NANO LETTERS
From Nanochannel-Induced Proton Conduction Enhancement to a Nanochannel-Based Fuel Cell
2005 Vol. 5, No. 7 1389-1393
Shaorong Liu,*,† Qiaosheng Pu,† Lin Gao,† Carol Korzeniewski,† and Carolyn Matzke‡ Department of Chemistry and Biochemistry, Texas Tech UniVersity, Lubbock, Texas 79409, and Sandia National Laboratories, Albuquerque, New Mexico Received April 17, 2005; Revised Manuscript Received May 25, 2005
ABSTRACT The apparent proton conductivity inside a nanochannel can be enhanced by orders of magnitude due to the electric double layer overlap. A nanochannel filled with an acidic solution is thus a micro super proton conductor, and an array of such nanochannels forms an excellent proton conductive membrane. Taking advantage of this effect, a new class of proton exchange membrane is developed for micro fuel cell applications.
As the dimensions of a microfluidic channel enter the nanometer regime, unique properties emerge. Examples of these properties include parabolic and reduced electroosmotic flow,1,2 increased viscosity,3 decreased dielectric constant,3 rapid current decrease,4,5 ion enrichment and ion depletion,6 peculiar meniscus shape,7 etc. In this work, we report another nanochannel-induced effectsthe proton conduction enhancement. Taking advantage of this effect, we also describe the development of a new class of proton conductive membrane, an array of nanochannels, for fuel cell applications. Electric double layers (EDLs) will overlap in a nanochannel. Based on the Gouy-Chapman model, the EDL thickness is inversely proportional to the square root of the electrolyte concentration.8 For a glass-water interface with electrolyte concentrations varying from 10-2 to 10-6 M, the EDL thickness changes from 3 to 300 nm.8,9 Therefore, the EDLs on the opposite sides of a nanochannel will overlap when the channel dimension is in the nanometer regime. One of the consequences of this overlap is the increased counterion concentration and decreased co-ion concentration in the nanochannel.6 It is the increased counterion concentration that results in the enhancement of the counterion conduction. Obviously, this effect strengthens with the extent of EDL overlap. To determine the effect of the channel depth on proton conduction enhancement, we fabricated a nanochannel device as presented in Figure 1. A standard photolithographic process and a Cr photomask were used for nanogroove * Corresponding author E-mail:
[email protected]. † Texas Tech University. ‡ Sandia National Laboratories. 10.1021/nl050712t CCC: $30.25 Published on Web 06/07/2005
© 2005 American Chemical Society
fabrication. The photomask contained 55 parallel lines that were 1-mm-long and 100-µm-wide and evenly spaced with a center-to-center distance of 200 µm. For fabrications of nanogrooves with depths of e1 µm, the process was similar to that described previously.6 Briefly, a borofloat glass wafer (Precision Glass and Optics, Santa Ana, CA) was cleaned, dried, and then coated with a thin layer of photoresist (Shipley1818, Shipley, Santa Clara, CA) using an EC101D spinner (Headway, Garland, TX). The photoresist was then soft-baked at 90 °C for 15 min. The channel pattern from the photomask was then transferred to the photoresist film using an ABM aligner (ABM, San Jose, CA). After the exposed photoresist was dissolved in a developer solution and the wafer was rinsed, the remaining photoresist on the wafer was hard-baked at 150 °C for 2 h. Ater the wafer was cooled, it was immersed in a 12% hydrofluoric acid solution at ambient temperature for a given period of time to etch the nanogrooves onto the wafer. After the residual photoresist was removed using a piranha solution (4 parts of 96% sulfuric acid and 1 part of 33% hydrogen peroxide), the wafer was rinsed with water, dried with nitrogen, and the groove depth was measured on an Alpha Step 200 profilometer (Tencor Instruments, Mountain View, CA). For production of grooves with depths of >1 µm, a sacrificial layer of Cr/Au was used to facilitate the groove etching. The procedure was pretty routine and it has been described in the literature.10,11 After four through-holes were drilled at the ends of the U-grooves, referring to Figures 1a and 1b, the structured wafers were aligned and thermally bonded together at 600 °C for 3 h.
Figure 2. Conductance normalization. (a) Normalization for microfabricated channels; and (b) normalization for circular capillaries. Figure 1. Schematic diagram of the nanochannel device. (a) Two U-shaped channels (1-mm-wide and 100-µm-deep) were micromachined on the bottom surface of a silica substrate, and four through-holes were drilled at the ends of the U-channels. (b) An array of 55 parallel nanochannels (1-mm-long and 100-µm-wide) with a depth between 50 nm to 50 µm was etched on the top surface of another silica substrate. (c) After the two substrates were aligned and bonded, the two U-channels were connected by the nanochannels.
The groove depths measured on the profilometer were taken as the channel depths after bonding. A bonded chip was diced across the channels and the channel depths were measured under a scanning electron microscope (SEM).6 No significant differences were observed between the original profilometric groove depths and the post-bonding microscopically measured groove depth, within the measurement uncertainties.6 Capillaries were also used in this experiment for the conductance measurements. The lengths of the capillaries were identical (10 mm), while the diameters were different (5, 50, 75, 100, and 250 µm). For the data presentations in Figures 3 and 4, the capillary diameters were taken as the channel depths. All channel surfaces (including the capillary inner walls) were derivatized with -SO3H groups by a two-step reaction. First, a toluene solution containing 10% mercaptopropyltrimethoxysilane and 0.1% acetic acid was flushed through the nanochannels for 12 h, which attached -SH groups to the silica surface. Then, an aqueous solution containing 3% H2O2 and 1 M HNO3 was forced through the nanochannels for 18 h, converting the -SH groups to -SO3H groups. The conductance data in various channels were measured using either 1 mM or 10 µM HClO4 solution. Because the channels used for measurements had different cross-section profiles (see Figure 2), the measured conductance data were normalized for proper comparison. Referring to Figure 2, a 1390
Figure 3. HClO4 conductance as a function of channel depth. The diamond symbols indicate the conductance data obtained using 1 mM HClO4, while the circular symbols represent the conductance value obtained using 10 µM HClO4. The data points from d ) 50 nm to 50 µm were measured with microfabricated channels, and those from d ) 5 µm to 250 µm were measured with capillaries. The middle two points (d ) 5 and 50 µm) were averages of the microchannel and capillary data. When capillaries were used, their inner walls were derivatized with -SO3H, and their diameters were used as the channel depth.
normalized channel was created for each actual channel used in this experiment. All normalized channels had the same width (100 µm) and length (1 mm) but maintained the depths of the actual channels. The conductance data obtained from the actual channels were converted to the equivalent values in the normalized channels. Figure 3 presents the normalized conductance as a function of the channel depth. The conductance decreases with the channel depth linearly from 250 µm to 1.5 µm, representing the property of a bulk solution. From 1.5 µm to 50 nm, the conductance decrease decelerates progressively, deviating from the normal behavior of a bulk solution. Figure 4 exhibits the effect of channel depth on the proton conductivity. The proton conductivity was calculated directly from the conductance data presented in Figure 3 using the Nano Lett., Vol. 5, No. 7, 2005
Figure 4. Effect of channel depth on H+ apparent conductivity. The diamond symbols represent the conductivity values obtained using 10 µM HClO4, while the circular symbols indicate the conductivity data obtained using 1 mM HClO4. All data in this Figure were converted directly from the data in Figure 3.
proton concentration in the bulk solution, that is, [H+] ) [HClO4]. Since the proton concentration in the nanochannel was increased due to the EDL overlap, these conductivities are thus referred to as the apparent proton conductivities. In deep channels (d g 5 µm), the apparent proton conductivity was close to its theoretical value in a bulk dilute solution. These results also indicated that the test solutions were not contaminated (e.g., by absorbing the ambient NH3, CO2, etc.). When a solution is at µM concentration level, care must be taken to avoid solution contaminations. In shallow channels (d e 0.5 µm), however, the apparent proton conductivity increases considerably with the decrease of channel depth. In a 50-nm-deep channel filled with 10 µM perchloric acid, the apparent proton conductivity is enhanced by a factor of 30, while in the same channel filled with 1 mM perchloric acid, it is enhanced by a factor of 10. It is interesting to notice that good linear relationships exist between log(apparent conductivity) and log(d) from d ) 50 nm to 1500 nm. A linear relationship of log(apparent conductivity) ) -0.753 log(d) + 1.28
(1)
was obtained with a linear coefficient of r2 ) 0.999, when 10 µM perchloric acid was used as the test solution. By extrapolating this relationship to 5-nm-deep channels, it is predicted that the apparent proton conductivity will be enhanced by a factor of 170. When 1 mM perchloric acid was used as the test solution, the linear relationship changed to log(apparent conductivity) ) -0.728 log(d) + 0.874
(2)
with a linear coefficient of r2 ) 0.998. Extrapolation of this relationship predicts a 55-fold conductivity enhancement. It is worth mentioning that the apparent proton conductivity started increasing at a channel depth of ∼1-2 µm. Based on the theoretical models,8,9 the EDL thicknesses were estimated to be approximately 10 nm in 1 mM perchloric acid and 100 nm in 10 µM perchloric acid. Therefore, EDL overlap should not have occurred in these micrometer-deep Nano Lett., Vol. 5, No. 7, 2005
channels. There are two potential explanations to this observation: (a) EDL overlap is not necessary for the proton conduction enhancement to occur, and (b) the surface derivatization has increased the zeta potential, which in turn extended the EDL thickness. To examine whether the surface derivatization had increased the zeta potential, we measured the electroosmotic flows that are proportional to the zeta potential in both derivatized and underivatized capillaries. Based on the electroosmotic flow (EOF) data (∼1 × 10-4 cm2 V-1 s-1), the zeta potential in a derivatized capillary filled with 1 mM perchloric acid was comparable to that in an underivatized capillary filled with 1 mM HAc-NaAc solution (pH 4.7). This suggests that the surface derivatization did not increase the zeta potential under the experimental conditions. Therefore, we assumed preliminarily that the proton conduction enhancement happened before EDL overlap occurred. With the traditional definition of the thickness of an EDL, EDL overlap is not necessary for the proton conduction enhancement effect. To explain this, let us recall the expression of the potential in an EDL region.8 φ ) φ0e-κx
(3)
where φ0 is the potential at the solid surface, and φ is the potential at a distance of x from the solid surface. These potentials are measured with respect to the bulk solution, and their values will be negative when the counterions are protons. The proton concentration is directly related to the potential. [H+] ) [H+]0e-qφ/kT
(4)
where [H+]0 is the proton concentration in the bulk solution, q is the charge of the electron, k is the Boltzmann constant, T is the absolute temperature, and [H+] is the proton concentration at potential φ. The thickness of the EDL is defined as λ)
1 κ
(5)
With this definition, significant potential (0.37φ0) is still present at the “edge” (x ) λ) of an EDL. Based on eqs 3 and 4, if the proton concentration at a silica channel surface is 10[H+]0, it will be 2.3[H+]0 at x ) λ (the edge of the EDL), 1.37[H+]0 at x ) 2λ, and 1.12[H+]0 at x ) 3λ. That is, the increased proton concentration region extended much further than the thickness of an EDL. Therefore, the proton conduction enhancement effect will occur outside the traditionally defined EDL boundaries. Additionally, the apparent proton conductivity is the average of the proton conductivity over the entire crosssectional area. As long as the EDL regions occupy a significant portion of the cross-sectional area, they will contribute the proton conduction enhancement. An increase 1391
in the apparent proton conductivity will be detected, even if EDL overlap did not occur. The enhanced proton conduction in nanochannels is important to fuel cell applications. The increasing societal demand for portable electrically powered devices has resulted in a surge of research on micro fuel cells.12,13 A key component in this type of fuel cell is a proton conductive membrane. The basic requirements of the membrane are high proton conduction, good mechanical/thermal strength, and low fuel crossover.14,15 As we have presented above, a nanochannel filled with an acidic solution is a tiny super proton conductor. An array of such nanochannels forms an excellent proton conductive membrane. If 1-mm-long nanochannels are employed (equivalent to a 1-mm-thick membrane), the mechanical/thermal strength of the membrane will be improved dramatically, compared to micrometerthick Nafion membranes.13 Due to the diminished diffusion16 and reduced EOF2 in the nanochannels, nanochannels are effective in suppressing fuel crossover. An excellent feature of this nanochannel-based proton exchange membrane is that, by decreasing the nanochannel depth, the apparent proton conductivity will increase and the fuel crossover will decrease. Therefore, reducing the nanochannel depth is an effective means to promote the performance of a nanochannel-based proton exchange membrane. To demonstrate the feasibility of such a nanochannel-based proton exchange membrane for fuel cell applications, we utilized the same device as presented in Figure 1, with a nanochannel depth of 50 nm. The fuel was an aqueous solution containing 1.0 M methanol in 1.0 mM H2SO4, and the oxidant was a 1 mM KMnO4 solution. In conventional fuel cells, high concentrations of acids in the fuel solutions are used to improve the proton conduction. Using a nanochannel-based proton conductive membrane, high concentrations of acids are not necessary because of the enhanced proton conductivity in nanochannels. In fact, high concentrations of acids are not desired since they will reduce the EDL thickness and hence the degree of the proton conduction enhancement. Referring to Figure 1, as the fuel was introduced into one U-channel, the nanochannels were automatically filled with the fuel by capillary action. The other U-channel was then filled with the oxidant. A Ru/Pt (Pt deposited with Ru) electrode was inserted into the fuel solution while a Pt electrode was placed in the oxidant solution. After the device was warmed to 60 °C, the two electrodes were connected externally through a variable resistor, and the cell voltage and current were measured as a function of the external resistance. Figure 5 presents the polarization curve and the power output performance of this nanochannel-based fuel cell. A maximum power density of about 130 mW/cm2 was obtained. It should be noted that the active cross-sectional area of the nanochannels was used for the calculations of the current and power densities. Since the nanochannels were separated by the same distance as the nanochannel width, these densities will be reduced by 50% if the cross-sectional area of the nanochannels and the walls between the nanochannels were used for the calculations. The current and power densities can be amplified by 1392
Figure 5. Performance of a nanochannel-based fuel cell. Fuel: 1.0 mM methanol in 1.0 mM H2SO4; Oxidant: 1.0 mM KMnO4. The test was performed at 60 °C. The current density was calculated using the active cross-sectional area of the nanochannels.
decreasing the length of the nanochannels (the distance between the two U-channels), but the expense will be an increased fuel crossover. In summary, we have, for the first time, described and systematically investigated the nanochannel-induced proton conduction enhancement effect and demonstrated that this effect exists before EDL overlap occurs. The apparent proton conductivity in a nanochannel can be enhanced by orders of magnitude, relative to that in a bulk solution. We have also developed a new class of proton exchange membranesan array of nanochannelssfor micro fuel cell development. Compared to a polymeric electrolyte membrane, the nanochannel-based proton exchange membrane has three major advantages: (i) it provides high proton conductivity, (ii) it has improved mechanical strength, and (iii) it is capable of operating at elevated temperatures. In addition, due to common materials and manufacturing processes, a nanochannel-based fuel cell can be monolithically integrated with other micro/nanofluidic devices and microelectromechanical systems, enabling high-performance systems on a single platform. One practical limitation of a nanochannel-based fuel cell with the current one-dimension-array configuration is the limited power output (3 nW for the device presented in Figure 1c), although the power density is high. This limitation can be improved by increasing the number of the parallel channels. For example, 105 nanochannels with a width of 100 µm and a length of 1 mm can be arranged in a 10 × 10 cm2 area, and hundreds of this device can then be stacked together. The entire assembled fuel cell will occupy a space of 10 × 10 × 10 cm3 and generate a power of >100 mW. Another way to address this issue is by utilizing two-dimension-array nanochannel membranes such as the nanoporous membranes produced by the track-etched method17-20 and anodic synthesis.21 Pore densities of as high as 1011 pores/cm2 have been achieved.22 There is an ongoing project in our group toward increasing the fuel cell power output, and the results will be published later. Acknowledgment. The authors thank Dr. Henryk Temkin in the Department of Electrical Engineering and Dr. Jordan Nano Lett., Vol. 5, No. 7, 2005
Berg in the Department of Chemical Engineering at Texas Tech University for valuable discussions. Some of the devices used in this experiment were fabricated in the Nano Tech Center at Texas Tech University. The project is partially supported by the Texas Tech Interdisciplinary SEED fund. References (1) Rice, C. L.; Whitehead, R. J. Phys. Chem. 1965, 69, 4017-4024. (2) Ramsey, M. J. Proceedings of the µTAS 2002 Symposium; Baba, Y., Shoji, S., van den Berg, A., Eds.; Kluwer Academic: Dordrecht, The Netherlands, 2002; pp 3-7. (3) Hibara, A.; Saito, T.; Kim, H.; Tokeshi, M.; Ooi, T.; Nakao, M.; Kitamori, T. Anal. Chem. 2002, 74, 6170-6176. (4) Kobayashi, Y.; Martin, C. R. Anal. Chem. 1999, 71, 3665-3672. (5) Lazar, I. M.; Karger, B. L. Anal. Chem. 2002, 74, 6259-6268. (6) Pu, Q.; Yun, J.; Temkin, H.; Liu, S. Nano Lett. 2004, 4, 10991103. (7) Tas, N. R.; Mela, P.; Kramer, T.; Berenschot, J. W.; van den Berg, A. Nano Lett. 2003, 3, 1537-1540. (8) Bard, A. J.; Faulkner, L. R. Electrochemical methods: fundamentals and applications; John Wiley and Sons: New York, 2001; pp 546549. (9) Wallingford, R. A.; Ewing, A. G. AdV. Chromatogr. 1989, 29, 1-76. (10) Liu, S. J. Chromatogr. A 2003, 1013, 57-64.
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