• Energy Research

Abstract The use of single wall nanohorns (SWNH) as electrocatalyst support has proved to increase the performance of polymer electrolyte membrane-based fuel cells. In order to investigate in more detail such behavior, the electrochemical characterization of SWNH based electrodes was performed. The use of SWNH in vapour phase high temperature direct methanol fuel cells (HT-DMFC) was also addressed. Cyclic voltammetry experiments have indicated a higher electrochemical activity towards methanol electro-oxidation and a higher tolerance to carbonaceous species accumulation for a SWNH based electrode than for carbon black and commercial corresponding ones. Carbon black electrode presented a better performance than SWNH one for oxygen reduction reaction at low current densities while, at higher overvoltages, SWNH electrode performed better. The exact role of the improved performance of SWNH based electrodes is yet not clear but may be related to a higher water vapour adsorption or electrode morphology. Vapour phase HT-DMFC operation showed the improved performance of the SWNH electrode in agreement with previous works and with the electrochemical characterization performed during this work; despite the higher ohmic resistance observed in comparison with the carbon black based electrode. Moreover, SWNH based electrode showed improved fuel cell stability during longer operation times.

Abstract This work compares the hydrogen purity and recovery produced by a methanol steam reforming (MSR) packed bed membrane reactor (PBMR) equipped with a membrane selective to hydrogen (Pd-Ag) and with a membrane selective to carbon dioxide (porous membrane filled with ionic liquids-ILs). A 3-dimensional non-isothermal PBMR model was developed in Fluent (Ansys™) for simulating a PBMR equipped with these two types of membranes and simulating a conventional packed bed reactor (PBR). For the development PBMR models a MSR mechanistic kinetic model was fitted to experimental reaction rates of a commercial catalyst (BASF RP60). The results indicated that selective hydrogen removal from the reaction medium originates a significant increase in the methanol conversion, while the carbon dioxide removal has a smaller effect. CO 2 -PBMR showed to be more efficient in terms of energy consumption than H 2 -PMBR. The simulation results showed also that ILs membranes must have a minimum permeance of ⩾1 x 10 −6 mol s −1 m −2 Pa −1 and CO 2 /H 2 selectivity of ⩾200 at 473 K to be attractive for this type of applications. The advantages and limitations of each reactor configuration are discussed based on experimental and simulated data.

A fuel cell is an exothermic device that wastes ca. 50% of the input chemical energy while methanol steam-reforming (MSR) reaction is endothermic. The integration of a low temperature methanol steam-reforming cell (MSR-C) with a high temperature polymer electrolyte membrane fuel cell (HT-PEMFC) in a combined stack arrangement allows the thermal integration of both reactors. A novel bipolar plate of poly(p-phenylene sulfide) (PPS) featuring the fuel cell flow field in one side and the reformer flow field in the other was designed, built and assessed. For the first time are reported high current densities (>0.5 A cm-2) with the integrated system running at 453 K. The system was also ran for more than 100 h at 453 K, at 0.3 A cm-2, with a methanol conversion of>90%. It was observed some degradation of the membrane electrode assembly (MEA) due to the continuous presence of methanol in the reformate stream. Electrochemical impedance spectroscopy (EIS) analyses revealed an overall increase of the resistances. The self-thermal sustainability of the combined device was only reached for >0.75 A cm-2 due to the poor thermal insulation of the combined reactor.

Abstract In this work, the methanol steam reforming catalyst was considered into the anodic compartment of a high temperature polymer electrolyte fuel cell (HT-PEMFC), where reforming and electrochemical, reactions occur simultaneously. To avoid the anode electro-catalyst poisoning by methanol, a Pd-Ag membrane, with a thickness of a few micrometres, was considered between the reforming catalyst and the membrane electrode assembly. A 3-dimensional non-isothermal simulator was developed in Fluent (Ansys™) considering a packed bed membrane reactor cell (PBMR-C) combined with a HT-PEMFC in a single unit. The performance of the combined unit depends on the permeability, selectivity and stability of Pd-Ag membrane at 473 K. Therefore, a self-supported Pd-Ag membrane with a thickness of 4 μm, was produced with no defects by magnetron sputtering. The membrane showed a H2/N2 molar selectivity of ca. 5800 and permeability of 2.94 × 10–6 mol·m·s–1·m–2·bar−0.8 at 473 K. The novel PBMR-C/HT-PEMFC after proper validation was analysed by simulation, showing high performance, similar to the one obtained with a HT-PEMFC fed with hydrogen and allowed efficient heat integration between electrochemical and MSR reaction. The PBMR-C/HT-PEMFC also demonstrated to be very compact. The advantageous and limitations of the combined PBMR-C/HT-PEMFC unit are discussed based on the simulated results.

Abstract A dynamic one-dimensional isothermal phenomenological model was developed in order to describe the steady-state and transient behavior of high temperature polymer electrolyte membrane fuel cells (PEMFC). The model accounts for transient species mass transport at the bipolar plates and gas diffusion layers and the electric double layers charge/discharge. To record the impedance spectra, a small sinusoidal voltage perturbation was imposed to the simulator over a wide range of frequencies, and the resultant current density amplitude and phase were recorded. The steady-state behavior of the fuel cell, as well as the impedance spectra were obtained and compared to experimental data of two different fuel cells equipped with different MEAs based on phosphoric acid polybenzimidazole membrane. This approach is new and allows a deeper analysis of the controlling phenomena. The model fitted quite well the I–V curves for both systems, but fairly well the Nyquist plots. The differences observed in the Nyquist plots were attributed to proton resistance in the catalyst layer and the gas diffusion limitations to cross the phosphoric acid layer that coats the catalyst, phenomena not included in the proposed phenomenological model.

This work presents a crude bioethanol fueled integrated power system for backup and off-grid applications. The system is based on ethanol steam reformation to hydrogen that is used in polymer electrolyte fuel cells to produce electricity. We introduce the system design and overall system specifications, and report the experimental process of defining specifications for the produced hydrogen quality, a key variable affecting the final system cost, efficiency and durability. Results from development of individual subsystems are also presented together with discussion on the complete system integration.

This work aims at better understanding the activation process of phosphoric acid doped PBI-based MEA. The phenomena involved in the activation of Celtec® – P1000 MEA were studied based on polarization curves, AC impedance spectroscopy combined with equivalent circuit modelling and cyclic voltammetry analysis. It was concluded that galvanostatic activation procedure enhanced Celtec® – P1000 MEA performance by increasing the catalyst activity and by decreasing the ohmic resistance. Also, galvanostatic and potential cycling procedures were applied to an in-house prepared MEA; for the same activation time, the galvanostatic allowed a deeper activation of the in-house prepared MEA than the potential cycling activation method. It is accepted that the humidification of the reactants is not necessary for high temperature PEMFC based on phosphoric acid doped PBI membrane, since water production at the cathode should be enough to ensure high performance of the fuel cell. In this work it is described the behavior of a PEMFC based on an in-house prepared MEA, after activation at different temperatures and relative humidities. It is shown that water has an enhanced effect on ohmic resistance during the PEMFC operation but can also have a detrimental effect on the cathode resistance due to migration of phosphoric acid outside MEA.

Abstract A novel transient multi-dimensional non-isothermal multiphase model for simulating PEMFC was developed. A multiphase agglomerate catalyst model was considered for the cathode catalyst layer, while in the anode catalyst layer the effect of CO and CO2 presence was taken into consideration assuming two families of catalysts, Pt/C and Pt-Ru. The model predictions were compared to experimental data found in the literature and from an in-house PEMFC. The model was able to capture accurately the steady polarization curves of PEMFCs fed with hydrogen containing different amounts of CO and CO2. Moreover, the corresponding transient voltage was accurately simulated. The results indicated that even low CO concentration in the anode fuel, leads to a considerable degradation of the fuel cell output current density. Among the tested gas diffusion layers, the ones with the highest thickness showed worst performance of the PEMFC. Results showed, that high tortuosity and low contact angle (hydrophobicity) of the gas diffusion layer, decreases the performance of the PEMFC.