Superlattices and Microstructures 46 (2009) 291–296
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Metallic-semiconductor nanosystem assembly for miniaturized fuel cell applications Mihaela Miu ∗ , Mihai Danila, Teodora Ignat, Florea Craciunoiu, Irina Kleps, Monica Simion, Adina Bragaru, Adrian Dinescu National Institute for Research and Development in Microtechnologies (IMT-Bucharest), 077190, Romania
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Article history: Available online 9 December 2008 Keywords: Electro-catalyst-membrane Nanostructured silicon SiO2 nanoparticles Platinum nanoparticles
abstract In this paper, a new metallic-semiconductor nanosystem was studied for potential integration in a micro fuel cell as a proton exchange membrane (PEM)/electro-catalyst assembly using specific processes to nano-micro-electromechanical system (N/MEMS) technology. In order to increase the active area of Si based support layers for catalyst particles, two types of technologies have been developed: (i) an electrochemical etching process leading to a nanostructurated silicon layer; (ii) a precipitation technique based on controlled hydrolysis of a silicon alkoxide leading to SiO2 nanoparticles. The pores/fibrils of porous silicon have comparable dimensions with silica nanoparticles, being tens of nanometers, and present a similar chemical map of the internal walls due to the oxidation process achieved by exposure to ambient air. A Pt precursor solution (3.5 mM H2 PtCl6 ) was used for catalyst chemical deposition, and the experimental samples have been analyzed from the morphological and compositional point of view using microscopic (SEM) and compositional (EDX, XRD) techniques. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, the fuel cell technology has been adopted in portable electronics with the demand for uninterrupted power sources and the miniaturization of standard devices became a necessity without negative influences on operational factors [1]. Semiconductor technology is the main micro fabrication route used to reduce the standard structure of fuel cells to micrometer sizes. For miniaturization of direct methanol fuel cells (DMFC), silicon (Si) is a suitable material since
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0749-6036/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.spmi.2008.10.040
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it can be micropatterned using a variety of processes from semiconductor technology leading to desired features. The standard design of fuel cells comprises an anode part, a cathode part and a proton exchange membrane (PEM) sandwiched in-between the anode and the cathode, usually built on 2D geometries and assembled into 3D shapes. The membrane electro-catalyst assembly (MEA) represents the central compartment, where the electrochemical reactions take place, and also the ionic charges produced are conducted between the electrodes and thereby complete the cell’s electric circuit [2]. The electrochemical reaction of the fuel cell involves at least three species: gas, ions and electrons. Despite great efforts to find alternative efficient low-price catalysts, platinum and platinum based alloys remain the only ones capable of exhibiting full catalytic activities if the all three species are sufficiently supplied, generating high rates of chemical reactions at relatively low operating temperatures [3,4]. Our approach is to improve the deposition efficiency and the retention of Pt particles developing a high-surface-area supporting substrate. In later papers the researchers have been mainly focused on incorporating the Pt particles supported on hygroscopic oxides into the membrane to obtain low reactant crossover and resistance at the same time [5]; another research direction in this area was the fabrication of nanostructurated supporting layers/membranes – inorganic [6], polymeric [7], or even composites [8] – which contain the specific active sites for metallic nanoparticle attachment. 2. Experimental results and discussion
• Due the envisaged application on development of fully integrated miniaturized DMFC on Si, we have studied the Pt deposition on two types of Si based insulated catalyst supports: (i) oxidized nanostructured Si and (ii) silica (SiO2 ) nanoparticles. (i) Although more sophisticated techniques, as ICP-DRIE or EBL have been lately used in nanoscale fabrication to create columns/pillars on a Si substrate [9], electrochemical etching remains a simple process allowing the increase of the geometry surface area because it leads to a porous structure where the pore’s dimensions and shapes are determined by the process parameters. High aspect ratio Si fibrils are potentially good supporting platforms to host the noble metal based electrocatalysts leading an increase of the electroactive area explained by the presence of particle formed on the sidewalls of silicon columns. Moreover, the hydride terminated freshly etched porous silicon (PS) surface is readily oxidized, by simple storage in the atmosphere, leaving hydroxyl terminated nanoscale pores which may provide suitable channels for proton transport and in addition may serve as a diffusion barrier preventing larger fuel molecules from going through membrane [10]. Heavily phosphorous-doped n-type Si wafers, with a 1–5 m cm resistivities, have been used as substrate for nanostructured Si layers to preserve the necessary electrical characteristics. In order to study the influence of substrate’s crystallographic orientation in a metal deposition process, similar porosification processes have been set up on (100) and (111) Si wafers using an electrolyte solution with 25% HF concentration for 30 min to obtain PS layers with 50–60 µm thickness. The electrochemical etching of highly doped Si in aqueous electrolytes always produces mesopores, ranging from 10 nm to 50 nm diameters, depending on the current density. The pore formation models [11] predict the morphology dependence on substrate crystallographic orientation; therefore, assuming the h100i as pore growing direction, similar substrate orientation determines a straight geometry, and respectively branched one for (111)-Si, where an additional etching is developed perpendicular to surface, directly towards the current flow and consequently the source of the holes which become more clear with an increase of the electrical parameter. The plan view (a) and cross section (b) SEM images of N17 (S4) sample are presented in Fig. 1 and reveal the branched morphology of pores with diameters in the range of 20–30 nm. (ii) On the other hand, SiO2 nanoparticles in the same range size as PS fibrils have been obtained by a precipitation technique based on the controlled hydrolysis of a silicon alkoxide (tetraethylorthosilicate — TEOS) in a mixture of ethanol containing aqueous ammonia with the mass ratio TEOS:NH4 OH:C2 H5 OH = 4:2:50 (ml); the time of reaction 24 h. In order to further prevent nanoparticle agglomeration, ethylene glycol (EG) was used as a stabilizing agent being known its efficiency in metal nanoparticle synthesis [12]. The SEM images from Fig. 2 reveal two domains of dimensions
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Fig. 1. SEM images of nanoporous layer realized on (111)–Si: (a) plan view; (b) cross section.
Fig. 2. The SEM images of spherical SiO2 particles obtained by precipitation technique (a) the marker confirms the 91.1 nm diameter of particles; (b) the marker confirms the 18.2 nm diameter of particles.
for spherical particles: the first type, with 90–100 nm diameters — Fig. 2(a), and the second one, with diameters around 10–20 nm — Fig. 2(b). A detailed description of process parameters used to fabricate the nanostructurated Si based substrates for catalyst loading, further analyzed in the present paper, is presented in Table 1. • The deposition of catalyst layer/particles is one of the key processes for producing a feasible MEA: it should adhere strongly to the membrane in order to reduce ohmic losses and to support high mechanical stresses produced during the operation [13]. Moreover, it has been demonstrated that a metallic nanoparticle array presents an enhanced electro-activity comparing with macroelectrodes due to their high surface-to-volume ratio [14]. A platinum (Pt) precursor solution was used to achieve the chemical attachment of catalysts on nanostructurated Si based supports by simple storage of the samples for different periods of time, from one hour to five days. The PS layers were stored in 3.5 mM H2 PtCl6 solution, and the respectively, the SiO2 and H2 PtCl6 (3.7 mg of Pt/ml) in ethanolic solution
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Table 1 Experimental conditions. Catalyst support
Sample
Process parameters for catalyst support fabrication
Process parameters for Pt loading
Analyses/observation Pt nanoparticles average diameter (nm)/content
NanoPS
N16 (S1) N17 (S4)
n+ -(111) Si; 25% HF; 50 mA/cm2 5–10 nm pore diameter (SEM) n+ - (111) Si; 25% HF; 100 mA/cm2 15–20 nm pore diameter(SEM) n+ -type Si (100); 25% HF; 50 mA/cm2 20–25 nm pore diameter(SEM) n+ -type Si (100);25% HF; 100 mA/cm2 25 –30 nm pore diameter(SEM) Technique based on controlled hydrolysis of TEOS:NH4 OH:C2 H5 OH = 4:2:50 (ml)
1 day immersion in Pt precursor solution
∼10.1 nm (SAXS) 10–12 nm (in–plane X-ray spectrum) ∼18.5 nm (SAXS) 18–20 nm (in–plane X-ray spectrum)
N06
N07
SiO2 nanoparticles
ab
∼26.8 nm (SAXS) ∼29.4 nm (SAXS) The Pt content is improved by a reduction of SiO2 nanoparticle size (EDX)
Fig. 3. (a) The SEM image of Pt nanoparticles deposited on PS surface; (b) Size distribution of Pt nanoparticles obtained from the SAXS pattern.
were mixed at a 1:1 the volumetric ratio [15]. Besides its simplicity, the catalyst loading by liquid phase represents a suitable method for both types of samples: for example, regarding PS layers, the surface tension coefficient favors the ethanol based solution to go inside the pores as soon as it reaches the PS-Si surface, and respectively, the dispersion of silica particles in solution initiates the whole surface exposure to metal nucleation. The morphological analyses of the N06 sample from Fig. 3(a) that shows the homogeneous distribution of Pt nanoparticles on the PS surface has been further confirmed by small angle X-ray scattering (SAXS) measurements. SAXS technique is very precisely for analyzing small particle used to measure a wide range of nanoparticle sizes (from 1 to several tens of nanometers) [16] and Fig. 3(b) represents the result of the fit of the experimental data with a spherically shaped particle model, since the ellipsoidal shaped model does not produce a good fit. Assuming the 30 nm pore size of the PS sample support, the mean particle diameter of 26.8 nm for spherical distribution give us an indication that the deposition process starts inside the pores and is followed by a nucleation of particles on surfaces.
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Fig. 4. EDX spectra for nanoPt on 90 nm SiO2 nanoparticles (a) and for 10 nm SiO2 nanoparticles (b) respectively.
Fig. 5. (a) In-plane X-ray spectra of samples S1 (5–10 nm pores) and S4 (10–15 nm pores); (b) X-ray spectra for Pt impregnated in PS/Si (100) – N07 – and respectively PS/Si (111) – N17 – test samples.
Regarding the Pt content measured on the each type of SiO2 particles, the EDAX analyses presented in Fig. 4 reveal that reducing of SiO2 nanoparticle size leads to improvement of Pt distribution corresponding to increase of surface/volume ratio of catalyst. The X-ray data recorded in different experimental configuration, as in–plane X-ray diffraction and wide angle X-ray diffraction methods (WAXD) complete the analyses with information about the crystalline structure of the metallic particles. The Bruker AXS D8 diffractometer in θ –2θ angular scanning using a Cu X-ray tube (X-ray Kα wavelength of λK α = 1.540609 Å)has been used with an operating power of U = 40 kV, I = 40 mA, and a 2θ step of 0, 05◦ ; the contiguous X-ray emission spectra of the X-ray tube were filtered using a Ni absorber filter. The in-plane X-ray spectra of Pt nanoparticles loaded on PS/Si-(111) presented in Fig. 5(a) show that the Pt (111) is predominant texture, being a new confirmation that crystallization of Pt took place at the beginning inside the pores, and then continues on the surface, but it always preserves the preferred orientation i.e. the texture of the substrate. The crystalline Pt content of sample S4 is almost double than that of sample S1 and the corresponding crystallite medium size in the direction parallel with the wafer surface is also double in sample S4 compared with sample S1, from around 18–20 nm in S4 to 10–12 nm in S1 (medium crystallite sizes are calculated using the Scherrer equation; crystallite size is inversely proportional with the width of the diffraction line FWHM), in good agreement with the pore diameter. On the other hand, the nature of Pt nanoparticles deposited on PS supports fabricated using the same process parameters for different substrates crystallographic orientation has been compared and 1
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the XRD patterns corresponding to N07 and N17 are presented in Fig. 5 (b). We can conclude the (100) oriented Si substrate produces a higher crystalline content and larger (111) Pt crystallite medium size, but a pronounced (111) texture on planes parallel with the surface is observed on (111) n-Si substrate. 3. Conclusions Two types of Si based catalyst support nanomaterials have been studied for further integration in a miniaturized DMFC: (i) oxidized porous silicon layers; (ii) silica nanoparticles. A simple chemical deposition process was used for Pt impregnation and it was demonstrated that the substrate structures influence the dimensions and also the crystalline structures of metallic particles. Besides of all above mentioned advantages related to Pt catalyst nanostructuration, the silicon oxide assimilated in both structures, determine good membrane hydration due to the imbedded hygroscopic Pt–SiO2 catalysts. The advantages of porous structure is that it allows Pt to grow on surface and also inside the nanostructured layer and also that it can be subjected to additional functionalisation/grafting processes to mimic the supposed structure of Nafion membranes – the standard membrane used in fuel cells which is inadequate for MEMS technology – in order to obtain significant proton conductivity. Acknowledgment The authors gratefully acknowledge the support of the Romanian Ministry of Education and Research through the contract no. 11.023, 11.024 (PN II programme) and respectively through the contract no. ID-883 (PN II –IDEI programme). References [1] C.S. Kelly, G.A. Deluga, W.H. Smyrl, Electrochem. Solid-State Lett. 3 (2000) 407. [2] L. Carrette, K.A. Friedrich, U. Stimming, Fuel Cells 1 (1) (2001) 5. [3] W. Goddard III, B. Merinov, A. van Duin, T. Jacob, M. Blanco, V. Molinero, S.S. Jang, Y.H. Jang, Molecular Simulation 32 (3–4) (2006) 251. [4] H.X. Huang, S.X. Chen, C. Yuan, J. Power Sources 175 (2008) 168. [5] H. Hagihara, H. Uchida, M. Watanabe, Electrochim. Acta 51 (2006) 3979. [6] C. Feng, Z. Xiao, P.C.H. Chan, I.M. Hsing, Electrochem. Commun. 8 (2006) 1235. [7] Y. Zhang, H. Zhang, X. Zhu, L. Gang, C. Bi, Y. Liang, J. Power Sources 165 (2007) 786. [8] P. Jannasch, Current Opinion in Colloid and Interface Science 8 (2003) 96. [9] S.-C Yao, X. Tang, C.-C Hsieh, Y. Alyousef, M. Vladimer, G.K. Fedder, C.H. Amon, Energy 31 (2006) 636. [10] K.L. Chu, S. Gold, V. Subramanian, C. Lu, M.A. Shannon, R.I. Masel, J. Microelectromech. Syst. 15 (3) (2006). [11] H. Foll, M. Christophersen, J. Carstensen, G. Hasse, Mater. Sci. Eng. R 280 (2002) 1. [12] Y. Wang, J. Ren, K. Deng, L. Gui, Y. Tang, Chem. Mater. 12 (2000) 1622. [13] R. O’Hayer, S.J. Lee, S.W. Cha, F.B. Prinz, J. Power Sources 109 (2002) 483. [14] A. Kundu, J.H. Jang, J.H. Gil, C.R. Jung, H.R. Lee, S.-H. Kim, B. Ku, Y.S. Oh, J. Power Sources 170 (2007) 67. [15] X. Zhu, H. Zhang, Y. Liang, Y. Zhang, Electrochem. Solid-State Lett. 9 (2006) A49. [16] H. Song, R.M. Rioux, J.D. Hoefelmeyer, R. Komor, K. Niesz, M. Grass, P. Yang, G.A. Somorjai, J. Am. Chem. Soc. 128 (2006) 3027.
