Walter Mérida

Non-planar architecture for proton exchange membrane fuel cells

Abstract PEMFCs traditionally rely on the use of sculpted graphite bi-polar plates and planar MEAs. A PEMFC design based on a non-planar electrode–membrane assembly and using non-conventional collector plates is described in this paper. The design achieves high volumetric power densities while maintaining low contact resistance and structural integrity. Several new methods for supporting the MEA, for collecting current and for sealing are described. A technique and hardware developed for assembling the non-planar membrane–electrode assemblies are described and the performance of two prototype cells based on the new design is examined. Preliminary results demonstrating significant gains in volumetric power density are presented.

Enhanced hydrogen production from indirectly heated, gasified biomass, and removal of carbon gas emissions using a novel biological gas reformer

We propose an enhanced integrated hydrogen production system that includes biological processes. Biomass gasi cation, achieved through the periodic combustion and pyrolysis of solid organic waste (under anaerobic conditions), results in a “producer-gas” stream consisting predominantly of carbon monoxide, carbon dioxide and hydrogen. This producer gas is typically used as a fuel in high temperature combustion. In the modi ed process, the producer gas is used to generate electricity using a combination of high-temperature (Solid oxide) and low-temperature (Proton exchange membrane) fuel cells. Carbon monoxide is reformed to additional H2 using a biological system; an anaerobic bacterium, Rubrivivax gelatinosus CBS that can enzymatically convert CO and H2O into CO2 and H2. R. gelatinosus CBS can also sequester CO2 as biomass. While the heating value does not vary signi cantly between the two streams, we propose that a larger hydrogen fraction can increase the value of this fuel, especially in the context of fuel cell applications. ? 2003 Published by Elsevier Ltd on behalf of the International Association for Hydrogen Energy.

An Alternative Approach to Evaluate the Wettability of Carbon Fiber Substrates

The wettability of carbon fiber substrate plays an important role in a vast number of electrochemical energy production and storage technologies. Here, we report an alternative approach to evaluate the relative wettability for three substrates with the solid-liquid (S-L) interfacial area as the wettability parameter. We applied electrochemical techniques to quantify the S-L interfacial area and obtained the relative wettability on for three substrates with varying fiber morphology. This work proposes and validates a methodology to experimentally measure the substrate wettability and elucidates important aspects of the relevant wetting phenomena. Our results indicate that the wettability of carbon fiber substrate is affected by the liquid intrusion resulting from the instability of the Cassie-Baxter wetting state and that the contact angle is not dependent on the S-L interfacial area under the droplet. The present technique can be used to characterize the surface wettability of a wide range of conductive surfaces with irregular and multiscale surface roughness features.

Monolithic Regenerator Technology for Low Temperature (4 K) Gifford-McMahon Cryocoolers

A two-stage Gifford-McMahon (GM) cryocooler has been selected to produce and maintain the low temperatures required by the superconducting magnet system in an active magnetic regenerative liquefier (AMRL). The operation of practical AMRLs requires relatively large magnetic fields (e.g., 8 T). Currently, these fields can only be produced via low-temperature superconducting magnets that typically operate at liquid helium temperatures (4.2 K).

Water Permeation Through Catalyst-Coated Membranes

Water permeabilities, driven by concentration or pressure gradients, through NRE211 and catalyst-coated membranes are reported, and the effect of the catalyst layer (∼ 18 μm thick, 30 wt % Nafion ionomer, carbon-supported Pt, 0.4 mg Pt cm ―2 ) on membrane water permeation is deconvoluted. For the system studied, water permeation is limited by the bulk membrane; the effect of the catalyst layer was insignificant and, despite catalyst layers being deposited on the membranes surface, it does not influence the rates of sorption or desorption at membrane interfaces.

Understanding the wettability of rough surfaces using simultaneous optical and electrochemical analysis of sessile droplets

The interaction of a droplet with a solid surface is characterized by two parameters, the contact angle and the wetted area under the droplet. The Cassie-Baxter and the Wenzel modes make predictions on the interfacial area by comparing the contact angles on smooth surfaces (the intrinsic wettability) with those on rough surfaces (the apparent wettability). In these models, the actual wetted area is used as a fitting parameter. In this work, we highlight the significance of determining the actual wetted area under the droplet and the limitation of using only the contact angle to represent the wetting behavior of a surface. Our experimental studies were performed on hydrophilic carbon surfaces where a combination of optical measurements (contact angle and hysteresis) along with an electrochemical approach was employed. An electrochemical method was used to determine the true wetted area using a droplet of aqueous electrolyte on the surface. The interfacial area was then used to correlate wetting behavior to that of the model predictions. We examined the impact of electrolyte concentration and potential sweep rate in our evaluation of the wetted area. Our results show that, for a rough hydrophilic surface, the decrease in contact angles with increasing solid-liquid interfacial areas is not always valid, as generally predicted by the Wenzel and the Cassie-Baxter models.

Modeling and control design of a Vienna rectifier based electrolyzer

Hydrogen production is an interesting alternative of storing energy. Electrolyzers produce hydrogen through water electrolysis; the resulting hydrogen is later used to generate electricity by using fuel cells, that reverse the process. Electrolyzers use rectifiers to convert the grid ac voltage into dc voltage for supplying the electrolyzer cells. Previous research used a rectification process based on conventional rectifiers (diode- or thyristor-based) which draw non-sinusoidal current from the main grid. This requires increased filtering to prevent power quality problems and equipment malfunctioning/failure. In addition, previous literature assumed simplified models for the power electronics converters and lacked a detailed control system. The Vienna rectifier is a non-regenerative converter that produces sinusoidal currents with low losses due to the reduced number of active switches. This manuscript proposes using the Vienna rectifier as an interface to connect electrolyzers to the ac grid. The dc voltage applied to the electrolyzer is regulated by using another dc-dc converter, which is selected to be a synchronous buck converter for simplicity and maximum efficiency. In this paper, the models of the Vienna rectifier, synchronous buck converter, and the electrolyzer are developed along with their respective controls. The control system has the ability to function in two operation modes for the overall reference: hydrogen production and power demand. The first one is adequate for grid-connected operation and the later for off-grid operation. Simulation results are given to show the validity of the proposed procedures.

An Empirical Model for Proton Exchange Membrane Fuel Cell Diagnostics

Individual cell voltage monitoring (CVM) has been the de facto technique to detect a general failure within a fuel cell stack. However, discerning between different types of failures is challenging because different failure modes can produce the same observed change in dc potential. This work discusses failure detection and isolation in proton exchange membrane fuel cells (PEMFCs). One specific approach based on electrochemical impedance spectroscopy (EIS) is used to illustrate these concepts. The relevant EIS measurements were made on fuel cell stack at different typical operating temperatures, covering the current density range 0.1 to 1.0 A cm -2 , and the frequency range 0.1 to a few hundred kHz. The collected spectra were modelled with an equivalent circuit whose time constants corresponded to ideal and Warburg components. Despite the qualitative limitations of this model, the measurements can discern between two failure modes.

Implications of transportation electrification in metro vancouver

The fleet composition forecasted to the year 2020 for the new portion of light and medium duty vehicles (LMDV) in Metro Vancouver is analyzed. Sixteen scenarios of zero tailpipe emission Electric Vehicle (EV) penetration in the new fleet were investigated. These scenarios focus on electricity and hydrogen. The study assesses the efficiency of EV technologies, quantifies the electric transportation energy demand, and summarises the implications of using electricity to power the transportation sector. The analysis shows that from the zero emission vehicle (ZEV) options, battery electric vehicles (BEV) have better energy efficiency. The electricity demand ranges from 32 to 248 GWh yr-1 across the scenarios. The larger numbers are due to the hydrogen production via electrolysis. If the required energy were produced via renewable energy sources, e.g., solar or wind, the life cycle greenhouse gas emissions could decrease up to 96%.