Analytical models provide useful information for researchers to study fuel cell function. In this paper, it’s aimed to present a 2D analytical model for direct ethanol fuel cell (DEFC) performance. The model included equations inside diffusion layer, catalyst layer, and Ethanol cross-over through membrane, which all have been solved. Analytical model has been validated by some experimental trials. The results showed that there is proper agreement between experimental and analytical curves. Furthermore, by increasing current density, cathodic over potential will remain zero but anodic over potential will increase up to certain value. The model showed that Ethanol concentration changes almost linearly inside anode channel.
Fuel cells are new power sources which produce electricity without any noise or environmental contamination. Fuel cells are used nowadays for rural, military portable and station applications [
1) Fluid flows in the steady state.
2) Fuel cell temperature is constant in the active area and chemical reaction takes place under constant temperature.
3) Reactants diffusion and transportation in catalyst layer along y direction is not considered.
4) Due to low diffusion layer thickness before its size long channel length, reactants concentration change across diffusion layer is ignored.
5) Reactants diffusion inside diffusion layer along y direction has been ignored because of low diffusion layer thickness before its length.
6) Because the thickness of membrane is so smaller than its thickness, reactants diffusion along y direction is ignored.
7) Due to smaller depth of ethanol and water transporttation channel than its length, reactants concentration change along x direction inside the channel has been neglected.
8) Pressure drop inside channel is neglected.
9) Fluid flows at constant velocity inside the channels.
Anode and cathode overall reaction is as follows
Ethanol concentration inside anodic channel, could be mentioned as below
Whereas is mass flux from anodic channel through diffusion layer. Based on Fick’s law, we can write the following equation for mass flux, whereas is diffusion coefficient of ethanol through diffusion layer
It can be noticed that for 12 M electron production, 1 M ethanol is consumed. Furthermore ethanol crossover lead to a part of ethanol permeate through membrane, so
Current density can be written as below
In which is anodic over potential and is crossover from membrane as below
The equations for cathode are similar to those for the anode, so
, current density is
That is cathodic over potential. Equations (2) and (3) can be written for cathode, thus
For oxygen concentration variation inside channel
At last, for fuel cell voltage and current density, following equations is determined
That is current density, membrane thickness, membrane conductivity, ideal electromotive force, and is electromotive difference rate. (Difference of exchanged gas moles between two sides of reaction in (1)) and (number of exchanged electrons) are constants which are –1 and 12 for (DEFC) respectively. Other symbols are listed in
In this solution, ethanol concentration in cathode layer in neglected (zero) and ethanol is linearly distributed, thus
and are substituted with, respectively, and then will be found
By substituting Equation (7) in Equation (9) and integrating from 0 to, oxygen concentration in catalyst layer is
By assuming and substituting Equation (8) in Equation (15)
Whereas is defined as follows
With regard to Equations (7), (8), (10) and (16) the below equation is acquired
whereas is
At last, by integrating Equation (18) and assuming, oxygen concentration inside anode channel will be found
Concentration of oxygen in the catalyst layer can be determined by substituting (20) into (16)
Substituting Equation (21) into (8) and integrating
Such trend can be implemented for anode side, thus Ethanol concentration variation inside anode channel could be written as follows
Ethanol concentration distribution in the catalyst layer is
Ethanol concentration average in the catalyst layer, by integrating (24) through to is
Average current density
, , are variants as below
Finally, by using Equations (14), (25), and (26), following equations between anode over potential and cur rent density is attained
Using Equations (14), (22), (25), following equation between cathode over potential and current density could be presented
By using Equations (11) and (12) and combining them with Equations (30) and (31) polarization curves will be obtained. It should be mentioned that these two equations are solved by numerical methods
After In this section analytical results will be compared with experiments. These experiments are performed under certain condition.
Pt/Ru/C catalyst was used for anode side and Pt/C black for cathode side. Catalyst loading on both sides was 4 mg/cm2. and Nafion 117 was used as membrane and flow channel wide and depth was 1mm. Cathode and anode flow channel pattern was 5 parallel and 2 parallel serpentine respectively. The cell was humidified by hot water for 2hours and activated by 1 M ethanol. Active area was 10 × 10 cm2 and back plates were made of aluminum.
For checking analytical model, because of some undefined coefficients, (assumed parameters in
Coefficients of analytical model are gathered in
Equation (23) foretells ethanol concentration variation inside anode channel exponentially, but based on
Over potential variation both for anode and for cath-
ode can be estimated Based on proposed analytical model. With regard to attained curve for anode over potential versus current density, by increasing current density, anodic over potential will increase, but for cathodic over potential, by increasing current density, cathodic over potential will remain approximately zero (Figures 7 and 8). These results are both for 0.5 M and for 0.25 M and match cathodic over potential results of G. Andreadis.
In this paper by an analytical 2D model, (DEFC) performance was predicted. This model is capable of estimating polarization curves up to 0.5 M. This model is precise in the first and second zone (Activation and Ohmic loss region), but in the third zone (Concentration loss region) because of neglecting concentration loss and increasing inlet ethanol concentration, model error will increase and it will have more difference with experimental curves. Based on model, ethanol concentration varies almost linearly inside anodic channel. By increasing current density cathodic over potential remains zero but anodic over potential will increase up to certain value.