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Adequate water management represents one of the main challenges in the design and operation of polymer electrolyte membrane fuel cells. In this work, the influence of inlet gas humidification on cell performance is investigated by in-situ current density measurements obtained using the segmented cell approach. Particular attention is paid to the combined effect of cell temperature and relative humidity of the anode and cathode feed streams. When operated at 80 • C and low humidity conditions, the cell is seen to undergo a severe voltage decline that is not observed at 60 • C. The analysis shows that the variation with temperature of the water uptake rate of the gaseous streams plays a key role in determining the observed differences in performance stability. In the case of 60 • C operation, the water uptake rate of the cathode stream at 50% inlet relative humidity is roughly 30% of its value at 80 • C at the same humidification level, resulting in a significantly lower drying capacity. A simple balance of water model, able to explain the observed cell behavior, is finally presented and discussed. Energy demand has become one of the most serious concerns of modern society due to the problems related with greenhouse gas emissions and the depletion of fossil fuels. In this context, hydrogen is expected to play an important role as future energy vector, with polymer electrolyte membrane fuel cells (PEMFCs) being the leading candidates to provide efficient and clean electric energy conversion during the XXI century. Recently, significant progress has been made toward meeting the challenging cost and performance targets required for the widespread use of PEMFCs, specifically in the automotive industry. 1The state-of-the-art of polymer electrolyte membrane fuel cell technology is based on perfluorosulfonic acid (PFSA) polymer membranes operating at a typical temperature between 60• C and 80 • C. 2 Since the ionic conductivity of PFSA membranes depends on the water content of the membrane, 3,4 water management is one of the most important issues for successful operation, high performance, and good durability of PEMFCs. Excess inlet gas humidification as well as condensation processes within the cell are likely to produce the accumulation of liquid water in the porous electrodes and gas diffusion media (effect known as flooding), thereby decreasing cell performance. On the other hand, an insufficient level of gas humidification lowers the ionic conductivity of the membrane and also results in a performance reduction.Numerous studies have investigated the operation of PEMFC under dry conditions in order to simplify operation. 5,6 Early work to demonstrate stable performance for PEMFC using dry or slightly humidified gases has been reported by Büchi et al. 5 Strategies for operating polymer electrolyte fuel cells include also the reduction of humidification of both reactant gases 7-10 or the dry operation of the cathode 11,12 or anode 13 sides. In the last decade, a wide variety of diagnostic and visualization tools have b...

Starvation, flooding, and dry‐out phenomena occur in polymer electrolyte membrane fuel cells (PEMFCs), due to heterogeneous local conditions, material inhomogeneity, and uneven flow distribution across the single cell active area and in between the individual cells. The impact of the load level and air feed conditions on the performance was identified for individual single cells within a 10‐cell stack. Analysis of the current density distribution across the active area at the cell level was correlated with electrochemical impedance spectroscopy to enable operando fault diagnostic without any impact of the applied analytical tools on the single cell behavior. Moreover, the combination of both technologies allows in‐depth analysis of fault mechanisms in fuel cell single cells with improved sensitivity. Current density distribution and the quantitative assessment of the performance homogeneity demonstrated high sensitivity to small humidity changes and allow the detection of critical events, such as dry‐out in single cells. Impedance analysis is more sensitive regarding polarization and diffusion limitations and allows detection of cell flooding. The combination of both techniques is required for reliable identification of air starvation faults.

A 1D across-the-channel model is proposed for the anode of a Direct Ethanol Fuel Cell. The complex kinetics of the multi-step ethanol oxidation reaction is described using the reaction mechanism proposed by Meyer et al. [Electrochim. Acta, 56, 4299 (2011)], which considers free and adsorbed intermediate species on a Pt-based binary catalyst. The adsorbed species are modeled using coverage factors to account for the blockage of the active reaction sites on the catalyst surface. The reactions rates are described by Butler-Volmer equations, including the effect of ethanol and acetaldehyde crossover. A genetic algorithm is employed for determining the reaction constants for several catalyst types using polarization curves obtained from literature sources. By adjusting the reaction constants, different catalyst layers can be modeled and their selectivities can be partially reproduced. The discrepancies between the experimental and numerical results at low current densities suggest that Meyer’s mechanism could be improved by adding acetaldehyde to the adsorbed intermediates.

Water management represents one of the main challenges in the design and operation of PEMFCs. The influence of inlet gas humidification on cell performance is analyzed using in-situ diagnostic tools, such as cyclic voltammetry and segmented cell current density measurements, supported by post-mortem ex-situ investigations. Particular attention is paid to the effect of low humidity conditions in both cathode and anode, under which the cell is observed to suffer severe voltage decline. A simple onedimensional water balance model is proposed to contribute to the understanding of the various operation regimes observed in PEMFCs under medium-to-low humidification conditions.

This paper presents an isothermal, single-phase model for direct ethanol fuel cells. The ethanol electrooxidation reaction is described using a detailed kinetic model that is able to predict anode polarization and product selectivity data. The anode kinetic model is coupled to a one-dimensional (1D) description for mass and charge transport across the membrane electrode assembly, which accounts for the mixed potential induced in the cathode catalyst layer by the crossover of ethanol and acetaldehyde. A simple 1D advection model is used to describe the spatial variation of the concentrations of the different species as well as the output and parasitic current densities along the flow channels. The proposed 1D+1D model includes two adjustable parameters that are fitted by a genetic algorithm in order to reproduce previous experimental data. The calibrated model is then used to investigate the consumption of ethanol and the production, accumulation and consumption of acetaldehyde along the flow channels, which yields the product selectivity at different channel cross-sections. A parametric study is also presented for varying ethanol feed concentrations and flow rates. The results obtained under ethanol starvation conditions highlight the role of acetaldehyde as main free intermediate, which is first produced and later consumed once ethanol is fully depleted. The detailed kinetic description of the ethanol oxidation reaction enables the computation of the four efficiencies (i.e., theoretical, voltage, faradaic, end energy utilization) that characterize the operation of direct ethanol fuel cells, thus allowing to present overall fuel efficiency vs. cell current density curves for the first time.

A 1D+1D model for liquid-feed direct ethanol fuel cells with detailed ethanol electro-oxidation kinetics is presented. Onedimensional convective transport along the flow channels is coupled to a one-dimensional description for species transport across the MEA accounting for the effect of the electrochemical reactions and the mixed potential due to ethanol and acetaldehyde crossover. The model, validated against previous experimental data, provides the variation of the concentrations of the free species and the output and parasitic current densities along the flow channels. The downstream evolution of ethanol and acetaldehyde affects the anode reaction rates and the ethanol and acetaldehyde crossover fluxes, which in turn impacts on the cathode reaction rates. Polarization curves obtained at different positions along the flow channels show the effect of ethanol consumption and acetaldehyde upstream production and downstream consumption. Power density, parasitic current, fuel utilization and product selectivity curves are presented and discussed for different ethanol feed concentrations and flow rates.

Fuel cell modeling is an inherently multiphysics problem. As a result, scientists and engineers trained in different areas are required to work together in this field to address the complex physicochemical phenomena involved in the design and optimization of fuel cell systems. This multidisciplinary approach forces researchers to become accustomed to new concepts. Electrochemical processes, for example, constitute the heart of a fuel cell. Accurate modeling of electrochemical reactions is therefore essential to successfully predict the performance of these devices. However, becoming familiar with the complex concepts of electrochemistry can be an arduous task for those who approach the study of fuel cells from fields other than chemical engineering. This process can extend over time and requires careful reading of many textbooks and papers, the most illuminating ones being hidden to the newcomer in a plethora of recent publications on the subject. The authors, who engaged in the study of fuel cells coming from the field of mechanical engineering, had to travel this road once and, with this contribution, would like to make the journey easier for those who come behind. As an illustrative example, the thermodynamic and electrochemical principles reviewed in this chapter are applied to a complex electrochemical system, the direct ethanol fuel cell (DEFC), reviewing recent work on this problem and suggesting future research directions.