A novel proton exchange membrane based on sulfo functionalized porous silicon for monolithic integrated micro direct methanol fuel cells
Introduction
The demand of durable and integrated power sources increases radically with rising functionalities and decreasing sizes of portable devices. Among the various fuel 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], [4], [5], [6], [7], [8] and their related components [9], [10], [11], [12], [13], [14], [15], [16], [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 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], [20], [21], cross-linked polyethylene (CLPE) [22], polyimide [23], [24], [25], poly(ethylene-co-tetrafluoroethylene) (ETFE) [26], [27], [28], and porous silicon [29], [30], [31], [32], [33], [34], [35], [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. Millions of hydroxyl groups (OH) can be attached on the walls of porous silicon nanoholes, which can crosslink together with SiOHs in organics via silanization process. For the other skeleton materials, the chemical grafting process need extra cross-linking agent [19], [20], [21], [22], [23], [24], [25] or radiation [26], [27], [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 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.
Section snippets
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
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 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
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
Acknowledgements
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).
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.
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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 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.