A Facile Microfluidic Hydrogen Peroxide Fuel Cell with High

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A Facile Microfluidic Hydrogen Peroxide Fuel Cell with High Performance: Electrode Interface and Power-Generation Properties Yang Yang, Yishen Xue, Fei Huang, Heng Zhang, Kai Tao, Ruirong Zhang, Qiang Shen, and Honglong Chang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00943 • Publication Date (Web): 18 Sep 2018 Downloaded from http://pubs.acs.org on September 26, 2018

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ACS Applied Energy Materials

A Facile Microfluidic Hydrogen Peroxide Fuel Cell with High Performance: Electrode Interface and Power-Generation Properties Yang Yanga,b,*, Yishen Xuea, Fei Huanga, Heng Zhanga, Kai Taoa,b, Ruirong Zhanga,b, Qiang Shena,b, Honglong Chang a,b,* a

Ministry of Education Key Laboratory of Micro/Nano Systems for Aerospace,

School of Mechanical Engineering, Northwestern Polytechnical University, Xi’an710072, China b

Unmanned System Research Institute, Northwestern Polytechnical University, Xi’an

710072, China

* Yang Yang-Ph.D., Corresponding author E-mail: [email protected]; * Honglong Chang-Ph.D., Corresponding author E-mail: [email protected]; Address: 127 West Youyi Road, Xi’an, Shaanxi, 710072, P.R. China Tel/Tax: +86 029-8849-2841

1

ACS Paragon Plus Environment

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract To simplify the miniature fuel cell structure and flow mode, here we report a microfluidic fuel cell running on H2O2 as both fuel and oxidant under acidic conditions. Prussian blue coating on carbon paper serves as the cathode side while the anode is made of three-dimensional flow-through Ni foam. The fuel cell achieves a power density in excess of 0.58 ± 0.13 W m-2 at a current density of 3.68 ± 0.1 A m-2, and an open circuit potential of 0.65 V. Importantly, the Ni foam shows a corrosion in H2O2-catalyzing after a long-term operation, which is seriously neglected by most of previous H2O2-running fuel cells. SEM images and XPS spectra demonstrate a gradient corrosion occurs in the three-dimensional flow-through porous Ni-foam electrode. The corrosion degree of Ni foam gradually aggravates along the vertical direction, which is caused by the gradient accumulation of H2O in the porous electrode. The protection methods including surface coating a protection layer and doping some more reactive metals have been proposed to improve the system commercialization.

Keywords: Microfluidic; Miniature fuel cells; Nickel catalysis; Hydrogen peroxide； Prussian blue; Porous electrode; Corrosion 2


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1. Introduction Modern micro-electro-mechanical systems (MEMS) and applications are trending toward tiny, light weight and flexible vogues to suit particular environments. To power these MEMS, micro-scale energy harvesting and storage accessories based on piezoelectric, electrostatic and electromagnetic micro harvesters, micro batteries, super capacitors and fuel cells open up many opportunities for our choice.1-2 Among them, microfluidic fuel cells (MFCs) employ the laminar-flow pattern between anolyte and catholyte at two sides of a microchannel to eliminate the physical membranes selected in most fuel cell configurations, while simultaneously yielding a high power output.3-7 It successfully addresses the issues related to the membranes such as manufacturing cost, internal resistance and fabrication bulkiness. Despite above cited promising features, the fuel and oxidant have to be separated to form a confined liquid-liquid interface where diffusive mixing is limited. Once the interface is unstable or even destroyed, the power output will deteriorate and further retard such device in practical applications.8-9 Meanwhile, the flow-pattern controlling is so laborious that increasing the complexity of the system, and further lowering the total efficiency. In response to this issue, a number of efforts have been consequently shifted to ensure the interface stability, including introducing an extra electrolyte,10 optimization MFC device fabrication,11-12 and controlling the flow pattern.13 The optimization of fuel/oxidant choice is considered as one of the most practical pathways to maintain the interface stability. Hydrogen peroxide is a carbon-free energy carrier that can serve as both fuel and oxidant in the fuel cell.14 It can be easily accessed from the oxygen in the air by two-electron reduction process or the water by two-electron oxidation process. Both of the starting materials used for producing

hydrogen

peroxide

are

earth

abundant.15

Recently,

various

environmental-friendly approaches have been demonstrated in the reports. For 3



example, Yamada et al., utilized the photovoltaic solar cell to produce the H2O2 from reducing oxygen at 0.5 V.16 The amount of H2O2 reached 1.46×10-5 mol during the 11 hour operation with a current efficiency of nearly 100%. Besides the easy accessibilities, none of environmental detrimentally products will be produced by H2O2 unlike carbonaceous fuel because H2O2 only decomposes to water and oxygen.16 The major issue related to H2O2 is the system stability raised by the corrosive environments, since H2O2 is a powerful oxidizer. Nevertheless, H2O2 is still one of the most promising fuel-oxidant choices especially considering the desire to commercialize the MFC technology. Previous studies have rendered the availability of serving H2O2 as sole reactant in operating MFC device. For example, Hasegawa et al. first constructed a MFC using decomposition of H2O2 in sodium hydroxide solution and sulfuric acid solution as fuel and oxidant, and proved its comparability to that of a conventional air-breathing direct-methanol fuel cell.17 Shyu et al. further revealed the power-generation capacity of H2O2-fed MFCs at different volumetric flow rates and channel-widths.18 They studied the bubble behavior in microchannel under different flow conditions, and revealed bubble growth played a pivotal role in the cell performance. In addition to offering insights into the underlying principles of H2O2-fed MFC devices, much efforts have been evolved in the improvement of cell performance. Ha et al. designed a microchannel with a groove pattern to facilitate the mass transportation and mitigate the bubble accumulated on the electrode surface.19 Compared to those of common planar electrodes, the maximum power density has been improved by 13.9% by optimizing the electrode structure. Nevertheless, all of the H2O2-fed MFCs exploited basic-acidic or alkaline-acidic bipolar electrolytes. Two disadvantages are accompanied with the bipolar system. First, it is imperative that the fuel and oxidant should be separated to avoid the potential neutralization of H+ and OH- in microchannel. Second, noble-metal catalysts like platinum, gold and palladium-iridium alloy are always selected to catalyze the H2O2 decomposition. These noble metals undoubtedly have a high catalytic activity, but will bring an conspicuous capital cost to the whole device.14 Worse still, the evolution of oxygen 4


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from the H2O2 decomposition at the cathode increases some operational difficulties in the fuel cell device.20 Therefore, we need develop a compact H2O2 decomposition system, and improve the cost effectiveness of total device. Here, we highlighted a strategy to fabricate the MFC by employing Ni foam as a three-dimensional (3D) flow-through porous electrode, and Prussian blue (PB) coated on carbon paper serving as the cathode to simplify the device architecture. The power-generation properties of this MFC were examined and compared to those of conventionally flat architectures by constructing power density and polarization curves. To show the electrode stability in the acidic environment, microscopic images and XPS spectra were collected and analysed after the device operation.

2. Experimental Section 2.1 Preparation and Characterizations of Electrode Materials The cathode was fabricated by spraying the PB catalyst solution on carbon paper (denoted as PB-CP, HCP120, Hesen Ltd., Shanghai, China), which was made by adding 4 mL isopropanol and 1.2 mL DI water from Millipore (Direct Q) to 24 mg of PB. The solution was agitated in an ultrasound water bath for 30 minutes. Subsequently, 100 µL Nafion solution (5 at. %) was added to the mixture and again agitated for 30 minutes. The paste was brushed on the electrode and air dried at 80 oC in an oven (DHG-9011A, Shanghai JingHong Inc., China). The catalyst loading can be optimized by controlling the number of brushing process. For the anode, Ni foam was carefully washed by using acetone, 1 M HCl, ethanol and DI water to remove the impurities before used.21 All chemical reagents were analytical grade and purchased from Sigma-Aldrich (Sigma-Aldrich Corp., St. Louis, MO). After finishing the assembling of electrodes, their morphologies were probed by the scanning electron microscopy (SEM, VEGA 3LMU, TESCAN). The 3D structure 5



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of Ni-foam electrode was visualized by micro Computed Tomography (micro CT, skyscan1172, Bruker Inc., Germany). The composition of catalyst was determined by powder X-ray diffraction (XRD, Rigaku Americas Miniflex Plus powder diffractometer) and X-ray photoelectron spectroscopy (XPS, ESCALAB 250 Xi XPS System). The XRD pattern of Ni foam was collected using Co Kα radiation, and XPS spectra was measured using monochromatic Al Kα radiation (1486.6 eV) with an operating power of 150 W. The XPS peaks was aligned and the compositions were computed by XPS PEAK4.1 software. 2.2 Assembly and Set-up of MFCs Fig. 1 presents a schematic illustration of the MFC. The PB-CP and 3D Ni foam were used as cathode and anode, respectively. The reactions at the electrodes are given by as followings: Anode: → + 2 + 2 ; = 0.68

[1]

Cathode: + 2 + 2 → 2 ; = 1.77

[2]

Total: 2 → 2 +

[3]

The theoretical cell voltage for the total reaction is 1.09 V. A chamber with dimensions of 20 mm in length, 2.5 mm in width and 1 mm in height was engraved on a polymethylmethacrylate (PMMA) plate by a laser beam to serve as the microchannel. To test the MFC performance in a continuous-mode operation, 0.33 M hydrogen peroxide diluted in 0.067 M HCl solution was continuously supplied into the porous Ni foam. The syringe pump operated at a pull-mode with a steady feeding volumetric flow rate of 150 µL min-1 (Pump 11 Elite, Harvard Apparatus, MA). The pH value of the solutions was measured to ~1.2 by pH meter (PHS-25, INESA Instruments Inc., Shanghai, China). After injecting H2O2 solution into the anode, it passed into the microchannel and flowed over the surface of cathode. At the cathode, it was reduced to H2O while discharged at the bottom of the plant. H2O2 acted as the 6


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sole reactant and HCl served as the supporting electrolyte. A Ag/AgCl (saturated KCl solution, + 0.197 V vs. standard hydrogen electrode) was used as a reference electrode. All the experiments were carried out under ambient pressure and room temperature.

Figure 1. Schematic diagram illustrating the configuration of MFC device with the PB-CP cathode and 3D Ni-foam anode.

2.3 Electrochemical Characterizations In order to characterize the MFCs’ performance, the power-density and polarization curves were plotted by using chronoamperometry method by setting a stepwise voltage (V) from open circuit voltage (OCP) to 0.0 V. The values of OCP were monitored by running the MFC at the open circuit state for 10 minutes. At each step, the current density (I) was obtained until a steady state was reached, and power density (P) was calculated as P=V×I. Both I and P were normalized to the surface of the cathode (~0.5 cm2). We tested the cell performance in triplicate to ensure a good reproducibility. For each measurement, we used newly prepared electrodes and flushed microchannel to assemble the devices. All electrochemical measurements were conducted using an electrochemical work station (CHI 660E, CH Instruments Inc., Shanghai, China). 7



3. Results and Discussion 3.1 Power-generation property We investigated the MFC power-generation properties through constructing polarization and power-density curves by setting the stepwise voltage. The values of power density were calculated normalized to the area of cathode when the device reached a steady state. As shown in Fig.2 (a), it delivered a maximum power density (MPD) of 0.39 W m-2 at a current density of 1.95 A m-2. The maximum current density (MCD) was measured to be 4.75 A m-2. We fabricated three distinct devices to collect the deviation of these values [Fig.2 (b)]. An average value of 0.58 ± 0.13 W m-2 and 4.82 ± 0.068 A m-2 was obtained for MPD and MCD, which is capable of powering some niche devices such as digital thermometer,22 switch23 and drug pumps.24 Significantly, the device output a 5.8-folds higher power density than previous device under a similar reaction system,15 and a comparable power density with other state-of-art microfluidic energy suppliers,25-28 but render the most versatile way to realize it (Table 1). The cell performance was obtained at the simplest cell design resulting from the H2O2 as the sole reactant. A low feeding rate (~150 µL min-1) can easily satisfy the needs of normal operation comparing with other MFCs working at a much higher feeding rates (300-3000 µL min-1), although they may deliver a greater power densities.4, 29-32 In brief, the device successfully worked with the most compact design and low reactant-feeding rates.

8


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Figure 2. (a) Power-density and polarization curves of MFC; (b) Comparison of MPD and MCD collected from three parallel trials. Current and power densities were calculated normalized to the geometric surface area of electrode.

Table 1. Performance comparison between this work and reported MFC devices Fuel

Oxidant

Electrode area (cm2)

Volumetric flow velocity (µL min-1)

Power density (W m-2)

References

Hydrogen peroxide

Hydrogen peroxide

0.5

150

0.58

This work

Glucose

Oxygen

0.3

1000

0.32

25

Ethanol

Oxygen

0.1

10

0.05

26

Glucose

Oxygen

0.28

300

0.26

27

Acetate

ferricyanide

1

1

0.33

28

3.2 Cathode characteristics The morphology and structure of PB-CP electrode were first determined by scanning electron microscopy (SEM) inspections. As Fig.3 (a) shown, pristine CP was 9



made of carbon-fibre yarns which were closely packed together and exhibited a relatively rough surface. There was no particles or clusters observed on the surface of yarns. Upon refluxing in the catalyst ink, CP was gradually covered by PB particles. Small agglomerates of the catalysts mainly adhered to the hard carbon ridges. By repeating the loading process, the mass loading of PB on CP reached at 5 mg cm-2, as shown in Fig.3 (b). Most vacancies between adjacent yarns were filling with the PB clusters, but some gaps with diameters of tens of micrometers to hundreds of micrometers could still be observed. These gaps may benefit the transportation of H2O2 and H2O interior the cathode.33

Figure 3. SEM images of (a) pristine CP, (b) PB-CP before reaction, (c) reacted after 0.5 h and (d) reacted after 2 h. 10


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We further observed the morphology of the PB-CP by ex situ post-running cell performance measurements, which characterized the steady properties of PB particles during the H2O2 reduction reaction process. As shown in Fig.3 (c-d), no obvious corrosion or even detachment was observed after the operation for 0.5 h and 2 h, in which CP was still uniformly covered by PB particles. It suggests the PB is stable in catalyzing the H2O2 reduction reaction in acidic medium. 3.3 Anode characteristics Similarly, we compared the morphologies of Ni foam before and after reaction, as the SEM images shown in Fig.4. Blank Ni foam presented a very clean and smooth surface before catalyzing the H2O2 oxidation [Fig.4 (a)]. After the reaction, the color of Ni foam turned from bright white to dark, as the optical images evidenced in insets of Fig.5 (b). The change of macro profile may be due to the instability of Ni foam in acidic medium. To test our hypothesis, we investigated the morphological variations of Ni mesh along the vertical direction [denoted in the left-bottom insets of Fig.4 (b-d)]. The whole surface of the Ni foam became rough near the entrance of H2O2 solution, and some hairline cracks were beginning to be observed as shown in Fig.4 (b). As the reactant further penetrate, large amounts of cracks dominated the surface of Ni foam, and formed a porous structure on the scaffold of Ni mesh [Fig.4 (c)]. The high-resolution SEM image obviously indicated some pores arise on the surface of Ni-foam scaffold [as shown in the top-right inset of Fig.4 (c)]. At the spots close to the flow channel, pores with a diameter larger than 2 µm were observed. Meanwhile, the width and thickness of surface cracks are much greater than those of previous observation points [Fig.4 (d)]. These SEM inspections corroborate our findings, showing that the Ni foam is unstable in H2O2 solution. We further observed the 3D structure of reacted Ni-foam electrode using micro CT (see the Fig.S1 in supplementary materials). The surface of Ni-scaffold exhibited a shading of colours, 11



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primarily lighter near the entrance. Gradient colours representing the morphological variations of Ni mesh were also visualized across the vertical direction. The colour change of Ni mesh in H2O2 solution was previously noticed by Yamada et al., who also exploited the Ni metal as the anode and run the system in the acidic environment, but the further explanation of Ni mesh corrosion still wasn’t given.34 The poor stability and durability of Ni metal in catalysing the H2O2 may dramatically lower the device performance and operational time. In addition to offering insights into the corrosion of Ni in catalysing the H2O2-oxidation reaction, SEM images have deterministically revealed a gradient Ni corrosion in the 3D macro-porous structure. We consider it could be due to gradient distributions of reactants and reaction products including H+, O2 and H2O. Previous study has demonstrated that the Ni passivation reaction is supremely faster in the presence of solution than that in the gaseous environment.35 A Ni-based passive oxide film can be formed according to the following equation:35

Ni + ⇌ %& + 2 + 2

[4]

NiO can be further oxidized to other two types of higher oxide, Ni3O4 and Ni2O3, according to the following equations:35

3NiO + ⇌ %&) * + 2 + 2

[5]

2%&) * + ⇌ 3%& ) + 2 + 2

[6]

As the Eq. [4-6] shown, the existence of H2O is essential to trigger the Ni passivation or oxidation reaction. In the porous structure, gas easily accumulates at the top of 3D structure due to the effect of buoyancy lift. A much higher concentration of oxygen is usually observed close to the cover plate.36 For fluid-velocity, it is typically increased along the flow direction as a result of the low permeability of the Ni form, and an increased pressure distribution.37 A high percentage of H2O will be typically presented at the bottom of the electrodes, whereas a severe accumulation of 12


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bubbles occupies the area of solution at the top of the electrodes. Ni scaffolds are more easily to contact with H2O close to the microchannel. Therefore, a gradient passivation for Ni form in the 3D porous electrode was observed.

Figure 4. SEM images collected from the (a) pristine Ni foam and (b-d) reacted Ni-foam electrode along the flow direction. Scale bares are 20 µm. Insets in Figures indicate the high resolution

(top-right corner) and schematics of 3D porous Ni foam (left-bottom corner) before reaction (a), and the corresponding observation spots of electrode after reaction (b, c, d).

To foster the understanding of the underlying mechanistic information of Ni catalysis in H2O2 oxidation, we further collected the XRD pattern and XPS spectra from the pristine Ni foam and used electrode. As shown in Fig.5 (a-b), two major peaks centred at 52.3o and 61.2o were observed for both cases, belonging to Ni metal (PDF no.04-0850).38 No other diffraction peaks and obvious peak shifts were 13



collected in the XRD patterns. There is no obvious difference between two samples in the XRD survey, which doesn’t agree with the optical observations (as shown in insets of Fig.5) and possibly due to several reasons including the trace amount of samples, small diameters of Ni products and amorphous magnetism.39 For further investigating the Ni surface information, XPS spectrum was used to detect the chemical states of bonded elements. A series of peaks corresponding to nickel, oxygen, chlorine and other species were clearly observed in the survey spectrum (see the Fig.S2 in supplementary materials). There was no significant change in the Ni 2p and O 1s peaks after the MFC operation, whereas the concentrations of Ni metal and oxygen-containing groups (e.g. NixOy, -OH) changed a lot. As shown in Fig.6 (b), visible peaks at 853.2 eV and 872.5 eV clearly appeared in the Ni 2p3/2 and Ni 2p1/2 spectra after the H2O2-oxidation reaction. As suggested in these results, it was found that Ni 2p3/2 position was quite different from those of metallic Ni (~852.3 eV) and Ni2O3 (~856.7 eV). The peak was more close to that of NiO (~853.4 eV), and the peak of Ni 2p1/2 was also close to NiO (~872.7 eV).40-42 At the same time, a peak centred at 529.7 eV was observed in the XPS core level O 1s spectrum [Fig.6 (d)]. The peak was also referred to as the production of NiO, which was in agreement with the reaction Eq. [4].43 These results demonstrate the vulnerability of Ni in catalyzing the H2O2-oxidation reaction. But there was no peak associated with the higher oxide like Ni3O4 and Ni2O3. This situation could be expected because a much higher potential of 0.35 V and 1.47 V is needed to trigger the formation of higher oxide in the same pH environment.35 The reactions occurred at the anode couldn’t supply such high potentials. Table 1 specifically summarized the content change of various elements and functional groups including Ni, C-O and -OH. As suggested in the table, the content of NiO increased from 0% to 1.97%, and the Ni metal was hard to be detected after the MFC operation. The absence of Ni metal is inconsistent with the XRD pattern, which could be due to the limited detection area for the XPS method (less than 5 nm). As showed in the SEM and micro-CT image, the surface of Ni-foam was reacted by H2O2 and H2O. The surface of Ni metal without corrosion is hardly 14


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detected by the XPS method. In the test of XRD technique, it can successfully detect the whole composition of sample that not limit to the surface. The part of Ni-foam that still not totally reacted by H2O2 and H2O can still be detected by the XRD technique. Table 2 also indicated the content of C-O decreased from 32.90% to 22.99%, but the total content of oxygen basically remained unchanged. Based on the variation of oxygen content, we considered the element of oxygen in NiO was stemmed from breaking the carbon/oxygen single bond. Notably, the elements of carbon and chlorine were observed before reaction, which were due to the impurities in the pristine Ni foam. After operating the device for a long term, the content of chlorine increased by 5-folds. The boost of chlorine content is considered resulting from the attached HCl in the Ni electrode during operation. The XPS spectra foster the basic understanding of reaction mechanism of Ni-anode behaviour in microfluidic hydrogen peroxide fuel cell operated under acidic conditions.

Figure 5. Schematic illustration showing the XRD pattern for Ni-foam electrode: (a) before reaction; (b) after reaction. Inset: Optical Ni-foam images before and after reaction.

Table 2. Composition variation of the electrode before and after reaction. Sample (at. %)

Ni metal

NiO

Ni(OH)2

Cl-

15


C-O

C=O

-OH


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Before

3.96

－

38.68

2.45

32.90

3.50

6.63

After

－

1.97

36.10

11.73

22.99

5.58

5.36

Figure 6. High resolution Ni 2p, O 1s and Cl 2p XPS spectra of Ni-foam electrode before reaction (a, c, e) and after reaction (b, d, f). Solid lines and dash lines represent the experimental data and corresponding fitting curves created by simulating the experimental data using XPSpeak software.

4. Conclusion 16


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In this work, we successfully fabricated a microfluidic H2O2 fuel cell incorporating PB-CP and 3D flow-through Ni mesh as cathode and anode materials, respectively. The H2O2 solution crossed-flow through the porous electrode into the microchannel and act as both fuel and oxidant. Unlike the already-established microfluidic fuel cells, the separation between fuel and oxidant was not required and the flow-type was simplified to a single-fluid mode. The MFC achieved a volumetric power density of 0.58 ± 0.13 W m-2 with OCP value of 0.65 V, which were comparable to other types of micro power sources. More importantly, SEM images and XPS spectra first demonstrated the Ni foam was unstable in H2O2-catalyzing in an acid medium, which was neglected by previous studies. A gradient corrosion in the flow-through porous Ni-foam electrode was observed, and the corrosion appeared more severe at the bottom. Nevertheless, considering the desire to commercialize the miniature fuel cell technology, H2O2 is still one of the most promising candidates serving as the sole reactant. Incremental gains in the protection of Ni-based electrode should continue to be made through increase the erosion resistance such as surface coating an inhibition layer like polytetrafluoroethylene (PTFE). The Ni-mesh can be wet proofed by the layer of PTFE to avoid its contact with water and oxygen. Besides, chemically modified the Ni mesh such as regulating the elementary content distribution and doping some more reactive metals (e.g. Mn, Fe) may be other pathways to protect the electrode.

Associated Content Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at XXX.

Details on the 3D images and high-resolution XPS analyses of Ni-foam. 17



Author Information Corresponding Authors Yang Yang*-Ph.D., E-mail: [email protected]; Honglong Chang**-Ph.D., Professor E-mail: [email protected];

ORCID Yang Yang: 0000-0002-6572-9068 Honglong Chang: 0000-0003-0400-3658

Notes The authors declare no competing financial interest.

Acknowledgement The authors acknowledge the financial support from the National Natural Science Foundation of China (No. 51706184), the 111 Project (No. B13044), the key laboratory of low-grade energy utilization technologies and system (No. LLEUTS-201813), and the Fundamental Research Funds for the Central Universities (No. 3102017OQD011). We would like to thank the Analytical & Testing Center of Northwestern Polytechnical University for XRD test. We also thank Dr. Zhibo Ma for SEM images acquisition.

References (1) Chu, S.; Cui, Y.; Liu, N. The Path Towards Sustainable Energy. Nat. Mater. 2017, 16, 16-22. (2) Modestino, M. A.; Rivas, D. F.; Hashemi, S. M. H.; Han, G.; Psaltis, D. The Potential for Microfluidics in Electrochemical Energy Systems. Energy Environ. Sci. 2016, 9, 3381-3391. (3) Ferrigno, R.; Stroock, A. D.; Clark, T. D.; Mayer, M.; Whitesides, G. M. Membraneless Vanadium Redox Fuel Cell Using Laminar Flow. J. Am. Chem. Soc. 2002, 124, 12930-12931.

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TOC 190x114mm (150 x 150 DPI)



Figure 1. Schematic diagram illustrating the configuration of MFC device with the PB-CP cathode and 3D Nifoam anode. 150x109mm (150 x 150 DPI)


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Figure.2 (a) Power-density and polarization curves of MFC; (b) Comparison of MPD and MCD collected from three parallel trials. 169x79mm (300 x 300 DPI)



Figure 3. SEM images of (a) pristine CP, (b) PB-CP before reaction, (c) reacted after 0.5 h and (d) reacted after 2 h. 172x157mm (300 x 300 DPI)


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Figure 4. SEM images collected from the (a) pristine Ni foam and (b-d) reacted Ni-foam electrode along the flow direction. 183x143mm (150 x 150 DPI)



Figure 5. Schematic illustration showing the XRD pattern for Ni-foam electrode: (a) before reaction; (b) after reaction. 312x140mm (150 x 150 DPI)


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Figure.6 High resolution Ni 2p, O 1s and Cl 2p XPS spectra of Ni-foam electrode before reaction (a, c, e) and after reaction (b, d, f). 171x235mm (300 x 300 DPI)


A Facile Microfluidic Hydrogen Peroxide Fuel Cell with High

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