The construction of integrated Si-based micro proton exchange membrane fuel cells with improved performances

Nano Energy 61 (2019) 604–610

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The construction of integrated Si-based micro proton exchange membrane fuel cells with improved performances

T

Yingjian Yua,c,d,1, Yucheng Wangb,1, Shan Zhanga, Pengyang Zhangb, Shi Xuea, Yannan Xiee,∗, Zhiyou Zhoub,∗∗, Jing Lia,∗∗∗, Junyong Kanga a Collaborative Innovation Center for Optoelectronic Semiconductors and Efficient Devices, Department of Physics/Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen, Fujian, 361005, China b State Key Lab of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian, 361005, China c Department of Physics Science and Technology, Kunming University, Kunming, Yunnan, 650214, China d National Laboratory of Solid State Microstructures, Nanjing University, Nanjing, 210093, China e Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, Nanjing, 210023, Jiangsu, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Si-based flow fields PEMFCs Dry etching Hierarchical structure Integration

The integrated micro proton exchange membrane fuel cells (μPEMFCs) were fabricated by constructing the hierarchical dot-type flow fields on Si wafers and then being compacted with the carbon black as gas diffusion layer (GDL) and Pt/C catalyst layer, which presents the simplicity and good device-integrity by replacing the traditional carbon paper or cloth as GDL and omitting the hot-press processing. The Si-compatible processes, including the template assembly of SiO2 spheres, dry etching, channel dicing, were elaborately manipulated to optimize the wafer-scale hierarchical flow field and thus industriously improve the fluids transport and gas distribution. Ultimately, the peak power of over 1W/cm2 was accomplished when the graphite bipolar plates were replaced by the hierarchical Si-based flow fields alone, while the maximum power of 354 mW/cm2 was successfully demonstrated in the integrated Si-based μPEMFCs with measured size of 4 cm2. This work not only broadens the application of PEMFCs to micro/nano-electro-mechanical systems (M/NEMS) or other Si-based electronic devices, but also provides guidelines for fabricating other metal-air batteries such as Zn-air batteries and Al-air batteries.

1. Introduction Given high power density and environmentally friendly, e.g. low emission and noise, proton exchange membrane fuel cells (PEMFCs) have been considered to be promising in applications of transportation, combined heat and power (CHP) systems, and portable electronic devices [1–10]. Usually, PEMFCs consist of some pivotal components including bipolar plates and membrane-electrode-assembly (MEA) [2]. In traditional designs, bipolar plates, acting as reactants distributors and current collectors, [11–15] are usually made of graphite, metal sheets or graphite-polymer composites that may be heavy or hard to machine [16–19]. Meanwhile, to fabricate MEA, catalyst inks containing ionomer and solvent are often coated on gas diffusion layer (GDL) like carbon paper or cloth through decal [20], spraying [21] or brushing

[22], and then the proton electrolyte membrane (PEM) is inserted between anode and cathode layers, followed by the hot pressing [23]. The whole procedures are complex and not suited to mass production in micro/nano-electro-mechanical systems (M/NEMS) due to the uncertainty in processing [24]. Moreover, the torturous porosity formed in the conventional catalyst layer may reduce the effective diffusivities of fluids and mass transfer [22]. To solve the dilemma facing with PEMFCs, a novel approach should be explored to redesign and reconstruct a PEMFC. Nowadays, the dramatic developing of M/NEMS promotes the demand for Si-compatible processes to fabricate Si-based bipolar plates with miniature flow-fields, which would be possible for further direct integration with catalysts and even replacement of the conventional MEA [25–30]. In some previous works on bipolar plates for micro fuel cells, researchers attempted to process flow fields on

∗

Corresponding author. Corresponding author. ∗∗∗ Corresponding author. E-mail addresses: [email protected] (Y. Xie), [email protected] (Z. Zhou), [email protected] (J. Li). 1 These two authors contributed equally. ∗∗

https://doi.org/10.1016/j.nanoen.2019.05.014 Received 21 February 2019; Received in revised form 25 April 2019; Accepted 6 May 2019 Available online 07 May 2019 2211-2855/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. Schematic showing the comparison between (a) the common Si-based flow field and (b) the hierarchical Si-based flow field with multi-function.

assembled by a spin-coating process containing a low-speed rotation at ∼500 rpm for 20 s and a subsequent high-speed rotation at ∼850 rpm for 30 s, as shown in Fig. S1. After coated with SiO2 template, the prepared Si wafer was etched in a SENTECH SI500 ICP system under an RF power of 200 W at 20 °C. The gases combination of SF6 and O2 with flow rates of 20 sccm and 40 sccm were applied to etch the Si wafer vertically. After the dry etching process, the SiO2 micro-spheres were removed by cleaning in 10 wt% the NaOH solution for 10 min and then washed by DI water, and the Si MRs can be obtained. Furthermore, the prepared Si wafer was diced and the hierarchical Si-based flow field was constructed. Finally, the Si wafer was cut into square pieces with the size of 2 cm × 2 cm for further electrochemical measurements.

silicon wafer by photolithography and etching; however, the cell performances without appropriate conduction layers were not satisfactory due to the high cell resistance [31,32]. Meanwhile, the dimensions of flow fields fabricated by photolithography are usually larger than 100 μm [33,34]. Since the electronic devices are becoming more and more miniaturized, tiny structures (e.g. < 10 μm) are usually necessary to distribute the fluids uniformly. The lack of more tiny structures may also hinder the further catalysts integration due to the low electrochemical surface area, as shown in Fig. 1a. Attempts to meliorate the MEA have also been reported like porous silicon, which could be used as a catalyst support or a PEM; however, the integration of catalysts are still challenging because the random porous configurations probably restrict the depth of deposition [35]. In this work, novel designs of the miniature hierarchical flow fields and integrated micro proton exchange membrane fuel cells (μPEMFCs) were realized on Si wafers. As good mass transfers are critical issues for sufficient and efficient electrochemical conversion of the fuel, the vertical dot-type flow fields were fabricated by template-assisted dry etching cooperating with the dicing. In this hierarchy, the nicks fabricated by the dicing act as distributors of fluids; meanwhile, the Si micro rods (MRs) would support the catalysts and provide sufficient space for the gas distribution, as illustrated in Fig. 1b. Consequently, the peak power of over 1W/cm2 was reached by utilizing the hierarchical Sibased flow fields, which was positively affected by the etching time and dicing. Furthermore, the commercial Pt/C catalysts (the load of Pt was 0.2 mg/cm2) and carbon black as GDL were directly dropped onto the hierarchical flow fields and the integrated Si-based μPEMFCs were constructed. Eventually, a highest peak power of 354 mW/cm2 was measured, which is comparable to the best performances in the Si-based μPEMFC (measure size > 0.2 cm2) as far as we know [28,31,32,36–46]. This work proposed a novel design of the hierarchical Si-based flow fields, which can distribute fluids and further act as a platform for the construction of integrated Si-based μPEMFCs to replace the traditional carbon paper or cloth as gas diffusion layer. The uniqueness of this work is that the wafer-scale (2 or 4 inches) Si-compatible standardized processing is conducted, so that the sizes of cells can vary in a wide range from 0.5 cm to 5 cm to meet different demands, which would facilitate their applications in MEMS devices and provide new possibilities for the design of small electronic devices [47,48].

2.2. Fabrication of integrated Si-based μPEMFCs To integrate GDL on Si-based flow fields, the ink containing 20 mg carbon black (KJ600), 4 mL isopropanol, 60 mg PTFE emulsion (15 wt %) was dropped directly onto the processed Si wafers. Then, the catalyst ink containing commercial Pt/C catalysts, H2O, isopropanol and Nafion solution (5 wt%) was sprayed onto the GDL. The load of Pt for cathode was 0.2 mg/cm2. Finally, the Nafion membrane (Dupont NRE 212) was nipped by two processed Si wafers and the integrated Si-based μPEMFC was constructed. 2.3. Morphologies and structures characterization The morphologies of the Si MRs were investigated using the SU70 thermal field emission scanning electron microscope (SEM). The acceleration voltage for SEM is 10 kV and the working distance (WD) is 5 mm. The transmission electron microscope (TEM) images of the Pt/C catalyst were obtained by JEM-2100 HRTEM and the accelerating voltage is 200 kV. The X-ray diffraction (XRD) pattern of the Pt/C catalyst was characterized by Panalytical X'pert PRO using Cu-Kα radiation (λ = 1.5406 Å). 2.4. Preparation for electrochemical testing For the electrochemical testing of hierarchical Si-based flow fields, 20 nm Cr/200 nm Au were consecutively sputtered on both sides of the wafer by an Explorer-14 magnetron sputtering system to decrease the ohmic resistance. Then, the prepared Si wafers were fixed into the specially-made devices for O2/H2 transfer. The membrane electrode assembly (MEA) was fabricated by hot-pressing a Nafion membrane (Dupont NRE 212) nipped by commercial Pt/C papers and appropriate size of gaskets at 135 °C for 2 min with the pressure of 5 MPa. For the electrochemical testing of integrated Si-based μPEMFCs, the μPEMFCs were fixed in the specially-made devices mentioned above.

2. Experimental section 2.1. Fabrication of hierarchical flow fields made from Si wafer As shown in the scheme in Fig. 2, to fabricate channels for gas transport, the Si wafer was diced by a DISCO DAD321 dicing saw, named as single structure. The n-type Si < 100 > wafers have a thickness of ∼800 μm. To construct the hierarchical structure, the Si micro-rod (MR) arrays were prepared by the SiO2 template method followed by inductive coupled plasma (ICP) etching. The 40 wt% SiO2 microsphere suspension with the diameter of ∼10 μm was spin-coated on the Si wafer with a hydrophilic surface. The SiO2 template was

2.5. Electrochemical testing Polarization curves were measured by an 850e Fuel Cell Test System (Arbin Instrument Corporation). Humidified O2 or air and H2 with a flow rate of 0.2 L/min were fed and the whole cell was kept at 80 °C. 605

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Fig. 2. Scheme: Illustration of the fabrication processes for the Si-made flow field: (i and v) dicing of the processed Si wafer, (ii) SiO2 microsphere template assembly, (iii) ICP etching employing SF6/O2 gases to fabricate the Si MR arrays, and (iv) template removed by the NaOH solution cleaning. (a) The photograph of the Si wafer with hierarchical structures; the top-view (b) and side-view (c) SEM images of Si-based hierarchical flow fields; (d) the top-view SEM image of Si MRs; section-view SEM images of Si MRs etched for (e) 1200s, (f) 2000s and (g) 3000s.

After a steady open circuit voltage (OCV) was obtained, the polarization performance was tested. The test of electrochemical impedance spectroscopy (EIS) was carried out by a Princeton 2263 potentiostat and the analysis was conducted by ZPlot/ZView software. During the EIS test, the H2 and O2 were fed to the anode and cathode at the voltage of 0.6 V with an AC amplitude of 10 mV and a frequency range of 0.1 Hz–100 kHz. 3. Results and discussion The photograph of the Si wafer with hierarchical structure can be seen in Fig. 2a. The size of the square piece is 2 cm × 2 cm, and the Cr/ Au conducting layers have already been sputtered on both sides of the piece. The hierarchical flow fields were fabricated by dicing Si wafers with Si MRs halfway as visualized in Fig. 2b. The distance between the nicks is 1 mm and the width of the nicks is ∼200 μm. Meanwhile, the depths of the nicks are ∼400 μm, as shown in Fig. 2c. The nicks can transport gas reactants and remove excessive liquid water. Besides the nicks, the hierarchical flow fields contain the Si MR arrays as exhibited in Fig. 2d. The configuration of the Si MR arrays is similar to dot-type flow fields, in which the gases can be uniformly distributed [49]. With the protection of SiO2 micro spheres template, the depth of Si MRs increases from ∼6.6 μm to 15 μm (Fig. 2e–g) when the etching time rises from 1200 s to 3000 s. The clearance among MRs also increases apparently as the SF6/O2 gases would etch both vertically and horizontally. The etching time cannot be too long in case the Si MRs are over etched. The large surface areas of the Si MRs indicate their potential applications in M/NEMS and integration with catalysts via Sicompatible processes like the magnetron sputtering [50–52], pulsed laser deposition (PLD) [53–55], atomic layer deposition (ALD) [56–58] and spraying [59–61]. To investigate how the clearances among Si MRs would influence the properties of PEMFCs, Si-based flow fields with only Si MRs of various depths were tested, as exhibited in Fig. 3. Along with the prolonging of etching time (from 0 s to 3000 s), the depths of Si MRs increase and the peak powers rise from 159 mW/cm2 to 524 mW/cm2. It can be demonstrated that the deeper-etched Si MRs with more sufficient clearances are beneficial to the mass transfer and provide more reactants to the GDL, thus enhancing the peak powers of the PEMFCs. Moreover, it is worth mentioning that the voltages of the devices are

Fig. 3. The performances of PEMFCs fabricated by different etching time (without nicks by dicing).

related to the electron and mass transfer. In the low current density (< 0.125 A cm−2), the voltages are mainly decided by the electron transfer. For the devices with etching times between 0 and 1200s, the gap between the Si MRs is small and the Cr/Au conduction layer cannot be fully deposited onto the Si surface, which leads to higher resistances and voltages. In the high current density region, the voltages are more sensitive to the mass transfer. For the cases of 2000 and 3000 s etching, the Si wafers are etched deeper, which are favorable for the mass transfer. As a result, in the low current density region, the voltages of the devices with etching times between 0 and 1200 s are higher than those of the samples with 2000 and 3000 s etching; while in the high current density region, the reverse happens. In addition, due to the special design of the Si micro rods flow field, the gas distribution is more homogeneous. So at the low voltage region large current density was generated and thus the high power density in devices can be realized. After taking the stability of the configuration of Si MRs and the electrochemical performances into consideration, the etching time would be fixed at 3000 s for the following tests and analyses. The electrochemical performances of the Si-based flow field with single and hierarchical structures (dicing depth = 400 μm) are shown in Fig. 4. Expectedly, the Si MRs in the hierarchical flow fields 606

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Fig. 4. (a) The polarization curves and (b) power densities of Si-based flow fields with single and hierarchical structures (dicing depth = 400 μm).

significantly affect the properties of PEMFCs. As shown in Fig. 4a, samples with single structures generated a current density of ∼1.7 A cm−2 at the voltage of 0.4V and have an average peak power of 658 mW/cm2. For comparison, a higher current density of 2.5 A cm−2 can be realized in PEMFCs with hierarchical structures, and the average peak power density is achieved to be ∼958 mW/cm2, as seen in Fig. 4b. It is well known that, polarization performance at low current density was mainly controlled by kinetic properties of ORR catalysts and the difference at large current density region was mainly attributed to different oxygen-transport conditions. Combined with the difference of these two flow channel structures, it is rational to conclude that the improved mass-transport properties of hierarchical Si flow channel were mainly achieved by the introduction of Si MRs. The nicks only provide basic requirements for gas transport and the movement of excessive products, as shown in Fig. 1a. The addition of Si MRs on nicks enriches the paths for gases distribution, which promote the mass transfer when ORR and HOR happens (Fig. 1b). To strengthen the argument, the single and hierarchical samples with the dicing depth of 200 μm were also tested with the results illustrated in Fig. S2. It can be found that the hierarchical Si-based flow field has a higher maximum power density of 885 mW/cm2 than the corresponding single sample. Furthermore, as shown in Fig. S3, the samples with the single and hierarchical structures were tested when the air and pure O2 were imported in the cathode, respectively. For the sample with the single flow field, the peak power density is 367 mW/cm2 when the air was introduced into the cathode. After the air was replaced by pure O2, the peak power density reaches 664 mW/cm2. For a comparison, when the sample with the hierarchical flow field is applied, the peak power density rises more obviously under the O2 atmosphere, enhancing from 368 mW/cm2 to 952 mW/cm2. The hierarchical flow field helps to distribute the O2 homogeneously and improve the mass transport. Thus, among the samples with hierarchical structures as shown in Fig. 4b, the highest peak power density of 1004 mW/cm2 can be accomplished, verifying the effectiveness of the rational designs of the hierarchical Sibased flow fields. To make the whole procedure more suitable to mass production and standardization, the construction of integrated Si-based electrode were realized by taking the Si-based flow fields (single and hierarchical) as platforms to load catalysts, as shown in Fig. 5a and b. The SEM images of integrated hierarchical Si-based electrode can be visualized in Fig. S4. Pt/C catalysts and porous carbon almost distribute among the Si MRs (Figs. S5 and S6) and large channels still exist since the porous carbon and catalysts do not fulfill the nicks. The X-ray diffraction (XRD) and transmission electron microscope (TEM) (Fig. S7) characterized the Pt particles in the diameter of ∼5 nm with the orientation of (111) plane. Unlike conventional gas diffusion electrode (GDE) including gas diffusion layer (Carbon paper) and microporous layer (MPL), which makes gas distribute uniformly across the whole catalyst layer, integrated Si electrode discards these two components for the smaller thickness and higher volume power density. Luckily, the specific hierarchical configuration includes not only large channels (nicks) but also tiny space among Si MRs. Large channels (nicks) provides enough room

Fig. 5. The diagrams of integrated Si-based PEMFCs with (a) single structures and (b) hierarchical structures; (c) The polarization curves and power densities of integrated Si-based PEMFCs with single and hierarchical structures. The inset shows the performance of the Dicing-400 sample.

for gas transport while tiny space among Si MRs make gas uniformly distributed across the whole catalyst layer (Fig. 5b). However, in the case of single Si electrode, gas (O2/H2) transport can be achieved through large channels (nicks) but gas (O2/H2) distribution is inhomogeneous in the whole catalyst layer, which will result in severe concentration losses (Fig. 5a). After the integration of Si-based electrode, the Si-based μPEMFC were constructed simply by fixing two Sibased electrode with a Nafion membrane without the hot-pressing, as shown in Fig. S8. The thickness of the Si-based μPEMFC is less than 2 mm, and the active area is ∼4 cm2. The electrochemical performances of different Si-based μPEMFCs are shown in Fig. 5c. Expectedly, the sample with hierarchical structures has a much higher cell voltage and current density, and exhibits the highest peak power of 354 mW/ cm2, which is 12 times higher than that of the sample with only single structures. As far as we know, it is one of the highest peak power in Sibased μPEMFC with active area larger than 0.2 cm2, referring to the specific comparison as listed in Table 1 [28,31,32,36–46]. The satisfactory properties result from the rational designs of hierarchical Sibased flow fields, successful replacing the conventional GDLs (carbon paper or cloth) by integrating the carbon black directly onto the hierarchical Si-based flow fields. The power densities of the integrated Sibased μPEMFC could be furtherly improved by optimizing the 607

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Table 1 The comparison of the performances of micro-PEMFCs. Author

Dimensions (active area)

Gas Conditions

Peak power density (mW cm-2)

Itsuki et al. [28] Yu et al. [31] Lee et al. [32] Lee et al. [39] Peng et al. [40] Ogura et al. [41] Kouassi et al. [43] Kim et al. [44] Yu et al. [45] Yamazaki et al. [46] This work

4 mm * 4 mm 5 cm2 10 mm * 10 mm 10 mm * 10 mm 1 cm * 1 cm

H2: 10 sccm O2: 5 sccm H2: 50 ml/min O2: 50 ml/min Inlet: 100 kPa H2: 16.1 ml/min O2: 857 ml/min H2: 20 ml/min O2: 40 ml/min

240 194.3 42 174.6 26 420 90 275 190.4 37 354

1 cm2 H2: 50 ml/min H2: 40 ml/min O2: 60 ml/min

10 mm * 3 mm 1 cm * 1 cm 2 cm * 2 cm

H2: 200 ml/min O2: 200 ml/min

hierarchical Si-based flow fields and meliorating the integration processes of GDLs and catalysts. To further investigate the mechanism, the EIS test was employed as shown in Fig. S9. Ws represents the finite length Warburg-short circuit terminus and the expression of the impedance is shown below:

Z=

R×tanh( i× T× ω) p ( i× T× ω) p

Materials and Application, the Fundamental Research Funds for the Central Universities (Grant No. 20720160089 and 20720170041), and the Start-Up Fund from Nanjing University of Posts and Telecommunications (Grant No. NY218151). Appendix A. Supplementary data

(1)

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.nanoen.2019.05.014.

In the expression, i is the current density, T is the time constant, ω is the frequency and p is the exponent. The fitted Ws-R represents the resistance to proton conduction in the MEA. R1, R2 and CPE-1 represent the electronic resistance in the MEA, charge-transfer resistance in the catalyst layer and double-layer capacitance, respectively. As listed in Table S1, it can be found that all kinds of the resistance mentioned above are smaller in the hierarchical structure than those in the single structure, evidencing that the hierarchical structure is also beneficial for the proton and electron transfer.

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4. Conclusion In conclusion, the novel integrated Si-based μPEMFCs were constructed by the fabrication of hierarchical Si-based flow fields and integration of the GDL and catalysts. The hierarchical Si-based flow fields were fabricated by a Si-compatible process. After the introduction of the etching cooperating with dicing to improve the mass transfer and gases distribution, the highest peak power of over 1W/cm2 is realized. Furthermore, the Si MRs were employed as platforms to prepare integrated μPEMFCs. It can be demonstrated that the Si MRs increase the contact area between catalysts and the Nafion membrane and provide more voids for gases distribution. Finally, the highest peak power of 354 mW/cm2 has been realized, which is comparable to the best performances in the Si-based μPEMFCs with the active area larger than 0.2 cm2. This work provides guidelines for the construction of novel integrated Si-based μPEMFCs, and the design mentality may also be applied in other metal-air batteries. Acknowledgement This work is financially supported by the National Basic Research Program of China (Grant No. 2015CB932301), National Natural Science Foundation of China (Grant No. 61675173, 61505172, U1405253, and 61601394), Natural Science Foundation of Fujian Province of China (Grant No. 2017H6022, 2018J01102, and 2016J01319), Science and Technology Program of Xiamen City of China (Grant No. 3502Z20161223 and 3502Z20144079), Natural Science Foundation of Guangdong Province (Grant No. 2018B030311002), Science Foundation of Yunnan Provincial Education Department (Grant No. 2018JS392), Science Foundation of National Laboratory of Solid State Microstructures (Grant No. M31036), the Talents Introduction Project of Kunming University (Grant No. YJL18008), the Open Project Program of the Fujian Province Key Laboratory of Semiconductor 608

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Dr. Yingjian Yu is currently a Lecturer in Department of Physics Science and Technology at Kunming University. He received his B.S. degree in Materials Science and Engineering from Xiamen University and Ph.D. degree in Microelectronics and Solid Electronics from Xiamen University. His research interests include fuel cells, lithium/ sodium ion batteries, semiconductor-air batteries and phononic crystals.

Yucheng Wang received his bachelor's degree at Hubei University in 2014. He is currently a Ph.D. student in physical chemistry under the supervisor of Prof. Zhi-You Zhou and Shi-Gang Sun at Xiamen University. His research interests are non-noble precious metal oxygen reduction reaction catalysts and fuel cells.

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Y. Yu, et al. Prof. Yannan Xie is currently a Professor in Institute of Advanced Materials at Nanjing University of Posts and Telecommunications. He received his B.S. degree in Applied Physics from Nanjing University of Science and Technology and Ph.D. degree in Microelectronics from Xiamen University. His research interests focus on nanogenerators, self-powered systems, and energy harvesting technology.

Prof. Jing Li is currently a Professor in Pen-Tung Sah Institute of Micro-Nano Science and Technology at Xiamen University. She received her B.S. degree in Analytical Chemistry from Xiamen University, M.S. degree inMaterials Science from National University of Singapore, and Ph.D. degree in Physics from Xiamen University. Her research interests include optoelectronic materials and devices, nanomaterials for energy storage or conversion, plasmonic nanostructures for sensing,wide-band-gap semiconductors, and solar cells.

Zhiyou Zhou is a professor at the Department of Chemistry, Xiamen University, China. He received his B.Sc. Degree in 1998 and Ph.D. in 2004 from Xiamen University. His current research interests include electrocatalysis, nanomaterials, fuel cells, and electrochemical in situ FTIR spectroscopy.

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