Hydrogen fuel cells, mechanism

Considering their possible applications in fuel cells, hydrogen sensors, electro-chromic displays, and other industrial devices, there has been an intensive search for proton conducting crystals. In principle, this type of conduction may be achieved in two ways a) by substituting protons for other positively charged mobile structure elements of a particular crystal and b) by growing crystals which contain a sufficient amount of protons as regular structure elements. Diffusional motion (e.g., by a vacancy mechanism) then leads to proton conduction. Both sorts of proton conductors are known [P. Colomban (1992)]. 

A. A. Kulikovsky. Direct methanol-hydrogen fuel cell The mechanism of functioning. 

The fundamental start/stop mechanism was first reported by Reiser et al. [ 11], and occurs when one part of the anode flow-field is partially filled with hydrogen and another part is filled with air, a situation which occurs during the start-up of a fuel cell (hydrogen displacing air in the anode flow-field) or during shutdown (air... 

The oxygen reduction reaction (ORR) at the PEM fuel cell cathode is a multielectron, multistep reaction with a sluggish kinetics thus, a catalyst is generally required to accelerate the reaction. At present, platinum (Pt)-based catalysts are the most practical catalysts for the ORR in PEM fuel cells. The mechanism of the Pt-catalyzed ORR has been an active research area for about the past 40 years [21-24]. Yet, despite numerous studies, the detailed mechanism remains elusive. Figure 6.2 illustrates the simplified mechanism [24]. On Pt, the oxygen reduction reaction can proceed along several pathways for example, a "direct" four-electron reduchon to water, a two-electron pathway to hydrogen peroxide, and a "series" pathway with two- and four-electron reduction to water. 

Ilinich O, Ruettinger W, Liu X et al (2007) CU-AI2O3-CUAI2O4 water-gas shift catalyst for hydrogen production in fuel cell applications mechanism of deactivation under start-stop operating conditions. J Catal 247 112-118... 

In general, a fuel cell converts gaseous hydrogen and oxygen into water, electricity (and, inevitably, some heat) via the following mechanism, shown in Figure 2 ... 

It is to be noted that the difference in dissipation of energy between the thermal energy demanded by organic chemical hydrides and the mechanical energy by compression or liquefaction of hydrogen is quite significant because the former is supplied as waste heat, whereas the latter is lost at the site of hydrogen utilization for fuel cells or ICEs. 

Hydrogen fuel cells advances in transportation and power I Michael Frank Hordeski. 2009 by The Fairmont Press. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publisher. 

In GM s Hy-wire hydrogen powered concept vehicle, there is a fuel cell for the power source and electronics replace mechanical parts in the steering and braking systems. The driver looks through a large, sloped windshield that covers space usually taken up by an engine. There is no dashboard, instrument panel, steering wheel or pedals, only a set of adjustable footrests. 

The General Motors Sequel fuel cell concept car holds enough fuel for 300 miles. It fits the seven kilograms of hydrogen into an 11-inch thick skateboard chassis. The Sequel has been called a crossover SUV. Since mechanical components are replaced by electrical parts, interior layouts can be more open with more space in smaller vehicles. 

A fascinating point, especially to physical chemists, is the potential theoretical efficiency of fuel cells. Conventional combustion machines principally transfer energy from hot parts to cold parts of the machine and, thus, convert some of the energy to mechanical work. The theoretical efficiency is given by the so-called Carnot cycle and depends strongly on the temperature difference, see Fig. 13.3. In fuel cells, the maximum efficiency is given by the relation of the useable free reaction enthalpy G to the enthalpy H (AG = AH - T AS). For hydrogen-fuelled cells the reaction takes place as shown in Eq. (13.1a). With A//R = 241.8 kJ/mol and AGr = 228.5 under standard conditions (0 °C andp = 100 kPa) there is a theoretical efficiency of 95%. If the reaction results in condensed H20, the thermodynamic values are A//R = 285.8 kJ/ mol and AGR = 237.1 and the efficiency can then be calculated as 83%. 

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