Electrochemistry

Volume 14

Craig Banks, Steven McIntosh

• Publisher: Royal Society of Chemistry
• Year: 2017
• eBook ISBN: 9781788011501
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

Providing the reader with an up to date digest of the most important research currently carried out in the field, Electrochemistry Volume 14 is compiled and written by leading experts from across the globe. Coverage includes chapters on the use of metal organic frameworks as a precursor for electrocatalytic centre supports to enhance the oxygen reduction process in low temperature fuel cell systems, electrocatalysis for ethanol electrooxidation in alkaline media, and new polymer electrolyte and electrocatalysts for direct alcohol fuel cells. This volume is a key reference for researchers providing a timely overview of this exciting and developing area.

Newer polymer electrolytes and electrocatalysts for direct alcohol fuel cells

P. Sridhar^*a, S. D. Bhat,^b and A. K. Sahu,^b

DOI: 10.1039/9781782622727-00102

^a M/s Mesha Energy Solutions Pvt. Ltd.93A, Industrial Suburb, 2nd stage, Yeshwanthpur, Bengaluru, 560 022, India.E-mail: [email protected]

^b Central Electrochemical Research Institute (CECRI)-Madras Unit, CSIR Madras Complex, Chennai, 600 113, India

1 Introduction

Fuel Cells are known for their higher energy conversion efficiency. Polymer Electrolyte Fuel Cells (PEFCs) are preferred in comparison with the other types of fuel cells due to their lower operating temperature, ease of electrolyte management etc. Hydrogen as a fuel in PEFCs has its own limitation in terms of generation and storage. Among several types of fuel cells, direct alcohol fuel cells are gaining tremendous interest as energy source for portable application, since liquid fuel without the external reforming system could stave off storage and safety issues.^1,2 Certain hydrogen-carrying organic fuels such as methanol, ethanol, propanol, ethylene glycol and diethyl ether are also considered for fuelling PEFCs directly. The use of methanol/ethanol as fuel has several advantages in comparison to hydrogen: it is a cheap liquid fuel, easily handled, transported, and stored, and with a high theoretical energy density.³

The specific energy of hydrogen compressed at 700 bar is 142 MJ kg⁻¹ while the specific energies for methanol and ethanol are 22.7 and 29.7 MJ kg⁻¹, respectively, while gasoline contains about 45 MJ kg⁻¹.⁴ This sets the basis for sourcing methanol and ethanol as fuels in PEFCs. However, there are certain technical issues that are being addressed to overcome the obstacles for their commercialisation. The critical ones are (i) the polymer electrolyte and (ii) the catalysts used for the oxidation of alcohols at the anode and that for the reduction of oxygen at the cathode. Perfluorosulphonic acid membrane commercially available as Nafion manufactured by M/s DuPont Inc., USA is presently being widely used as the polymer electrolyte for Direct Methanol and Direct Ethanol fuel cells. Though thinner versions like Nafion 211, 212 are available, the preferred one being Nafion 117 on account of less methanol/ethanol permeability through the latter in comparison with the former. The ionic conductivity of these membranes are in the order of 10⁻² S cm⁻¹ that makes them attractive inspite of higher alcohol permeability through these membranes. Higher permeability of the alcohol through the membranes results not only in loss of fuel available for oxidation, but interferes in the reduction of oxygen at the cathode resulting in the reduced conversion efficiency. This has made the researchers to look for alternative electrolytes. The polymer membrane electrolytes investigated for their application in Direct Alcohol Fuel Cells have been exhaustively reviewed by Horacio R. Corti.⁵

The real challenge is in designing a catalyst for methanol or ethanol oxidation, which involves transfer of six or twelve electrons for the oxidation to go to completion. Similar is the case in designing a catalyst for oxygen reduction that is insensitive for oxidation of alcohol. Designing such a catalyst will be helpful as permeability of alcohol through the polymer electrolyte may not be completely eliminated but could only be mitigated by proper design of the membrane electrolyte.

Alkaline Direct alcohol fuel cells (ADAFCs) have attracted increasing interest over the past decade because of their favourable reaction kinetics in alkaline media, higher energy densities achievable and the easy handling of the liquid fuels. Unlike in acidic media, the kinetics of both methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR) in alkaline media is much faster allowing the use of non-precious metal catalysts to reduce the cost of the fuel cell.

Excellent review articles are available in the literature covering PEMs and electrocatalysts for Direct Ethanol Fuel Cell.^6–11

The present chapter is organized in two major sections, each section dealing with the development of polymer membrane electrolyte and, anode and cathode catalysts for Direct Alcohol Fuel Cells. The section on Polymer Membrane Electrolyte focuses on recent advances that have been made under Nafion and sulfonated polyether ether ketone (sPEEK) composite membranes containing inorganic additives viz. Zeolites, Mesoporous materials, Clays and carbon nanostructures in the light of the potential that is seen in these composites for their ability to reduce the alcohol cross over in addition to improved ionic conductivity and durability and also on development of Anion Exchange Membrane (AEMs) for Alkaline Direct Methanol Fuel Cells (ADMFCs).

In the section under Electrocatalysts for DAFCs, the recent advances that have been made under methanol tolerant ORR catalysts for DMFCs and ADMFCs, methaol oxidation catalysts for DMFCs, ethanol tolerant ORR catalysts and ethanol oxidation catalysts for DEFCs are presented.

2 Proton conducting polymer electrolytes for DAFCs

In following sections, different types of proton exchange membranes (PEMs) based on Nafion and sPEEK blend and composites incorporated with inorganic additives are discussed for application in DAFCs.

2.1 Nafion based polymer electrolytes for DMFCs

2.1.1 Organic–inorganic composite polymer-electrolyte membranes for DMFCs.

Organic–inorganic composite membranes comprising Nafion with inorganic materials such as silica, mesoporous zirconium phosphate (MZP) and mesoporous titanium phosphate (MTP) are fabricated and evaluated as PEM for DMFCs. For Nafion–silica composite membrane, silica is impregnated into Nafion matrix as a sol by a novel water hydrolysis process precluding the external use of an acid.¹² Instead, the acidic nature of Nafion facilitates in situ polymerization reaction with Nafion leading to a uniform composite membrane. The rapid hydrolysis and polymerization reaction while preparing zirconia and titania sols leads to uncontrolled thickness and volume reduction in the composite membranes, and hence is not conducive for casting membranes. Nafion–MZP and Nafion–MTP composite membranes are prepared by mixing pre-formed porous MZP and MTP with Nafion matrix. MZP and MTP are synthesised by co-assembly of a tri-block co-polymer, namely pluronic-F127, as a structure-directing agent, and a mixture of zirconium butoxide/titanium isopropoxide and phosphorous trichloride as inorganic precursors. The use of tri-block co-polymers as structure-directing agents enables the synthesis of mesoscopically ordered materials exhibiting a narrow pore-size distribution after surfactant removal. MZP and MTP reported here are surface-functionalized solid-superacid-proton conducting materials as well as inorganic fillers with high affinity to absorb water which helps fast proton-transport across the electrolyte membrane and makes it suitable for DMFCs. Specifically designed composite membranes increase water-uptake properties with reduced methanol permeability ameliorating the DMFC performance in comparison to a DMFC using commercially available Nafion-117 membrane while operating under identical conditions. Methanol release kinetics is also studied by volume-localized NMR spectroscopy (employing “point resolved spectroscopy”, PRESS), the results clearly demonstrating that the incorporation of inorganic fillers in Nafion retards the methanol release kinetics under osmotic drag. Appreciable proton conductivity with reduced methanol permeability across the composite membranes leads to improved performance of DMFCs in relation to commercially available Nafion-117 membrane.

2.1.2 Diffusion of CH₃OH from membrane sac to 2 M CD₃OD in water.

Permeation of methanol from membrane sacs – each saturated with 2 M CH₃OH in D₂O of Nafion, Nafion impregnated with 10 wt% SiO₂, Nafion with 5 wt% MTP and Nafion with 5 wt% MZP, to surrounding 2 M CD₃OD in water was recorded at different time intervals by PRESS spectroscopy.¹³ It may be noted that ‘shimming’ the voxel of interest is facilitated by water in the surrounding medium, although this signal is finally suppressed by standard outer volume s...

Table of contents

• Cover
• Title
• Copyright
• Preface
• Author biographies
• Contents
• Borohydride electro-oxidation on metal electrodes: structure, composition and solvent effects from DFT
• Recent progress in the development of anion exchange membranes for electrochemical devices
• Anodic materials for electrooxidation of alcohols in alkaline media
• Newer polymer electrolytes and electrocatalysts for direct alcohol fuel cells
• Application of Metal Organic Framework (MOF) in the electrocatalytic process