     

Reduced concentration polarization and enhanced steam throughput conversion with a solid oxide electrolysis cell supported on an electrode with optimized pore structure

Conversion of steam to hydrogen in solid oxide electrolysis cells (SOECs) is largely limited by concentration polarization occurring in the supporting cathodes. The present study was thus aimed at reducing the concentration polarization and increasing the steam conversion by optimizing the pore structure of the cathodes. Button cells supported on cathodes with straight pore structure showed much improved performance than those with tortuous pore structure especially under steam-lean conditions (1.19 vs. 0.69 A?cm^?2 at 1.3 V, 750 °C and 10% vol. % H₂O). Impedance spectroscopy analysis indicated that the straight pore structure in the cathodes allowed for facile transport of steam, thus effectively mitigating the concentration polarization. The relation between the electrolysis current density and steam concentration determined from the button cells was used to simulate the electrolysis performance of single cells. The simulation predicts that single cells with the straight pore structure in the cathodes can achieve high steam throughput conversion even at high steam feeding rate.     

The effect of flow distributors on the liquid water distribution and performance of a PEM fuel cell

Water management is one of the important factors which determine the performance of a Proton Exchange Membrane (PEM) fuel cell using hydrogen as fuel. For developing efficient water management systems, it is important to know the potential locations of formation and the nature of distribution of liquid water in the fuel cell. In the present study a PEM fuel cell with three different types of flow distributors are modeled and numerically simulated to find out the water formation and distribution characteristics. The model is validated by comparing the simulated polarization curve to experimental data. It is found that the type of flow distributor used plays a major role in determining the distribution of liquid water in the cell. A parallel flow distributor exhibits poor water removal capabilities whereas a serpentine flow distributor exhibits better water removal. A mixed flow distributor is found to give better water distribution characteristics compared to the parallel and serpentine distributors. Further the effect of liquid water formation and distribution on the species transport, temperature distribution and current generation are also investigated.     

Development and impact of sandwich wettability structure for gas distribution media on PEM fuel cell performance

Water management is one of the key issues related to the performance, durability and cost of polymer electrolyte membrane fuel cells (PEMFCs); and the wettability of gas distribution media (GDM) is critical to the water management. In this study, a novel design is developed for GDM, referred to as sandwich wettability GDM. After being coated with a silica particle/poly(dimethylsiloxane) (PDMS) composite, the GDM has superhydrophobic surfaces with a contact angle of 162 ± 2°, but hydrophilic internal pores. Water droplets (10 μl) can roll off the tilted surface of the coated GDM at an angle of 5°, and can also be drawn into the pores of the coated GDM in 10 min when it is horizontal. The surface morphology, roughness and pore structures of GDMs are characterized by profilometry, scanning electron microscopy, and porosimetry. The measured internal pore size of the coated GDM is around 7.1 μm, and shows low energy resistance to gas transport. Performance testing indicates that the PEMFC equipped with sandwich wettability GDMs offers the best performance compared to those with raw GDM (untreated with surface coating), conventional GDM (with microporous layer) coated with PTFE or hydrophilic GDM (coated with silica particles).     

Synthesis of three-dimensional Pd nanospheres decorated with a Pt monolayer for the oxygen reduction reaction

Pd@Pt nanoparticles with a three-dimensional (3D) nanospherical shape have been prepared using a two-step approach in which the Pd nanospheres are synthesized controllably by a solvothermal method, and then a Pt monolayer is deposited on the Pd nanospheres using an underpotential deposition process. We systematically investigate (i) the influences of temperature and additive on the morphology of the Pd nanoparticles and (ii) the electrochemical activity of Pd@Pt for the oxygen reduction reaction (ORR). The Pd@Pt nanoparticles exhibit enhanced activity towards the ORR. The mass activity of Pt in the Pd@Pt nanospheres (1.03 A mgPt⁻¹) is 3.3 times higher than that of commercial Pt/C (0.24 A mgPt⁻¹). The catalyst enhanced activity may result from the 3D structure of the Pd nanospheres and the monolayer dispersion of Pt on the surface of the nanoparticles.     

Effect of silver nano-fluid on pulsating heat pipe thermal performance

This paper presents preliminary experimental results on using copper tube having internal and external diameter with 2.4 mm and 3 mm, respectively, to carry out the experimental pulsating heat pipe. The working fluids include the silver nano-fluid water solution and pure water.In order to study and measure the efficiency, we compare with 20 nm silver nano-fluid at different concentration (100 ppm and 450 ppm) and various filled ratio (20%, 40%, 60%, 80%, respectively), also applying with different heating power (5 W, 15 W, 25 W, 35 W, 45 W, 55 W, 65 W, 75 W, 85 W, respectively). According to the experimental result in the midterm value (i.e. 40%, 60%) of filled ratio shows better. In the majority 60% of efficiency is considered much better. The heat dissipation effect is analogous in sensible heat exchange, 60% has more liquid slugs that will turn and carry more sensible heat, so in 60% of filled ratio, heat dissipation result is better than 40%, and the best filled fluid is 100 ppm in silver nano-fluid.Finally, we observed through the measurement comparison in thermal performance with pure water. When the heating power is 85 W, the average temperature difference and the thermal resistance of evaporator and condenser are decreased by 7.79 °C and 0.092 °C/W, respectively.     

Effect of concentration,obstacles, and ignition location on the explosion overpressure of hydrogen-air in a closed-vessel

The report deals with the investigation of explosion safety parameters of hydrogen-air mixtures in a 17.17 L cylindrical closed-vessel with different concentrations, obstacles, and ignition locations. The experimental data including the maximum explosion pressure, laminar burning velocity, and corresponding flame radius were confirmed by using GASEQ code and theoretical calculation, respectively. The report shows the orifice plate reduced the maximum explosion pressure of the low-concentration hydrogen (φ＜20% v/v), while the maximum explosion pressure of high-concentration hydrogen (φ＞20% v/v) was increased, and the oscillation of the explosion pressure in the closed-vessel was obvious. The effect of the ignition location on the maximum explosion pressure was related to the interaction between the flame instability and the orifice plate for the φ = 30% v/v hydrogen-air mixture.     

Effect of channel and rib width on transport phenomena within the cathode of a proton exchange membrane fuel cell

A two-phase, half cell, non-isothermal model of a proton exchange membrane fuel cell has been developed. The model geometry includes a gas diffusion layer, a micro-porous layer and a catalyst layer along with the interfaces for channel, land and membrane. The effect of channel and rib width on transport phenomena has been examined. The model was run with saturated gas feed at different operating current densities and cell temperatures. The results show that increasing the channel to rib width ratio does not have any effect on the total amount of liquid saturation, however, its distribution is significantly affected under the channel and rib region within the porous layers. The degree of supersaturation and undersaturation extends, but, the supersaturation region shrinks and the undersaturation region extends with increase in channel to rib width ratio. It is concluded that the transport mechanism within the cathode is a highly coupled phenomena which interlinks local distribution of temperature, liquid saturation and the relative humidity.     

Temperature distribution and performance of ground-coupled multi-heat pump systems for a greenhouse

The primary objective of a greenhouse is to produce good plant-growth conditions such as temperature and humidity. One of the hot issues for the greenhouse is to provide an appropriate heating system which can achieve favorable temperature condition and save energy. In this study, the performance of a ground-coupled multi-heat pump system for the greenhouse heating was investigated. The ground-coupled multi-heat pump system was composed of GLHX (ground loop heat exchanger) and multi-heat pump unit which had one outdoor unit and two or more indoor units. The temperature distribution within the greenhouse using the ground-coupled multi-heat pump system was represented relatively uniform comparing to when the conventional heating system and GCHP system were adopted, because the capacity of each indoor unit could be changed linearly according to the variation of load. The temperature difference between the maximum and minimum temperatures and the standard deviation of inside temperature for the greenhouse were 2.1 °C and 1.2 °C, respectively. It is necessary to develop the multi-heat pump unit which can be operated with high performance at relatively low temperature setting conditions. The system COP of the ground-coupled multi-heat pump unit decreased greatly at part load condition due to relatively high power consumption of the ground circulation pump. Therefore, it is suggested that a control algorithm of the ground circulation flow rate has to be developed to maximize energy saving by applying the ground-coupled multi-heat pump system to the greenhouse.     

Measurements of current and water distribution for a micro-PEM fuel cell with different flow fields

Time-dependent measurements for the mapping of current distribution via a segmented flow field plate approach in a micro-proton exchange membrane (PEM) fuel cell were conducted and the effects of flow field configuration were studied and discussed under fixed operating conditions. The results show that, among four flow fields studied herein, the interdigitated flow channel has the most uniform transient current distribution with a much higher water content at an early phase (say t < 0.5 h) than those of the other three channels: serpentine, mesh and parallel, indicating an adequate oxygen concentration of the airflow on the cathode. In addition, the effect of water content on current distribution was also examined and discussed. It was found that the volume of water in flow channels could reach a steady value of 45% for all four flow fields after a 3-h operation.     

Three dimensional channel effect on the flow characteristics and the performance of hydrogen Knudsen compressors

As a new type of the micro fluidic device, Knudsen compressor can provide the potential utilizations on the hydrogen transport in the micro systems. Considering actual structure of the compressor is three-dimensional, flow characteristic studies are the key issue for the performance predictions. Firstly, the model of three-dimensional Knudsen compressor is built, and the validity of the model is proved by comparison with the experimental result. Secondly, the flow behaviors in the three-dimensional model is investigated, and the distributions of pressure and velocity are investigated. Also, the performance of the hydrogen Knudsen compressor in two-dimensional structure and three-dimensional structure are compared and discussed. Thirdly, the three-dimensional hydrogen Knudsen compressors with different width are analyzed, and the pressure increase in different cases of the hydrogen Knudsen compressors are studied.