Evaluation of TiO2 as catalyst support in Pt-TiO2/C composite cathodes for the proton exchange membrane fuel cell

Anatase TiO₂ is evaluated as catalyst support material in authentic Pt-TiO₂/C composite gas diffusion electrodes (GDEs), as a different approach in the context of improving the proton exchange membrane fuel cell (PEMFC) cathode stability. A thermal stability study shows high carbon stability as Pt nanoparticles are supported on TiO₂ instead of carbon in the Pt-TiO₂/C composite material, presumably due to a reduced direct contact between Pt and C. The performance of Pt-TiO₂/C cathodes is investigated electrochemically in assembled membrane-electrode assemblies (MEAs) considering the added carbon fraction and Pt concentration deposited on TiO₂. The O₂ reduction current for the Pt-TiO₂ alone is expectedly low due to the low electronic conductivity in bulk TiO₂. However, the Pt-TiO₂/C composite cathodes show enhanced fuel cell cathode performance with growing carbon fraction and increasing Pt concentration deposited on TiO₂. The proposed reasons for these observations are improved macroscopic and local electronic conductivity, respectively. Electron micrographs of fuel cell tested Pt-TiO₂/C composite cathodes illustrate only a minor Pt migration in the Pt-TiO₂/C structure, in which anatase TiO₂ is used as Pt support. On the whole, the study demonstrates a stable Pt-TiO₂/C composite material possessing a performance comparable to conventional Pt–C materials when incorporated in a PEMFC cathode.     

Pore network modeling of cathode catalyst layer of proton exchange membrane fuel cell

In this paper, a pore network model is developed to investigate the coupled transport and reaction processes in the cathode catalyst layer (CCL) of proton exchange membrane fuel cell (PEMFC). The developed model is validated by comparing the predicted polarization curve with the experimental data, and the parametric studies are carried out to elucidate the effects of CCL design parameters. With the decrease of the CCL thickness and the Nafion content, the cell voltage reduces at the low current density but increases when the current density is higher. The cell performance is also improved by increasing the proton conductivity of the Nafion film in the CCL. As compared to the CCL of uniformly distributed Nafion, the CCL with the Nafion volume decreasing along the thickness direction exhibits better performance at the high current density.     

Improved stability of TiO2 modified Ru85Se15/C electrocatalyst for proton exchange membrane fuel cells

The electrocatalytic stability of the carbon supported Ru₈₅Se₁₅ nanoparticles has been improved by the modification of titanium dioxide for proton exchange membrane fuel cells (PEMFCs). Transmission electron microscopy (TEM), X-ray diffraction (XRD) measurements and inductively coupled plasma-atomic emission spectroscopy (ICP-AES) are applied for characterizing Ru₈₅Se₁₅/C and titanium dioxide modified Ru₈₅Se₁₅/C (Ru₈₅Se₁₅/TiO₂/C) electrocatalysts. Electrochemical measurements and single cell tests are conducted for the evaluation of the electrocatalysts. The results indicate that Ru₈₅Se₁₅/TiO₂/C electrocatalyst, presenting similar initial oxygen reduction reaction (ORR) activity with Ru₈₅Se₁₅/C, reveals better electrochemical stability. The final potential of Ru₈₅Se₁₅/TiO₂/C is 137 mV higher than that of Ru₈₅Se₁₅/C at 2 mA cm⁻² after the electrochemical durability test. Moreover, in the single cell stability test Ru₈₅Se₁₅/TiO₂/C also shows comparable initial performance with Ru₈₅Se₁₅/C, but better final performance. Therefore, the Ru₈₅Se₁₅/C is expected to be used as an effective cathode electrocatalyst for PEMFCs by TiO₂ modification on the carbon support.     

Dodecylamine-modified carbon supports for cathode in proton exchange membrane fuel cells

In this investigation, hydrophobic dodecylamine-modified carbon supports are prepared for proton exchange membrane fuel cells by organic synthesis. Well-dispersed Pt-Ru nanoparticles, with a narrow size distribution, are then deposited on the dodecylamine-modified carbon supports by methanol reduction to serve as cathodic catalysts. These dodecylamine-modified catalysts are separately mixed with either a commercial catalyst or unmodified catalyst to provide hydrophobic channels to convey the reaction gas to the active sites in the catalyst layer. The best cathode composite catalyst, containing 20-40 wt% of modified-catalyst, gives approximate 30% increase in the maximum power density, comparing to E-TEK catalyst (125 mW cm⁻²). The increase in the maximum power density is attributed to higher activity and lower resistance. This result is discussed in the context of AC-impedance and proton conductivity analysis.     

Effects of anatase TiO2 with different particle sizes and contents on the stability of supported Pt catalysts

Pt/TiO₂-C catalyst with TiO₂ and carbon black as the mixed support has been synthesized by the microwave-assisted polyol process (MAPP). Effects of anatase TiO₂ with different particle sizes and contents on the stability of supported Pt catalysts have been systematically studied. X-ray diffraction (XRD), transmission electron microscopy (TEM), cyclic voltammograms (CV), and accelerated potential cycling tests (APCT) have been carried out to present the influence degree. The experimental results indicate that the original electrochemically active specific surface areas (ESA) of the catalysts decrease with the increase of mean particle sizes of TiO₂ and TiO₂ contents. However, the activity of Pt/TiO₂-C-20 is very close to that of Pt/TiO₂-C-5 and the stability of Pt/TiO₂-C-20 is the best after 1000 cycles APCT, illustrating that the optimized particle size of TiO₂ in Pt/TiO₂-C catalyst is 20 nm. Furthermore, the stability of the catalysts increase with the increase of TiO₂ contents in the mixed support. Taking into account both the activity and stability of various Pt/TiO₂-C catalysts, the optimized particle size of TiO₂ is 20 nm and the optimal TiO₂ content existed in the mixed support is 40%.     

Dynamic behavior of liquid water transport in a tapered channel of a proton exchange membrane fuel cell cathode

A numerical model of a proton exchange membrane fuel cell (PEMFC) cathode with a tapered channel design has been developed in order to examine the dynamic behavior of liquid water transport. Three-dimensional, transient simulations employing the level-set method (available in COMSOL 3.5a, a commercial finite element method software) have been used to explicitly track the liquid-gas interface. A liquid water droplet suspended in the center of the channel, 2 mm from the channel entrance, is subjected to airflow in the bulk of the channel. Three different cases have been studied: 1) hydrophobic bottom wall representing the gas diffusion layer and hydrophilic channel top and side walls, 2) all walls are partially wetted i.e. having a contact angle of 90°, 3) a hydrophilic bottom wall and hydrophobic top and side walls. The results show that tapering the channel downstream helps in water exhaust due to increased airflow velocity. A bottom wall, although hydrophilic, results in fast removal of water droplet as compared to partially wetted and hydrophobic bottom surfaces.     

Effect of sulfonated carbon nanofiber-supported Pt on performance of Nafion-based self-humidifying composite membrane for proton exchange membrane fuel cell

In the present study, the Nafion^®-based self-humidifying composite membrane (N-SHCM) with sulfonated carbon nanofiber-supported Pt (s-Pt/CNF) catalyst, N-s-Pt/CNF, is successfully prepared using the solution-casting method. The scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) images of N-s-Pt/CNF indicate that s-Pt/CNF is well dispersed in the Nafion^® matrix due to the good compatibility between Nafion^® and s-Pt/CNF. Compared with those of the non-sulfonated Pt/CNF-containing N-SHCM, N-Pt/CNF, the properties of N-s-Pt/CNF, including electronic resistivity, ion-exchange capacity (IEC), water uptake, dimensional stability, and catalytic activity, significantly increase. The maximum power density of the proton exchange membrane fuel cell (PEMFC) fabricated with N-s-Pt/CNF operated at 50 °C under dry H₂/O₂ condition is about 921 mW cm⁻², which is approximately 34% higher than that with N-Pt/CNF.     

A facile synthesis of Pd/C cathode electrocatalyst for proton exchange membrane fuel cells

A carbon supported palladium (Pd/C-NaBH₄-NH₃) catalyst was synthesized via modified sodium borohydride reduction method using ammonia as the complexing reagent. The Pd/C catalysts were characterized by means of powder X-ray diffraction (XRD), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). Rotating disk electrode (RDE), cyclic voltammetry (CV), electrochemical impedance spectra (EIS) and single cell measurements were employed to evaluate the activities of the catalysts. The as-prepared catalysts with face-centered cubic (fcc) structure are uniformly dispersed on the carbon supports. Twinned and polycrystalline structures are observed in the HRTEM image of Pd/C-NaBH₄-NH₃. The results indicate that the Pd/C-NaBH₄-NH₃ catalyst shows high activity for the oxygen reduction reaction. Single cell with Pd/C-NaBH₄-NH₃ as the cathode displays a maximum power density of 508 mW cm⁻². The favorable performance of the Pd/C-NaBH₄-NH₃ catalyst may be attributed to the uniformly dispersed nanoparticles and more crystalline lattice defects.     

Development of a proton exchange membrane fuel cell cogeneration system

A proton exchange membrane fuel cell (PEMFC) cogeneration system that provides high-quality electricity and hot water has been developed. A specially designed thermal management system together with a microcontroller embedded with appropriate control algorithm is integrated into a PEM fuel cell system. The thermal management system does not only control the fuel cell operation temperature but also recover the heat dissipated by FC stack. The dynamic behaviors of thermal and electrical characteristics are presented to verify the stability of the fuel cell cogeneration system. In addition, the reliability of the fuel cell cogeneration system is proved by one-day demonstration that deals with the daily power demand in a typical family. Finally, the effects of external loads on the efficiencies of the fuel cell cogeneration system are examined. Results reveal that the maximum system efficiency was as high as 81% when combining heat and power.     

Durable ultra-low-platinum ionomer-free anode catalyst for hydrogen proton exchange membrane fuel cell

An ultra-low-platinum catalyst based on finely dispersed platinum (Pt) deposited on a highly porous complex microporous layer was investigated as a candidate of durable anode catalyst for hydrogen oxidation reaction (HOR) in proton exchange membrane fuel cells. Etching of teflonated and nitridized base carbon substrate in oxygen plasma and simultaneous deposition of cerium oxide were applied to increase active surface area and electrochemical activity of the platinum nanocatalyst. Ultra-low loadings of Pt (between 0.85 and 8.5 μg cm⁻²) deposited by magnetron sputtering on this substrate were assembled with Nafion 212 membrane and commercially available Pt/C cathodes (300-400 μg cm⁻² Pt). Such membrane electrode assembly (MEA) with extremely low Pt content at anode can deliver high output power densities, reaching 0.95 W cm⁻² or 0.65 W cm⁻² with only 1.7 μg cm⁻² of Pt, using H₂ as fuel and pure O₂ or air as an oxidant, respectively. Although electrocatalysts with highly dispersed active metals are known to often suffer from irreversible degradation, the above MEAs proved to be very stable when the cell was subjected to a durability test under heavy duty conditions of on/off cycling. The system with lower Pt content is more prone to water flooding which can, however, be eliminated by maintaining better control over the fuel humidity. Average decay of the cell voltage less than 50 μV h⁻¹ was obtained in the cycling regime, while excellent stability <10 μV h⁻¹ is achievable under the static load of 0.4 A cm⁻².     

Enhanced hydrogen generation by hydrolysis of LiBH4 doped with multiwalled carbon nanotubes for micro proton exchange membrane fuel cell application

LiBH₄ has a high hydrogen storage capacity and could potentially serve as a superior hydrogen storage material. However, during the hydrolysis process for hydrogen generation, the agglomeration of the hydrolysis product of LiBH₄ limits its full utilization. In order to completely release the stoichiometric amount of H₂ from LiBH₄ hydrolysis, multiwalled carbon nanotubes (MWCNTs) were doped with LiBH₄ by mechanical milling. The results show that MWCNT carried LiBH₄ can slowly react with water vapor at room temperature which is 25 °C lower than the reaction temperature of neat LiBH₄. Agglomeration can be avoided when the addition of MWCNTs exceeds 7 wt.%, which results in a complete hydrolysis process. The total hydrogen capacity is 7.5 wt.%. The enhanced hydrolysis of LiBH₄ can be attributed to the MWCNTs which increased the contact areas between LiBH₄ and water and created gas channels for hydrogen diffusion. The performance of a micro proton exchange membrane fuel cell connected to MWCNT-doped LiBH₄ powder packed-bed reactor was examined. The result demonstrates that doping with MWCNTs enhanced the hydrogen generation of LiBH₄ hydrolysis. MWCNT-doped LiBH₄ can be applied as hydrogen source of fuel cells.     

Potential dependence of sulfur dioxide poisoning and oxidation at the cathode of proton exchange membrane fuel cells

Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were employed to investigate changes in the cathode after the introduction of SO₂. The decay in performance of the proton exchange membrane fuel cell (PEMFC) was ascribed to the increasing of the charge transfer resistance (R_ct) caused by the loss of the electrochemical surface area (ECA). The results show that the oxidation and adsorption behaviors of SO₂ depended closely on the potential. Adsorbed sulfur began to be oxidized over 0.9 V and could be oxidized completely with CV maximum potentials up to 1.05 V or higher. At about 0.65 V, the adsorbed SO₂ was probably in a molecular state, which could be reduced in the range of 0.65–0.05 V. Some SO₂ molecules occupied initially two Pt sites through S and O. One of the two sites could be released after the reduction. The increasing of ECA due to reduction could lessen the impact of SO₂ on the PEMFC performance at voltages below 0.65 V.     

Optimization of cathode microporous layer materials for proton exchange membrane fuel cell

The microporous layer (MPL) of diffusion medium has an important impact on the water management ability of proton exchange membrane fuel cells. In this study, six kinds of carbon black were used to prepare the cathode MPL. The thickness, conductivity, pore structure, hydrophobicity, and surface microstructure of MPL were characterized. The single cell was prepared and electrochemical tests were performed. The results showed that the single cell prepared by Acetylene black (ACET) and Vulcan XC-72R has a considerable power generation performance. In addition, polyvinylidene fluoride hexafluoropropylene copolymer P(VDF-HFP) was used to replace Polytetrafluoroethylene (PTFE) as hydrophobic binder. MPL with different P(VDF-HFP) contents were prepared, and the single cell performance was investigated. The results showed that all the single cells prepared by P(VDF-HFP) were worse than that of PTFE. This study provides an important reference for further improving the performance of fuel cells from the perspective of material optimization with MPL.     

Hydrothermal synthesis of Pt/MWCNTs nanocomposite electrocatalysts for proton exchange membrane fuel cell systems

A hydrothermal method for preparation of size-controlled Pt nanoparticles dispersed highly on multiwalled carbon nanotubes (Pt/MWCNTs) has been studied to optimize the effective parameters (temperature, time, pH and stirring rate) using Taguchi method. The synthesized Pt/MWCNTs nanocomposite samples were characterized through X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray fluorescence (XRF) analyses to identify mean Pt nanoparticles size and Pt content. The analysis of the primary experimental data and demonstration of the main effect trend of each parameter showed that a reaction temperature of about 140 °C, a reaction period of 5 h, a slightly basic reaction pH (∼9) and a stirring rate of 500 rpm are the optimum process conditions which give a low mean Pt nanoparticles size (2.8 nm) and a high Pt content (19.4 wt.%) simultaneously. Cyclic voltammetry (CV) analysis showed that under optimum conditions the synthesized sample gives a specific surface area of 99 m² g⁻¹. Obtaining the polarization curves for the two fabricated membrane electrode assemblies (MEAs) using the optimized catalyst and a commercial Pt/C catalyst (10 wt.%, Aldrich) with Pt loading of 0.4 mg cm⁻² demonstrated that the catalyst prepared under optimum conditions shows a considerably better performance.     

Fabrication and investigation of SiO2 supported sulfated zirconia/Nafion self-humidifying membrane for proton exchange membrane fuel cell applications

A self-humidifying composite membrane based on Nafion^® hybrid with SiO₂ supported sulfated zirconia particles (SiO₂–SZ) was fabricated and investigated for fuel cell applications. The bi-functional SiO₂–SZ particles, possessing hygroscopic property and high proton conductivity, were homemade and as the additive incorporated into our composite membrane. X-ray diffraction (XRD) and Fourier infrared spectrum (FT-IR) techniques were employed to characterize the structure of SiO₂–SZ particles. Scanning electronic microscopy (SEM) and energy dispersive spectroscopy (EDS) measurements were conducted to study the morphology of composite membrane. To verify the advantages of Nafion^®/SiO₂–SZ composite membrane, the IEC value, water uptake, proton conductivity, single cell performance and areal resistance were compared with Nafion^®/SiO₂ membrane and recast Nafion^® membrane. The single cell employing our Nafion^®/SiO₂–SZ membrane exhibited the highest peak power density of 0.98 W cm⁻² under dry operation condition in comparison with 0.74 W cm⁻² of Nafion^®/SiO₂ membrane and 0.64 W cm⁻² of recast Nafion^® membrane, respectively. The improved performance was attributed to the introduction of SiO₂–SZ particles, whose high proton conductivity and good water adsorbing/retaining function under dry operation condition, could facilitate proton transfer and water balance in the membrane.     

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.     

From microstructure to the development of water and major reaction sites inside the catalyst layer of the cathode of a proton exchange membrane fuel cell

Microstructures of various sizes and shapes are fabricated on the surface of the catalyst layer (CL) of the cathode of a PEMFC, adjacent to the micro porous layer (MPL). Three major experimental results are: (1) performance is improved by up to 60% and the percentage of the increase is the same as that of the increase in interface area of CL and MPL; (2) the cell suffers no significant performance loss when Pt loading of the cathode is reduced from 1 to 0.25 mg cm⁻² and; (3) transient responses in periodical linear sweep tests show an obvious performance “jump” for all the cathodes with microstructures when approaching steady state, but none for others. Based on observations, a proposal related to the development of water and, consequently, the major reaction sites in the CL is made: there is a general water “surface” inside the CL. Major electrochemical reactions occur above (on the MPL side) of this surface and within a limited height. The surface will “move” from the membrane toward the MPL as more water is produced. The vapor generation rate (current load) relative to the removal rate of the rest of the cell components will determine the steady state position of this water surface.     

Nafion/PTFE/silicate membranes for high-temperature proton exchange membrane fuel cells

Fuel cell performance of membrane electrode assemblies (MEAs) prepared from poly(tetrafluoroethylene)/Nafion/silicate (PNS) membrane and Nafion-112 membrane were investigated. Due to the low conductivity of PTFE and silicate, PNS had a higher proton resistance than Nafion-112. However, in this work we show that PNS performs better than Nafion-112 for a high current density operation with a low inlet gas humidity. As the PEMFCs were operated at with 100% RH, the results showed the maximum power density (PD_max) of PNS was: at with both H₂ and O₂ flow rates of 300 ml/min, and at with H₂ flow rate of 360 ml/min and O₂ flow rate of 600 ml/min, which were much higher than the at of Nafion-112 with both H₂ and O₂ flow rates of 300 ml/min. The PD_max of PNS was: , , and at as the operating temperature and inlet gas humidity were set at with 67.7% RH, with 46.8% RH, and with 33.1% RH, respectively. However, no output power was detected for Nafion-112 MEA when the cell was operated at a temperature higher than and an inlet gas humidity lower than 67.7% RH. The high PEMFC performance of PNS at high current density and low humidity is attributed to the presence of silicate in the PNS membrane, which enhances water uptake and reduces electro-osmosis water loss at a high current density.     

Self-humidification of a PEM fuel cell using a novel Pt/SiO2/C anode catalyst

In this work, a membrane electrode assembly (MEA) for proton exchange membrane fuel cell (PEMFC) operating under no external humidification has been successfully fabricated by using a composite Pt/SiO₂/C catalyst at the anode. In the composite catalyst, amorphous silica, which originated from the hydrolysis of tetraethyl orthosilicate (TEOS), was immobilized on the surface of carbon powder to enhance the stability of silica and provide a well-humidified surrounding for proton transport in the catalyst layer. The characteristics of silica in the composite catalyst were investigated by XRD, SEM and XPS analysis. The single cell tests showed that the performance of the novel MEA was comparable to MEAs prepared using a standard commercial Pt/C catalyst with 100% external humidification, when both were operated on hydrogen and air. However, in the absence of humidification, the MEA using Pt/SiO₂/C catalyst at the anode continued to show excellent performance, while the performance of the MEA containing only the Pt/C catalyst rapidly decayed. Long-term testing for 80 h further confirmed the high performance of the non-humidified MEA prepared with the composite catalyst. Based on the experimental data, a possible self-humidifying mechanism was proposed.     

Dynamic performance for a kW-grade air-cooled proton exchange membrane fuel cell stack

In this study, a kW-grade air-cooled proton exchange membrane fuel cell (PEMFC) stack with a dead-end anode (DEA) operation is designed and manufactured. The gravity-assisted drainage principle is applied for the stack to design the wettability of gas diffusion layers (GDLs) and the anode channel geometry, which can help the liquid water that diffuses to the anode to drain out of the anode porous electrode and move down the anode channel outlets. As a result, the stack can stably operate in a long purge interval of 268 s and in a short purge time of 2 s. In addition, using this design, only four small-power fans are employed to pump air to the cathode to provide oxygen for the electrochemical reaction and cool the stack. With a constant load current of 30, 45, or 60 A, the stack output voltage is experimentally tested at various cathode air flow rates (CAFRs). The local temperatures (60 measurement points) inside the stack and the pressure differences across anode channels are also monitored to understand heat dissipation and the back diffusion of liquid water. In a wide range of operating conditions, the designed stack possesses superior and stable voltage output characteristics with relatively uniform temperature distributions. The measured maximum output power is 3.83 kW, and the parasitic powers of fans are only 80～112 W.