New approaches towards novel composite and multilayer membranes for intermediate temperature-polymer electrolyte fuel cells and direct methanol fuel cells

Highlights

• Reviews composite membranes & pros/cons of multilayer membrane (MMs) prep techniques.

• Sulphonated GO as filler has higher performance than GO in composite membranes.

• Current MMs focussed on DMFC. MMs (with improved tolerance) critical for IT-PEFC.

• Next step for MMs development-combine composite (novel polymer-filler) & MM concept.

• Ionic liquids fillers have potential for use in IT-PEFC -slowly gaining momentum.

Abstract

This review analyses the current and existing literature on novel composite and multilayer membranes for Polymer Electrolyte Fuel Cell applications, including intermediate temperature polymer electrolyte fuel cell (IT-PEFC) and direct methanol fuel cell (DMFC) systems. It provides a concise scrutiny of the vast body of literature available on organic and inorganic filler based polymer membranes and links it to the new emerging trend towards novel combinations of multilayered polymer membranes for applications in DMFC and IT-PEFC. The paper carefully explores the advantages and disadvantages of the most common preparation techniques reported for multilayered membranes such as hot-pressing, casting and dip-coating and also summarises various other fresh and unique techniques employed for multilayer membrane preparation.

Introduction

In the last few decades, the mounting concerns over environmental issues and a consequent bend of socio-economic policies towards greener alternatives have led to an extensive research into the development of fuel cell economy. Fuel cells are electrochemical devices that use hydrogen (or hydrogen based chemicals) as fuel, which combines with oxygen to produce electricity, water and heat. The hydrogen feeds the anode (where it is oxidized), producing protons and electrons in the presence of a catalyst (usually platinum). An electrically insulating electrolyte allows protons to pass through to the cathode and blocks the electrons, enabling a current flow in the external circuit. At the cathode electrons re-combine with the protons and the oxygen supplied at the cathode to produce water. The schematic of the well-known anodic and cathodic reactions in a fuel cell are shown in Fig. 1a. The advantages of fuel cells over the existing fossil fuel based systems (internal combustion engines) are i) no generation of harmful, greenhouse gases like SO_x, NO_x, CO₂ and CO; ii) higher efficiency and iii) reduced sound pollution. However, drawbacks such as the high cost of component materials, storage and production of high purity hydrogen, scaling up and long-term durability and performance of the individual components as well as that of the system still remain to be addressed more effectively to enable large-scale commercialisation [1], [2], [3], [4].

There are many kinds of fuel cells, which operate on different fuels (acting as a source of hydrogen) and have varying operating temperatures, covering a whole range from 50 °C up to 1000 °C. Fuel cells are commonly classified on the basis of their electrolyte [5] according to which they can be divided into five main groups: Alkaline Fuel Cell (AFC), Phosphoric Acid Fuel Cell (PAFC), Polymer Electrolyte Fuel Cell (PEFC), Molten Carbonate Fuel Cell (MAFC) and Solid Oxide Fuel Cell (SOFC). PEFC can be further sub-divided into three groups, the general PEFCs feed on hydrogen, Direct Methanol Fuel Cell (DMFC) and Direct Ethanol Fuel Cell (DEFC). Direct alcohol fuel cells (DMFC and DEFC) are similar to the PEFC in operation but as the names suggest, they feed on methanol and ethanol, respectively. PEFC (including DMFC and DEFC) are preferentially used in small devices and in transport applications that require quick start-up and do not require very high power. Among the various components of a PEFC is the proton exchange membrane (PEM) that plays the most vital role, i.e. separating the anode from the cathode, which enables the current flow through an external circuit. The performance of a PEM strongly affects the durability and efficiency of the fuel cell system. The most important properties required for an ideal PEM can, therefore, be outlined as (1) electrically insulating, (2) good proton conductivity, and (3) impermeable to gases and/or fuel to prevent gas/fuel crossover.

Proton exchange membranes are a sub-category of ion exchange membranes, which have been used quite successfully in diverse industries and applications (such as dialyses, electrolyser and desalination of water, among others) [6], [7], [8] before the advent of PEFC. Perfluorinated sulphonic acid (PFSA) polymers, usually fluorinated copolymers of tetrafluorethylene-co-sulphonic acid monomers with high thermal and chemical stability, are the most successfully used commercial membranes for PEFC and alcohol fuel cells. The PFSA membranes have two phases: a hydrophobic phase consisting of tetrafluorethylene (PTFE) forming the backbone (Fig. 1b, block m) which provides mechanical resistance, and a hydrophilic phase consisting of side chains having sulphonic acid group (Fig. 1b, block n) which is responsible for the proton transport [9], [10]. The proton conductivity is due to the main carbon chain, which separates the side chains, thus forming the polar and non-polar regions. Proton transport through the electrolyte membrane can occur by the Grotthuss (also called hopping mechanism) and vehicular (diffusion) mechanism [10], [11]. According to the Grotthuss mechanism, protons that are linked to the sulphonic groups combine with water molecules in the hydronium (H₃O⁺) form. A proton of the hydronium is then transferred to another water molecule bonded to a nearby sulphonic acid group (see Fig. 1c), and these water molecules form the ‘Water Bridge’. Thus, the proton hopping occurs through the network of hydrogen bonds. Higher the water molecule content attached to sulphonic acid group (λ), closer the water molecules are to each other, resulting in faster proton transport (facilitated between the SO₃^- groups) through the membrane. The λ is defined as the number of water molecules per sulphonic acid group. On the other hand, according to the vehicular mechanism, proton in the hydronium H₃O⁺ ion, due to electrochemical differences diffuses with water molecules through the membrane [2]. Although it is possible that both mechanisms are active simultaneously, Grotthuss mechanism is considered to be the preferred and faster mechanism [12], [13].

The most successful commercial PFSA membrane is Nafion^®, developed in the early 1970s by DuPont. Chemical structure of Nafion^® consists of PTFE sequences without and with side chains of perfluoroether, that end in sulfonic acid groups (–SO₂OH) (as the fluoro 3,6-dioxo 4,6-octane sulphonic acid shown in Fig. 1b). Other companies also made their own Nafion-like membranes, such as Flemion^® from Asahi Glass Company Ltd and Dow membrane, from Dow Company [2]. Nonetheless, in spite of the extensive studies on polymer membranes to substitute Nafion, no reported membrane has so far achieved a performance comparable to that of Nafion in PEFC. Although, some other membranes may exhibit certain proprieties better than Nafion, so far none seem to have the optimum balance between all the properties that is demonstrated by Nafion^®. However, Nafion^® still suffers from various drawbacks like degradation due to dry conditions during start stop cycles, and due to fuel crossover, especially in DMFC systems, leading to loss of cell performance as revealed by various long-term studies. Moreover, recent studies [14], [15], [16] investigating performance of membranes in intermediate temperature (100–120 °C) PEFC (or IT-PEFC), suggest the need to develop new membranes which could solve specific problems faced by PEMs in this kind of environment. Operating at higher temperatures not only enables faster reaction kinetics for hydrogen oxidation and oxygen reduction but also a) enables better catalytic activity due to reduced poisoning from CO and other gases and b) facilitates easier water management and elimination [17]. However, the operation of IT-PEFC at temperatures between 100 and 120 °C means the PEM needs to be more tolerant, especially towards low humidity conditions, in order to maintain proton transport and prevent membrane degradation [18], [19]. While proton transport in hydrated membrane is commonly explained through the formation and cleavage of hydroniums bonds, thermodynamically this route is not the most favourable. There are two widely accepted structures involving the hydronium: the Zundel (H₅O₂⁺) cation and Eigen (H₉O₄⁺) cation complex (Fig. 1d). In the Zundel H₅O₂⁺ complex, two water molecules in symmetric hydrogen bond share the proton equally. In the Eigen solvated structure, the hydronium ion is at the centre of the H₉O₄⁺ complex and is strongly bonded to three neighbouring water molecules via hydrogen bonds [20]. Both complexes represent the ideal structures in a more general hydrogen bond network and define the diffusion of the hydrogen-bond structure in which the excess proton is transported/tunnelled back and forth. It is thought that both the complexes transform into each other and act as donors of protons by the formation and cleavage of hydrogen bonds. These bonds are not as strong as those in simple hydronium structures and therefore enable a faster proton transfer [20].

Proton diffusion, although independent of transport mechanism, increases as temperature and membrane hydration level (λ) increase. Feng and Voth [21] in a modelling study investigating transport of protons have revealed that for λ values between 6 and 15 and for temperatures between 25 °C and 67 °C, the Grotthuss mechanism has a higher diffusion ratio than that of vehicular mechanism. As the temperature and value of λ increase, Grotthuss mechanism becomes more dominant. The activation energy available for proton transport drops for λ values less than 10 due to the different solvation structure at this level of hydration. Beyond this exception, the activation energy increases with λ being almost the same as in pure water when λ is 15. The indispensability of water to proton transport, as established by the various proton transport and diffusion mechanisms, underlines the importance of incorporation of hydrophilic groups in Nafion or any other PEMs to enable higher proton conductivity especially at elevated temperatures. Therefore, if a hydrophilic group is added, the water retention would improve resulting in better proton conductivity. This is the concept behind all composite membranes. Another new concept under investigation to achieve better PEMs, is the use of multilayer membrane. A multilayer design can bring together layers of different polymers each bringing its unique characteristic (mechanical strength, non-permeability, better water retention, etc.) property to the composed multilayer membrane, eliminating the need for compromising one property for another when choosing a PEM material.

This review focuses on the new developments in composite and multilayer membranes for IT-PEFC and DMFC, highlighting the various unique and novel approaches towards multilayer membrane development. The composite membranes section covers both organic and inorganic filler membranes. However, since membranes with organic fillers have been extensively discussed in previous works [22], [23], [24], [25], this paper highlights only the more recent work on organic fillers and provides a more detailed discussion on inorganic fillers. This is followed by extensive analysis of the recent reports on multilayer membranes for use in IT-PEFC and DMFC, speculating the future possibilities due to this paradigm shift in our perspective of PEMs.