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Evaluation and coupling of a membraneless nanofluidic device for low-power applications
Journal of Power Sources 307 (2016) 244e250
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Evaluation and coupling of a membraneless nanofluidic device for low-power applications ~ iga a, A.U. Cha vez-Ramírez b, M.P. Gurrola b, E. Ortiz-Ortega a, C. Farias-Zun J. Ledesma-García a, *, L.G. Arriaga b n de Investigacio n y Posgrado, Universidad Auto noma de Quer Facultad de Ingeniería, Divisio etaro, Centro Universitario, Cerro de las Campanas, Quer etaro, Qro., C.P. 76010, Mexico b n y Desarrollo Tecnolo gico en Electroquímica, Pedro Escobedo, Qro., C.P. 76703, Mexico Centro de Investigacio a
h i g h l i g h t s The The The The The
construction of a membraneless nanofluidic fuel cell is introduced. evaluation of nanofluidic fuel cell is made with a home testing station. device is composed of fiberglass with flow-through porous electrode. nanofluidic fuel cell is coupled with a microelectronic interface. complete power device supplies energy to low-power devices.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 August 2015 Received in revised form 14 December 2015 Accepted 17 December 2015 Available online xxx
This work presents the construction and evaluation of a membraneless nanofluidic fuel cell made with fiberglass using flow-through porous electrodes based on Toray paper, coupled with a microelectronic interface to supply energy to low-power demand applications. The device performance is optimized for different operating conditions related with flow rate, stoichiometry and concentration and employing formic acid as fuel. Evaluation tests were performed with a homemade testing station using a commercial varying resistance. © 2016 Elsevier B.V. All rights reserved.
Keywords: Membraneless nanofluidic fuel cell Microelectronic interface Toray paper based electrode
1. Introduction The trend for miniaturization of technology is continuing in a variety of gadgets, such as cell phones, laptops, implants, etc. Miniaturization allows to incorporate higher amount of functions at a major processing speed; however, power source remains as the main challenge since current batteries do not meet the continuous energy demand of today's devices. Microfluidic Fuel Cells (MFC) have been recently investigated as clean power sources alternative to batteries, due their high power density and the capability to supply energy for extended periods [1e4]. Several fuels have been used in MFC such as formic acid, methanol and glucose [4e9];
* Corresponding author. E-mail address: [email protected] (J. Ledesma-García). http://dx.doi.org/10.1016/j.jpowsour.2015.12.091 0378-7753/© 2016 Elsevier B.V. All rights reserved.
however, the use of formic acid presents considerable advantages for portable applications because of its high energy density (2086 kW L1) and low potential for pollution compared to common materials found in conventional batteries [10,11]. In MFCs that use oxygen as an oxidant and formic acid as a fuel, the theoretical potential difference generated by their oxidationereduction reactions is 1.48 V vs NHE; however, the theoretical potential cannot be reached experimentally due to losses caused by parasitic reactions and design issues in the microfluidic fuel cell [12,13]. The development of new electrocatalytic materials that promote the redox reaction had contributed to increase the potential difference and hence improve cell performance. Finally, fluid inlet channels with different architectures as well as the location of the electrodes have been tested to optimize the streamline interface between the fuel and oxidant [14e17]. MFCs employing formic acid as the fuel and oxygen as the oxidant are limited by the
M.P. Gurrola et al. / Journal of Power Sources 307 (2016) 244e250
availability of oxygen with a low diffusivity (2 105 cm2 s1), thus limiting mass transfer [18e26]. E. Ortiz-Ortega et al. developed electrodes (anode and cathode) based on porous materials such as Toray Paper and Carbon Nanofoam (average pore size ~ 10e100 nm) where a bunch of micro/nano-streams was formed by the effect of the solution flowing through the pores [27]. The energy produced for these devices is not presented in the format required by the most of electronic devices on the market. For this reason, DC/DC boosters are employed as electronic interfaces. These devices have been fabricated and tested for use as low power sources, such as microbial fuel cells in which the typical voltage of 0.3 V was increased to 3 V [28,29]. Nevertheless, these studies failed to integrate the energy producer and its booster for an electronic application. This work presents a novel direct formic acid nanofluidic fuel cell (NFC) fabricated in fiberglass using Toray Paper based electrodes. The microdevice combined an O2-saturated solution and the O2 from air as the oxidant. The fuel cell performance was evaluated in a homemade test station (controlled resistance variation) and manipulating the fuel concentration, the flow rates and the stoichiometric (oxidant: fuel). In order to adapt the NFC output voltage for low power applications (3 V), the micro power source was coupled to a microelectronic interface, allowing the operation of small electronic devices, such as a LED.
245
2.3. Construction of the nanofluidic fuel cell The NFC construction consisted of two substrates composed of fiberglass (width 1.5 mm) with a superficial thin layer of copper used to print the electronic circuit. The top substrate had two inlets for the reagents (fuel and oxidant) and a window on the cathode electrode to allow oxygen to be taken from the air; meanwhile, the bottom substrate had only one outlet to release the reaction products. Both plates were fabricated using a mini-router CNC (Computer Numerical Control). The microchannel and gaskets were fabricated of a silicone elastomer film (Silastic®, Dow Corning, prepared using an Elcometer® Film Applicator with a final thickness of 600 mm approx.) and traced using a Silhouette® cutting plotter. Pd/Vulcan XC-72 as anode and Pt/Vulcan XC-72 as cathode (20 and 30%, respectively, from E-TEK) were deposited on the electrode matrix by the spray technique employing an airbrush until the base metal loading was 0.6 mg. The electrocatalyst ink was prepared with 120 ml of isopropyl alcohol per milligram of catalyst and sonicated for 30 min. Then, 14 ml of Nafion (5%, diluted in isopropyl alcohol, Electrochem®) per milligram of catalyst was added to the solution and sonicated again for 30min. The current collectors were cut from commercial foil (Alumark 50). Finally, double-faced tape was placed on the back of the substrate to link all of the pieces together with only hand pressure, eliminating the need for screws (Fig. 1a).
2. Experimental setup 2.4. Construction of the membraneless nanofluidic device (MND) 2.1. Electrical conductivity measurements and physicochemical characterization Electrical conductivity measurements were carried out by a four-point method with an Agilent 4338B miliohmmeter [30,31]; the cell was evaluated by a porous 3D electrode (Toray Paper) using a geometric area of 59.4 mm2. A SEM cross-sectional image was taken using a JEOL JSM-7401F field emission scanning electron microscope (FE-SEM).
2.2. Electrochemical measurements The electrochemical evaluation in half-cell configuration was completed by preparing electrocatalyst inks using commercial Pt black (from E-TEK), 120 mL of isopropyl alcohol and 14 mL of Nafion® per milligram of catalyst. The inks were then sonicated for 1 h and deposited on the electrode matrix by a spray technique employing an airbrush until an effective Pt metal loading of 1.5 mg cm2 was achieved. The electrochemical characterization consisted on Cyclic Voltammetry (CV) to acquire the electrochemical profile for the electrochemical surface area (ESA) estimation in a Biologic VSP Potentiostat/Galvanostat. A solution of 0.5 M H2SO4 (J. T. Baker, 99.7%) was used as the supporting electrolyte and was purged with nitrogen gas before the each measurement. Cyclic voltammograms were obtained in a conventional three-electrode electrochemical cell using Pt/Toray Paper (the electrode dimensions were length: 20 mm, width: 2 mm and height: 0.1 mm; the geometric area was 0.02 cm2) as the working electrode, a Hg/Hg2SO4 in a solution of 0.5 M H2SO4 as the reference electrode and a graphite rod as the counter electrode. An electrochemical profile was obtained at scan rates of 10 mV1 in a potential range between 0.3 V and 1.4 V; the area under the curve was evaluated to determine the electrochemical surface area (ESA) and roughness factor (S) [32,33]. The electrochemical measurement was recorded and reported versus the Normal Hydrogen Electrode (NHE) at room temperature. All solutions were prepared with deionized water DI (r 18 MU cm).
The energy source (Fig. 1c) was like that of the NFC as previously described, coupled to a microelectronic interface (MI). The MI (Fig. 1b) contained a booster model TS 3310 (Touchstone Semiconductors) that convert the NFC output voltage from 0.83 V to 3 V (the commercial value for low power applications) with a low current consumption of 3.75 mA cm2 and a surface mounted inductor (SMI) of 10 mH (Taiyo Yuden) as the accumulator. In the inlet and outlet, a surface mounted capacitor (SMC) of 0.1 mF was added in addition to two 10 mF filters (Murata) (Fig. 1b). A minirouter CNC was employed to build the MI. The MND was connected to a green LED (SMD, Chicago 160 Fcd at 560 nm) that operated in a power range of 0.5 mA cm2 to 50 mA cm2. 2.5. Construction of the home testing station To evaluate the MND in terms of actual energy consumption, all tests were performed at room temperature. First, the homemade test station (Fig. 2) was coupled to the MND with a commercial varying resistance (from 0 KU to 5 KU with a maximum power of 0.5 W). According to a polarization curve based on previous studies [1,10,12], the resistances were of 2360U, 1370 U, 654 U, 346 U, 192 U, 110 U, 62 U, 33 U and 14 U. 2.6. Evaluation of the MND performance The anolyte and catholyte streams were fed through the nanoporous electrodes with a flow rate from 1 to 12 ml h1 and varying stoichiometry of 1:1, 2:1, 3:1 and 1:2 (catholyte: anolyte) using syringe pumps (NE-4000, New Era Pump Systems Inc.) at 0.1, 0.5, 1.5, 3, 5 M of formic acid and combined with a O2-saturated solution and O2 from the air, in 0.5 M H2SO4 solutions. 3. Results and discussion The electrical conductivity data of the Toray Paper based electrode was obtained by the four-point method; for comparison purposes, the value for Carbon Nanofoam was also obtained. The
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M.P. Gurrola et al. / Journal of Power Sources 307 (2016) 244e250
Fig. 1. a) Schematic representation of the nanofluidic fuel cell, b) Schematic representation of the microelectronic interface and c) the membraneless nanofluidic device.
Fig. 2. Scheme of the homemade test station.
electrical conductivity was 159.42 103 Sm1 and 142.85 103 Sm1 for the Toray Paper and Carbon Nanofoam, respectively. The Carbon Nanofoam conductivity is higher than the Toray Paper (roughly 10.4%), even when the Carbon Nanofoam has a higher surface area (400e800 m2 g-1) [33,34] and is around 2000x greater than the Toray Paper (190 103 m 2 g1) [35e38]. The SEM image of the Toray Paper electrode is shown in Fig. 3a, with interconnected fibres that maintain a good electrical conductivity of the electrode. 3.1. Electrochemical measurements Characterization of the two electrodes were carried out (Toray paper and carbon nanofoam) in half-cell. Fig. 3b shows the
Voltammetric profiles in acid media for Pt/Toray Paper and Pt/ Carbon Nanofoam electrodes. In the case of the Carbon nanofoam, the peaks related to adsorption and desorption of hydrogen are not observed clearly, similarly the peak related to oxidation of Pt are not observed, this is attributable to the large capacitance of the carbon nanofoam. By contrast, the reactions associated with the surface of Pt in acid media are not limited by the presence of the Toray paper. According to classical studies, three regions can be observed. The first is the oxygen region in a potential positive area between 0.5 V and 1.4 V during the positive sweep, before the oxygen evolution. A hydrated oxide of Pt is formed. Then, in the centre, the region appears in the electrical double layer where only the capacitive processes occur. Finally, the potential region of hydrogen is observed approximately 0 Ve0.3 V. The electrochemical surface area (ESA) of Pt was calculated from the charge (QH ¼ 210 mC cm2) required to oxidize a complete monolayer of the adsorbed hydrogen atoms [32] in the electrochemical profile was 100.56 cm2, in contrast, for Carbon nanofoam was not possible to obtain this information. According to electrical conductivity and electrochemical profile, Toray paper was decided to be employed as electrode for the MND tests. 3.2. Evaluation of the MND performance for Toray paper electrodes The polarization and power curves presented in Fig. 4a show a comparison between the Biologic VSP Potentiostat/Galvanostat and the homemade test station; the curves obtained by the test station are reliable for performing realistic measurements for device optimization. Conversely, the impedance spectra (Fig. 4b) show the total resistance of the MND for the Toray Paper at 0.7 U cm-2. The
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247
Fig. 3. a) SEM micrograph for the Toray Paper and b) Cyclic voltamperograms of the Pt/ Toray Paper and Carbon Nanofoam electrodes in 0.5 M H2SO4 at a 10 mV s1scan rate in an N2-saturated atmosphere.
results indicate that the Toray paper electrodes present low total resistance according to the electrical conductivity obtained by the four -point method, observing a good cell performance. 3.3. Evaluation of the concentration effect on the MND performance for Toray paper The polarization and power curves (Fig. 5a) show the behaviour effect with the formic acid concentration versus performance. The highest power density obtained was around of 75 mW cm-2 at 3 M and the current density associated was 250 mA cm-2. However, the MND shows a good tolerance to CO and CO2 poisoning at high formic acid concentrations (5 M), observing a performance attenuation around 25% (roughly 100 mA cm-2) with respect to the maximum achieved. 3.4. Evaluation of the stoichiometric effect on the MND performance In agreement with the stoichiometry of the overall reaction of the cell, two molecules of formic acid and one molecule of oxygen are necessary, however the limited availability of oxygen molecules on the electrode surface causes mass limited factors. This is challenging when modifying the stoichiometric of the feed inlets (Fig. 5b). The performance of the MND increases from 60 mW cm-2 to 85 mW cm-2 when the flow cathodic stream rises 3 times because of the increasing oxygen source. In contrast, when the flow anodic stream increases, the performance drops drastically to
Fig. 4. a) Comparison of the polarization and power curves between the Biologic VSP Potentiostat/Galvanostat and the homemade test station; b) impedance spectra of MND using Pd/Toray Paper (anode) and Pt/Toray paper (cathode) (500 kHze100 mHz as the open circuit potential) and 3 M formic acid at a flow rate of 6 ml h1.
10 mW cm-2 because of the low oxygen disposition. 3.5. Evaluation of the flow rate effect on the MND performance for Toray paper The CO and CO2 formed during the formic acid oxidation reaction reduce the electro active area due to the CO adsorption and CO2 bubbles accumulated in the channels. The polarization and power curves in Fig. 5c show the performance versus the flow rate from 1 ml h-1 to 12 ml h-1. The power increases considerably, from 40 mW cm-2 to 100 mW cm-2, respectively to the increase in flow rate. The flow rate increase assists in removing certain amounts of by-products from the reactions, thus decreasing the poisoning effect and improving the mass transport. 3.6. Optimal operational condition of the MND This work evaluates the performance of the proposed MND design at different operational conditions to reach an output
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Fig. 5. Polarization and power curves for of Pd/Toray Paper (anode) and Pt/Toray paper (cathode) at a) different formic acid concentrations at and a 6 ml h1 flow rate 6 ml h1evaluated in a the homemade test station; b) with a stoichiometric effect at 3 M of formic acid and the flow cathodic and anodic streams (1:1, 2:1, 3:1 and 1:2) and c) with the flow rate effect at 3 M of formic acid in the homemade test station.
voltage useful in commercial low-power devices. In all of the previous tests (Fig. 5a, b and 5c), the change in the OCP values was negligible; however, a current increase in the function of the operational conditions was observed. This current change could be associated with the ButlereVolmer equation (Eq. (1)):
ared Fh aoxd Fh o n RT RT i ¼ i0 exp exp
(1)
It was assumed that the electronic transfer coefficients were equal in all cases due to the commercial electrocatalyst employed. Likewise, R and T were considered constants due to the tests performed at room temperature. F is Faraday's constant, and h was considered to be a constant at a specific voltage value in the polarization curve. Therefore, only the exchange current (Eq. (2)) value was modified because it depends on the concentration to be in agreement with the following equation:
i0 ¼ nFk0 Cexp
aFh RT
(2)
where, ko is called the standard rate constant. The concentration change was associated with the different operational conditions applied; consequently, several short-circuit current values were attained that were reflected in the MND performance. The optimal stoichiometry operational condition for the
proposed MND was 3:1 (cathode: anode), involving 18:6 ml h1 at 3 M. The polarization curve was divided into two zones, the first zone or high voltage zone was close to OCP and low current and according to the OCP minimum values reported in the microfluidic fuel cell using formic acid as fuel (Fig. 6a) [39]. The second zone or the high power zone was located where the voltage and current were high enough to achieve maximum power (Fig. 6b). The MND had a microelectronic interface with a booster and required 0.83 V at 3.75 mA cm2 to deliver an output voltage of 3 V. Then, inside the first zone (Fig. 6a), the minimum input voltage values were required for the MND to operate [18]. Table 1 shows the current values for the MND at 0.83 V for different operational conditions (concentration, stoichiometric, flow rate); it was observed that in conditions of 0.1 M, 0.5 M and 1 ml h1, the current was not large enough to overpass the resistance of the MND and the device was not able to operate. On the other hand, the MND required a current of at least 4.5 mA cm-2 to start operating and to feed a low-power device, such as a green LED. For the second zone (Fig. 6b), the supplied voltage was below the minimum input voltage required for the MND (0.83 V), however the current was considerably greater than the first zone (100e350 mA cm2) and delivered more power. However, as perspective a nanofluidic stack fuel cell could be developed and connected in series to increase the power output to operate under the second zone condition and to supply energy to low power device such as digital thermometers, glucometers, remote controls,
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249
Fig. 7. Operation Voltage measured for the MND coupled to the LED device.
voltage indicates the voltage required for the microelectronic interface to become operational. At 0.8 V corresponds 30 mA cm-2, this value was within the power range of the green LED. Then, the LED lighting was kept “on” all the test, meaning that the proposed MND was capable to operate for long periods without any change in its performance cell, therefore, this stability made it an ideal candidate as alternative power source for low power devices. 4. Conclusions
Fig. 6. Polarization curve of the Pd/Toray Paper (anode) and Pt/Toray paper (cathode) in two performance zones: a) high voltage zone and b) high power zone, both at the optimal evaluation conditions.
Table 1 Current values of the MND at the minimum input voltage required for the microelectronic interface to operate (0.83 V).
Concentration (M)
Stoichiometric Scan rate (ml h1)
Operation Condition
I/mA
0.1 0.5 1.5 3 M/1: 1/6 ml h1 5 2:1 3:1 1 12
64 70 152 390 237 90 524 24 378
In summary, the design of a homemade test station was able to evaluate the fuel cell performance during real tasks avoiding the use of sophisticated equipment for microfluidic fuel cell evaluation as a perspective of development a microsystem for balance of plant. The microfluidic fuel cell was optimized by the concentration, flow rate and stoichiometric effects with values about 3 M, 6 ml h1 and 3:1, respectively. The internal octagonal channel design promotes homogeneous feed of reactants along the electrodes and minimizing the charge transfer distance between electrodes, thus reducing the ohmic losses and increasing the overall performance. The design and construction of a membraneless nanofluidic device composed of fiberglass with the flow-through porous electrode architecture of Toray paper was successfully coupled to a microelectronic interface. These integration allowed it to connect to a MND (with a commercial output voltage of 3 V) coupled to a LED. The results can be used as a basis for future design of a device capable of delivering enough power for low power applications. Acknowledgements The authors thank the Mexican Council for Science and Technology (CONACYT) for financial support through project CB-2014tica del 01-242787, project 248511 and through Red Tema geno (252003). Hidro References
calculators, automobile controls, digital pregnancy tests, and so on. 3.7. Evaluation of the MND coupled to an LED device Finally, the MND was coupled to a LED device to evaluate the stability over a 20 h period (Fig. 7). The attenuation in the operating
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Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Evaluation and coupling of a membraneless nanofluidic device for low-power applications ~ iga a, A.U. Cha vez-Ramírez b, M.P. Gurrola b, E. Ortiz-Ortega a, C. Farias-Zun J. Ledesma-García a, *, L.G. Arriaga b n de Investigacio n y Posgrado, Universidad Auto noma de Quer Facultad de Ingeniería, Divisio etaro, Centro Universitario, Cerro de las Campanas, Quer etaro, Qro., C.P. 76010, Mexico b n y Desarrollo Tecnolo gico en Electroquímica, Pedro Escobedo, Qro., C.P. 76703, Mexico Centro de Investigacio a
h i g h l i g h t s The The The The The
construction of a membraneless nanofluidic fuel cell is introduced. evaluation of nanofluidic fuel cell is made with a home testing station. device is composed of fiberglass with flow-through porous electrode. nanofluidic fuel cell is coupled with a microelectronic interface. complete power device supplies energy to low-power devices.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 August 2015 Received in revised form 14 December 2015 Accepted 17 December 2015 Available online xxx
This work presents the construction and evaluation of a membraneless nanofluidic fuel cell made with fiberglass using flow-through porous electrodes based on Toray paper, coupled with a microelectronic interface to supply energy to low-power demand applications. The device performance is optimized for different operating conditions related with flow rate, stoichiometry and concentration and employing formic acid as fuel. Evaluation tests were performed with a homemade testing station using a commercial varying resistance. © 2016 Elsevier B.V. All rights reserved.
Keywords: Membraneless nanofluidic fuel cell Microelectronic interface Toray paper based electrode
1. Introduction The trend for miniaturization of technology is continuing in a variety of gadgets, such as cell phones, laptops, implants, etc. Miniaturization allows to incorporate higher amount of functions at a major processing speed; however, power source remains as the main challenge since current batteries do not meet the continuous energy demand of today's devices. Microfluidic Fuel Cells (MFC) have been recently investigated as clean power sources alternative to batteries, due their high power density and the capability to supply energy for extended periods [1e4]. Several fuels have been used in MFC such as formic acid, methanol and glucose [4e9];
* Corresponding author. E-mail address: [email protected] (J. Ledesma-García). http://dx.doi.org/10.1016/j.jpowsour.2015.12.091 0378-7753/© 2016 Elsevier B.V. All rights reserved.
however, the use of formic acid presents considerable advantages for portable applications because of its high energy density (2086 kW L1) and low potential for pollution compared to common materials found in conventional batteries [10,11]. In MFCs that use oxygen as an oxidant and formic acid as a fuel, the theoretical potential difference generated by their oxidationereduction reactions is 1.48 V vs NHE; however, the theoretical potential cannot be reached experimentally due to losses caused by parasitic reactions and design issues in the microfluidic fuel cell [12,13]. The development of new electrocatalytic materials that promote the redox reaction had contributed to increase the potential difference and hence improve cell performance. Finally, fluid inlet channels with different architectures as well as the location of the electrodes have been tested to optimize the streamline interface between the fuel and oxidant [14e17]. MFCs employing formic acid as the fuel and oxygen as the oxidant are limited by the
M.P. Gurrola et al. / Journal of Power Sources 307 (2016) 244e250
availability of oxygen with a low diffusivity (2 105 cm2 s1), thus limiting mass transfer [18e26]. E. Ortiz-Ortega et al. developed electrodes (anode and cathode) based on porous materials such as Toray Paper and Carbon Nanofoam (average pore size ~ 10e100 nm) where a bunch of micro/nano-streams was formed by the effect of the solution flowing through the pores [27]. The energy produced for these devices is not presented in the format required by the most of electronic devices on the market. For this reason, DC/DC boosters are employed as electronic interfaces. These devices have been fabricated and tested for use as low power sources, such as microbial fuel cells in which the typical voltage of 0.3 V was increased to 3 V [28,29]. Nevertheless, these studies failed to integrate the energy producer and its booster for an electronic application. This work presents a novel direct formic acid nanofluidic fuel cell (NFC) fabricated in fiberglass using Toray Paper based electrodes. The microdevice combined an O2-saturated solution and the O2 from air as the oxidant. The fuel cell performance was evaluated in a homemade test station (controlled resistance variation) and manipulating the fuel concentration, the flow rates and the stoichiometric (oxidant: fuel). In order to adapt the NFC output voltage for low power applications (3 V), the micro power source was coupled to a microelectronic interface, allowing the operation of small electronic devices, such as a LED.
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2.3. Construction of the nanofluidic fuel cell The NFC construction consisted of two substrates composed of fiberglass (width 1.5 mm) with a superficial thin layer of copper used to print the electronic circuit. The top substrate had two inlets for the reagents (fuel and oxidant) and a window on the cathode electrode to allow oxygen to be taken from the air; meanwhile, the bottom substrate had only one outlet to release the reaction products. Both plates were fabricated using a mini-router CNC (Computer Numerical Control). The microchannel and gaskets were fabricated of a silicone elastomer film (Silastic®, Dow Corning, prepared using an Elcometer® Film Applicator with a final thickness of 600 mm approx.) and traced using a Silhouette® cutting plotter. Pd/Vulcan XC-72 as anode and Pt/Vulcan XC-72 as cathode (20 and 30%, respectively, from E-TEK) were deposited on the electrode matrix by the spray technique employing an airbrush until the base metal loading was 0.6 mg. The electrocatalyst ink was prepared with 120 ml of isopropyl alcohol per milligram of catalyst and sonicated for 30 min. Then, 14 ml of Nafion (5%, diluted in isopropyl alcohol, Electrochem®) per milligram of catalyst was added to the solution and sonicated again for 30min. The current collectors were cut from commercial foil (Alumark 50). Finally, double-faced tape was placed on the back of the substrate to link all of the pieces together with only hand pressure, eliminating the need for screws (Fig. 1a).
2. Experimental setup 2.4. Construction of the membraneless nanofluidic device (MND) 2.1. Electrical conductivity measurements and physicochemical characterization Electrical conductivity measurements were carried out by a four-point method with an Agilent 4338B miliohmmeter [30,31]; the cell was evaluated by a porous 3D electrode (Toray Paper) using a geometric area of 59.4 mm2. A SEM cross-sectional image was taken using a JEOL JSM-7401F field emission scanning electron microscope (FE-SEM).
2.2. Electrochemical measurements The electrochemical evaluation in half-cell configuration was completed by preparing electrocatalyst inks using commercial Pt black (from E-TEK), 120 mL of isopropyl alcohol and 14 mL of Nafion® per milligram of catalyst. The inks were then sonicated for 1 h and deposited on the electrode matrix by a spray technique employing an airbrush until an effective Pt metal loading of 1.5 mg cm2 was achieved. The electrochemical characterization consisted on Cyclic Voltammetry (CV) to acquire the electrochemical profile for the electrochemical surface area (ESA) estimation in a Biologic VSP Potentiostat/Galvanostat. A solution of 0.5 M H2SO4 (J. T. Baker, 99.7%) was used as the supporting electrolyte and was purged with nitrogen gas before the each measurement. Cyclic voltammograms were obtained in a conventional three-electrode electrochemical cell using Pt/Toray Paper (the electrode dimensions were length: 20 mm, width: 2 mm and height: 0.1 mm; the geometric area was 0.02 cm2) as the working electrode, a Hg/Hg2SO4 in a solution of 0.5 M H2SO4 as the reference electrode and a graphite rod as the counter electrode. An electrochemical profile was obtained at scan rates of 10 mV1 in a potential range between 0.3 V and 1.4 V; the area under the curve was evaluated to determine the electrochemical surface area (ESA) and roughness factor (S) [32,33]. The electrochemical measurement was recorded and reported versus the Normal Hydrogen Electrode (NHE) at room temperature. All solutions were prepared with deionized water DI (r 18 MU cm).
The energy source (Fig. 1c) was like that of the NFC as previously described, coupled to a microelectronic interface (MI). The MI (Fig. 1b) contained a booster model TS 3310 (Touchstone Semiconductors) that convert the NFC output voltage from 0.83 V to 3 V (the commercial value for low power applications) with a low current consumption of 3.75 mA cm2 and a surface mounted inductor (SMI) of 10 mH (Taiyo Yuden) as the accumulator. In the inlet and outlet, a surface mounted capacitor (SMC) of 0.1 mF was added in addition to two 10 mF filters (Murata) (Fig. 1b). A minirouter CNC was employed to build the MI. The MND was connected to a green LED (SMD, Chicago 160 Fcd at 560 nm) that operated in a power range of 0.5 mA cm2 to 50 mA cm2. 2.5. Construction of the home testing station To evaluate the MND in terms of actual energy consumption, all tests were performed at room temperature. First, the homemade test station (Fig. 2) was coupled to the MND with a commercial varying resistance (from 0 KU to 5 KU with a maximum power of 0.5 W). According to a polarization curve based on previous studies [1,10,12], the resistances were of 2360U, 1370 U, 654 U, 346 U, 192 U, 110 U, 62 U, 33 U and 14 U. 2.6. Evaluation of the MND performance The anolyte and catholyte streams were fed through the nanoporous electrodes with a flow rate from 1 to 12 ml h1 and varying stoichiometry of 1:1, 2:1, 3:1 and 1:2 (catholyte: anolyte) using syringe pumps (NE-4000, New Era Pump Systems Inc.) at 0.1, 0.5, 1.5, 3, 5 M of formic acid and combined with a O2-saturated solution and O2 from the air, in 0.5 M H2SO4 solutions. 3. Results and discussion The electrical conductivity data of the Toray Paper based electrode was obtained by the four-point method; for comparison purposes, the value for Carbon Nanofoam was also obtained. The
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Fig. 1. a) Schematic representation of the nanofluidic fuel cell, b) Schematic representation of the microelectronic interface and c) the membraneless nanofluidic device.
Fig. 2. Scheme of the homemade test station.
electrical conductivity was 159.42 103 Sm1 and 142.85 103 Sm1 for the Toray Paper and Carbon Nanofoam, respectively. The Carbon Nanofoam conductivity is higher than the Toray Paper (roughly 10.4%), even when the Carbon Nanofoam has a higher surface area (400e800 m2 g-1) [33,34] and is around 2000x greater than the Toray Paper (190 103 m 2 g1) [35e38]. The SEM image of the Toray Paper electrode is shown in Fig. 3a, with interconnected fibres that maintain a good electrical conductivity of the electrode. 3.1. Electrochemical measurements Characterization of the two electrodes were carried out (Toray paper and carbon nanofoam) in half-cell. Fig. 3b shows the
Voltammetric profiles in acid media for Pt/Toray Paper and Pt/ Carbon Nanofoam electrodes. In the case of the Carbon nanofoam, the peaks related to adsorption and desorption of hydrogen are not observed clearly, similarly the peak related to oxidation of Pt are not observed, this is attributable to the large capacitance of the carbon nanofoam. By contrast, the reactions associated with the surface of Pt in acid media are not limited by the presence of the Toray paper. According to classical studies, three regions can be observed. The first is the oxygen region in a potential positive area between 0.5 V and 1.4 V during the positive sweep, before the oxygen evolution. A hydrated oxide of Pt is formed. Then, in the centre, the region appears in the electrical double layer where only the capacitive processes occur. Finally, the potential region of hydrogen is observed approximately 0 Ve0.3 V. The electrochemical surface area (ESA) of Pt was calculated from the charge (QH ¼ 210 mC cm2) required to oxidize a complete monolayer of the adsorbed hydrogen atoms [32] in the electrochemical profile was 100.56 cm2, in contrast, for Carbon nanofoam was not possible to obtain this information. According to electrical conductivity and electrochemical profile, Toray paper was decided to be employed as electrode for the MND tests. 3.2. Evaluation of the MND performance for Toray paper electrodes The polarization and power curves presented in Fig. 4a show a comparison between the Biologic VSP Potentiostat/Galvanostat and the homemade test station; the curves obtained by the test station are reliable for performing realistic measurements for device optimization. Conversely, the impedance spectra (Fig. 4b) show the total resistance of the MND for the Toray Paper at 0.7 U cm-2. The
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Fig. 3. a) SEM micrograph for the Toray Paper and b) Cyclic voltamperograms of the Pt/ Toray Paper and Carbon Nanofoam electrodes in 0.5 M H2SO4 at a 10 mV s1scan rate in an N2-saturated atmosphere.
results indicate that the Toray paper electrodes present low total resistance according to the electrical conductivity obtained by the four -point method, observing a good cell performance. 3.3. Evaluation of the concentration effect on the MND performance for Toray paper The polarization and power curves (Fig. 5a) show the behaviour effect with the formic acid concentration versus performance. The highest power density obtained was around of 75 mW cm-2 at 3 M and the current density associated was 250 mA cm-2. However, the MND shows a good tolerance to CO and CO2 poisoning at high formic acid concentrations (5 M), observing a performance attenuation around 25% (roughly 100 mA cm-2) with respect to the maximum achieved. 3.4. Evaluation of the stoichiometric effect on the MND performance In agreement with the stoichiometry of the overall reaction of the cell, two molecules of formic acid and one molecule of oxygen are necessary, however the limited availability of oxygen molecules on the electrode surface causes mass limited factors. This is challenging when modifying the stoichiometric of the feed inlets (Fig. 5b). The performance of the MND increases from 60 mW cm-2 to 85 mW cm-2 when the flow cathodic stream rises 3 times because of the increasing oxygen source. In contrast, when the flow anodic stream increases, the performance drops drastically to
Fig. 4. a) Comparison of the polarization and power curves between the Biologic VSP Potentiostat/Galvanostat and the homemade test station; b) impedance spectra of MND using Pd/Toray Paper (anode) and Pt/Toray paper (cathode) (500 kHze100 mHz as the open circuit potential) and 3 M formic acid at a flow rate of 6 ml h1.
10 mW cm-2 because of the low oxygen disposition. 3.5. Evaluation of the flow rate effect on the MND performance for Toray paper The CO and CO2 formed during the formic acid oxidation reaction reduce the electro active area due to the CO adsorption and CO2 bubbles accumulated in the channels. The polarization and power curves in Fig. 5c show the performance versus the flow rate from 1 ml h-1 to 12 ml h-1. The power increases considerably, from 40 mW cm-2 to 100 mW cm-2, respectively to the increase in flow rate. The flow rate increase assists in removing certain amounts of by-products from the reactions, thus decreasing the poisoning effect and improving the mass transport. 3.6. Optimal operational condition of the MND This work evaluates the performance of the proposed MND design at different operational conditions to reach an output
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Fig. 5. Polarization and power curves for of Pd/Toray Paper (anode) and Pt/Toray paper (cathode) at a) different formic acid concentrations at and a 6 ml h1 flow rate 6 ml h1evaluated in a the homemade test station; b) with a stoichiometric effect at 3 M of formic acid and the flow cathodic and anodic streams (1:1, 2:1, 3:1 and 1:2) and c) with the flow rate effect at 3 M of formic acid in the homemade test station.
voltage useful in commercial low-power devices. In all of the previous tests (Fig. 5a, b and 5c), the change in the OCP values was negligible; however, a current increase in the function of the operational conditions was observed. This current change could be associated with the ButlereVolmer equation (Eq. (1)):
ared Fh aoxd Fh o n RT RT i ¼ i0 exp exp
(1)
It was assumed that the electronic transfer coefficients were equal in all cases due to the commercial electrocatalyst employed. Likewise, R and T were considered constants due to the tests performed at room temperature. F is Faraday's constant, and h was considered to be a constant at a specific voltage value in the polarization curve. Therefore, only the exchange current (Eq. (2)) value was modified because it depends on the concentration to be in agreement with the following equation:
i0 ¼ nFk0 Cexp
aFh RT
(2)
where, ko is called the standard rate constant. The concentration change was associated with the different operational conditions applied; consequently, several short-circuit current values were attained that were reflected in the MND performance. The optimal stoichiometry operational condition for the
proposed MND was 3:1 (cathode: anode), involving 18:6 ml h1 at 3 M. The polarization curve was divided into two zones, the first zone or high voltage zone was close to OCP and low current and according to the OCP minimum values reported in the microfluidic fuel cell using formic acid as fuel (Fig. 6a) [39]. The second zone or the high power zone was located where the voltage and current were high enough to achieve maximum power (Fig. 6b). The MND had a microelectronic interface with a booster and required 0.83 V at 3.75 mA cm2 to deliver an output voltage of 3 V. Then, inside the first zone (Fig. 6a), the minimum input voltage values were required for the MND to operate [18]. Table 1 shows the current values for the MND at 0.83 V for different operational conditions (concentration, stoichiometric, flow rate); it was observed that in conditions of 0.1 M, 0.5 M and 1 ml h1, the current was not large enough to overpass the resistance of the MND and the device was not able to operate. On the other hand, the MND required a current of at least 4.5 mA cm-2 to start operating and to feed a low-power device, such as a green LED. For the second zone (Fig. 6b), the supplied voltage was below the minimum input voltage required for the MND (0.83 V), however the current was considerably greater than the first zone (100e350 mA cm2) and delivered more power. However, as perspective a nanofluidic stack fuel cell could be developed and connected in series to increase the power output to operate under the second zone condition and to supply energy to low power device such as digital thermometers, glucometers, remote controls,
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Fig. 7. Operation Voltage measured for the MND coupled to the LED device.
voltage indicates the voltage required for the microelectronic interface to become operational. At 0.8 V corresponds 30 mA cm-2, this value was within the power range of the green LED. Then, the LED lighting was kept “on” all the test, meaning that the proposed MND was capable to operate for long periods without any change in its performance cell, therefore, this stability made it an ideal candidate as alternative power source for low power devices. 4. Conclusions
Fig. 6. Polarization curve of the Pd/Toray Paper (anode) and Pt/Toray paper (cathode) in two performance zones: a) high voltage zone and b) high power zone, both at the optimal evaluation conditions.
Table 1 Current values of the MND at the minimum input voltage required for the microelectronic interface to operate (0.83 V).
Concentration (M)
Stoichiometric Scan rate (ml h1)
Operation Condition
I/mA
0.1 0.5 1.5 3 M/1: 1/6 ml h1 5 2:1 3:1 1 12
64 70 152 390 237 90 524 24 378
In summary, the design of a homemade test station was able to evaluate the fuel cell performance during real tasks avoiding the use of sophisticated equipment for microfluidic fuel cell evaluation as a perspective of development a microsystem for balance of plant. The microfluidic fuel cell was optimized by the concentration, flow rate and stoichiometric effects with values about 3 M, 6 ml h1 and 3:1, respectively. The internal octagonal channel design promotes homogeneous feed of reactants along the electrodes and minimizing the charge transfer distance between electrodes, thus reducing the ohmic losses and increasing the overall performance. The design and construction of a membraneless nanofluidic device composed of fiberglass with the flow-through porous electrode architecture of Toray paper was successfully coupled to a microelectronic interface. These integration allowed it to connect to a MND (with a commercial output voltage of 3 V) coupled to a LED. The results can be used as a basis for future design of a device capable of delivering enough power for low power applications. Acknowledgements The authors thank the Mexican Council for Science and Technology (CONACYT) for financial support through project CB-2014tica del 01-242787, project 248511 and through Red Tema geno (252003). Hidro References
calculators, automobile controls, digital pregnancy tests, and so on. 3.7. Evaluation of the MND coupled to an LED device Finally, the MND was coupled to a LED device to evaluate the stability over a 20 h period (Fig. 7). The attenuation in the operating
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