methanol has the potential to be an efficient fuel for direct methanol fuel cells ( dmfcs ) with many applications ranging from small portable devices to large stationary power plants .
however , one major challenge that dmfc is facing is the fragility of the proton exchange membrane which is very thin in a dmfc .
the thickness is typically less than 100  m . a tiny crack or defect could fail the whole fuel cell .
ionic liquids ( ils ) can be a replacement of the conventional proton exchange membrane to overcome the risk of fragility .
used as the electrolyte , ionic liquids possess many advantages such as high ionic conductivity , chemical stability , and resistance to high temperature .
the current generation ionic liquids are stable even in the environment of moisture and air , making them ideal for general use as electrochemical media [ 3 , 4 ] . in developing il - based dmfc , the concentration of methanol must be controlled and monitored at several locations : the fuel feed , the electrolyte , the electrode assembly , and so forth .
it is critical to develop a fast sensing technique for methanol in the environment of ionic liquids . in this paper
the technique is based on potential step analysis and is reliable even when water is present in the ionic liquids .
chemicals and electrodes were purchased directly from the manufacturers or from venders such as fisher scientific and vwr : 1-butyl-3-methylimidazolium tetrafluoroborate ( basf quality , 98% ) from aldrich , methanol ( reagent grade acs ) from pharmco - aaper , phosphate buffer saline ( 25x ) from thermo scientific , l(+)-ascorbic acid ( 99% ) from acros , potassium tetrachloroplatinate ( ii ) 98% from aldrich , sulfuric acid 2.0  n from labchem , and gold disk electrode ( 2  mm dia . ) from ch instruments .
1-butyl-3-methylimidazolium tetrafluoroborate ( bmimbf4 ) is one of a few ionic liquids with relatively small viscosity , but it is still very viscous when compared to aqueous solutions .
the presence of trace amount of less viscous methanol may reduce the viscosity or increase the conductivity significantly . therefore , our most straightforward thinking was to sense methanol by the change in conductivity .
our thought was confirmed as the conductivity of bmimbf4 showed a linear relationship with the concentration of methanol in it . as shown in figure 1
, the conductivity would be a good indicator of methanol content if there was no interference of water .
secondly , water is produced on the cathode and it could cross over to the electrolyte and to the anode ( 2 ) .
thirdly , ionic liquids including bmimbf4 absorb water from the environment and the water content in bmimbf4 could reach as high as 0.29  wt% : 
 ( 1)anode :  ch3oh+h2oco2 + 6h++6e 
 ( 2)cathode :  32o2 + 6h++6e3h2o 
  water will also affect the conductivity of ionic liquids as reported before .
for this reason , the conductivity sensing method has little use in a dmfc . a practical method must be water - resistant .
we found a potential step method with pt - nanoparticle - coated au - nanoporous film ( pgnf ) was a reliable sensing method for methanol even when water was present .
methanol was oxidized on pgnf electrode when the potential step was applied ( 1.9  v versus ag / agcl with saturated kcl solution ) .
the current became smooth after 1 second and it was still measurable with amplitude of 175  a after 2 seconds .
our previous study has found that the adsorption / desorption and double layer charging were dominant within the first 2 seconds of potential step . in the following report ,
the potential was fixed at 1.9  v versus ag / agcl ( saturated kcl ) and the sampling time at 2 seconds . under the small
a / v ( electrode area to electrolyte volume ) condition and with a planar electrode , the current in a single potential step chronoamperometry can be predicted according to the cottrell equation : 
 ( 3)it = nfad01/2c01/2t1/2 . 
  in ( 3 ) , n is the number of electrons involved in redox reaction , f is the faraday constant , a is the electrode area , d 
 0 is the diffusion constant , c 
 0 is the bulk concentration , and t is the sampling time .
if t is fixed in the potential step analysis , the sampling current i(t ) should be directly proportional to the concentration c 
 0 or the concentration of methanol in bmimbf4 .
the current at the sampling time ( 2 seconds ) at various concentrations of methanol in bmimbf4 is shown in figure 3 .
the sampling current did not change too much when the methanol concentration was smaller than 1  m. desorption / adsorption and double layer charging were predominant at the sampling time when the concentration was low .
when the concentration was above 1  m , the electrooxidation of methanol became the predominant process and the sampling current had a linear relationship with the methanol concentration .
the potential step method described above not only had a good linearity but also was waterproof . when water was added to the methanol solution in bmimbf4 , it did not interfere with the oxidation of methanol .
the only effect of water was that it changed the molarity of methanol in bmimbf4 .
as shown in figure 4 , the linearity was not destroyed by the presence of water , even when the water content was as high as 2.4  m.
conductivity monitoring was a reliable method in sensing methanol in ionic liquids , but its use was limited to the situation without the presence of water . a potential step method with properly selected potential
step and sampling time could be a more practical method to analyze methanol concentration in ionic liquids especially when the methanol concentration was high so that the sampling current was predominantly contributed by the methanol oxidation .
 the development of direct methanol fuel cells required the attention to the electrolyte . 
 a good electrolyte should not only be ionic conductive but also be crossover resistant . 
 ionic liquids could be a promising electrolyte for fuel cells . 
 monitoring methanol was critical in several locations in a direct methanol fuel cell . 
 conductivity could be used to monitor the methanol content in ionic liquids . 
 the conductivity of 1-butyl-3-methylimidazolium tetrafluoroborate had a linear relationship with the methanol concentration . 
 however , the conductivity was significantly affected by the moisture or water content in the ionic liquid . on the contrary 
 , potential step could be used in sensing methanol in ionic liquids . 
 this method was not affected by the water content . 
 the sampling current at a properly selected sampling time was proportional to the concentration of methanol in 1-butyl-3-methylimidazolium tetrafluoroborate . 
 the linearity still stood even when there was 2.4  m water present in the ionic liquid . 
1. Introduction
2. Materials and Methods
3. Results and Discussion
4. Conclusions