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A novel proton exchange membrane based on sulfo functionalized porous silicon for monolithic integrated micro direct methanol fuel cells
Accepted Manuscript Title: A novel proton exchange membrane based on sulfo functionalized porous silicon for monolithic integrated micro direct methanol fuel cells Authors: Mei Wang, Litian Liu, Xiaohong Wang PII: DOI: Reference:
S0925-4005(17)31192-9 http://dx.doi.org/doi:10.1016/j.snb.2017.06.173 SNB 22644
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
17-4-2017 15-6-2017 26-6-2017
Please cite this article as: Mei Wang, Litian Liu, Xiaohong Wang, A novel proton exchange membrane based on sulfo functionalized porous silicon for monolithic integrated micro direct methanol fuel cells, Sensors and Actuators B: Chemicalhttp://dx.doi.org/10.1016/j.snb.2017.06.173 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A novel proton exchange membrane based on sulfo functionalized porous silicon for monolithic integrated micro direct methanol fuel cells
Mei Wang a, b, Litian Liu a, Xiaohong Wang a * a
Institute of Microelectronics, Tsinghua National Laboratory for Information Science and
Technology, Tsinghua University, Beijing 100084, P.R. China b
School of Computer and Communication Engineering, Zhengzhou University of Light
Industry, Zhengzhou, 450000, P.R. China
* Corresponding author: Xiaohong Wang, Ph.D. Institute of Microelectronics, Tsinghua University Beijing, 100084, China Phone: 86-10-62798432 Fax: 86-10-62771130 Email: [email protected]
Highlights A novel solid PEM based on sulfo functionalized porous silicon is presented. Chemical grafting method is designed to prepare the high proton-conductive PEM. The PEM shows higher proton conductivity and lower resistance than Nafion® 117. A monolithic integrated micro fuel cell based on porous silicon is obtained for the first time. 1
Abstract A novel sulfo functionalized porous silicon membrane is presented, which can be used as proton exchange membrane (PEM) for monolithic integrated micro direct methanol fuel cells (μDMFCs). The porous silicon membrane is functionalized with sulfonic acid groups (-SO3H) to induce proton conductivity via chemical grafting method. Electrical impedance analyses demonstrate that the sulfo functionalized porous silicon membranes exhibit excellent proton exchange capability, achieving a proton conductivity of 0.082 S/cm, which is 1.37 times larger than the value acquired on the most widely used Nafion® 117 membrane (0.060 S/cm). With the in-situ synthesis of 3D platinum nanoflowers on porous silicon presented in our previous work, a monolithic integrated micro fuel cell based on porous silicon is obtained for the first time, which achieves a power density of 5.5 mWcm-2. As such, we believe this work is more attractive for optimizing the integration and sizes of silicon-based μDMFCs.
Keywords: Proton exchange membrane (PEM), sulfo functionalized porous silicon, chemical grafting method, monolithic integrated micro direct methanol fuel cells (monolithic integrated μDMFCs)
1 Introduction The demand of durable and integrated power sources increases radically with rising functionalities and decreasing sizes of portable devices. Among the various fuel 2
cells, the micro direct methanol fuel cell (µDMFC) is considered as the most promising candidate power source, with the advantages of high energy density, low pollution, room temperature operation, simple and safe handling [1-2]. For the application of µDMFCs, their sizes and costs have to be comparable to the lithium ion batteries, which are the most widely used power sources for portable devices. Silicon-based µDMFCs have drawn much attention because they can make use of the micro fabrication techniques developed for integrated circuit (IC) and micro electromechanical systems (MEMS) to optimize their sizes and costs. In recent years, varieties of silicon-based µDMFCs [3-8] and their related components [9-17] have been developed. A typical silicon-based µDMFC generally consists of an anode, a cathode, and a membrane electrode assembly (MEA) sandwiched in the middle. The MEA is the most important component of µDMFC, which is made up of diffusion layer, catalyst layer and proton exchange membrane (PEM). Due to the thickness of each stacked electrode and the inter-electrode space, in addition to complicated fabrication and assembly process, such silicon-based µDMFCs require a large volume, which results in small volumetric power densities. Therefore, the power density for such fuel cells is commonly reported in terms of electrode surface area instead of volume, which is often the case for other power sources [18]. In order to address the issue, a monolithic integrated µDMFC is a promising way, even the biggest challenge is incompatibility of PEM and electrode plate fabricated by silicon. Nafion® membranes, the most popular PEM for DMFCs, are organic polymer, which lead to several limits in the applications of silicon-based μDMFCs: 1) they are 3
incompatible with micro fabrication techniques, in addition to the electrode plate; 2) their shapes vary in response to their water content due to the swelling of the polymer, thus the catalysts often fall off in practical operations; 3) their thicknesses are nearly 200 μm subject to durability and reactant crossover, which increase the resistances and sizes and go against the performance and miniaturization of µDMFCs. Therefore, developing a new type of PEM has been a key issue for monolithic integrated µDMFCs. One common approach is introducing proton-conductive molecules into porous solid skeleton materials which have high specific surface area and good mechanical stability. The solid skeleton material is used to control the deformation and decrease the thickness of PEM, demonstrating an improved performance of µDMFCs. Some materials have been used as a skeleton to fill or graft electrolyte, such as poly(tetrafluoroethylene) (PTFE) [19-21], cross-linked polyethylene (CLPE) [22], polyimide [23-25], poly(ethylene-co-tetrafluoroethylene) (ETFE) [26-28], and porous silicon [29-36]. Among the various solid skeleton materials, porous silicon is considered to be best candidate in silicon-based micro fuel cells due to its inherent properties, including 1) porous silicon is compatible with micro fabrication techniques and silicon electrode plate; 2) the route of proton transmission can be heavily shorten in porous silicon, compared with PTFE, CLPE, and ETFE, etc., consequently, the proton exchange resistance can be diminished significantly. That is because the holes of porous silicon can be in form of vertical through-holes by adjusting the parameters of anodization; 3) the chemical grafting can be facilely realized by single-step silanization process, while that is relatively complex for other skeleton materials. 4
Millions of hydroxyl groups (-OH) can be attached on the walls of porous silicon nanoholes, which can crosslink together with Si-OHs in organics via silanization process. For the other skeleton materials, the chemical grafting process need extra cross-linking agent [19-25] or radiation [26-28], etc., which need relatively complex treating processes; 4) porous silicon has high specific surface area on the order of hundreds of m2/g, therefore, more sulfo can be grafted on the walls of porous silicon nanoholes, as a result, the proton conductivity can be improved. Due to above reasons, developing a PEM based on porous silicon shows great potentials for optimizing the sizes of silicon-based µDMFCs and designing a novel monolithic integrated µDMFC. We proposed a strategy of monolithic integrated micro fuel cells based on porous silicon, as shown in Fig. 1. The monolithic integrated micro fuel cell combines PEM, Pt nanocatalysts and current collector layer together. PEM is achieved by sulfo functionalized porous silicon membrane and 3D Pt nanoflowers are synthesized in situ onto it as nanocatalysts. Ag nanowires film serves as current collector layer. Porous silicon serves as PEM and catalyst support simultaneously. Compared with the classical µDMFCs with “sandwich” structure, the monolithic integrated µDMFC shows several key advancements, including 1) simplifying the structure of fuel cells, 2) compatible with micro fabrication techniques and suitable for batch production, 3) solid PEM with little deformation and high proton conductivity, 4) in-situ synthesis of 3D Pt nanoflowers on a fuel cell body without the extra coating process. 3D platinum nanoflowers have been synthesized in situ on porous silicon in our previous work [37]. However, designing of a PEM based on 5
porous silicon is still a challenging work, probably because there are several key issues which have not been solved, consisting of 1) fabrication of the porous silicon membrane with vertical through-holes; 2) chemical grafting sulfo in the walls of porous silicon nanoholes; 3) achieving high proton conductivity by controlling the chemical grafting process.
Herein, we propose a novel solid PEM based on sulfo functionalized porous silicon. We start by presenting the fabrication process of the PEM based on porous silicon, followed by characterizations in order to demonstrate its morphology and performance. Finally, we demonstrate a monolithic integrated µDMFC based on sulfo functionalized porous silicon for the first time, combining in-situ synthesis of 3D platinum nanoflowers.
2 Experimental 2.1 Chemicals and materials 4 inch silicon wafer was purchased from MCL Electronic Materials, Ltd. 3-mercaptopropyltrimethoxysilane (MPTMS, (CH3O)3SiCH2CH2CH2SH, ≥ 98 wt. %) was bought from Nanjing Jingtianwei Chemical Co., Ltd. Benzene (≥ 99.5 wt. %) was obtained from Beijing Chemical Works. Glacial acetic acid (GAA, ≥ 99.5 wt. %) was got from Modern Oriental (Beijing) Technology Development Co., LTD. Ethyl alcohol (≥ 99.8 wt. %), methanol (≥ 99.8 wt. %), hydrofluoric acid (40 wt. %), hydrogen peroxide (40 wt. %), sulfuric acid (98 wt. %) and nitric acid (70 wt. %) were purchased from Tianjin Fengchuan Chemical Reagent Science and Technology 6
Co., Ltd. Nafion® 117 membrane and Nafion solution (5%) were bought from DuPont Co. The chemical regent H2PtCl6·6 H2O was got from Sinopharm Chemical Regent Co., Ltd. The Ag nanowires (length of 20~60 µm and diameter of 40~60 nm) dispersion with a 20 mg/mL concentration was got from XFNANO Materials Tech Co., Ltd. (Nanjing, China). Millipore water (18.2 MΩ·cm) provided by a Milli-Q Labo apparatus (Merck Millipore Ltd.) was used in all experiments.
2.2 Fabrication of PEM based on sulfo functionalized porous silicon 2.2.1 Fabrication of porous silicon membranes An antimony-doped 0.02 Ω cm n+-type (100) oriented silicon wafer was used to process silicon membranes [33]. Thermal oxide (100 nm) and LPCVD Si3N4 (160 nm) were deposited on both sides of the wafer as the mask of KOH etching and anodization. Double-side lithography was introduced to transfer the pattern and KOH etching followed. The morphology, pore size, and porosity of porous silicon can be controlled during the anodization process. In this paper, anodization was conducted in ethanolic HF solution (HF: ethyl ethanol = 2:1) at a constant current density of 200 mA/cm2 for 15 min, to obtain the porous silicon membranes with proper pore size and porosity. Inductively coupled plasma (ICP) was introduced to etch the residual silicon at the back side and unordered porous silicon at the frontal side, to fabricate the porous silicon membranes with vertical through-holes. Eventually, the regular porous silicon membranes with the thickness of 100 μm were achieved.
7
2.2.2 Functionalization of porous silicon membranes Proton conductivity, which directly correlates to the corresponding cell performance, could be the most significant characteristic of PEM for µDMFCs. The proton conductivity of sulfonated polymers is highly dependent on their contents of sulfonic acid groups. The property of porous silicon makes it feasible to graft more -SO3H groups because of its high specific surface area and good surface property. We notice that the silane coupling agent can be facilely grafted onto the walls of porous silicon nanoholes via silanization process in the presence of trace water [38-41], therefore a high proton-conductive PEM based on porous silicon can be developed by chemical grafting method. MPTMS was selected as a grafting material because it is terminated with mercapto groups, which can be oxidized to sulfonic acid groups easily. Porous silicon membranes were functionalized with sulfonic acid groups via chemical grafting method, which was done in four steps: 1) Hydrophilization process: hydroxyl groups (-OH) were attached to walls of porous silicon nanoholes by hydrophilization process in a mixed solution of H2SO4 and H2O2 at the volume ratio of 3:1 for 30 minutes at 60 ºC. Simultaneously, Si-O-CH3 groups in MPTMS molecule hydrolyzed to form Si-OH groups in mixed benzene solutions, with the MPTMS concentration of 30 wt. % and GAA concentration of 5 wt. %. 2) Adsorption process: Si-OH groups in hydrolyzed MPTMS were absorbed onto the walls of porous silicon nanoholes by chemical impregnation of hydrophilic porous silicon membrane from the mixed benzene solutions; 3) Grafting process: mercapto groups were grafted 8
via dehydration condensation reaction in the mixed benzene solution with ultrasonically activating for 48 hours; 4) Sulfonation process: mercapto groups were oxidize to sulfonic acid groups in an 30 wt. % HNO3 aqueous solution for 3.5 hours with ultrasound at room temperature. Eventually, a functionalized porous silicon membranes were obtained, which were proton-conductive as a result of sulfonic acid terminal, shown as Fig. 2.
2.3 Fabrication of monolithic integrated µDMFCs The fabrication process of the monolithic integrated µDMFC divided into four steps [42]: 1) Fabrication of porous silicon membranes; 2) in-situ synthesis of 3D Pt nanflowers on porous silicon membranes using by the method proposed in our previous work [37]; 3) functionalization of porous silicon membranes using the above method; 4) coating collecting layer based on Ag nanowires. The collecting layer was formed by dropping Ag nanowires dispersion on the PEM and baking 30 min at 50 ºC to dry. Finally, the monolithic integrated µDMFC prototype was obtained after assembly using by PMMA holders.
2.4 Instrumentation Various tests had been done to describe the PEMs based on sulfo functionalized porous silicon and the monolithic integrated µDMFC. The morphology characterizations were investigated by scanning electron microscopy (SEM). SEM measurements were performed on a HITACHI S-5500 operating at an accelerating 9
voltage of 5 kV. The componential characterizations of functionalized porous silicon membranes were analyzed by energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), respectively. EDS analysis was executed with a INCA-450. FTIR measurements were taken by a Nicolet 6700 in transmittance mode with resolution 0.5 cm−1. XPS analysis was carried out using a PHI Quantera with the Al Kα radiation (1486.7 eV), then the binding energy was calibrated with C 1s (284.8 eV). The proton conductivities of PEMs and the performance of the monolithic integrated µDMFC were characterized using by a Solartron 1287 electrochemical interface coupled with a Solartron 1260B frequency response analyzer at room temperature. Nafion® 117 membrane, for comparison, was pretreated by hydrogen peroxide aqueous solution (5 wt. %) at 80 ◦C for 1 h, followed by immersion in distilled water for 1 h. Then Nafion® 117 membrane was treated with sulfuric acid aqueous solution (0.5 M) at 80 ◦C for 1 h, followed by immersion in the distilled water for 1 h. Finally, Nafion® 117 membrane was immersed in the distilled water 24 h for activating.
4 Results and discussion 4.1 Characterization of PEM based on sulfo functionalized porous silicon 4.1.1 Morphological characterizations The SEM images of the porous silicon membrane before and after grafting process are shown as Fig. 3. It can be seen that the bare porous silicon membrane shows distinct nanohole structure with the bore diameter of 50 nm from the SEM 10
images in Fig. 3 (a) and (b), which are the frontal view and cross-sectional view of porous silicon membrane before grafting process. The porosity of the porous silicon membrane obtained by weight method is 79 %. When grafting process is applied, there are many organic molecules attached onto the surface of porous silicon membrane shown as Fig. 3 (c) and (e), and the walls of porous silicon nanoholes as shown in Fig. 3 (d) and (f). The thickness of most domains are much larger than one monolayer thickness of MPTMS, which is about 0.7 nm [43], and are considered as MPTMS polymer. Although a few MPTMS monolayers can be seen, the surface and the walls of porous silicon membrane are almost completely covered with continuous MPTMS polymers. The MPTMS polymers formed via dehydration condensation reaction indicate that MPTMS are grafted successfully.
4.1.2 Componential characterizations After treatment of porous silicon membrane with the MPTMS solution and drying in air at room temperature, mercapto groups should be attached to the walls of porous silicon nanoholes, which were then sulfonated to -SO3H groups to induce proton conductivity. SEM images only provide morphology property of the functionalized porous silicon membrane, but for proper analysis of its componential characterizations, EDS, XPS and FTIR were carried out. EDS were performed on selected areas of the porous silicon membranes before and after grafting process. The results are shown as Fig. 4. It can clearly see that there is no sulfur element attached to the walls of porous silicon nanoholes before grafting 11
process, shown as Fig. 4(a). When grafting process is applied, sulfur element is detectable, shown as Fig. 4(b). As we know, that porous silicon membranes were functionalized with sulfonic acid groups via chemical grafting method, which was done in four steps: 1) Hydrophilization process, 2) Adsorption process, 3) Grafting process, 4) Sulfonation process. Before the grafting process, there are three steps: fabrication of porous silicon membrane, hydrophilization process and adsorption process. After every single step, the porous silicon membrane was cleaned with deionized water, so the sample didn’t have sulfur element before grafting process. When the grafting process was applied, Si-OH groups in hydrolyzed MPTMS were absorbed onto the walls of porous silicon nanoholes and then mercapto groups were grafted via dehydration condensation reaction, as a result, the specific sulfur element was introduced. Therefore, the sulfur element in EDS result proves that -SH groups are grafted at the porous silicon nanoholes successfully. FTIR can be used to characterize the composition information of a sample, because different materials have various impacts on infrared absorption. The background sample of FTIR was handled in two steps. Firstly, the porous silicon membrane was treated with hydrophilization process, which was the same as the as-prepared samples. Secondly, the hydrophilic porous silicon membrane was immersed in the pure benzene solution with ultrasonically activating for 48 hours. Using the sample treated by the above method as background, the FTIR measurements of the porous silicon membrane after grafting process and after sulfonation process were carried out. The results are shown as Fig. 5. The FTIR 12
spectrum demonstrates a presence of the transmittance peak at 2560 cm−1 after grafting process, corresponding to S-H bond vibration of mercaptane, shown as Fig. 5 (a). Further sulfonation was done to convert -SH groups to -SO3H groups using by HNO3. FTIR spectrum in Fig. 5 (b) shows an absence of peak at 2560 cm−1, meaning that mercapto groups vanished from the walls of nanoholes. XPS characterizations confirm the distribution of -SH groups and -SO3H groups on the walls of porous silicon nanoholes to characterize the grafting and sulfonation process. Sulfur 2s binding energy displays its functional group, peak at 226.7 eV corresponds to -SH groups, while peak at 230.5 eV corresponds to -SO3H groups. Fig. 6 displays a representative cross-sectional XPS spectrum of the functionalized porous silicon membrane before (a) and after (b) sulfonation process. It can be seen from Fig. 6 (a) that there are a strong peak at 226.7 eV after grafting process, indicating that -SH groups were attached at the walls of porous silicon nanoholes. When sulfonation process is applied, -SH groups are oxidated to -SO3H groups, therefore, the sulfur 2s peak transfers to 230.5 eV, shown in Fig. 6 (b).
4.1.3 Proton conductivity Proton conductivity depends on the number of sulfonic acid groups inside the nanoholes of the functionalized porous silicon membrane, which has been measured using AC impedance method range from 0.1 Hz to 1 MHz. The intersection of the curve and the real axis in the Nyquist Z plots of impedance is the body resistance of the membrane (RM). The proton conductivity (σ) of the PEMs based on sulfo 13
functionalized porous silicon is calculated using the equation:
d
(1)
RM A
Where d is the thickness of the membrane, and A is the cross-sectional electrode contact area. The Nyquist Z plot of the PEM based on sulfo functionalized porous silicon, is shown as Fig. 7 (a). As a comparison, The Nyquist Z plot of Nafion® 117 membrane was also measured, shown as Fig. 7 (b). The σ of PEM based on sulfo functionalized porous silicon is 0.082 S/cm, which is 1.36 times the value of Nafion® 117 membrane. The higher proton conductivity of PEM based on sulfo functionalized porous silicon is attribute to millions of sulfonic acid groups attached onto the walls of porous silicon nanoholes. The sulfonic acid groups will hydrolyze in water, thus the walls of porous silicon nanoholes will be negatively charged and the protons can transfer between neighboring oxygen atoms at the gradient of potential and concentration, resulting in a larger proton conductivity.
4.1.4 The proton resistance per unit area The proton resistance per unit area (r) is of more significance for applications in a practical μDMFC, because it will significantly affect the resistance and furthermore affect the performance of a μDMFC. The proton resistance per unit area can be obtained from Eq. (2): r
d
(2)
=R M A
Where d is the thickness of the membrane, and A is the cross-sectional electrode 14
contact area. The proton resistances per unit area of the PEMs based on sulfo functionalized porous silicon are listed in Table 1. The PEMs based on sulfo functionalized porous silicon have r values which are much lower than that of the Nafion® 117 membrane because of their smaller thickness and higher proton conductivity, so the ohmic polarization can be reduced heavily, which can improve the performance of silicon-based μDMFCs greatly. It can be seen the porous silicon membrane without functionalization demonstrates a relatively high r value, that means high proton conductivity of the PEM based on sulfo functionalized porous silicon is due to chemical grafting. A comparison was done to explain the potentials of the PEMs based on sulfo functionalized porous silicon, and the result is shown in Table 2. It can be seen that the solid PEMs based on porous silicon have smaller deformation, thinner thickness than Nafion® 117 membrane, furthermore, they can be compatible with micro fabrication techniques and suitable for batch fabrication. Therefore, the PEMs based on sulfo functionalized porous silicon reported in this work are more attractive for optimizing the performance and sizes and of fuel cells and furthermore for the commercialization of silicon-based μDMFCs.
4.2 Characterization of the monolithic integrated µDMFC SEM images of a monolithic integrated µDMFC based on sulfo functionalized porous silicon membrane are shown as Fig. 8. Fig 8(a) is the cross-sectional view and 15
Fig 8(b) (c) (d) are the high resolution SEM images of sulfo functionalized porous silicon membrane, Pt nanoflowers and Ag nanowires, respectively. As we can see, the 3D Pt nanoflowers are assembled on the PEM based on sulfo-functionalized porous silicon membrane, with the size of about 200 ~ 400 nm. A uniform film fabricated by Ag nanowires are capping on the top of Pt nanoflowers serving as a collecting layer, with a square resistance of 0.25Ω/□. The novel monolithic integrated μDMFC prototype based on sulfo functionalized porous silicon membrane was shown as Fig. 9 (a). When filled with 2M methanol, a constant voltage was applied and the output current was monitored for a period of 50 s until the final steady-state value was recorded, and the I-V curve was obtained. The results (Fig.9 b) clearly show that the μDMFCs achieve an open circuit voltage of 0.3 V, a maximum power density of 5.5 mW/cm2 and a maximum current density of 80 mA/cm2. When the μDMFC prototype worked at the maximum power density, the operating time was nearly 100 min. Volumetric peak power density is an important index, which is often used to compare the performance of various power sources. However, µDMFCs require a large volume due to the thickness of each stacked electrode, the inter-electrode space, complicated fabrication and assembly process, which results in small volumetric power densities. Therefore, the power density for such fuel cells is commonly reported in terms of electrode surface area instead of volume, which is often the case for other power sources. Table 3 lists out performances of the as-prepared monolithic integrated µDMFC and other reported µDMFCs, comparing their peak power density normalized to both active area and volume. From the results, we can see that although 16
the peak power density of the monolithic integrated µDMFC normalized to active area has no obvious advantage, it demonstrates better performance than most of passive µDMFCs in terms of volume, which is 36.18 mW/cm3. That’s because of PEM based on sulfo functionalized porous silicon and 3D Pt nanoflowers prepared by in-situ synthesis, which results in a compact structure of monolithic integrated µDMFC.
5 Conclusions A novel solid PEM based on sulfo functionalized porous silicon is demonstrated using by chemical grafting method. The porous silicon membrane is functionalized with MPTMS, and then the mercapto groups in MPTMS are oxidated to sulfonic acid groups for inducing proton conductivity. This solid PEM has higher proton conductivity, lower thickness and proton resistance compared to Nafion® 117 membrane and can make full use of the MEMS technologies to optimize the sizes and costs of fuel cells. With the in-situ synthesis of 3D platinum nanoflowers on porous silicon, a monolithic integrated micro fuel cell based on porous silicon is obtained for the first time. The monolithic integrated μDMFC can output stable power density with passive fuel supply, which shows great potentials for optimizing the size and performance of µDMFCs, furthermore, are quite promising for integrated micro systems.
Acknowledgment 17
This work is supported by the National Natural Science Foundation of China (No. 61474071), 973 program (No. 2015CB352100) and Doctoral Research Foundation of Zhengzhou University of Light Industry (No. 2015BSJJ055).
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[30] K. L. Chu, M. A. Shannon, R. I. Masel, An improved miniature direct formic acid fuel cell based on nanoporous silicon for portable power generation batteries, fuel cells, and energy conversion, J. Electrochem. Soc. 153 (2006) A1562-A1567. [31] T. Pichonat, B. G. Manuel, D. Hauden, A new proton-conducting porous silicon membrane for small fuel cells, Chem. Eng. J. 101 (2004) 107-111. [32] T. Pichonat, B. G. Manuel, Mesoporous silicon-based miniature fuel cells for nomadic and chip-scale systems, Microsyst. Technol. 12 (2006) 330-334. [33] M. Wang, X. H. Wang, S. Wu, X. Guo, Z. M. Tan, L. T. Liu, Nano porous silicon membrane with channels for micro direct methanol fuel cells, IEEE NEMS 2011, Feb. 20-23, 2011, Kaohsiung, Taiwan, 968-971. [34] T. Pichonat, B. G. Manuel, Realization of porous silicon based miniature fuel cells, J. Power Sources 154 (2006) 198-201. [35] T. Pichonat, B. G. Manuel, A porous silicon-based ionomer-free membrane electrode assembly for miniature fuel cells, Fuel cells 5 (2006) 323-325. [36] B. Sciacca, S. D. Alvarez, F. Geobaldo, M. J. Sailor, Bioconjugate functionalization of thermally carbonized porous silicon using a radical coupling reaction, Dalton Trans. 39 (2010) 10847-10853. [37] M. Wang, X. H. Wang, J. N. Li, L. T. Liu, In situ synthesis of 3D platinum nanoflowers on porous silicon for monolithic integrated micro direct methanol fuel cells, J. Mater. Chem. A 1 (2013) 8127-8133. [38] N. D. Meeks, S. Rankin, D. Bhattacharyya, Sulfur-functionalization of porous silica particles and application to mercury vapor sorption, Ind. Eng. Chem. Res. 22
49 (2010) 4687-4693. [39] T. Duvdevani, M. Philosoph, M. Rakhman, D. Golodnitsky, E. Peled, Novel composite proton-exchange membrane based on silica-anchored sulfonic acid (SASA), J. Power Sources 161 (2006) 1069-1075. [40] M. H. Hu, S. Noda, T. Okubo, Y. Yamaguchi, H. Komiyama, Structure and morphology of self-assembled 3-mercaptopropyltrimethoxysilane layers on silicon oxide, Appl. Surf. Sci. 181 (2001) 307-316. [41] J. Singh, J. E. Whitten, Adsorption of 3-mercaptopropyltrimethoxysilane on silicon oxide surfaces and adsorbate interaction with thermally deposited gold, J. Phys. Chem. C 112 (2008) 19088-19096. [42] M. Wang, Y. X. Lu, L. T. Liu, X. H. Wang, A monolithic integrated micro direct methanol fuel cell based on sulfo functionalized porous silicon, PowerMEMS 2006, Dec. 6-9, 2016, Paris, France, 012017. [43] X. Huang, H. Huang, N. Wu, R. Hu, T. Zhu, Z. Liu, Investigation of structure and chemical states of self-assembled Au nanoscale particles by angle-resolved X-ray photoelectron spectroscopy, Surf. Sci. 459 (2000) 183-190. [44] J. V. Larsen, B. T. Dalslet, A. C. Johansson, C. Kallesøe, E.V. Thomsen, Micro direct methanol fuel cell with perforated silicon-plate integrated ionomer membrane, J. Power Sources 257 (2014) 237-245. [45] N. Hashim, S. K. Kamarudin, W. R. W. Daud, Design, fabrication and testing of a PMMA-based passive single-cell and a multi-cell stack micro-DMFC, Int. J. Hydrogen Energy 34 (2009) 8263-8269. 23
[46] D. S. Falcão, J. P. Pereira, C. M. Rangel, A. M. F. R. Pinto, Development and performance analysis of a metallic passive micro-direct methanol fuel cell for portable applications, Int. J. Hydrogen Energy 40 (2015) 5408-5415. [47] J. Cao, Z. Zou, Q. Huang, T. Yuan, Z. Li, B. Xia, H. Yang, Planar air-breathing micro-direct methanol fuel cell stacks based on micro-electronic–mechanical-system technology, J. Power Sources 185 (2008) 433-438. [48] Y.M. Zhu, X.H. Wang, Y.A. Zhou, L.T. Liu, A novel assembly method for micro direct methanol fuel cells using multi-layer bonding technique, Transducers 2011, Jun. 5-9, Beijing, China, 2618-2621. [49] Y. H. Seo, Y. H. Cho, Micro direct methanol fuel cells and their stacks using a polymer electrolyte sandwiched by multi-window microcolumn electrodes, Sens. Actuators A Phys. 150 (2009) 87-96. Biographies
Mei Wang received her B.S. degree in 2009 from College of Electronic Science & Engineering, Jilin University and Doctor’s degree in 2015 from Institute of Microelectronics, Tsinghua University. Now she is a lecturer in Zhengzhou University of Light Industry, China. She is engaged in fuel cells and electrochemical sensors.
Litian Liu received the B. Eng. degree and Doctor’s degree in Tsinghua University. Currently he is a professor and works in Institute of Microelectronics, Tsinghua University. His research interests are focused on microelectronic devices and their 24
fabrication techniques.
Xiaohong Wang received the B. Eng. degree and M.S. degree in Southeast University. She received her Doctor’s degree in Department of Precision Instrument at Tsinghua University in 1998. Now she is a professor in Tsinghua University. Her current research is super-capacitors, energy harvesters and fuel cells.
25
Figure captions
Fig. 1 Schematic of a monolithic integrated µDMFC based on sulfo functionalized porous silicon membrane. The monolithic integrated µDMFC combines PEM, Pt nanocatalysts and current collector layer together. PEM is achieved by sulfo functionalized porous silicon membrane and 3D Pt nanoflowers are synthesized in situ onto it as nanocatalysts. Ag nanowires film serves as current collector layer.
26
Fig. 2 Chemical grafting process of the porous silicon membrane: (1) Hydrophilization process: hydroxyl groups were attached to walls of porous silicon nanoholes by hydrophilization process; (2) Adsorption process: Si-OH groups in hydrolyzed MPTMS absorbed onto the walls of porous silicon nanoholes; (3) Grafting process: mercapto groups were grafted onto the walls of porous silicon nanoholes through dehydration condensation reaction; (4) Sulfonation process: mercapto groups were oxidize to sulfonic acid groups.
27
Fig. 3 SEM images of the porous silicon membrane before and after grafting process: frontal view (a) and cross-sectional view (b) of porous silicon membrane before grafting process, frontal view (c), cross-sectional view (d) and high magnification SEM images (e and f) of porous silicon membrane after -SH groups functionalized in a mixed benzene solution containing 30 wt. % MPTMS and 5 wt. % GAA.
28
Fig. 4 SEM images of the porous silicon membrane before (a) and after (b) grafting process and their corresponding EDS profiles.
29
Fig. 5 The FTIR spectrum of the porous silicon membrane after grafting process (a) and the FTIR spectrum of the porous silicon membrane after sulfonation process for oxidizing -SH groups to -SO3H groups (b).
30
Fig. 6 The cross-sectional XPS spectrum of the porous silicon membrane after –SH grafting process (dash dot) and after sulfonation process (solid). Sulfur 2s binding energy displays its functional group, peak at 226.7 eV corresponds to -SH groups, while peak at 230.5 eV corresponds to -SO3H groups.
Fig. 7 The Nyquist Z plots of the PEM based on sulfo functionalized porous silicon (a) and that of Nafion® 117 membrane.
31
Fig. 8 SEM images of a monolithic integrated µDMFC based on sulfo functionalized porous silicon membrane: cross-sectional view (a); high resolution SEM image of sulfo functionalized porous silicon membrane (b), Pt nanoflowers (c) and Ag nanowires (d).
32
Fig. 9 The monolithic integrated μDMFC prototype based on sulfo functionalized porous silicon membrane (a) and its corresponding I-V curves (b), operating time curve recorded at 50 mA/cm2 using 2M methanol (c).
Table 1 The proton resistances per unit area of the PEMs based on sulfo functionalized porous silicon at 25 ◦C and the cross-sectional electrode contact area of 0.196 cm2. Membrane
d (cm)
RM (Ω)
σ (S/cm)
r (cm2/S)
PEM based on sulfo
0.01
0.62
0.082
0.122
0.01
288.59
1.77*10-4
56.564
functionalized porous silicon porous silicon membrane
33
without functionalization Nafion® 117 membrane
1.83*10-2
1.55
0.060
0.304
Table 2 Comparison of PEMs based on sulfo functionalized porous silicon with Nafion® 117 membrane Membrane
as-prepared
Thickness change, % Linear expansion, %
Thickness
Compatibility
increase (from 50 %
increase (from 50 %
(μm)
with micro
RH to water soaked
RH to water soaked
fabrication
at room temperature)
at room temperature)
techniques
0
0
100
compatible
10 a
183
incompatible
PEMs Nafion® 117 10 a membrane a
The data is obtained from Dupont company.
Table 3 Comparison of the monolithic integrated µDMFC prototype with other reported µDMFCs Reference Fuel supply
DMFC
Total Size
Active
Peak
Volumetric
Structure
(without
area (cm2)
power
peak power
density
density
(mW/cm2)
(mW/cm3)
5.50
36.18
mode
This paper
holders)
Passive Monolithic 1.9cm×1.6cm integrated
×0.05cm 34
1.00
Larsen et
Passive Monolithic 1cm×0.75cm×
al. [44]
integrated
Hashim
Passive Sandwich
et al. [45] Falcão et
Passive Sandwich
Passive DMFC
[49]
1.00
2.20
0.29
2.25
19.20
24.69
8.64
17.50
28.65
2.25
20.29
40.45
1.00
0.031
0.15
0.6cm 3.0cm×2.5cm
~3.5cm×2.5c
Active
6.1cm×3.2cm
Stacks
×0.27cm
Sandwich
2.7cm×2.2cm
[48] Seo et al.
2.44
m×0.2cm
[47] Zhu et al.
2.50
×1.0cm
al. [46] Cao et al.
0.44
×0.19cm Active
Sandwich
1.23cm×1.43c m×0.12cm
35
S0925-4005(17)31192-9 http://dx.doi.org/doi:10.1016/j.snb.2017.06.173 SNB 22644
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
17-4-2017 15-6-2017 26-6-2017
Please cite this article as: Mei Wang, Litian Liu, Xiaohong Wang, A novel proton exchange membrane based on sulfo functionalized porous silicon for monolithic integrated micro direct methanol fuel cells, Sensors and Actuators B: Chemicalhttp://dx.doi.org/10.1016/j.snb.2017.06.173 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A novel proton exchange membrane based on sulfo functionalized porous silicon for monolithic integrated micro direct methanol fuel cells
Mei Wang a, b, Litian Liu a, Xiaohong Wang a * a
Institute of Microelectronics, Tsinghua National Laboratory for Information Science and
Technology, Tsinghua University, Beijing 100084, P.R. China b
School of Computer and Communication Engineering, Zhengzhou University of Light
Industry, Zhengzhou, 450000, P.R. China
* Corresponding author: Xiaohong Wang, Ph.D. Institute of Microelectronics, Tsinghua University Beijing, 100084, China Phone: 86-10-62798432 Fax: 86-10-62771130 Email: [email protected]
Highlights A novel solid PEM based on sulfo functionalized porous silicon is presented. Chemical grafting method is designed to prepare the high proton-conductive PEM. The PEM shows higher proton conductivity and lower resistance than Nafion® 117. A monolithic integrated micro fuel cell based on porous silicon is obtained for the first time. 1
Abstract A novel sulfo functionalized porous silicon membrane is presented, which can be used as proton exchange membrane (PEM) for monolithic integrated micro direct methanol fuel cells (μDMFCs). The porous silicon membrane is functionalized with sulfonic acid groups (-SO3H) to induce proton conductivity via chemical grafting method. Electrical impedance analyses demonstrate that the sulfo functionalized porous silicon membranes exhibit excellent proton exchange capability, achieving a proton conductivity of 0.082 S/cm, which is 1.37 times larger than the value acquired on the most widely used Nafion® 117 membrane (0.060 S/cm). With the in-situ synthesis of 3D platinum nanoflowers on porous silicon presented in our previous work, a monolithic integrated micro fuel cell based on porous silicon is obtained for the first time, which achieves a power density of 5.5 mWcm-2. As such, we believe this work is more attractive for optimizing the integration and sizes of silicon-based μDMFCs.
Keywords: Proton exchange membrane (PEM), sulfo functionalized porous silicon, chemical grafting method, monolithic integrated micro direct methanol fuel cells (monolithic integrated μDMFCs)
1 Introduction The demand of durable and integrated power sources increases radically with rising functionalities and decreasing sizes of portable devices. Among the various fuel 2
cells, the micro direct methanol fuel cell (µDMFC) is considered as the most promising candidate power source, with the advantages of high energy density, low pollution, room temperature operation, simple and safe handling [1-2]. For the application of µDMFCs, their sizes and costs have to be comparable to the lithium ion batteries, which are the most widely used power sources for portable devices. Silicon-based µDMFCs have drawn much attention because they can make use of the micro fabrication techniques developed for integrated circuit (IC) and micro electromechanical systems (MEMS) to optimize their sizes and costs. In recent years, varieties of silicon-based µDMFCs [3-8] and their related components [9-17] have been developed. A typical silicon-based µDMFC generally consists of an anode, a cathode, and a membrane electrode assembly (MEA) sandwiched in the middle. The MEA is the most important component of µDMFC, which is made up of diffusion layer, catalyst layer and proton exchange membrane (PEM). Due to the thickness of each stacked electrode and the inter-electrode space, in addition to complicated fabrication and assembly process, such silicon-based µDMFCs require a large volume, which results in small volumetric power densities. Therefore, the power density for such fuel cells is commonly reported in terms of electrode surface area instead of volume, which is often the case for other power sources [18]. In order to address the issue, a monolithic integrated µDMFC is a promising way, even the biggest challenge is incompatibility of PEM and electrode plate fabricated by silicon. Nafion® membranes, the most popular PEM for DMFCs, are organic polymer, which lead to several limits in the applications of silicon-based μDMFCs: 1) they are 3
incompatible with micro fabrication techniques, in addition to the electrode plate; 2) their shapes vary in response to their water content due to the swelling of the polymer, thus the catalysts often fall off in practical operations; 3) their thicknesses are nearly 200 μm subject to durability and reactant crossover, which increase the resistances and sizes and go against the performance and miniaturization of µDMFCs. Therefore, developing a new type of PEM has been a key issue for monolithic integrated µDMFCs. One common approach is introducing proton-conductive molecules into porous solid skeleton materials which have high specific surface area and good mechanical stability. The solid skeleton material is used to control the deformation and decrease the thickness of PEM, demonstrating an improved performance of µDMFCs. Some materials have been used as a skeleton to fill or graft electrolyte, such as poly(tetrafluoroethylene) (PTFE) [19-21], cross-linked polyethylene (CLPE) [22], polyimide [23-25], poly(ethylene-co-tetrafluoroethylene) (ETFE) [26-28], and porous silicon [29-36]. Among the various solid skeleton materials, porous silicon is considered to be best candidate in silicon-based micro fuel cells due to its inherent properties, including 1) porous silicon is compatible with micro fabrication techniques and silicon electrode plate; 2) the route of proton transmission can be heavily shorten in porous silicon, compared with PTFE, CLPE, and ETFE, etc., consequently, the proton exchange resistance can be diminished significantly. That is because the holes of porous silicon can be in form of vertical through-holes by adjusting the parameters of anodization; 3) the chemical grafting can be facilely realized by single-step silanization process, while that is relatively complex for other skeleton materials. 4
Millions of hydroxyl groups (-OH) can be attached on the walls of porous silicon nanoholes, which can crosslink together with Si-OHs in organics via silanization process. For the other skeleton materials, the chemical grafting process need extra cross-linking agent [19-25] or radiation [26-28], etc., which need relatively complex treating processes; 4) porous silicon has high specific surface area on the order of hundreds of m2/g, therefore, more sulfo can be grafted on the walls of porous silicon nanoholes, as a result, the proton conductivity can be improved. Due to above reasons, developing a PEM based on porous silicon shows great potentials for optimizing the sizes of silicon-based µDMFCs and designing a novel monolithic integrated µDMFC. We proposed a strategy of monolithic integrated micro fuel cells based on porous silicon, as shown in Fig. 1. The monolithic integrated micro fuel cell combines PEM, Pt nanocatalysts and current collector layer together. PEM is achieved by sulfo functionalized porous silicon membrane and 3D Pt nanoflowers are synthesized in situ onto it as nanocatalysts. Ag nanowires film serves as current collector layer. Porous silicon serves as PEM and catalyst support simultaneously. Compared with the classical µDMFCs with “sandwich” structure, the monolithic integrated µDMFC shows several key advancements, including 1) simplifying the structure of fuel cells, 2) compatible with micro fabrication techniques and suitable for batch production, 3) solid PEM with little deformation and high proton conductivity, 4) in-situ synthesis of 3D Pt nanoflowers on a fuel cell body without the extra coating process. 3D platinum nanoflowers have been synthesized in situ on porous silicon in our previous work [37]. However, designing of a PEM based on 5
porous silicon is still a challenging work, probably because there are several key issues which have not been solved, consisting of 1) fabrication of the porous silicon membrane with vertical through-holes; 2) chemical grafting sulfo in the walls of porous silicon nanoholes; 3) achieving high proton conductivity by controlling the chemical grafting process.
Herein, we propose a novel solid PEM based on sulfo functionalized porous silicon. We start by presenting the fabrication process of the PEM based on porous silicon, followed by characterizations in order to demonstrate its morphology and performance. Finally, we demonstrate a monolithic integrated µDMFC based on sulfo functionalized porous silicon for the first time, combining in-situ synthesis of 3D platinum nanoflowers.
2 Experimental 2.1 Chemicals and materials 4 inch silicon wafer was purchased from MCL Electronic Materials, Ltd. 3-mercaptopropyltrimethoxysilane (MPTMS, (CH3O)3SiCH2CH2CH2SH, ≥ 98 wt. %) was bought from Nanjing Jingtianwei Chemical Co., Ltd. Benzene (≥ 99.5 wt. %) was obtained from Beijing Chemical Works. Glacial acetic acid (GAA, ≥ 99.5 wt. %) was got from Modern Oriental (Beijing) Technology Development Co., LTD. Ethyl alcohol (≥ 99.8 wt. %), methanol (≥ 99.8 wt. %), hydrofluoric acid (40 wt. %), hydrogen peroxide (40 wt. %), sulfuric acid (98 wt. %) and nitric acid (70 wt. %) were purchased from Tianjin Fengchuan Chemical Reagent Science and Technology 6
Co., Ltd. Nafion® 117 membrane and Nafion solution (5%) were bought from DuPont Co. The chemical regent H2PtCl6·6 H2O was got from Sinopharm Chemical Regent Co., Ltd. The Ag nanowires (length of 20~60 µm and diameter of 40~60 nm) dispersion with a 20 mg/mL concentration was got from XFNANO Materials Tech Co., Ltd. (Nanjing, China). Millipore water (18.2 MΩ·cm) provided by a Milli-Q Labo apparatus (Merck Millipore Ltd.) was used in all experiments.
2.2 Fabrication of PEM based on sulfo functionalized porous silicon 2.2.1 Fabrication of porous silicon membranes An antimony-doped 0.02 Ω cm n+-type (100) oriented silicon wafer was used to process silicon membranes [33]. Thermal oxide (100 nm) and LPCVD Si3N4 (160 nm) were deposited on both sides of the wafer as the mask of KOH etching and anodization. Double-side lithography was introduced to transfer the pattern and KOH etching followed. The morphology, pore size, and porosity of porous silicon can be controlled during the anodization process. In this paper, anodization was conducted in ethanolic HF solution (HF: ethyl ethanol = 2:1) at a constant current density of 200 mA/cm2 for 15 min, to obtain the porous silicon membranes with proper pore size and porosity. Inductively coupled plasma (ICP) was introduced to etch the residual silicon at the back side and unordered porous silicon at the frontal side, to fabricate the porous silicon membranes with vertical through-holes. Eventually, the regular porous silicon membranes with the thickness of 100 μm were achieved.
7
2.2.2 Functionalization of porous silicon membranes Proton conductivity, which directly correlates to the corresponding cell performance, could be the most significant characteristic of PEM for µDMFCs. The proton conductivity of sulfonated polymers is highly dependent on their contents of sulfonic acid groups. The property of porous silicon makes it feasible to graft more -SO3H groups because of its high specific surface area and good surface property. We notice that the silane coupling agent can be facilely grafted onto the walls of porous silicon nanoholes via silanization process in the presence of trace water [38-41], therefore a high proton-conductive PEM based on porous silicon can be developed by chemical grafting method. MPTMS was selected as a grafting material because it is terminated with mercapto groups, which can be oxidized to sulfonic acid groups easily. Porous silicon membranes were functionalized with sulfonic acid groups via chemical grafting method, which was done in four steps: 1) Hydrophilization process: hydroxyl groups (-OH) were attached to walls of porous silicon nanoholes by hydrophilization process in a mixed solution of H2SO4 and H2O2 at the volume ratio of 3:1 for 30 minutes at 60 ºC. Simultaneously, Si-O-CH3 groups in MPTMS molecule hydrolyzed to form Si-OH groups in mixed benzene solutions, with the MPTMS concentration of 30 wt. % and GAA concentration of 5 wt. %. 2) Adsorption process: Si-OH groups in hydrolyzed MPTMS were absorbed onto the walls of porous silicon nanoholes by chemical impregnation of hydrophilic porous silicon membrane from the mixed benzene solutions; 3) Grafting process: mercapto groups were grafted 8
via dehydration condensation reaction in the mixed benzene solution with ultrasonically activating for 48 hours; 4) Sulfonation process: mercapto groups were oxidize to sulfonic acid groups in an 30 wt. % HNO3 aqueous solution for 3.5 hours with ultrasound at room temperature. Eventually, a functionalized porous silicon membranes were obtained, which were proton-conductive as a result of sulfonic acid terminal, shown as Fig. 2.
2.3 Fabrication of monolithic integrated µDMFCs The fabrication process of the monolithic integrated µDMFC divided into four steps [42]: 1) Fabrication of porous silicon membranes; 2) in-situ synthesis of 3D Pt nanflowers on porous silicon membranes using by the method proposed in our previous work [37]; 3) functionalization of porous silicon membranes using the above method; 4) coating collecting layer based on Ag nanowires. The collecting layer was formed by dropping Ag nanowires dispersion on the PEM and baking 30 min at 50 ºC to dry. Finally, the monolithic integrated µDMFC prototype was obtained after assembly using by PMMA holders.
2.4 Instrumentation Various tests had been done to describe the PEMs based on sulfo functionalized porous silicon and the monolithic integrated µDMFC. The morphology characterizations were investigated by scanning electron microscopy (SEM). SEM measurements were performed on a HITACHI S-5500 operating at an accelerating 9
voltage of 5 kV. The componential characterizations of functionalized porous silicon membranes were analyzed by energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), respectively. EDS analysis was executed with a INCA-450. FTIR measurements were taken by a Nicolet 6700 in transmittance mode with resolution 0.5 cm−1. XPS analysis was carried out using a PHI Quantera with the Al Kα radiation (1486.7 eV), then the binding energy was calibrated with C 1s (284.8 eV). The proton conductivities of PEMs and the performance of the monolithic integrated µDMFC were characterized using by a Solartron 1287 electrochemical interface coupled with a Solartron 1260B frequency response analyzer at room temperature. Nafion® 117 membrane, for comparison, was pretreated by hydrogen peroxide aqueous solution (5 wt. %) at 80 ◦C for 1 h, followed by immersion in distilled water for 1 h. Then Nafion® 117 membrane was treated with sulfuric acid aqueous solution (0.5 M) at 80 ◦C for 1 h, followed by immersion in the distilled water for 1 h. Finally, Nafion® 117 membrane was immersed in the distilled water 24 h for activating.
4 Results and discussion 4.1 Characterization of PEM based on sulfo functionalized porous silicon 4.1.1 Morphological characterizations The SEM images of the porous silicon membrane before and after grafting process are shown as Fig. 3. It can be seen that the bare porous silicon membrane shows distinct nanohole structure with the bore diameter of 50 nm from the SEM 10
images in Fig. 3 (a) and (b), which are the frontal view and cross-sectional view of porous silicon membrane before grafting process. The porosity of the porous silicon membrane obtained by weight method is 79 %. When grafting process is applied, there are many organic molecules attached onto the surface of porous silicon membrane shown as Fig. 3 (c) and (e), and the walls of porous silicon nanoholes as shown in Fig. 3 (d) and (f). The thickness of most domains are much larger than one monolayer thickness of MPTMS, which is about 0.7 nm [43], and are considered as MPTMS polymer. Although a few MPTMS monolayers can be seen, the surface and the walls of porous silicon membrane are almost completely covered with continuous MPTMS polymers. The MPTMS polymers formed via dehydration condensation reaction indicate that MPTMS are grafted successfully.
4.1.2 Componential characterizations After treatment of porous silicon membrane with the MPTMS solution and drying in air at room temperature, mercapto groups should be attached to the walls of porous silicon nanoholes, which were then sulfonated to -SO3H groups to induce proton conductivity. SEM images only provide morphology property of the functionalized porous silicon membrane, but for proper analysis of its componential characterizations, EDS, XPS and FTIR were carried out. EDS were performed on selected areas of the porous silicon membranes before and after grafting process. The results are shown as Fig. 4. It can clearly see that there is no sulfur element attached to the walls of porous silicon nanoholes before grafting 11
process, shown as Fig. 4(a). When grafting process is applied, sulfur element is detectable, shown as Fig. 4(b). As we know, that porous silicon membranes were functionalized with sulfonic acid groups via chemical grafting method, which was done in four steps: 1) Hydrophilization process, 2) Adsorption process, 3) Grafting process, 4) Sulfonation process. Before the grafting process, there are three steps: fabrication of porous silicon membrane, hydrophilization process and adsorption process. After every single step, the porous silicon membrane was cleaned with deionized water, so the sample didn’t have sulfur element before grafting process. When the grafting process was applied, Si-OH groups in hydrolyzed MPTMS were absorbed onto the walls of porous silicon nanoholes and then mercapto groups were grafted via dehydration condensation reaction, as a result, the specific sulfur element was introduced. Therefore, the sulfur element in EDS result proves that -SH groups are grafted at the porous silicon nanoholes successfully. FTIR can be used to characterize the composition information of a sample, because different materials have various impacts on infrared absorption. The background sample of FTIR was handled in two steps. Firstly, the porous silicon membrane was treated with hydrophilization process, which was the same as the as-prepared samples. Secondly, the hydrophilic porous silicon membrane was immersed in the pure benzene solution with ultrasonically activating for 48 hours. Using the sample treated by the above method as background, the FTIR measurements of the porous silicon membrane after grafting process and after sulfonation process were carried out. The results are shown as Fig. 5. The FTIR 12
spectrum demonstrates a presence of the transmittance peak at 2560 cm−1 after grafting process, corresponding to S-H bond vibration of mercaptane, shown as Fig. 5 (a). Further sulfonation was done to convert -SH groups to -SO3H groups using by HNO3. FTIR spectrum in Fig. 5 (b) shows an absence of peak at 2560 cm−1, meaning that mercapto groups vanished from the walls of nanoholes. XPS characterizations confirm the distribution of -SH groups and -SO3H groups on the walls of porous silicon nanoholes to characterize the grafting and sulfonation process. Sulfur 2s binding energy displays its functional group, peak at 226.7 eV corresponds to -SH groups, while peak at 230.5 eV corresponds to -SO3H groups. Fig. 6 displays a representative cross-sectional XPS spectrum of the functionalized porous silicon membrane before (a) and after (b) sulfonation process. It can be seen from Fig. 6 (a) that there are a strong peak at 226.7 eV after grafting process, indicating that -SH groups were attached at the walls of porous silicon nanoholes. When sulfonation process is applied, -SH groups are oxidated to -SO3H groups, therefore, the sulfur 2s peak transfers to 230.5 eV, shown in Fig. 6 (b).
4.1.3 Proton conductivity Proton conductivity depends on the number of sulfonic acid groups inside the nanoholes of the functionalized porous silicon membrane, which has been measured using AC impedance method range from 0.1 Hz to 1 MHz. The intersection of the curve and the real axis in the Nyquist Z plots of impedance is the body resistance of the membrane (RM). The proton conductivity (σ) of the PEMs based on sulfo 13
functionalized porous silicon is calculated using the equation:
d
(1)
RM A
Where d is the thickness of the membrane, and A is the cross-sectional electrode contact area. The Nyquist Z plot of the PEM based on sulfo functionalized porous silicon, is shown as Fig. 7 (a). As a comparison, The Nyquist Z plot of Nafion® 117 membrane was also measured, shown as Fig. 7 (b). The σ of PEM based on sulfo functionalized porous silicon is 0.082 S/cm, which is 1.36 times the value of Nafion® 117 membrane. The higher proton conductivity of PEM based on sulfo functionalized porous silicon is attribute to millions of sulfonic acid groups attached onto the walls of porous silicon nanoholes. The sulfonic acid groups will hydrolyze in water, thus the walls of porous silicon nanoholes will be negatively charged and the protons can transfer between neighboring oxygen atoms at the gradient of potential and concentration, resulting in a larger proton conductivity.
4.1.4 The proton resistance per unit area The proton resistance per unit area (r) is of more significance for applications in a practical μDMFC, because it will significantly affect the resistance and furthermore affect the performance of a μDMFC. The proton resistance per unit area can be obtained from Eq. (2): r
d
(2)
=R M A
Where d is the thickness of the membrane, and A is the cross-sectional electrode 14
contact area. The proton resistances per unit area of the PEMs based on sulfo functionalized porous silicon are listed in Table 1. The PEMs based on sulfo functionalized porous silicon have r values which are much lower than that of the Nafion® 117 membrane because of their smaller thickness and higher proton conductivity, so the ohmic polarization can be reduced heavily, which can improve the performance of silicon-based μDMFCs greatly. It can be seen the porous silicon membrane without functionalization demonstrates a relatively high r value, that means high proton conductivity of the PEM based on sulfo functionalized porous silicon is due to chemical grafting. A comparison was done to explain the potentials of the PEMs based on sulfo functionalized porous silicon, and the result is shown in Table 2. It can be seen that the solid PEMs based on porous silicon have smaller deformation, thinner thickness than Nafion® 117 membrane, furthermore, they can be compatible with micro fabrication techniques and suitable for batch fabrication. Therefore, the PEMs based on sulfo functionalized porous silicon reported in this work are more attractive for optimizing the performance and sizes and of fuel cells and furthermore for the commercialization of silicon-based μDMFCs.
4.2 Characterization of the monolithic integrated µDMFC SEM images of a monolithic integrated µDMFC based on sulfo functionalized porous silicon membrane are shown as Fig. 8. Fig 8(a) is the cross-sectional view and 15
Fig 8(b) (c) (d) are the high resolution SEM images of sulfo functionalized porous silicon membrane, Pt nanoflowers and Ag nanowires, respectively. As we can see, the 3D Pt nanoflowers are assembled on the PEM based on sulfo-functionalized porous silicon membrane, with the size of about 200 ~ 400 nm. A uniform film fabricated by Ag nanowires are capping on the top of Pt nanoflowers serving as a collecting layer, with a square resistance of 0.25Ω/□. The novel monolithic integrated μDMFC prototype based on sulfo functionalized porous silicon membrane was shown as Fig. 9 (a). When filled with 2M methanol, a constant voltage was applied and the output current was monitored for a period of 50 s until the final steady-state value was recorded, and the I-V curve was obtained. The results (Fig.9 b) clearly show that the μDMFCs achieve an open circuit voltage of 0.3 V, a maximum power density of 5.5 mW/cm2 and a maximum current density of 80 mA/cm2. When the μDMFC prototype worked at the maximum power density, the operating time was nearly 100 min. Volumetric peak power density is an important index, which is often used to compare the performance of various power sources. However, µDMFCs require a large volume due to the thickness of each stacked electrode, the inter-electrode space, complicated fabrication and assembly process, which results in small volumetric power densities. Therefore, the power density for such fuel cells is commonly reported in terms of electrode surface area instead of volume, which is often the case for other power sources. Table 3 lists out performances of the as-prepared monolithic integrated µDMFC and other reported µDMFCs, comparing their peak power density normalized to both active area and volume. From the results, we can see that although 16
the peak power density of the monolithic integrated µDMFC normalized to active area has no obvious advantage, it demonstrates better performance than most of passive µDMFCs in terms of volume, which is 36.18 mW/cm3. That’s because of PEM based on sulfo functionalized porous silicon and 3D Pt nanoflowers prepared by in-situ synthesis, which results in a compact structure of monolithic integrated µDMFC.
5 Conclusions A novel solid PEM based on sulfo functionalized porous silicon is demonstrated using by chemical grafting method. The porous silicon membrane is functionalized with MPTMS, and then the mercapto groups in MPTMS are oxidated to sulfonic acid groups for inducing proton conductivity. This solid PEM has higher proton conductivity, lower thickness and proton resistance compared to Nafion® 117 membrane and can make full use of the MEMS technologies to optimize the sizes and costs of fuel cells. With the in-situ synthesis of 3D platinum nanoflowers on porous silicon, a monolithic integrated micro fuel cell based on porous silicon is obtained for the first time. The monolithic integrated μDMFC can output stable power density with passive fuel supply, which shows great potentials for optimizing the size and performance of µDMFCs, furthermore, are quite promising for integrated micro systems.
Acknowledgment 17
This work is supported by the National Natural Science Foundation of China (No. 61474071), 973 program (No. 2015CB352100) and Doctoral Research Foundation of Zhengzhou University of Light Industry (No. 2015BSJJ055).
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[46] D. S. Falcão, J. P. Pereira, C. M. Rangel, A. M. F. R. Pinto, Development and performance analysis of a metallic passive micro-direct methanol fuel cell for portable applications, Int. J. Hydrogen Energy 40 (2015) 5408-5415. [47] J. Cao, Z. Zou, Q. Huang, T. Yuan, Z. Li, B. Xia, H. Yang, Planar air-breathing micro-direct methanol fuel cell stacks based on micro-electronic–mechanical-system technology, J. Power Sources 185 (2008) 433-438. [48] Y.M. Zhu, X.H. Wang, Y.A. Zhou, L.T. Liu, A novel assembly method for micro direct methanol fuel cells using multi-layer bonding technique, Transducers 2011, Jun. 5-9, Beijing, China, 2618-2621. [49] Y. H. Seo, Y. H. Cho, Micro direct methanol fuel cells and their stacks using a polymer electrolyte sandwiched by multi-window microcolumn electrodes, Sens. Actuators A Phys. 150 (2009) 87-96. Biographies
Mei Wang received her B.S. degree in 2009 from College of Electronic Science & Engineering, Jilin University and Doctor’s degree in 2015 from Institute of Microelectronics, Tsinghua University. Now she is a lecturer in Zhengzhou University of Light Industry, China. She is engaged in fuel cells and electrochemical sensors.
Litian Liu received the B. Eng. degree and Doctor’s degree in Tsinghua University. Currently he is a professor and works in Institute of Microelectronics, Tsinghua University. His research interests are focused on microelectronic devices and their 24
fabrication techniques.
Xiaohong Wang received the B. Eng. degree and M.S. degree in Southeast University. She received her Doctor’s degree in Department of Precision Instrument at Tsinghua University in 1998. Now she is a professor in Tsinghua University. Her current research is super-capacitors, energy harvesters and fuel cells.
25
Figure captions
Fig. 1 Schematic of a monolithic integrated µDMFC based on sulfo functionalized porous silicon membrane. The monolithic integrated µDMFC combines PEM, Pt nanocatalysts and current collector layer together. PEM is achieved by sulfo functionalized porous silicon membrane and 3D Pt nanoflowers are synthesized in situ onto it as nanocatalysts. Ag nanowires film serves as current collector layer.
26
Fig. 2 Chemical grafting process of the porous silicon membrane: (1) Hydrophilization process: hydroxyl groups were attached to walls of porous silicon nanoholes by hydrophilization process; (2) Adsorption process: Si-OH groups in hydrolyzed MPTMS absorbed onto the walls of porous silicon nanoholes; (3) Grafting process: mercapto groups were grafted onto the walls of porous silicon nanoholes through dehydration condensation reaction; (4) Sulfonation process: mercapto groups were oxidize to sulfonic acid groups.
27
Fig. 3 SEM images of the porous silicon membrane before and after grafting process: frontal view (a) and cross-sectional view (b) of porous silicon membrane before grafting process, frontal view (c), cross-sectional view (d) and high magnification SEM images (e and f) of porous silicon membrane after -SH groups functionalized in a mixed benzene solution containing 30 wt. % MPTMS and 5 wt. % GAA.
28
Fig. 4 SEM images of the porous silicon membrane before (a) and after (b) grafting process and their corresponding EDS profiles.
29
Fig. 5 The FTIR spectrum of the porous silicon membrane after grafting process (a) and the FTIR spectrum of the porous silicon membrane after sulfonation process for oxidizing -SH groups to -SO3H groups (b).
30
Fig. 6 The cross-sectional XPS spectrum of the porous silicon membrane after –SH grafting process (dash dot) and after sulfonation process (solid). Sulfur 2s binding energy displays its functional group, peak at 226.7 eV corresponds to -SH groups, while peak at 230.5 eV corresponds to -SO3H groups.
Fig. 7 The Nyquist Z plots of the PEM based on sulfo functionalized porous silicon (a) and that of Nafion® 117 membrane.
31
Fig. 8 SEM images of a monolithic integrated µDMFC based on sulfo functionalized porous silicon membrane: cross-sectional view (a); high resolution SEM image of sulfo functionalized porous silicon membrane (b), Pt nanoflowers (c) and Ag nanowires (d).
32
Fig. 9 The monolithic integrated μDMFC prototype based on sulfo functionalized porous silicon membrane (a) and its corresponding I-V curves (b), operating time curve recorded at 50 mA/cm2 using 2M methanol (c).
Table 1 The proton resistances per unit area of the PEMs based on sulfo functionalized porous silicon at 25 ◦C and the cross-sectional electrode contact area of 0.196 cm2. Membrane
d (cm)
RM (Ω)
σ (S/cm)
r (cm2/S)
PEM based on sulfo
0.01
0.62
0.082
0.122
0.01
288.59
1.77*10-4
56.564
functionalized porous silicon porous silicon membrane
33
without functionalization Nafion® 117 membrane
1.83*10-2
1.55
0.060
0.304
Table 2 Comparison of PEMs based on sulfo functionalized porous silicon with Nafion® 117 membrane Membrane
as-prepared
Thickness change, % Linear expansion, %
Thickness
Compatibility
increase (from 50 %
increase (from 50 %
(μm)
with micro
RH to water soaked
RH to water soaked
fabrication
at room temperature)
at room temperature)
techniques
0
0
100
compatible
10 a
183
incompatible
PEMs Nafion® 117 10 a membrane a
The data is obtained from Dupont company.
Table 3 Comparison of the monolithic integrated µDMFC prototype with other reported µDMFCs Reference Fuel supply
DMFC
Total Size
Active
Peak
Volumetric
Structure
(without
area (cm2)
power
peak power
density
density
(mW/cm2)
(mW/cm3)
5.50
36.18
mode
This paper
holders)
Passive Monolithic 1.9cm×1.6cm integrated
×0.05cm 34
1.00
Larsen et
Passive Monolithic 1cm×0.75cm×
al. [44]
integrated
Hashim
Passive Sandwich
et al. [45] Falcão et
Passive Sandwich
Passive DMFC
[49]
1.00
2.20
0.29
2.25
19.20
24.69
8.64
17.50
28.65
2.25
20.29
40.45
1.00
0.031
0.15
0.6cm 3.0cm×2.5cm
~3.5cm×2.5c
Active
6.1cm×3.2cm
Stacks
×0.27cm
Sandwich
2.7cm×2.2cm
[48] Seo et al.
2.44
m×0.2cm
[47] Zhu et al.
2.50
×1.0cm
al. [46] Cao et al.
0.44
×0.19cm Active
Sandwich
1.23cm×1.43c m×0.12cm
35