Direct Alcohol Fuel Cells for Portable Applications

Fundamentals, Engineering and Advances

Alexandra M. F. R. Pinto, Vania Sofia Oliveira, Daniela Sofia Castro Falcao

• Publisher: Academic Press
• Year: 2018
• ISBN:
  9780128118986
• Topic: Technology & Engineering
• Subtopic: Renewable Power Resources
• Index: Technology & Engineering

Direct Alcohol Fuel Cells for Portable Applications: Fundamentals, Engineering and Advances presents the fundamental concepts, technological advances and challenges in developing, modeling and deploying fuel cells and fuel cell systems for portable devices, including micro and mini fuel cells. The authors review the fundamental science of direct alcohol fuel cells, covering, in detail, thermodynamics, electrode kinetics and electrocatalysis of charge-transfer reactions, mass and heat transfer phenomena, and basic modeling aspects. In addition, the book examines other fuels in DAFCs, such as formic acid, ethylene glycol and glycerol, along with technological aspects and applications, including case studies and cost analysis.

Researchers, engineering professionals, fuel cell developers, policymakers and senior graduate students will find this a valuable resource. The book's comprehensive coverage of fundamentals is especially useful for graduate students, advanced undergraduate students and those new to the field.

• Provides a comprehensive understanding of the fundamentals of DAFCs and their basic components, design and performance
• Presents current and complete information on the state-of-the-art of DAFC technology and its most relevant challenges for commercial deployment
• Includes practical application examples, problems and case studies
• Covers the use of other fuels, such as formic acid, ethylene glycol and glycerol

1

Introduction to direct alcohol fuel cells

Abstract

The aim of this chapter is to prepare the reader to the journey into the direct alcohol fuel cells (DAFCs) learning and deeper understanding. Therefore, a broad overview of the DAFC technology, as well as its definition, is provided.

A brief comparison between fuel cells and batteries or combustion engines is made as well as a comparison between the DAFCs and its direct rival—the hydrogen PEMFC. The working principles of the DMFCs and DEFCs are presented together with the description of the two different types of fuel and oxidant supply in active and passive systems. The main advantages and disadvantages of the DAFCs are explained, stressing out the great potential of application in the portable sector. An introduction to the markets and target applications is then offered with the presentation of several examples of products already under commercialization. Finally, the main challenges of this technology are presented.

Keywords

Fuel cell; Direct alcohol fuel cell; Direct methanol fuel cell; Direct ethanol fuel cell; Active DAFC; Passive DAFC; Alcohol crossover; Water management; Portable applications; Stationary applications

The success of fuel-cell-powered vehicles such as Toyota Mirai using proton exchange membrane fuel cells (PEMFCs) shows that this technology has already reached the commercialization readiness level expected for many years. The interest in fuel cells of all types has increased dramatically, due to high efficiencies, nonexistence of gaseous pollutants (sulfur dioxide and various nitrogen oxides), and simple design, making them an attractive alternative to batteries and internal combustion engines. Furthermore, the urgent need of energy storage toward a fully sustainable energy paradigm reinforces the importance of this technology, since fuel cells along with hydrogen production by electrolysis are expected to play a major role linking energy storage and power generation.

A fuel cell (FC) is simply defined as an electrochemical energy converter, transforming the chemical energy of a fuel directly into electrical energy. It works as long as the electroactive chemicals are provided to the cell. Hydrogen usually emerges as the best fuel candidate, but to the date, production, storage and distribution of this fuel are still very complex issues. Alternatively, the Direct Fuel Cells (DFCs) using liquid organic fuels have gained increasing importance. The most commonly used liquid fuels in DFCs are alcohols such as methanol and ethanol, which have a much higher volumetric energy density and are much easier to store, transport, and distribute than hydrogen. DFCs and in particular direct alcohol fuel cells (DAFCs) usually have a compact design and potentially can offer up to 10 times the energy density of rechargeable batteries. In addition, DFCs can be designed to operate at ambient temperature, which significantly reduces thermal management challenges for small systems. These advantages make the technology attractive to the rapid growing need for portable power sources, which should include micro and small DAFCs. In particular, the small cells have market potential in education, auxiliary power systems, and recreational and military applications. Further research and development is needed to achieve the miniaturization needed for small fuel cells be integrated into consumer portable electronics, which will certainly unlock this significant market [1]. Important niche markets for portable applications are in developing countries and rural areas where the grids are unreliable and people use mobile phones as a primary communication device. Small decentralized power systems are seen as a way to keep communications open even when the power goes out.

This chapter is the beginning of a journey to the world of the DAFC technology and its tremendous field of applications. A comparison with the direct rival—hydrogen polymer electrolyte membrane fuel cells (PEMFCs)—is made all over the chapter.

1.1 What is a direct alcohol fuel cell (DAFC)?

A fuel cell is like a factory treating raw materials and delivering products [2]. The plant will be running as long as raw materials are available, generating its main product, electricity. Unlike batteries, fuel cells are not consumed during the energy production, i.e., they do not discharge. Like batteries, fuel cells have a positive and negative electrode and an electrolyte. Fuel cells, like combustion engines, transform the energy stored in a fuel in useful forms of energy, such as electrical, but in a direct conversion process. In the combustion machine, the heat released in the combustion reaction is converted into mechanical and afterwards into electrical energy in a potentially complex and inefficient process. The direct conversion of chemical energy into electrical energy occurring in fuel cells arises from the existence of a physical barrier between reactants (the electrolyte), in a way that the transfer of electrons involved in the bonds reconfiguration through an external trajectory can be harnessed as electrical current.

Fig. 1.1A and B give a schematic representation of a hydrogen-oxygen fuel cell and a direct alcohol fuel cell.

Fig. 1.1 Schematic representation of (A) hydrogen-oxygen fuel cell and (B) direct alcohol fuel cell.

In the hydrogen/oxygen fuel cell, the hydrogen molecules are oxidized producing water and releasing waste heat. Hydrogen is oxidized at the anode producing protons and oxygen is reduced at the cathode. Protons are transported through the electrolyte—the polymer electrolyte membrane (PEM)—and the electrons proceed to the cathode via an external circuit.

Direct alcohol fuel cells produce electricity directly from the electro-oxidation of alcohols such as methanol or ethanol into water and other byproducts such as carbon dioxide (Fig. 1.1B). As in the H₂-O₂ PEM fuel cells, the heart of the DAFC is the membrane, conducting to the cathode the protons formed at the anode by the oxidation reaction of the alcohol molecules. The most common material used for the electrolyte (Nafion) is a polymer made of persulfonic acid groups with a Teflon backbone, which is not completely impermeable to the alcohol molecules (namely the smaller molecules of methanol), thus resulting in the crossover of some ...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Preface
• Acknowledgments
• 1: Introduction to direct alcohol fuel cells
• 2: Direct alcohol fuel cells basic science
• 3: Direct alcohol fuel cells (DAFCs) basic modeling
• 4: Experimental methods of characterization
• 5: Other fuels for direct fuel cells (DFCs)
• 6: Development of direct alcohol fuel cells components
• 7: Miniaturization of direct alcohol fuel cells: Microfabrication techniques and microfluidic architectures
• 8: Direct alcohol fuel cell stacks
• 9: Case studies—Portable applications of direct alcohol fuel cells
• 10: Status and research trends of direct alcohol fuel cell technology
• Index