2. EXPERIMENT
2.2 Experimental setup
In order to conduct the electrons, the gold wires are attached on the anode and cathode of membrane electrode assembly (MEA) by gold paste. Then, the MEA with gold wires is connected with a variable resistor, an ammeter (Keithley Model 6487) and a voltmeter as shown in Figure 2-7.
(a)
(b)
Fig. 2-7 (a) Schematic diagram and (b) Photograph of the experimental setup for characteristics of MEA.
Ammeter
Variable resistor
Voltmeter MEA
Chapter 3
Results and Discussions
3.1 SEM micrograph
3.1.1 Inverse pyramidal shaped silicon membrane
The inverse pyramid structures are made on silicon wafer after the wet etching process is performed on the front-side of the silicon wafer (step 2 in Figure 2-6). Results are shown in Figure 3-1. Figure 3-1(a) is the SEM cross-section imaging of a V-shaped groove on silicon wafer. To produce the nano-size pore, the tip of V-shaped groove must be sharp. Figure 3-1(b) shows the array of the V-shaped grooves arranged in order on the same sample.
In order to open the tip of the inverse pyramid (or V-shaped groove) and to form the V-shaped channels, the wet etching process is performed on the back-side of the wafer (step 5 in Figure 2-6). Figure 3-2(a) and 3-2(b) are images of the SEM cross-section and top view of a V-shaped channel, respectively. The depth of the V-shaped channel is around 250 µm and the angle between its base and lateral edge is 53.92°. This angle is very close to its theoretical value of 54.74° for (100) oriented silicon wafer. To reduce the fuel crossover through the membrane for the fuel cell application, the tip-opening diameter of all pores on the membrane was made to be smaller than 500 nm (see Figure 3-2(b)).
Fig.3-1. (a) SEM cross-section view of a V-groove after the anisotropic etch obtained using a water solution at 33% of KOH etch at 80 °C, while (b) refers to a sequence of more V-grooves defined on the same wafer.
(a)
(b)
(b) (a)
lateral edge length lateral edge
length
Fig.3-2. (a) Using 33% KOH solution etch wafer back at 80 °C to get pores and the size is around 400 nm (b) Top view of a 400 nm x 400 nm pore.
3.1.2 Proton exchange silicon membrane
Figure 3-3 is a SEM cross-section image showing the V-shaped channels filled with Nafion® prior to the reactive ion etching process for removing the excessive Nafion® from the membrane surface (step 6 in Figure 2-6).It is extremely difficult to cut the Nafion® filled membrane at the center of its V-shaped channel. During the cutting process, the Nafion® is peeled off and pushed out from the channel. In Figure 3-3, we were not able to cut the cross section of the channel at the center of its tip. Therefore, even though we have fabricated perfect V-shaped channels in the membrane (see Figure 3-2), the channels in Figure 3-3 seem like they are not all the way through the backside of membrane. However, we still can see the V-shaped Nafion® that is pushed out from and resting on the silicon membrane surface. The lateral edge length of V-shaped Nafion® is about 305 µm. When Figure 3-2(a) and Figure 3-3 are compared, we can notice that the lateral edge length of V-shaped Nafion® from Figure 3-3 is almost same as that of the V-shaped channel in Figure 3-2(a). For this reason, we believe that the Nafion® was able to diffuse down to the tip of the channel during the spin coating process.
The reactive ion etching process was able to remove the most of excessive Nafion® that are present on the membrane surface. After this reactive ion etching process, the filled Nafion® in the channel was not level with the silicon surface of the membrane frame. Instead, it formed a puddle in its center (step 7 in Figure 2-6).
The thickness of palladium layer and platinum layer are around 75 nm and 30 nm (estimated from atomic force microscope), respectively. Figure 3-4 is the photograph of membrane electrode assembly.
Fig.3-3. SEM cross-section imaging of a Nafion® filled inverse pyramid structure silicon membrane.
Nafion
®Silicon
Fig.3-4. Membrane electrodes assembly (MEA) after depositing the catalyst layers using E-gun evaporation process. Scale comparison with a penny.
3.2 Swelling test
In order to check the swelling condition, the reactive ion etched membrane was immersed into DI water. Figure 3-5(a) and 3-5(b) are the optical microscope images of the membrane before and after immersing it into the DI water for three days, respectively. Even though the swelling occurred, the palladium layer is still undamaged. As it was mentioned earlier, the filled Nafion® in the channel was not level with the silicon surface of the membrane frame. Therefore, the surface of Pd thin film was not flat; instead it was concave down at the center of the channel as shown in Figure 3-5(a). In Figure 3-5(a), the optical image is brighter at the center because its bottom is open and the Nafion® is transparent as the light passes through.
While this membrane hydrated in the DI water for three days, the filled Nafion® swelled in the V-shaped channel. A decrease in the contrast at the center of image in Figure 3-5(b) can be observed when compared to that of Figure 3-5(a). This indicates that the height difference in the channel decreases due to the swelling of the filled Nafion®. In other words, as the Nafion® swelled, it expanded and the depth of the puddle decreased as shown in Figure 3-5(b).
During the Nafion® swelling, the Pd catalyst layer did not rupture because its thickness is only 75nm and this thin Pd layer was flexible enough to sustain the pressure inflicted by the expanding Nafion®. Similar to the Pd layer, the Pt catalyst layer was very thin (30 nm) and it did not rupture during the hydration process (Not shown in Fig).
3.3 Current-voltage (I-V) characteristics and power density
Formic acid is used as the fuel to obtain current-voltage (I-V) characteristics and power density curve. The current-voltage (I-V) characteristics of the proposed membrane electrode assembly can provide valuable information about the mechanism as described in Section 1.4.2.
The current and voltage are measured by an external resistor and an ammeter in series and a voltmeter in parallel, respectively. Measurements are carried out at room temperature. The current-voltage (I-V) characteristics of the single membrane electrode assembly are operated in 5M formic acid/air breathing with and without 0.5M sulfuric acid as shown in Figure 3-6 for various value of resistance.
Original After
(a) (b)
Si Si
Original After
(a) (b)
Si
Si SiSi
Fig.3-5. The optical microscope imaging of (a) the original membrane (b) after immersing into DI water for three days. The schematic cross-section diagram as below.
Fig.3-6. 5M formic acid/air and 5M formic acid with sulfuric acid/air (a) cell potential and (b) power density curves.
(a)
(b) 0
0.1 0.2 0.3 0.4 0.5 0.6
0 20 40 60 80 100 120 140 160
Cell potential (V)
Current density (mA/cm2)
5M FA (10ml)
5M FA (10ml) + 0.5M H2SO4 (2.5ml)
0 5 10 15 20 25
0 20 40 60 80 100 120 140 160
Power density (mW/cm2)
Current density (mA/cm2)
5M FA (10ml)
5M FA (10ml) + 0.5M H2SO4 (2.5ml)
In Figure 3-6, the open circuit potential and current density are increased by adding few amount of sulfuric acid into formic acid. The probable reason is that the V-shaped channels of silicon membrane may not be filled with Nafion® entirely, so the proton conductivity is improved a lot after adding few amount of sulfuric acid. With the condition for adding sulfuric acid, we obtain a current density of about 145 mAcm−2 in minimal charge and 600 mV for open circuit potential, and the maximal power density can reach 23 mWcm-2.
The current-voltage (I-V) characteristics of the single membrane electrode assembly are operated in 5M, 8M, and 12M formic acid with 0.5M sulfuric acid/air breathing as shown in Figure 3-7.
Fig.3-7. 5M, 8M, and12M formic acid with sulfuric acid/air current density vs.
feed concentration: (a) cell potential and (b) power density curves.
0 12M FA (10ml) + 0.5M H2SO4 (2.5ml)
(b) 12M FA (10ml) + 0.5M H2SO4 (2.5ml)
(a)
Figure 3-7 shows the comparison of current-voltage curves and power density curves in different concentrations of formic acid. When the concentration of formic acid is increased, the open circuit potential is decreased. A possible explanation is that the dehydration of Nafion® membrane results in the increase of resistance in fuel cell, due to the fact that the higher feed concentrations are almost devoid of water.
Chapter 4
Conclusions and Future works
4.1 Conclusions
In this study, we successfully fabricate a silicon based porous membrane with the V-shaped nano-size channels for portable fuel cell applications. One of its advantages is the total compatibility with silicon microfabrication process; which will allow us to reduce the cell size furthermore and mass produce it at a low cost in the future. The inverse pyramid shaped channels in silicon membrane is advantageous to avoid surface tension and swelling problem. The micro fuel cell had been characterized by feeding formic acid with sulfuric acid and the max power density is around 23 mWcm-2.
4.2 Future works
We will functionalize the surface of each pore with acidic functional groups using various surface chemistry. By controlling the aspect ratio of our V-shaped channel and reducing its tip diameter down to the order of 10 nm, we hope to manage the fuel crossover problem for future micro fuel cells.
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