Hongmei Yu

Two-Phase Dynamic Modeling of PEMFCs and Simulation of Cyclo-Voltammograms

A mathematical model is developed that is based on a coupled system of partial differential equations. The model contains a dynamic and two-phase description of the proton exchange membrane fuel cell (PEMFC) and a membrane model that accounts for Schroeders paradox. The mass transport in the gas phase and in the liquid phase is considered as well as the phase transition between liquid water and water vapor. The transport of charges and the electrochemical reactions are part of the model. A potential sweep experiment is simulated using the mathematical model and measured using a test cell with an active area of 1 cm 2 . In this way, the dynamic effect of liquid water formation and transport on the current-voltage characteristic of the fuel cell is investigated. A hysteresis effect is found in the measured time-dependent current-voltage relation. The limiting current density is time-dependent. Qualitative agreement of simulated and measured results is achieved. An analysis of the observed hysteresis of the current-voltage characteristics, based on the modeling results, is given.

A study of the electrochemistry of nickel hydroxide electrodes with various additives

Nickel composite electrodes (NCE) with various additives are prepared by a chemical impregnation method from nitrate solutions on sintered porous plaques. The electrochemical properties, such as utilization of active material, swelling and the discharge potential of the nickel oxide electrode (NOE) are determined mainly through the composition of the active material and the characteristics of nickel plaques. Most additives (Mg, Ca, Sr, Ba, Zn, Cd, Co, Li and Al hydroxide) exert effects on the discharge potential and swelling of the NOE. Chemical co-precipitation with the addition of calcium, zinc, magnesium and barium hydroxide increases the discharge potential by more than 20 mV, but that with zinc hydroxide results in an obvious decrease of active-material utilization and that with calcium and magnesium hydroxide produces a larger increase of electrode thickness. The effects of anion additives are also examined. Less than 1% mol of NiS in the active material increases the discharge potential, Cadmium, cobalt and zinc hydroxide are excellent additives for preventing swelling of the NCE. Slow voltammetry (0.2 mV s(-1)) in 6 M KOH is applied to characterize the oxygen-evolving potential of the NCE. The difference between the oxygen-evolution potential and the potential of the oxidation peak for the NCE with additives of calcium, lithium, barium and aluminium hydroxide is at least + 60 mV.

CO tolerance electrocatalyst of PtRu-HxMeO3/C (Me = W, Mo) made by composite support method

CO tolerance electrocatalyst of PtRu-HxMeO3/C is prepared by a composite support method, which overcomes evident shortcomings of other preparing methods. According to the result of XRD and TEM, the catalyst shows high dispersion and the HxMeO3 exists in amorphous form. The electrooxidations of CO on the catalysts have been investigated in cyclic voltammetry, in which PtRu-HxMeO3/C shows lower CO ignition potential according to hydrogen and CO spill over effects, and the bifunctional mechanism is proved to be strengthened for the involvement of HxMeO3. The CO tolerances of catalysts are compared according to the PEMFC single cell performance and anode polarization curves operated with H2/50 ppm CO and H2/100 ppm CO.

Supported Noble Metals on Hydrogen-Treated TiO2 Nanotube Arrays as Highly Ordered Electrodes for Fuel Cells

Hydrogen-treated TiO2 nanotube (H-TNT) arrays serve as highly ordered nanostructured electrode supports, which are able to significantly improve the electrochemical performance and durability of fuel cells. The electrical conductivity of H-TNTs increases by approximately one order of magnitude in comparison to air-treated TNTs. The increase in the number of oxygen vacancies and hydroxyl groups on the H-TNTs help to anchor a greater number of Pt atoms during Pt electrodeposition. The H-TNTs are pretreated by using a successive ion adsorption and reaction (SIAR) method that enhances the loading and dispersion of Pt catalysts when electrodeposited. In the SIAR method a Pd activator can be used to provide uniform nucleation sites for Pt and leads to increased Pt loading on the H-TNTs. Furthermore, fabricated Pt nanoparticles with a diameter of 3.4 nm are located uniformly around the pretreated H-TNT support. The as-prepared and highly ordered electrodes exhibit excellent stability during accelerated durability tests, particularly for the H-TNT-loaded Pt catalysts that have been annealed in ultrahigh purity H2 for a second time. There is minimal decrease in the electrochemical surface area of the as-prepared electrode after 1000 cycles compared to a 68 % decrease for the commercial JM 20 % Pt/C electrode after 800 cycles. X-ray photoelectron spectroscopy shows that after the H-TNT-loaded Pt catalysts are annealed in H2 for the second time, the strong metal-support interaction between the H-TNTs and the Pt catalysts enhances the electrochemical stability of the electrodes. Fuel-cell testing shows that the power density reaches a maximum of 500 mWcm(-2) when this highly ordered electrode is used as the anode. When used as the cathode in a fuel cell with extra-low Pt loading, the new electrode generates a specific power density of 2.68 kWg(Pt) (-1) . It is indicated that H-TNT arrays, which have highly ordered nanostructures, could be used as ordered electrode supports.

Composite anode for CO tolerance proton exchange membrane fuel cells

Fuel of proton exchange membrane fuel cells (PEMFC) mostly comes from reformate containing CO. which will poison the fuel cell electrocatalyst. The effect of CO on the performance of PEMFC is studied in this paper. Several electrode structures are investigated for CO containing fuel. The experimental results show that thin-film catalyst electrode has higher specific catalyst activity and traditional electrode structure can stand for CO poisoning to some extent. A composite electrode structure is proposed for improving CO tolerance of PEMFCs. With the same catalyst loading. the new composite electrode has improved cell performance than traditional electrode with PtRu/C electrocatalyst for both pure hydrogen and CO/H-2. The EDX test of composite anode is also performed in this paper, the effective catalyst distribution is found in the composite anode

Comparative Study of PEM Fuel Cell Storage at − 20 ° C after Gas Purging

The effect of water removal and freeze/thaw cycles on proton exchange membrane (PEM) fuel cells was investigated by comparative study of 20 on/off and freeze/thaw cycles after purging by reactant gas with relative humidity 58.0% at 25°C. Within 20 cycles, the cell performance, electrochemical active surface areas, and electrochemical impedance spectra responses were analyzed by the mean squared deviation method. No performance decay caused by water freezing was observed, and the water amount in the cell was reduced to an extent by which freeze degradation was avoided.

Vertically Aligned FeOOH/NiFe Layered Double Hydroxides Electrode for Highly Efficient Oxygen Evolution Reaction

Employing a low-cost and highly efficient electrocatalyst to replace Ir-based catalysts for oxygen evolution reaction (OER) has drawn increasing interest in renewable energy storage. In this work, a vertically aligned FeOOH/NiFe layered double hydroxides (LDHs) nanosheets supported on Ni foam (VA FeOOH/NiFe LDHs-NF) is prepared as a highly effective OER electrode in alkaline electrolyte. The VA FeOOH/NiFe LDHs-NF represents nanosheet arrays on nickel foam with some interspace among them. The vertically aligned and interlayer-structured architecture is binder-free and contributes to facile strain relaxation, relieving the exfoliation of the catalysts layer caused by the oxygen evolution process. The as-prepared electrode shows current densities of 10 and 500 mA cm-2 at overpotentials of 208 and 288 mV, and good stability in a half-cell electrolyzer. Besides, the alkaline polymer electrolyte water electrolyzer (APEWE) with this electrode showed 1.71 V at 200 mA cm-2, and 2.041 V at 500 mA cm-2, exhibiting the corresponding energy efficiency of 86.0% and 72.0% (based on the lower heating value of hydrogen), which is better than the typical commercial alkaline water electrolyzer.

A Hard‐Template Method for the Preparation of IrO2, and Its Performance in a Solid‐Polymer‐Electrolyte Water Electrolyzer

Morphological control by SBA-15: The performance of catalysts for the oxygen evolution reaction (OER) depends strongly on their structural and morphological properties. An IrO(2) nanomaterial with a morphology suitable for the OER is prepared by using a synthetic scheme involving a zeolite template, and shows enhanced activity and stability compared to IrO(2) fabricated by the traditional Adams-fusion method.

Ethylene glycol adjusted nanorod hematite film for active photoelectrochemical water splitting

We reported a facile adjusted method for the synthesis of high surface area nanorod hematite film as a photoanode for application in water splitting. Crystalline hematite nanorods (EG-α-Fe2O3) are fabricated by electrodeposition in Fe(2+) precursor solution with the addition of ethylene glycol (EG) and followed by annealing at 450 °C. The nanorod hematite film fabricated by the modified electrodeposition approach exhibits a more uncompact structure than α-Fe2O3 obtained by directly electrodepositing on the FTO substrate. The optical and structural characteristics of the obtained film are also tested. The results infer that EG can tune the morphology of hematite and improve the photoabsorption in the visible light region due to its inducement of one-dimensional growth of crystal hematite. It also enhances the photoresponse activity of hematite in water splitting by improving the activities at the semiconductor/solution interface. The photocurrent density of EG-α-Fe2O3 nanorods increased to 0.24 mA cm(-2) at 1.4 V vs. RHE in 1 M KOH (pH = 13.6), almost 5 times higher than the original α-Fe2O3 (0.05 mA cm(-2), measured under the same conditions).

Behaviors of a proton exchange membrane electrolyzer under water starvation

Water starvation could be one of the reasons for proton exchange membrane (PEM) water electrolyzer degradation. In this paper, the water starvation phenomena of a unit cell in a PEM electrolyzer stack are investigated. The voltage, current density and temperature distribution are investigated in situ with a segmented electrolyzer. The results show that the voltage of the middle and outlet regions is higher than the inlet voltage, which illustrates that water starvation could occur simultaneously in different regions of the electrolyzer. The water stoichiometries have an important effect on the voltage distribution, current density distribution and temperature distribution at 0.5 A cm−2 and 60 °C. The electrochemical impedance spectra of different segments show that the cell resistance and charge transfer resistance gradually increase along the water flow direction when the water stoichiometry is 3. According to the flow regime map, the critical water stoichiometry for electrolysis is further discussed.