Commercially available proton exchange membranes such as Nafion do not meet the requirements for high power density direct methanol fuel cells, partly due to their high methanol permeability. The aim of this work is to develop a new class of high-proton conductivity membranes, with thermal and mechanical stability similar to Nafion and reduced methanol permeability. Nanocomposite membranes were produced by the in-situ sol-gel synthesis of silicon dioxide particles in preformed Nafion membranes. Microstructural modification of Nafion membranes with silica nanoparticles was shown in this work to reduce methanol crossover from 7.48x10-6 cm2s^-1 for pure Nafion® to 2.86 x10-6 cm2s^-1 for nanocomposite nafion membranes (Methanol 50% (v/v) solution, 75 degrees C). Best results were achieved with a silica composition of 2.6% (w/w). We propose that silica inhibits the conduction of methanol through Nafion by blocking sites necessary for methanol diffusion through the polymer electrolyte membrane. Effects of surface chemistry, nanoparticle formation and interactions with Nafion matrix are further addressed

Dicks, A. L.

Diniz da Costa, J. C.

Duke, M. C.

Ladewig, B. P.

Lu, G.

Martin, D. J.

Rodriguez, J. I. C.

University of Queensland eSpace

Nanocomposite Nafion-silica membranes for direct methanol fuel cells 
 
J.I. Garnica Rodriguez, B. Ladewig, A. Dicks, M. Duke, D. Martin, G. Q. Lu, J.C. Diniz da Costa 
 
ARC Centre for Functional NanoMaterials, School of Engineering, 
The University of Queensland, Brisbane, QLD 4072, Australia 
joedac@cheque.uq.edu.au 
 
 
Abstract 
Commercially available proton exchange membranes such as Nafion do not meet the requirements for 
high power density direct methanol fuel cells, partly due to their high methanol permeability.  The aim 
of this work is to develop a new class of high-proton conductivity membranes, with thermal and 
mechanical stability similar to Nafion and reduced methanol permeability.  Nanocomposite membranes 
were produced by the in-situ sol-gel synthesis of silicon dioxide particles in preformed Nafion 
membranes. Microstructural modification of Nafion membranes with silica nanoparticles was shown in 
this work to reduce methanol crossover from 7.48x10-6 cm2s-1 for pure Nafion® to 2.86 x10-6 cm2s-1 for 
nanocomposite nafion membranes (Methanol 50% (v/v) solution, 75°C). Best results were achieved 
with a silica composition of 2.6% (w/w). We propose that silica inhibits the conduction of methanol 
through Nafion by blocking sites necessary for methanol diffusion through the polymer electrolyte 
membrane. Effects of surface chemistry, nanoparticle formation and interactions with Nafion matrix 
are further addressed. 
 
Keywords: proton exchange membranes, in-situ sol-gel, functionalisation, proton conduction and 
methanol diffusion. 
 
1.Introduction 
 Fuel cells are electrochemical devices that convert chemical energy to electrical energy at an 
efficiency much higher than that of traditional heat engines. Among the different fuel cell technologies, 
the direct methanol fuel cell (DMFC), with methanol as the energy carrier, has one of the best 
application prospects as an energy converter for portable electronic devices [1]. However, polymer 
electrolyte membranes generally allow the methanol molecules to diffuse from the anode to the cathode 
side. This so called methanol crossover has a negative impact on the overall performance of the DMFC 
due to a reduction in fuel efficiency through wasted oxidation at the cathode, as well as catalyst 
poisoning [3]. Although diluted methanol in water mixtures and low operation temperatures are 
strategies used to reduce the effect of methanol crossover, these approaches have yet to deliver 
significant fuel cell performance improvements [4]. 
Modification of nafion polymeric matrix by the incorporation of nanoparticles forms part of a 
concerted effort by the scientific community to address this issue. Silica has been embedded into 
Nafion membranes through both solvent casting and in-situ synthesis from silicon alkoxide precursors. 
Dimitrova et al. [5] reported a three-fold increase in the proton conductivity of a 4.3% SiO2/Nafion 
composite membrane ( = 0.3 S.cm-1) over as-received Nafion 117 membrane, where the Silica was 
incorporated from Aerosil A380 fumed silica. Nafion-silica membranes formed from in-situ synthesis 
techniques have also shown comparable proton conductivity to that of Nafion. Jung et al. [6] claimed 
an optimum SiO2 content of approximately 12 wt%, having  = 0.08 S.cm-1 and delivering a current 
density of 650 mA.cm-2 at 0.5 V and 125 ºC in a DMFC. Similarly, the incorporation of 3–5 wt% TiO2 
in Nafion membranes has shown superior performance in high  temperature DMFCs, with a power 
density of 350 mW.cm-2 at 145 ºC with oxygen feed. The performance of the cell also increased with 
decreasing TiO2 particle size, thus further suggesting that the filler morphology influences the 
electrochemical properties of the composite membrane [7].  
Various phosphates have also been employed to alter the properties of Nafion, including 
calcium hydroxyphosphate, which at 5 wt% loading in recast Nafion gave  = 0.18 S.cm-1 [8] and 
zirconium phosphate, which at 23 wt% loading in a H2/O2 cell operated at 130 ºC and 3 bar, gave a 
fourfold increase in current density over a MEA based on commercial Nafion [9]. The addition of 
mordenite to Nafion showed only a slight increase in proton conductivity over that of Nafion at 
temperatures greater than 80 ºC, and a corresponding minor increase in H2/O2 cell performance [10]. 
Structural modification of Nafion polymer has also been reported in the literature.  Approaches have  
included grafting of styrene monomer on Nafion by means of supercritical CO2 impregnation in order 
to build a hydrophobic (methanol repellent) barrier (Sauk 2004), pore-filling type PEM using Teflon as 
substrate and acrylic acid-vinyl sulfonic acid copolymer as the proton conductive matrix (Yamaguchi 
2003), and Nafion modification with polyvinyl alcohol (Shao 2002). All these studies resulted in the 
reduction of the methanol permeation, but at the expense of reduced proton conductivity. 
This work investigates at the effect of silica modified Nafion membranes for application in 
DMFCs. The silica loading is studied in terms of proton conduction and methanol crossover.  
 
2. Experimental 
Silica nanoparticles were embedded into the hydrophilic clusters of Nafion using an in-situ sol-
gel technique. Nafion 117 membranes cut in squares with an approximate area of 4 cm2 were modified 
with silica following the technique developed by Ladewig (2004). In this technique, silica nanoparticles 
were embedded into the hydrophilic clusters of the perfluorinated membrane using a sol-gel process. 
After pre-treated, the membranes were dried at 100ºC and atmospheric pressure. Then, the membranes 
were stabilised in a media with a controlled relative humidity of 40% for 30min and then immersed on 
a solution of TEOS-Ethanol 1:8 (M/M) for 1min. After this step, the membranes were rinsed in pure 
ethanol in order to eliminate any build-up of silica on the surface of the membranes. The process was 
repeated several times to increased the silica loading in the Nafion matrix. 
Nafion-functionalised silica composite membranes were produced using the same technique, 
using 3-mercaptopropyl trimethoxysilane (MPTMS) in addition to tetraethyl orthosilicate (TEOS) as a 
silicon alkoxide precursor, in a molar ratio of 1:1:8 (MPTMS:TEOS:EtOH).  As shown in Fig. 1, 
treatment of the Nafion-functionalised silica composite membranes in 30% H2O2 at 60 °C for 1h was 
employed to oxidise the thiol groups to sulfonic acid groups [15]. 
 
 
Figure 1. Hydrolysis, condensation and oxidation of MPTMS 
 
The methanol and water permeation on the membranes was determined using a pervaporation 
system (Fig. 2). The polymer electrolyte membrane was hermetically clamped between two metallic 
bulbs, and fed at one side with a liquid stream (methanol-water solution) which concentration was 
maintained constant by means of a reflux using a peristaltic pump. At the other side, the permeate 
vapour was carried by a helium stream and collected in a glass condenser immersed in liquid nitrogen. 
The weight of permeate was determined using a digital scale and the concentration by means of gas 
chromatography. All measurements were made at 50 °C, with a feed concentration of 50 vol% or 
69 vol% for Nafion-silica and Nafion-sulfonated silica samples respectively. 
 
 
 
Figure 2. Permeability measurement set up 
 
 
The permeabilities of water and/or methanol through the membranes tested on the 
pervaporation experiments were calculated using of the following formula: 
 
 
          (1) 
 
 
where L is the membrane thickness, J is the flux of permeate through the membrane, A the active area 
of mass transfer, and C is the gradient of concentration between the feeding side and the permeate 
side. 
 The normal direction conductivity of the fully hydrated membranes at ambient temperature was 
measured by means of impedance spectroscopy on a Solartron 1260 (StandAlone mode, frequency 
sweep 10MHz to 1Hz). The membrane resistance was obtained by the difference between the measured 
resistance and the contribution of the short circuited electrodes, and can be easily calculated using the 
following formula: 
 
]/[ cmS
AR
L
m
=σ
           (2) 
 
where L is the thickness of the membrane, A the contact area between the gold electrodes and the 
membrane, and Rm is the real value of the electric resistance in the Nyquist plot generated by the 
Solartron 1260. The membrane resistance was obtained by the difference between the measured 
resistance and the contribution of the short circuited cells. 
 
 
 
]/[ 2 scm
CA
JLP
∆
=
3. Results and Discussion 
 
Generally speaking, the measured values of proton conductivity for both the Nafion-silica and 
the Nafion-sulfonated silica membranes were equivalent to or lower than that of Nafion 117 alone (0% 
Si loading), as shown in Figure 3.   
 
Figure 3. Proton conductivity as a function of inorganic content for Nafion-silica and Nafion-sulfonated 
silica membranes at 20oC. 
 
Slight improvements were seen for the Nafion-silica 1.1 wt% and Nafion-sulfonated silica 10 
wt% membranes, at 13.8 and 13.6 mS.cm-1 respectively.  It may be expected that, particularly at higher 
loadings of the sulfonated silica particles, the increased concentration of mobile protons in the 
composite matrix would lead to higher proton conductivity values.  This is made possible by the fact 
that, ideally, the proton conductivity is proportional to the proton concentration.  However this was not 
seen in this work, which may be due to a number of factors.  Interestingly, the Nafion-sulfonated silica 
composite membranes showed a peak in conductivity at around 10 wt%, which may provide some 
insight into the morphological structure of the composite membrane.  Proton transfer in Nafion is 
mediated by water molecules; the dry material has negligible proton conducting capacity as the 
separation distance of adjacent sulfonic acid groups is too great for unassisted transfer.   
However, as the material becomes hydrated, water molecules are able to act as intermediaries 
and facilitate proton hopping through the material.  In this light, it may be hypothesised that at lower 
loadings, the functionalised silica particles, with their reasonably large pendant sulfonic acid 
terminating propyl side chains, simply take up low energy positions within the polymer matrix, 
predominantly within the hydrophilic water-rich domains, and do little to impede or hinder proton 
transport.  However at approximately 10 wt% loading a significant peak in conductivity suggests that 
the concentration of additional sulfonic acid groups may be reaching the critical point at which they 
form a proton-conducting route, or begin to interact strongly with the sulfonic acid groups of the 
Nafion. 
The methanol permeability of the composite membranes is shown in Fig. 4.  Of immediate 
interest are the contradictory trends in permeability with increasing filler loading; for Nafion-silica an 
increase in permeability is noted with increasing filler loading, whilst for Nafion-sulfonated silica the 
opposite is true.  This suggests that, for the low silica loadings, the particles in fact cause the membrane 
structure to open up and allow a higher rate of methanol diffusion as compared to Nafion (0% silica 
loading).  The downward trend in permeability for Nafion-sulfonated silica composite membranes 
indicates that the inorganic particles, which are expected to occupy primarily the hydrophilic domains 
of the polymer matrix, are genuinely impeding the progress of diffusing methanol molecules.  
 
 
Figure  4. Methanol permeability of Nafion-silica and Nafion-sulfonated silica membranes as a 
function of inorganic content at 20oC. 
 
Silica Loading (wt%)
Se
le
c
tiv
ity
 
(m
S.
c
m
-
1 )/
(c
m
3 .
s
-
1 )
0 2 4 6 8 10 12 14 16 18 20
3.25
3.5
3.75
4
4.25
4.5
4.75
5
5.25
 
Figure  5. Selectivity results at 20oC. 
 
Recent studies [5, 16] proposed the selectivity factor which corresponds to the ratio between the 
proton conductivity () and the methanol permeability (P). Although this factor shows a relative 
performance of the DMFC without electricity generation, it is useful to compare simultaneously the 
proton and methanol-water transport properties of the membranes at different temperatures. In terms of 
DMFC, the reduction of methanol crossover (a positive effect) must be balanced against any loss (a 
negative effect) in proton conductivity. It is observed in Figure 5 that the 10 wt% sulfonated silica 
loading sample outperformed Nafion. The other membranes resulted with lower selecitivities than 
Nafion, thus no improvements in performance may be expected. 
  
Fig. 6. TEM Micrograph of Nafion-silica composite membrane 
 
Figure 6 shows the TEM micrograph of a 6 wt% Nafion-silica composite membrane. The silica 
particles are embedded in the Nafion matrix and range from small sizes (10nm) to large particle 
aggregates (~100 nm). These results suggest that the in-situ sol-gel process used generates varying 
chemical potentials in the Nafion matrix, thus leading to varying particles sizes. This point is further 
evidenced by the proton conduction results (Figure 3) as a function of the silica loading. The proton 
conductivity changes to values above and below those obtained for pure Nafion (0% silica loading). In 
principle, silica particles are neither particularly enhancing nor significantly impeding conductivity.  
 
Conclusions 
In situ sol-gel syntheses in the Nafion membrane showed no major improvements in terms of 
proton conduction and methanol crossover. However, functionalisation of particles by sulfonated silica 
particles at 10 wt% in Nafion allowed superior performance, in particular by a significant reduction in 
methanol crossover. Variation fo results may be related to particle size and aggregation which may 
change the regimes of proton conductivity and methanol diffusion. 
 
Acknowledgements 
The authors acknowledge the Australian Research Council for financial support. 
 
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Nanocomposite Nafion-Silica membranes for direct methanol fuel cells

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Nanocomposite Nafion-Silica membranes for direct methanol fuel cells

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