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Enhancing fuel transport in air-breathing microfluidic fuel cells by immersed fuel micro-jet
Journal of Power Sources 445 (2020) 227326
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Enhancing fuel transport in air-breathing microfluidic fuel cells by immersed fuel micro-jet Yuan Zhou a, b, Biao Zhang a, b, *, Xun Zhu a, b, **, Ding-Ding Ye a, b, Rong Chen a, b, Tong Zhang a, b, Xiao-Lian Gong a, b, Qiang Liao a, b a b
Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Ministry of Education, Chongqing University, Chongqing, 400030, China Institute of Engineering Thermophysics, School of Energy and Powering Engineering, Chongqing University, Chongqing, 400030, China
H I G H L I G H T S
� Part of fresh fuel is jetted towards the anode, enabling targeted fuel transport. � The micro-jet can reach and flow along the anode without significant crossover. � The fuel transfer limitation from fuel concentration boundary layer is mitigated. � The peak power density is improved 40.9% by the immersed fuel micro-jet. A R T I C L E I N F O
A B S T R A C T
Keywords: Microfluidic fuel cells Fuel concentration boundary layer Fuel transport Immersed micro-jet Cell performance
Air-breathing microfluidic fuel cells with flow-over planar anodes can facilitate system integration but suffer from the fuel concentration boundary layer over the anode, which significantly hinders the fuel transport and limit cell performance. A novel approach is proposed to actively replenish the fuel concentration boundary layer by immersed fuel micro-jet, where part of the fresh fuel is jetted perpendicular to the anode, enabling targeted fuel transfer enhancement. The immersed fuel micro-jet is visualized by fluorescence microscopy, and the microjet can reach and flow along the anode at optimal condition. The same cell architecture is tested in both the flowover and micro-jet modes. The electrochemical measurement and preliminary modelling results indicate that the fuel transfer limitation can be largely mitigated by fuel micro-jet, and the cell performance is enhanced accordingly. The micro-jet located at the middle of flow channel can balance the trade-off between replenish ment and benefitted anode area. The effect of total fuel flow rate and micro-jet/lateral flow rate ratio on the fuel transport and cell performance are also discussed in detail. As compared with the flow-over mode, the maximum performance improvement of 40.9% is achieved by the immersed fuel micro-jet, and the optimal power density reaches 119.3 mW cm 3.
1. Introduction Membraneless microfluidic fuel cells (MMFCs) hold great potential to fulfill the requirements for next-generation micro-power sources. The naturally formed liquid-liquid interface can not only segregate aqueous fuel and oxidant, but also allow transversal ion transfer, therefore serves as an alternative to expensive polyelectrolyte membrane. The removal of physical membrane also helps to diminish the complexity of cell
fabrication and assembly, as well as the requirement for storage and operation. In addition, the composition of fuel and oxidant can be tuned separately and extended to biochemical components (e.g., urea [1] and glycerol [2]), with little concern about bio-fouling, making MMFCs attractive for biomedical point-of-care applications [3]. In the past two decades, much attention has been paid to airbreathing microfluidic fuel cells (AMFCs), because the air-breathing cathode uses the oxygen in air as the low-cost and non-toxic oxidant,
* Corresponding author. Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Ministry of Education, Chongqing University, Chongqing, 400030, China. ** Corresponding author. Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Ministry of Education, Chongqing University, Chongqing, 400030, China. E-mail addresses: [email protected] (B. Zhang), [email protected] (X. Zhu). https://doi.org/10.1016/j.jpowsour.2019.227326 Received 16 August 2019; Received in revised form 4 October 2019; Accepted 16 October 2019 Available online 1 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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leveraging its higher concentration and diffusivity as compared to dis solved oxygen [4]. Recently, considerable progress has been made in the areas of catalyst [5–9], electrode configuration [10–13], unconventional reactants [1,14–16], and cell architecture [17–23]. In general, the anode architecture of AMFCs can be categorized into three types: flow-over architecture with planar electrode [24–26], flow-through architecture based on permeable porous electrodes [23,27,28], and three-dimensional architecture based on cylinder electrodes [22, 29–32]. Among them, the flow-over architecture is simple-structured and compatible with microfabrication process, and has minimal re quirements for assembly and sealing, making it integratable for practical applications [33–36]. In the flow-over architecture (Fig. 1a), the reactant flows in parallel to the anode, and the fuel transfer to the anode perpendicular to the mainstream. It has been demonstrated by both experiments and modelling that fuel concentration boundary layer can develop over the anode due to the rapid electrochemical reaction consumption and ineffective diffusive mass transport [24,34,37]. The existence of fuel concentration boundary layer can diminish the local concentration gradient, and lead to fuel transfer limitation and ultimately lower cell performance. Several strategies have been proposed to enhance fuel transport, including operation at higher flow rates and higher fuel concentration, as well as the development of novel cell architecture. Kjeang et al. [22] and Jayashree et al. [24] reported that the cell per formance can be improved as the thickness of concentration boundary layer was reduced by increasing the flow rate. Chang et al. [38] and Zhang et al. [37] presented similar results in modelling work. However, the maximum flow rate is limited by the hydrodynamic instability that can perturb the laminar flow [38]. In addition, the trade-off between output power density and fuel utilization is still a challenge. Similarly, the fuel transfer can be enhanced by increasing fuel concentration to improve the cell performance. Nevertheless, the use of high fuel con centration will facilitate fuel crossover and lower fuel utilization [4,39]. Hydrodynamic focusing has also been applied to MMFCs, where the reactant concentration gradient can be improved as the reactant stream was pressed thinner by properly adjusting the buffer flow rate [40]. Jayashree et al. [24] and Xuan et al. [41] demonstrated that hydrody namic focusing can be an effective method to improve the cell perfor mance (both output power density and fuel utilization rate) while
avoiding undesired fuel crossover, but at the expense of extra electrolyte consumption and parasitic pumping power [41]. Yoon et al. [21] pro posed active strategies to manipulate the depletion boundary layer. The fuel transport to the catalytic surfaces were enhanced by engineering the depletion zone by multiple periodically-placed outlets/inlets, but can decrease effective electrode area. In addition, Yoon et al. [21] and Marschewski et al. [17] utilized herringbone structure to generate sec ondary transverse flow and enhance convective reactant transport to the electrodes, further improved the cell performance. However, the herringbone pattern requires complicated microfabrication as well as precise control of flow rates to avoid severe convective mixing. In this paper, for the first time, we proposed a novel cell configura tion with immersed fuel micro-jet to replenish the fuel concentration boundary layer and enhance fuel transport. Different from the multiple inlets/outlets approaches, which require relatively complicated micro fluidic network, the present approach confined all functional compo nents into a rectangular flow channel, providing unique benefit for system integration. As a proof-of-concept, a part of fuel was fed in parallel to the anode for the generation of fuel concentration boundary layer, while the other part was jetted perpendicularly to the anode (See Fig. 1b). An air-breathing membraneless microfluidic fuel cell (AMFC) operated in alkaline electrolyte was used as the prototype. The immersed fuel micro-jet was visualized using fluorescent microscopy. In addition, the effects of immersed micro-jet, micro-opening location, flow rate ratio, and fuel concentration on the fuel transfer and cell performance were investigated and discussed in detail. 2. Experimental Fig. 1b illustrates the proposed AMFC with immersed fuel micro-jet. As a proof-of-concept, a portion of fuel was fed into the flow channel, and flowed in parallel to the anode, while the other portion was fed into the capillary tube, and was jetted from a micro-opening on the tube wall perpendicular towards the planar anode. Fig. 2 shows the detailed cell architecture. Polymethyl methacrylate (PMMA) plates fabricated by a laser cutter (VLS 3.60) were used for cell construction. The rectangular flow channel was 20.0, 1.5 and 2.5 mm in length, width and height, respectively. A Titanium capillary tube with an outer diameter of 0.5 mm was fixed in the center of the flow channel. The micro-opening on the tube wall with a diameter about 70 μm was fabricated by laser. As the baseline, the micro-opening was located in the middle of the flow channel, i.e., 10.0 mm from the flow channel inlet. A vent hole with the area of 0.3 cm2 was fabricated in the cover plate to allow air diffusion. Carbon paper (Toray 090) was used as the substrate for the planar anode and the air-breathing cathode. The anode was fabricated by electrode position as Zhang et al. [42] with the effective surface area of 0.3 cm2. The Pd catalyst and Nafion loading were about 5.0 and 0.3 mg cm 2,
Fig. 1. Schematic illustration of the (a) air-breathing MMFC with flow-over configuration, and (b) the proposed air-breathing MMFC with immersed fuel micro-jet. (Not to scale). The insert image shows the immersed fuel micro-jet can reach and flow along the anode at an ideal condition.
Fig. 2. Schematic illustration of the proposed cell architecture. 2
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respectively. The cathode catalyst layer was fabricated by spray. Pt black (Alfa Aesar) and Nafion were used as the cathode catalyst and ionomer, with the loading of 3.0 and 0.5 mg cm 2, respectively. Titanium foils were used as the current collectors for the anode and cathode. The immersed micro-jet was visualized by a fluorescent microscope (BX63, Olympus). Deionized (DI) water carrying fluorescent beads (mean diameter of 50 nm) was delivered into the capillary tube and was jetted from the micro-opening towards the anode, while beads-free DI water was fed into the flow channel as the lateral flow. For cell opera tion, the advantage of alkaline electrolyte has been demonstrated [30, 43,44], showing relatively superior reaction kinetics. More importantly, the two-phase perturbation to the mass transfer and reaction can be eliminated. In the present work, sodium formate and atmospheric oxy gen were used as the fuel and oxidant, respectively, and potassium hy droxide as the electrolyte and supporting electrolyte. The aqueous fuel and electrolyte were delivered by a syringe pump (Longer Pump) at identical flow rates. The cell performance was measured by an electro chemical workstation (CHI760E) at room temperature. The polarization curve was obtained by stepwise potentiostatic chronoamperometry from open circuit voltage (OCV) to 0.2 V at 0.1 V per step. The reported current density and power density were normalized to the flow channel volume (0.075 cm3, including the capillary tube). An Ag/AgCl reference electrode (saturated KCl, 0.198 V vs. SHE) was placed at the outlet to separate the electrode potential. For comparison, the flow-over mode in the same cell was used as a control, where all the fuel was fed into the fuel inlet and flowed in parallel to the anode. All measurements were repeated multiple times. The electrochemical impedance spectroscopy (EIS) measurement was performed on the CHI760E electrochemical workstation with the anode as the working electrode. Nyquist plots were recorded at the cell voltage of 0.3 V with the perturbation amplitude of 10 mV and the frequency from 50 kHz to 500 mHz. Prior to each EIS measurement, the cell was kept at potentiostatic control at 0.3 V until a stable discharge current was obtained, implying steady-state mass transfer was reached.
along the anode, enabling targeted and enhanced fuel transport. It’s expected that the micro-jet can replenish the fuel concentration boundary layer by continuously adding fresh fuel. Furthermore, the fully developed flow condition was also visualized at downstream (not shown), and the thickness of the fluorescent stream was about 680 μm, almost one-fold less than the thickness of fuel stream in the flow-over mode (about 1250 μm). The thinner fuel stream can also improve the fuel concentration gradient and enhance fuel transport, as Jayashree et al. demonstrated in the hydrodynamic focusing approach [24]. Also from Fig. 3b, the fluorescent beads illustrated the streamlines of the micro-jet, and weak vortex was observed between the tube and the micro-jet. This phenomenon was mainly due to the entrainment of the fast micro-jet flow, and might lead to the fuel-electrolyte mixing and fuel dilution in actual cell operation. In addition, it’s found that the micro-jet pattern was sensitive to the micro-jet flow rate. At slightly higher micro-jet flow rate (Qj ¼ 60 μL min 1), the micro-jet can rebound on the anode as shown in Fig. 3c, resulting in significant entrainment and two vortexes at both sides of the micro-jet. More importantly, the fluorescent solution were found to intrude into the cathodic region (i.e., above the capillary tube), implying the risk for fuel crossover might be improved in this condition. To evaluate the effect of immersed micro-jet on the fuel transport and cell performance, the cell either with micro-jet or in the flow-over mode were tested at the fuel concentration of 0.5 M. The total fuel flow rate was 200 μL min 1 in both cases, while the Qj and Ql were equal to 100 μL min 1 in the micro-jet case. Fig. 4a compares the cell perfor mance. For the flow-over mode, the polarization curve dropped rapidly above 221.6 mA cm 3, induced by the insufficient fuel transport through the fuel concentration boundary layer. On the contrary, the fuel transfer limitation was largely reduced in the micro-jet case, and higher per formance was obtained. The maximum power density was 83.1 mW cm 3, 25.0% higher than the flow-over mode. It’s also noted from Fig. 4a that the OCV reached 1.04 V, and the difference between both cases was less than 10 mV, indicating the immersed fuel micro-jet would not induce additional fuel crossover at this condition. From the viewpoint of hydrodynamics, the fluid flow in the thin capillary tube and the jetting from the micro-opening can induce large flow resistance. A 3dimensional computational fluid dynamics model (not reported here) was developed to estimate the pumping power. The numerical results confirmed that the micro-jet mainly contributed to the flow resistance. The required pumping power can be two orders of magnitude higher in the micro-jet case as compared with the flow-over mode, but was still less than 0.02% of the peak power output, suggesting this micro-jet approach would not suffer significant parasitic pumping power to lower the net output. Fig. 4b shows the anode and cathode potentials. The anode potential increased rapidly at high current density for the flow-over mode. Whereas the fuel micro-jet can continuously replenish the fuel concentration boundary layer to enhance the fuel transport, thereby reducing the fuel transfer limitation. The nearly identical
3. Results and discussion 3.1. The AMFC with immersed fuel micro-jet The immersed micro-jet was visualized by fluorescent microscopy, as shown in Fig. 3. The density and viscosity of DI water were similar to that of aqueous fuel and electrolyte, making this qualitative visualiza tion informative for cell operation. The flow rate of micro-jet (Qj) varied from 30 to 60 μL min 1, while the lateral flow rate (Ql) was kept constant at 50 μL min 1 for all cases. As shown, the micro-jet couldn’t reach the anode at low micro-jet flow rate of 30 μL min 1 (Fig. 3a), the interaction with the fuel concentration boundary layer could be minimum. Desired micro-jet pattern was obtained at the flow rate of 50 μL min 1 (Fig. 3b), where the fluorescent solution was jetted perpendicular to and flowed
Fig. 3. Visualization of the immersed micro-jet. The flow rate of micro-jet (Qj) was (a) 30, (b) 50 and (c) 60 μL min 1, while the lateral flow rate (Ql) was kept constant at 50 μL min 1. Deionized water with and without fluorescent beads (50 nm in diameter) were used in the micro-jet and lateral flows, respectively. 3
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Fig. 5. Nyquist plots of the EIS measurements for the AMFC with or without micro-jet at the cell voltage of 0.3 V.
low frequency is much smaller in the micro-jet case as compared with the flow-over mode, suggesting the mass transfer resistance was greatly reduced by the fuel micro-jet [47–49]. 3.2. Effect of micro-jet location As demonstrated by previous reports [21,37], the fuel concentration boundary layer can gradually develop along the anode. Its thickness was relatively thin near the inlet, making the anode reaction current density biased towards the inlet. In other words, the fuel transfer limitation can be predominant at downstream. To investigate the effect of micro-jet location, the micro-opening was positioned at 5, 10 and 15 mm from the flow channel inlet, respectively (see Fig. 6a). Fig. 6b shows the effect of micro-jet location on the cell performance at the fuel concentration of 0.5 M. As compared to the flow-over mode, all the cases with micro-jet were able to reduce the fuel transfer limi tation. When the micro-jet was located at 5 mm from the inlet, the cell performance was only slightly higher than the flow-over mode. On one hand, the fuel transfer limitation was insignificant near the inlet, making the replenishment effect minimum. On the other hand, although the jetted fuel stream flowed in the vicinity of the anode that can improve the local fuel flow rate (see Fig. 3b), the fuel concentration boundary layer still develops downstream after the micro-jet location, hindering downstream fuel transport. Therefore, the micro-jet located at 5 mm can only lightly improve the cell performance. For the case where the microjet was located at 15 mm, it’s interesting to note that the peak power density was only 63.5 mW cm 3, even lower than the flow-over mode. For effective replenishment, the fuel should be added to the region where the fuel concentration boundary layer is relatively thick. Furthermore, the downstream anode area that can benefit from the enhanced fuel transport should also be considered. For this 15-mm case, the anode forepart (before the micro-jet location) can suffer from the relatively thick fuel concentration boundary layer due to the one-fold lower flow rate of the lateral fuel stream (Ql) as compared with the flow-over mode, which couldn’t be fully compensated by the down stream performance improvement on the anode rearpart (after the micro-jet location). By contrast, the micro-jet located in the middle (10 mm) appeared to balance the trade-off of replenishment and benefitted anode area, thus leading to good cell performance. The maximum power density was 83.1 mW cm 3, 19.6% and 25.0% higher than the 5-mm case (69.5 mW cm 3) and the flow-over mode (66.5 mW cm 3), respectively. The electrode potentials are shown in Fig. 6c. As compared to the
Fig. 4. Cell performance comparison of the AMFC with or without immersed fuel micro-jet. 0.5 M HCOONa þ2.0 M KOH was used as the fuel, while 2.0 M KOH as the electrolyte. The total fuel flow rate was 200 μL min 1, where the Qj and Ql were equal to 100 μL min 1 in the micro-jet case, respectively. The electrolyte flow rate was 200 μL min 1.
cathode curve also suggested that the micro-jet approach would not induce additional fuel crossover. Fig. S1 and Fig. S2 (see the Supple mental Information) compare the fuel utilization for the micro-jet and flow-over cases at baseline condition and various fuel flow rates, respectively, both suggest the fuel utilization was improved by the fuel micro-jet. Furthermore, the preliminary modelling results also suggest that continuous fuel concentration boundary layer develops over the anode in the flow-over case, whereas it is replenished by the micro-jet successfully (shown in Fig. S3). The electrochemical impedance spectroscopy (EIS) has been demonstrated as a non-destructive diagnosis tool that can provide fundamental understanding of the electrochemical and mass transfer processes with little perturbation to the operation condition [45]. The EIS measurement was performed at the cell voltage of 0.3 V, corre sponding to the maximum power density. The Nyquist plot intercepts the real impedance axis at high frequency, representing the overall ohmic resistance of the cell. From Fig. 5, the overall ohmic resistance of the cell was slightly reduced in the micro-jet case. Given the fact that these two measurements were conducted on the same cell, the reduced ohmic resistance can be mainly attributed to the enhanced hydroxide ion transfer to the anode by the targeted micro-jet. In addition, two semicircles were observed at medium and low frequency, representing the charge transfer and mass transfer resistances, respectively [46]. As shown, the radius of the semicircle at medium frequency were almost identical for the flow-over and micro-jet cases. Whereas the semicircle at 4
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previous reports that the fuel transport is inherently limited by the fuel concentration boundary layer for the MMFC with flow-over anode [23, 37]. In the contrary, distinct performance improvement was obtained for all the cases with fuel micro-jet at increased total fuel flow rates. At the flow rate ratio of Qj:Ql ¼ 3:7, it’s interesting to note that the cell with micro-jet produced similar and even lower performance at 100 and 200 μL min 1 as compared to the flow-over mode. From the florescent visualization (Fig. 7b1 and 7b2), the micro-jet couldn’t reach the anode surface, suggesting that the replenishment to the fuel concentration boundary layer was minimal. Thus, the effective fuel transport was only contributed by the lateral fuel flow rate Ql. Specifically, at the total fuel flow rate of 100 μL min 1, since the fuel transport was predominated by the fuel concentration boundary layer in both cases, similar cell per formance was obtained. However, the fuel transport was enhanced at 200 μL min 1 in the flow-over mode, while the micro-jet case still suf fered from the lower lateral fuel flow rate Ql, resulting in lower power density. At 400 μL min 1, the micro-jet rebounded on the anode but didn’t reach the cathode even at downstream (Fig. 7b3), which enhanced the local fuel transport and lead to higher power density as compared with the flow-over mode. For the flow rate ratio of Qj:Ql ¼ 5:5, the micro-jet can always reach the anode, and the rebound couldn’t interact with the cathode (Fig. 7b4~7b6), thereby nearly linear power density improvement versus the fuel flow rate was obtained. For the flow rate ratio of Qj:Ql ¼ 7:3, significant rebounds were observed at all flow rates. However, it’s interesting to note that the fuel couldn’t reach the cathode even at downstream (Fig. 7b7~7b9). These phenomena were mainly due to the stratified electrolyte laminar flow near the cathode, which suppressed the transversal crossover and prevented the fuel from reaching the cathode [50]. In addition, it is also found that the cell with micro-jet produced similar maximum power densities at 100 and 200 μL min 1 as compared to the flow-over mode. The reason is similar to that described in section 3.2. For the Qj:Q1 ¼ 7:3 case, although the downstream (after the microjet location) performance was further improved due to stronger microjet, the upstream (before the micro-jet location) performance can suf fer from the relatively thick fuel concentration boundary layer, as the lateral fuel flow rate (Ql) is 2.3-fold lower than that in the flow-over mode. Furthermore, the fuel will intrude into the cathodic region at this large micro-jet flow rate as suggested by Fig. 3. Overall, the maximum power density reached about 103 mW cm 3 at 400 μL min 1 for the fuel flow rate ratio (Qj:Ql) of 5:5 and 7:3, that is the highest improvement (40.9%) as compared with the flow-over mode in the present study. It’s worthy of note that the multiple inlets design by Yoon et al. [21] achieved 56% improvement upon introducing three addi tional feeds of reactants. In this proof-of-concept study, the authors expect that the cell performance can be further improved by multiple micro-jets along the anode, which is currently under investigation by our group. Furthermore, the EIS measurements were performed at various flow conditions as shown in Fig. S4. The overall ohmic resistance was slightly reduced in all the micro-jet cases as compared with the flow-over mode, implying the convection induced by the micro-jet also contributed to the OH transport to the anode and thereby reduce the overall ohmic resistance.
Fig. 6. Effect of the micro-jet location on the cell performance. 0.5 M HCOONa þ2.0 M KOH was used as the fuel, while 2.0 M KOH as the electrolyte. The total fuel flow rate was 200 μL min 1, where the Qj and Ql were equal to 100 μL min 1 in the micro-jet case, respectively. The electrolyte flow rate was 200 μL min 1.
anode potential, the cathode potential were almost identical in all cases, again confirming that the micro-jet has minimal effects on the cathode. It’s also noted that the anode performance was improved in the 5- and 10-mm cases, mainly due to the enhanced replenishment to the fuel concentration boundary layer. However, the anode performance drop ped in the 15-mm case, and was even lower than the flow-over case, suggesting the overall fuel transfer was suppressed in this condition.
3.4. Effect of fuel concentration
3.3. Effect of fuel flow rate ratio
The effect of fuel concentration on the cell performance was inves tigated as shown in Fig. 8. Notably, the cell with fuel micro-jet out performed the flow-over mode at all fuel concentrations. At low fuel concentration of 0.25 M, distinct fuel transfer limitation was observed in the flow-over mode. Whereas the micro-jet approach achieved highest relative improvement about 30.0%, resulting from the enhanced fuel transport as discussed above. However, the relative improvement gradually diminished as the fuel concentration increased, since the fuel transfer limitation can be mitigated at high fuel concentration. The highest cell performance was obtained at fuel concentration of 1.0 M,
It’s expected that the fuel flow rate ratio (Qj:Ql) will affect the microjet condition and thereby the fuel transport and cell performance. Fig. 7a shows the effect of fuel flow rate ratio on the maximum power density. The micro-jet was positioned at the middle of the flow channel, i.e., 10 mm from the inlet. For the flow-over mode (Qj:Ql ¼ 0:10), the maximum power density improved by 39.2% from 52.5 to 73.1 mW cm 3 as the total fuel flow rate increased from 100 to 400 μL min 1. However, the improvement trended to plateau as the fuel flow rate increased from 200 to 400 μL min 1, in accordance with 5
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Journal of Power Sources 445 (2020) 227326
Fig. 7. (a) Effect of the fuel flow rate ratio (Qj:Ql) on the maximum power density. 0.5 M HCOONa þ2.0 M KOH was used as the fuel, while 2.0 M KOH as the electrolyte. The electrolyte flow rate was identical to the total fuel flow rate. The fuel micro-jet was located in the middle of the flow channel (10 mm from the inlet). (b) Fluorescent visualization of the micro-jet pattern at various flow rates and flow rate ratios. The images at downstream (about 15 mm from the inlet) were also shown for several cases with significant rebound.
where the maximum power density achieved 119.3 mW cm 3, but the relative improvement was reduced to 14.7%. The comparison of the maximum power density of this work with literature data from airbreathing microfluidic fuel cells using formate/formic acid or meth anol as the fuel in recent years was shown in Fig. S5, suggesting satis factory performance was achieved in this study.
fuel transport was correlated with the trade-off of the replenishment and the benefitted anode area, which appeared to achieve a balance at the middle of the flow channel. The effect of total fuel flow rate and microjet/lateral flow rate ratio on the fuel transport and cell performance were also investigated. At optimal conditions, the cell performance improvement of 40.9% was obtained by the immersed fuel micro-jet as compared to the flow-over mode, and the highest power density reached 119.3 mW cm 3. The proposed approach opened a new avenue for the manipulation of fluid flow and reactant transfer within microfluidic electrochemical reactors.
4. Conclusions Immersed fuel micro-jet was proposed to actively replenish the fuel concentration boundary layer along the flow-over planar anode. In this novel approach, part of the fresh fuel was jetted perpendicular to the anode, enabling targeted convective fuel transport. The visualization by fluorescent microscopy showed that the jetted stream reached and flowed along the anode at optimal condition, without distinct fuel crossover and mixing. From both experimental and preliminary modelling results, the fuel transfer limitation can be reduced by the fuel micro-jet, leading to improved cell performance. The enhancement of
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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[10] [11] [12]
[13] [14] [15] [16] [17]
[18]
Fig. 8. Effect of the fuel concentration on the maximum power density. 2.0 M KOH was added to the fuel as the supporting electrolyte, and was used as the cathodic electrolyte. The fuel flow rate was 200 μL min 1. For the micro-jet case, the fuel flow rate Qj ¼ Ql ¼ 100 μL min 1. The insert is the relative improvement of the micro-jet case vs. the flow-over mode.
[19] [20] [21] [22]
Acknowledgements
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The authors acknowledge the financial supports of the International Cooperation and Exchange of the National Natural Science Foundation of China (No. 51620105011), the National Natural Science Foundation of China (No. 51776026), the Program for Back-up Talent Development of Chongqing University (No. cqu2017hbrc1B06), the Innovation Sup port Foundation for Returned Overseas Scholars, Chongqing, China (cx2017058), the Project (No. 2018CDXYDL0001) supported by the Fundamental Research Funds for the Central Universities, and the “High Level Foreign Experts” program (No. G20190022001) funded by the Ministry of Science and Technology, P.R. China.
[24] [25] [26]
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Appendix A. Supplementary data
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Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227326.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Enhancing fuel transport in air-breathing microfluidic fuel cells by immersed fuel micro-jet Yuan Zhou a, b, Biao Zhang a, b, *, Xun Zhu a, b, **, Ding-Ding Ye a, b, Rong Chen a, b, Tong Zhang a, b, Xiao-Lian Gong a, b, Qiang Liao a, b a b
Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Ministry of Education, Chongqing University, Chongqing, 400030, China Institute of Engineering Thermophysics, School of Energy and Powering Engineering, Chongqing University, Chongqing, 400030, China
H I G H L I G H T S
� Part of fresh fuel is jetted towards the anode, enabling targeted fuel transport. � The micro-jet can reach and flow along the anode without significant crossover. � The fuel transfer limitation from fuel concentration boundary layer is mitigated. � The peak power density is improved 40.9% by the immersed fuel micro-jet. A R T I C L E I N F O
A B S T R A C T
Keywords: Microfluidic fuel cells Fuel concentration boundary layer Fuel transport Immersed micro-jet Cell performance
Air-breathing microfluidic fuel cells with flow-over planar anodes can facilitate system integration but suffer from the fuel concentration boundary layer over the anode, which significantly hinders the fuel transport and limit cell performance. A novel approach is proposed to actively replenish the fuel concentration boundary layer by immersed fuel micro-jet, where part of the fresh fuel is jetted perpendicular to the anode, enabling targeted fuel transfer enhancement. The immersed fuel micro-jet is visualized by fluorescence microscopy, and the microjet can reach and flow along the anode at optimal condition. The same cell architecture is tested in both the flowover and micro-jet modes. The electrochemical measurement and preliminary modelling results indicate that the fuel transfer limitation can be largely mitigated by fuel micro-jet, and the cell performance is enhanced accordingly. The micro-jet located at the middle of flow channel can balance the trade-off between replenish ment and benefitted anode area. The effect of total fuel flow rate and micro-jet/lateral flow rate ratio on the fuel transport and cell performance are also discussed in detail. As compared with the flow-over mode, the maximum performance improvement of 40.9% is achieved by the immersed fuel micro-jet, and the optimal power density reaches 119.3 mW cm 3.
1. Introduction Membraneless microfluidic fuel cells (MMFCs) hold great potential to fulfill the requirements for next-generation micro-power sources. The naturally formed liquid-liquid interface can not only segregate aqueous fuel and oxidant, but also allow transversal ion transfer, therefore serves as an alternative to expensive polyelectrolyte membrane. The removal of physical membrane also helps to diminish the complexity of cell
fabrication and assembly, as well as the requirement for storage and operation. In addition, the composition of fuel and oxidant can be tuned separately and extended to biochemical components (e.g., urea [1] and glycerol [2]), with little concern about bio-fouling, making MMFCs attractive for biomedical point-of-care applications [3]. In the past two decades, much attention has been paid to airbreathing microfluidic fuel cells (AMFCs), because the air-breathing cathode uses the oxygen in air as the low-cost and non-toxic oxidant,
* Corresponding author. Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Ministry of Education, Chongqing University, Chongqing, 400030, China. ** Corresponding author. Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Ministry of Education, Chongqing University, Chongqing, 400030, China. E-mail addresses: [email protected] (B. Zhang), [email protected] (X. Zhu). https://doi.org/10.1016/j.jpowsour.2019.227326 Received 16 August 2019; Received in revised form 4 October 2019; Accepted 16 October 2019 Available online 1 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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leveraging its higher concentration and diffusivity as compared to dis solved oxygen [4]. Recently, considerable progress has been made in the areas of catalyst [5–9], electrode configuration [10–13], unconventional reactants [1,14–16], and cell architecture [17–23]. In general, the anode architecture of AMFCs can be categorized into three types: flow-over architecture with planar electrode [24–26], flow-through architecture based on permeable porous electrodes [23,27,28], and three-dimensional architecture based on cylinder electrodes [22, 29–32]. Among them, the flow-over architecture is simple-structured and compatible with microfabrication process, and has minimal re quirements for assembly and sealing, making it integratable for practical applications [33–36]. In the flow-over architecture (Fig. 1a), the reactant flows in parallel to the anode, and the fuel transfer to the anode perpendicular to the mainstream. It has been demonstrated by both experiments and modelling that fuel concentration boundary layer can develop over the anode due to the rapid electrochemical reaction consumption and ineffective diffusive mass transport [24,34,37]. The existence of fuel concentration boundary layer can diminish the local concentration gradient, and lead to fuel transfer limitation and ultimately lower cell performance. Several strategies have been proposed to enhance fuel transport, including operation at higher flow rates and higher fuel concentration, as well as the development of novel cell architecture. Kjeang et al. [22] and Jayashree et al. [24] reported that the cell per formance can be improved as the thickness of concentration boundary layer was reduced by increasing the flow rate. Chang et al. [38] and Zhang et al. [37] presented similar results in modelling work. However, the maximum flow rate is limited by the hydrodynamic instability that can perturb the laminar flow [38]. In addition, the trade-off between output power density and fuel utilization is still a challenge. Similarly, the fuel transfer can be enhanced by increasing fuel concentration to improve the cell performance. Nevertheless, the use of high fuel con centration will facilitate fuel crossover and lower fuel utilization [4,39]. Hydrodynamic focusing has also been applied to MMFCs, where the reactant concentration gradient can be improved as the reactant stream was pressed thinner by properly adjusting the buffer flow rate [40]. Jayashree et al. [24] and Xuan et al. [41] demonstrated that hydrody namic focusing can be an effective method to improve the cell perfor mance (both output power density and fuel utilization rate) while
avoiding undesired fuel crossover, but at the expense of extra electrolyte consumption and parasitic pumping power [41]. Yoon et al. [21] pro posed active strategies to manipulate the depletion boundary layer. The fuel transport to the catalytic surfaces were enhanced by engineering the depletion zone by multiple periodically-placed outlets/inlets, but can decrease effective electrode area. In addition, Yoon et al. [21] and Marschewski et al. [17] utilized herringbone structure to generate sec ondary transverse flow and enhance convective reactant transport to the electrodes, further improved the cell performance. However, the herringbone pattern requires complicated microfabrication as well as precise control of flow rates to avoid severe convective mixing. In this paper, for the first time, we proposed a novel cell configura tion with immersed fuel micro-jet to replenish the fuel concentration boundary layer and enhance fuel transport. Different from the multiple inlets/outlets approaches, which require relatively complicated micro fluidic network, the present approach confined all functional compo nents into a rectangular flow channel, providing unique benefit for system integration. As a proof-of-concept, a part of fuel was fed in parallel to the anode for the generation of fuel concentration boundary layer, while the other part was jetted perpendicularly to the anode (See Fig. 1b). An air-breathing membraneless microfluidic fuel cell (AMFC) operated in alkaline electrolyte was used as the prototype. The immersed fuel micro-jet was visualized using fluorescent microscopy. In addition, the effects of immersed micro-jet, micro-opening location, flow rate ratio, and fuel concentration on the fuel transfer and cell performance were investigated and discussed in detail. 2. Experimental Fig. 1b illustrates the proposed AMFC with immersed fuel micro-jet. As a proof-of-concept, a portion of fuel was fed into the flow channel, and flowed in parallel to the anode, while the other portion was fed into the capillary tube, and was jetted from a micro-opening on the tube wall perpendicular towards the planar anode. Fig. 2 shows the detailed cell architecture. Polymethyl methacrylate (PMMA) plates fabricated by a laser cutter (VLS 3.60) were used for cell construction. The rectangular flow channel was 20.0, 1.5 and 2.5 mm in length, width and height, respectively. A Titanium capillary tube with an outer diameter of 0.5 mm was fixed in the center of the flow channel. The micro-opening on the tube wall with a diameter about 70 μm was fabricated by laser. As the baseline, the micro-opening was located in the middle of the flow channel, i.e., 10.0 mm from the flow channel inlet. A vent hole with the area of 0.3 cm2 was fabricated in the cover plate to allow air diffusion. Carbon paper (Toray 090) was used as the substrate for the planar anode and the air-breathing cathode. The anode was fabricated by electrode position as Zhang et al. [42] with the effective surface area of 0.3 cm2. The Pd catalyst and Nafion loading were about 5.0 and 0.3 mg cm 2,
Fig. 1. Schematic illustration of the (a) air-breathing MMFC with flow-over configuration, and (b) the proposed air-breathing MMFC with immersed fuel micro-jet. (Not to scale). The insert image shows the immersed fuel micro-jet can reach and flow along the anode at an ideal condition.
Fig. 2. Schematic illustration of the proposed cell architecture. 2
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respectively. The cathode catalyst layer was fabricated by spray. Pt black (Alfa Aesar) and Nafion were used as the cathode catalyst and ionomer, with the loading of 3.0 and 0.5 mg cm 2, respectively. Titanium foils were used as the current collectors for the anode and cathode. The immersed micro-jet was visualized by a fluorescent microscope (BX63, Olympus). Deionized (DI) water carrying fluorescent beads (mean diameter of 50 nm) was delivered into the capillary tube and was jetted from the micro-opening towards the anode, while beads-free DI water was fed into the flow channel as the lateral flow. For cell opera tion, the advantage of alkaline electrolyte has been demonstrated [30, 43,44], showing relatively superior reaction kinetics. More importantly, the two-phase perturbation to the mass transfer and reaction can be eliminated. In the present work, sodium formate and atmospheric oxy gen were used as the fuel and oxidant, respectively, and potassium hy droxide as the electrolyte and supporting electrolyte. The aqueous fuel and electrolyte were delivered by a syringe pump (Longer Pump) at identical flow rates. The cell performance was measured by an electro chemical workstation (CHI760E) at room temperature. The polarization curve was obtained by stepwise potentiostatic chronoamperometry from open circuit voltage (OCV) to 0.2 V at 0.1 V per step. The reported current density and power density were normalized to the flow channel volume (0.075 cm3, including the capillary tube). An Ag/AgCl reference electrode (saturated KCl, 0.198 V vs. SHE) was placed at the outlet to separate the electrode potential. For comparison, the flow-over mode in the same cell was used as a control, where all the fuel was fed into the fuel inlet and flowed in parallel to the anode. All measurements were repeated multiple times. The electrochemical impedance spectroscopy (EIS) measurement was performed on the CHI760E electrochemical workstation with the anode as the working electrode. Nyquist plots were recorded at the cell voltage of 0.3 V with the perturbation amplitude of 10 mV and the frequency from 50 kHz to 500 mHz. Prior to each EIS measurement, the cell was kept at potentiostatic control at 0.3 V until a stable discharge current was obtained, implying steady-state mass transfer was reached.
along the anode, enabling targeted and enhanced fuel transport. It’s expected that the micro-jet can replenish the fuel concentration boundary layer by continuously adding fresh fuel. Furthermore, the fully developed flow condition was also visualized at downstream (not shown), and the thickness of the fluorescent stream was about 680 μm, almost one-fold less than the thickness of fuel stream in the flow-over mode (about 1250 μm). The thinner fuel stream can also improve the fuel concentration gradient and enhance fuel transport, as Jayashree et al. demonstrated in the hydrodynamic focusing approach [24]. Also from Fig. 3b, the fluorescent beads illustrated the streamlines of the micro-jet, and weak vortex was observed between the tube and the micro-jet. This phenomenon was mainly due to the entrainment of the fast micro-jet flow, and might lead to the fuel-electrolyte mixing and fuel dilution in actual cell operation. In addition, it’s found that the micro-jet pattern was sensitive to the micro-jet flow rate. At slightly higher micro-jet flow rate (Qj ¼ 60 μL min 1), the micro-jet can rebound on the anode as shown in Fig. 3c, resulting in significant entrainment and two vortexes at both sides of the micro-jet. More importantly, the fluorescent solution were found to intrude into the cathodic region (i.e., above the capillary tube), implying the risk for fuel crossover might be improved in this condition. To evaluate the effect of immersed micro-jet on the fuel transport and cell performance, the cell either with micro-jet or in the flow-over mode were tested at the fuel concentration of 0.5 M. The total fuel flow rate was 200 μL min 1 in both cases, while the Qj and Ql were equal to 100 μL min 1 in the micro-jet case. Fig. 4a compares the cell perfor mance. For the flow-over mode, the polarization curve dropped rapidly above 221.6 mA cm 3, induced by the insufficient fuel transport through the fuel concentration boundary layer. On the contrary, the fuel transfer limitation was largely reduced in the micro-jet case, and higher per formance was obtained. The maximum power density was 83.1 mW cm 3, 25.0% higher than the flow-over mode. It’s also noted from Fig. 4a that the OCV reached 1.04 V, and the difference between both cases was less than 10 mV, indicating the immersed fuel micro-jet would not induce additional fuel crossover at this condition. From the viewpoint of hydrodynamics, the fluid flow in the thin capillary tube and the jetting from the micro-opening can induce large flow resistance. A 3dimensional computational fluid dynamics model (not reported here) was developed to estimate the pumping power. The numerical results confirmed that the micro-jet mainly contributed to the flow resistance. The required pumping power can be two orders of magnitude higher in the micro-jet case as compared with the flow-over mode, but was still less than 0.02% of the peak power output, suggesting this micro-jet approach would not suffer significant parasitic pumping power to lower the net output. Fig. 4b shows the anode and cathode potentials. The anode potential increased rapidly at high current density for the flow-over mode. Whereas the fuel micro-jet can continuously replenish the fuel concentration boundary layer to enhance the fuel transport, thereby reducing the fuel transfer limitation. The nearly identical
3. Results and discussion 3.1. The AMFC with immersed fuel micro-jet The immersed micro-jet was visualized by fluorescent microscopy, as shown in Fig. 3. The density and viscosity of DI water were similar to that of aqueous fuel and electrolyte, making this qualitative visualiza tion informative for cell operation. The flow rate of micro-jet (Qj) varied from 30 to 60 μL min 1, while the lateral flow rate (Ql) was kept constant at 50 μL min 1 for all cases. As shown, the micro-jet couldn’t reach the anode at low micro-jet flow rate of 30 μL min 1 (Fig. 3a), the interaction with the fuel concentration boundary layer could be minimum. Desired micro-jet pattern was obtained at the flow rate of 50 μL min 1 (Fig. 3b), where the fluorescent solution was jetted perpendicular to and flowed
Fig. 3. Visualization of the immersed micro-jet. The flow rate of micro-jet (Qj) was (a) 30, (b) 50 and (c) 60 μL min 1, while the lateral flow rate (Ql) was kept constant at 50 μL min 1. Deionized water with and without fluorescent beads (50 nm in diameter) were used in the micro-jet and lateral flows, respectively. 3
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Fig. 5. Nyquist plots of the EIS measurements for the AMFC with or without micro-jet at the cell voltage of 0.3 V.
low frequency is much smaller in the micro-jet case as compared with the flow-over mode, suggesting the mass transfer resistance was greatly reduced by the fuel micro-jet [47–49]. 3.2. Effect of micro-jet location As demonstrated by previous reports [21,37], the fuel concentration boundary layer can gradually develop along the anode. Its thickness was relatively thin near the inlet, making the anode reaction current density biased towards the inlet. In other words, the fuel transfer limitation can be predominant at downstream. To investigate the effect of micro-jet location, the micro-opening was positioned at 5, 10 and 15 mm from the flow channel inlet, respectively (see Fig. 6a). Fig. 6b shows the effect of micro-jet location on the cell performance at the fuel concentration of 0.5 M. As compared to the flow-over mode, all the cases with micro-jet were able to reduce the fuel transfer limi tation. When the micro-jet was located at 5 mm from the inlet, the cell performance was only slightly higher than the flow-over mode. On one hand, the fuel transfer limitation was insignificant near the inlet, making the replenishment effect minimum. On the other hand, although the jetted fuel stream flowed in the vicinity of the anode that can improve the local fuel flow rate (see Fig. 3b), the fuel concentration boundary layer still develops downstream after the micro-jet location, hindering downstream fuel transport. Therefore, the micro-jet located at 5 mm can only lightly improve the cell performance. For the case where the microjet was located at 15 mm, it’s interesting to note that the peak power density was only 63.5 mW cm 3, even lower than the flow-over mode. For effective replenishment, the fuel should be added to the region where the fuel concentration boundary layer is relatively thick. Furthermore, the downstream anode area that can benefit from the enhanced fuel transport should also be considered. For this 15-mm case, the anode forepart (before the micro-jet location) can suffer from the relatively thick fuel concentration boundary layer due to the one-fold lower flow rate of the lateral fuel stream (Ql) as compared with the flow-over mode, which couldn’t be fully compensated by the down stream performance improvement on the anode rearpart (after the micro-jet location). By contrast, the micro-jet located in the middle (10 mm) appeared to balance the trade-off of replenishment and benefitted anode area, thus leading to good cell performance. The maximum power density was 83.1 mW cm 3, 19.6% and 25.0% higher than the 5-mm case (69.5 mW cm 3) and the flow-over mode (66.5 mW cm 3), respectively. The electrode potentials are shown in Fig. 6c. As compared to the
Fig. 4. Cell performance comparison of the AMFC with or without immersed fuel micro-jet. 0.5 M HCOONa þ2.0 M KOH was used as the fuel, while 2.0 M KOH as the electrolyte. The total fuel flow rate was 200 μL min 1, where the Qj and Ql were equal to 100 μL min 1 in the micro-jet case, respectively. The electrolyte flow rate was 200 μL min 1.
cathode curve also suggested that the micro-jet approach would not induce additional fuel crossover. Fig. S1 and Fig. S2 (see the Supple mental Information) compare the fuel utilization for the micro-jet and flow-over cases at baseline condition and various fuel flow rates, respectively, both suggest the fuel utilization was improved by the fuel micro-jet. Furthermore, the preliminary modelling results also suggest that continuous fuel concentration boundary layer develops over the anode in the flow-over case, whereas it is replenished by the micro-jet successfully (shown in Fig. S3). The electrochemical impedance spectroscopy (EIS) has been demonstrated as a non-destructive diagnosis tool that can provide fundamental understanding of the electrochemical and mass transfer processes with little perturbation to the operation condition [45]. The EIS measurement was performed at the cell voltage of 0.3 V, corre sponding to the maximum power density. The Nyquist plot intercepts the real impedance axis at high frequency, representing the overall ohmic resistance of the cell. From Fig. 5, the overall ohmic resistance of the cell was slightly reduced in the micro-jet case. Given the fact that these two measurements were conducted on the same cell, the reduced ohmic resistance can be mainly attributed to the enhanced hydroxide ion transfer to the anode by the targeted micro-jet. In addition, two semicircles were observed at medium and low frequency, representing the charge transfer and mass transfer resistances, respectively [46]. As shown, the radius of the semicircle at medium frequency were almost identical for the flow-over and micro-jet cases. Whereas the semicircle at 4
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previous reports that the fuel transport is inherently limited by the fuel concentration boundary layer for the MMFC with flow-over anode [23, 37]. In the contrary, distinct performance improvement was obtained for all the cases with fuel micro-jet at increased total fuel flow rates. At the flow rate ratio of Qj:Ql ¼ 3:7, it’s interesting to note that the cell with micro-jet produced similar and even lower performance at 100 and 200 μL min 1 as compared to the flow-over mode. From the florescent visualization (Fig. 7b1 and 7b2), the micro-jet couldn’t reach the anode surface, suggesting that the replenishment to the fuel concentration boundary layer was minimal. Thus, the effective fuel transport was only contributed by the lateral fuel flow rate Ql. Specifically, at the total fuel flow rate of 100 μL min 1, since the fuel transport was predominated by the fuel concentration boundary layer in both cases, similar cell per formance was obtained. However, the fuel transport was enhanced at 200 μL min 1 in the flow-over mode, while the micro-jet case still suf fered from the lower lateral fuel flow rate Ql, resulting in lower power density. At 400 μL min 1, the micro-jet rebounded on the anode but didn’t reach the cathode even at downstream (Fig. 7b3), which enhanced the local fuel transport and lead to higher power density as compared with the flow-over mode. For the flow rate ratio of Qj:Ql ¼ 5:5, the micro-jet can always reach the anode, and the rebound couldn’t interact with the cathode (Fig. 7b4~7b6), thereby nearly linear power density improvement versus the fuel flow rate was obtained. For the flow rate ratio of Qj:Ql ¼ 7:3, significant rebounds were observed at all flow rates. However, it’s interesting to note that the fuel couldn’t reach the cathode even at downstream (Fig. 7b7~7b9). These phenomena were mainly due to the stratified electrolyte laminar flow near the cathode, which suppressed the transversal crossover and prevented the fuel from reaching the cathode [50]. In addition, it is also found that the cell with micro-jet produced similar maximum power densities at 100 and 200 μL min 1 as compared to the flow-over mode. The reason is similar to that described in section 3.2. For the Qj:Q1 ¼ 7:3 case, although the downstream (after the microjet location) performance was further improved due to stronger microjet, the upstream (before the micro-jet location) performance can suf fer from the relatively thick fuel concentration boundary layer, as the lateral fuel flow rate (Ql) is 2.3-fold lower than that in the flow-over mode. Furthermore, the fuel will intrude into the cathodic region at this large micro-jet flow rate as suggested by Fig. 3. Overall, the maximum power density reached about 103 mW cm 3 at 400 μL min 1 for the fuel flow rate ratio (Qj:Ql) of 5:5 and 7:3, that is the highest improvement (40.9%) as compared with the flow-over mode in the present study. It’s worthy of note that the multiple inlets design by Yoon et al. [21] achieved 56% improvement upon introducing three addi tional feeds of reactants. In this proof-of-concept study, the authors expect that the cell performance can be further improved by multiple micro-jets along the anode, which is currently under investigation by our group. Furthermore, the EIS measurements were performed at various flow conditions as shown in Fig. S4. The overall ohmic resistance was slightly reduced in all the micro-jet cases as compared with the flow-over mode, implying the convection induced by the micro-jet also contributed to the OH transport to the anode and thereby reduce the overall ohmic resistance.
Fig. 6. Effect of the micro-jet location on the cell performance. 0.5 M HCOONa þ2.0 M KOH was used as the fuel, while 2.0 M KOH as the electrolyte. The total fuel flow rate was 200 μL min 1, where the Qj and Ql were equal to 100 μL min 1 in the micro-jet case, respectively. The electrolyte flow rate was 200 μL min 1.
anode potential, the cathode potential were almost identical in all cases, again confirming that the micro-jet has minimal effects on the cathode. It’s also noted that the anode performance was improved in the 5- and 10-mm cases, mainly due to the enhanced replenishment to the fuel concentration boundary layer. However, the anode performance drop ped in the 15-mm case, and was even lower than the flow-over case, suggesting the overall fuel transfer was suppressed in this condition.
3.4. Effect of fuel concentration
3.3. Effect of fuel flow rate ratio
The effect of fuel concentration on the cell performance was inves tigated as shown in Fig. 8. Notably, the cell with fuel micro-jet out performed the flow-over mode at all fuel concentrations. At low fuel concentration of 0.25 M, distinct fuel transfer limitation was observed in the flow-over mode. Whereas the micro-jet approach achieved highest relative improvement about 30.0%, resulting from the enhanced fuel transport as discussed above. However, the relative improvement gradually diminished as the fuel concentration increased, since the fuel transfer limitation can be mitigated at high fuel concentration. The highest cell performance was obtained at fuel concentration of 1.0 M,
It’s expected that the fuel flow rate ratio (Qj:Ql) will affect the microjet condition and thereby the fuel transport and cell performance. Fig. 7a shows the effect of fuel flow rate ratio on the maximum power density. The micro-jet was positioned at the middle of the flow channel, i.e., 10 mm from the inlet. For the flow-over mode (Qj:Ql ¼ 0:10), the maximum power density improved by 39.2% from 52.5 to 73.1 mW cm 3 as the total fuel flow rate increased from 100 to 400 μL min 1. However, the improvement trended to plateau as the fuel flow rate increased from 200 to 400 μL min 1, in accordance with 5
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Fig. 7. (a) Effect of the fuel flow rate ratio (Qj:Ql) on the maximum power density. 0.5 M HCOONa þ2.0 M KOH was used as the fuel, while 2.0 M KOH as the electrolyte. The electrolyte flow rate was identical to the total fuel flow rate. The fuel micro-jet was located in the middle of the flow channel (10 mm from the inlet). (b) Fluorescent visualization of the micro-jet pattern at various flow rates and flow rate ratios. The images at downstream (about 15 mm from the inlet) were also shown for several cases with significant rebound.
where the maximum power density achieved 119.3 mW cm 3, but the relative improvement was reduced to 14.7%. The comparison of the maximum power density of this work with literature data from airbreathing microfluidic fuel cells using formate/formic acid or meth anol as the fuel in recent years was shown in Fig. S5, suggesting satis factory performance was achieved in this study.
fuel transport was correlated with the trade-off of the replenishment and the benefitted anode area, which appeared to achieve a balance at the middle of the flow channel. The effect of total fuel flow rate and microjet/lateral flow rate ratio on the fuel transport and cell performance were also investigated. At optimal conditions, the cell performance improvement of 40.9% was obtained by the immersed fuel micro-jet as compared to the flow-over mode, and the highest power density reached 119.3 mW cm 3. The proposed approach opened a new avenue for the manipulation of fluid flow and reactant transfer within microfluidic electrochemical reactors.
4. Conclusions Immersed fuel micro-jet was proposed to actively replenish the fuel concentration boundary layer along the flow-over planar anode. In this novel approach, part of the fresh fuel was jetted perpendicular to the anode, enabling targeted convective fuel transport. The visualization by fluorescent microscopy showed that the jetted stream reached and flowed along the anode at optimal condition, without distinct fuel crossover and mixing. From both experimental and preliminary modelling results, the fuel transfer limitation can be reduced by the fuel micro-jet, leading to improved cell performance. The enhancement of
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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[10] [11] [12]
[13] [14] [15] [16] [17]
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Fig. 8. Effect of the fuel concentration on the maximum power density. 2.0 M KOH was added to the fuel as the supporting electrolyte, and was used as the cathodic electrolyte. The fuel flow rate was 200 μL min 1. For the micro-jet case, the fuel flow rate Qj ¼ Ql ¼ 100 μL min 1. The insert is the relative improvement of the micro-jet case vs. the flow-over mode.
[19] [20] [21] [22]
Acknowledgements
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The authors acknowledge the financial supports of the International Cooperation and Exchange of the National Natural Science Foundation of China (No. 51620105011), the National Natural Science Foundation of China (No. 51776026), the Program for Back-up Talent Development of Chongqing University (No. cqu2017hbrc1B06), the Innovation Sup port Foundation for Returned Overseas Scholars, Chongqing, China (cx2017058), the Project (No. 2018CDXYDL0001) supported by the Fundamental Research Funds for the Central Universities, and the “High Level Foreign Experts” program (No. G20190022001) funded by the Ministry of Science and Technology, P.R. China.
[24] [25] [26]
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Appendix A. Supplementary data
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Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227326.
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