Alkali doped polyvinyl alcohol/graphene electrolyte for direct methanol alkaline fuel cells
Graphical abstract
A significant enhancement of the graphene/PVA composite membranes' transport properties are obtained with optimum graphene loadings; i.e., a ∼126% improvement of ionic conductivity and a ∼55% reduction of methanol permeability were achieved with a graphene loading of 0.7 wt%, resulting in a significant enhancement of the cell's performance (∼148% improvement in maximum power density).
Introduction
Proton exchange membrane fuel cells (PEMFCs) are being developed as a future power generation technology with the potential to deliver clean-at-the-point-of-use power. As well as their environmental friendliness they additionally offer high power densities, high energy conversion efficiencies, and low starting temperatures [1], [2]. Although PEMFCs have been widely researched in recent decades and are considered to be an important development, their inherent limitations in acidic conditions, due to: noble metal catalyst poisoning by carbon monoxide at low temperatures, complex water management, limited PEM working lifetime and high fuel permeability, together with the rarity and consequent high cost of platinum catalysts bring various limitations to PEMFCs [3], [4], [5]. In contrast to acidic fuel cells, alkaline fuel cells (AFCs) have several advantages that overcome some of the PEMFC's scientific and technological difficulties. These advantages include: (1) the enhancement of both fuel oxidation and oxygen reduction reaction kinetics, thereby allowing the use of non-noble metal catalysts (e.g. silver, nickel and palladium), (2) the metals suffering less corrosion and are thus more stable in alkaline environments, (3) improved water management, i.e. the electroosmotic drag transports water away from the cathode, and (4) low fuel permeability, due to hydroxide ion transport, from the cathode to the anode [5], [6], [7], [8], [9].
The anion exchange membrane (AEM) that plays a crucial role in separating fuel and oxygen (or air) while achieving simultaneous anion transfer is one of the key components in AEMFCs. AEMs have the following requirements: adequate mechanical strength, good thermal and chemical stability and suitable ionic conductivity. To meet such demands, most studies use polymeric materials containing quaternary ammonium groups, e.g. quaternized polysulfone, [10], [11], [12], [13] poly(2,6-dimethyl-1,4-phenylene oxide), [14] cardo polyetherketone (PEK-C), [15] poly(phenylene), [16] and radiation-grafted PVDF, ETFT and FEP, [17], [18], [19]. Unfortunately, the quaternized polymer is unstable in alkaline media at temperatures above 60 °C and at high KOH concentrations [20], [21]. Moreover, there are still some outstanding problems confronting the development of AEMs for AEMFC applications, such as, the chemical stability of the cationic groups attached to the AEMs, the lower conductivity of AEMs compared with PEMs, and their overall cost-effectiveness; thus, the development of cost-effective, easily prepared, high-performance AEMs remains an unmet need.
It is well known that incorporating inorganic fillers into polymer electrolytes can alter and improve their physical and chemical properties [22], [23], [24], [25]. In addition, the approach of using carbon nanofillers in the polymer electrolyte fuel cell's membrane has also led to a remarkable improvement in membrane performance – in terms of ionic conductivity, mechanical properties and methanol permeability [26], [27], [28], [29]. Membranes modified with these nanosized inorganic/carbon fillers have shown encouraging results in both PEMs and AEMs.
Graphene nanosheets are considered effective polymer fillers and have been incorporated into fuel cell polymer matrices. For example, a Nafion membrane, incorporating 0.5 wt% sulfonated graphene oxide (SGO), showed an enhancement in proton conductivity of ∼66% and a significantly reduced methanol permeability of ∼35% [30]. In our previous study, the incorporation of 0.5 wt% GO/poly(sodium-4-styrenesulfonate) modified graphene (PSS-G) into sulfonated polyimide (SPI) led to a significant improvement in selectivity (proton conductivity/methanol permeability), ∼1.5-fold greater than Nafion 117 and 7-fold greater than pristine SPI at 30 °C, while the tensile strength increased by 76% with the addition of 0.9 wt% of GO [31]. These phenomenon have also been observed in ionic liquid (IL)-based polymer electrolyte membranes with incorporated IL polymer modified graphene [PIL(NTFSI)-G] giving enhancements in ionic conductivity (257.4%) and mechanical properties (345% improvement in tensile strength and a near 25-fold increase in modulus at 150 °C) with a minimal loading of PIL(NTFSI)-G (0.5 wt%) [32]. However, the incorporation of graphene into AEMs has not been previously reported.
One key factor affecting the performance of graphene-based nanocomposite membranes is the dispersion of nanofillers in the polymer matrix. To successfully realize the reinforcing potential of graphene nanosheets, they must be fully exfoliated. GO can be easily dispersed in water; thus, hydrophilic polymers, or water-soluble polymers, are suitable polymer matrices for polymer/graphene nanocomposites, due to their dispersion properties. Poly(vinyl alcohol) (PVA) is a hydroxyl-rich, water-soluble, biocompatible and non-toxic polymer that is commonly used in fuel cells, drug delivery, coating materials, adhesives etc. [33]. When graphene/GO is well dispersed, at the molecular level, e.g. in PVA, it significantly enhances the resulting material's mechanical properties, thermal stability and electrical conductivity [34], [35], [36], [37], [38], [39].
Here, we used a simple technique to prepare graphene/PVA nanocomposite membranes by incorporating GO into a PVA matrix, using water as the processing solvent, followed by hydrazine hydrate reduction, film casting, and finishing by alkaline doping. In these PVA/graphene composite membranes, the graphene nanosheets are uniformly dispersed – resulting in continuous, well-connected tortuous ionic channels. This approach introduces sought after transport behaviour properties into the nanocomposite membranes, e.g. high ionic conductivity, low methanol permeability and low activation energy for ionic conduction. Additionally, the graphene nanosheets exhibit strong interfacial strength with the PVA matrix thereby conferring significantly enhanced mechanical properties.
Section snippets
Composite membrane preparation
Scheme 1 illustrates the preparation of PVA/graphene composite membranes, together with an inner structural model of PVA/graphene, at each stage after doping with KOH solution. PVA powder was dissolved in deionized (D.I.) water at 90 °C in a three-necked flask with mechanical stirring to form an aqueous solution (0.1 g mL−1). GO was prepared according to the method described in our previous study and purified by centrifugation [22], [31], [32]. The resulting concentrated GO was dilute to 5 mg mL
Morphology and structure
It is well known that ionic conductivity and methanol crossover are strongly influenced by the ionic clusters size, connectivity and functional groups. The physical modification of ionic channels by various fillers such as inorganic particles, [42] mesoporous materials, [23] clay or carbon nanotubes [31], [41], [43] leads to a dramatic decline in methanol crossover due to barrier and molecular sieve effects; moreover, the ionic conductivity may also be increased by the external surface forming
Conclusions
In this work, graphene was incorporated into PVA and the resulting membranes' transport properties were investigated. In these PVA/graphene AFC membranes, the graphene nanosheets were uniformly dispersed resulting in continuous, well-connected and tortuous ionic channels. Such improvements in transport properties result in significant improvements in ionic conductivity (∼126%) and reductions in methanol permeability (∼55%), resulting in a significant enhancement in the cell's performance (∼148%
Acknowledgements
The financial supports from the National Science Council (NSC) (101-3113-E-011-002, 100-2221-E-011-105-MY3), the Ministry of Economic Affairs (MOEA) (101-EC-17-A-08-S1-183), and the Top University Projects of Ministry of Education (MOE) (100H451401), as well as the facilities supports from the National Taiwan University of Science and Technology (NTUST) are acknowledged.
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