Investigating the Composition of Iron Salts on the Performance of

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Investigating the Composition of Iron Salts on the Performance of Microfluidic Fuel Cells Chunmei Liu, Qiang Liao, Xun Zhu, and Yang Yang Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 11 Jan 2018 Downloaded from http://pubs.acs.org on January 11, 2018

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Industrial & Engineering Chemistry Research

Investigating the Composition of Iron Salts on the Performance of Microfluidic Fuel Cells Chunmei Liu1,2,3*, Qiang Liao2, Xun Zhu2, Yang Yang4 1

Institute of Vehicle and Transportation Engineering, Henan University of Science and

Technology, Luoyang 471003, Henan province, China 2

Key Laboratory of Low-grade Energy Utilization Technologies and Systems (Chongqing

University), Ministry of Education of China, Chongqing University, Chongqing 400044, China 3

Collaborative Innovation Center of Machinery Equipment Advanced Manufacturing of

Henan Province, Henan University of Science and Technology, Luoyang 471003, Henan province, China 4

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

Mechanical Engineering, Northwestern Polytechnical University, Xi’an 710072, China

*

Corresponding author. Tel./Fax: +86 379 64230418. E-mail address: [email protected] (C.M. Liu) 1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 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:

Microfluidic fuel cells are a group of miniaturized fuel cells without

membranes for operation. Their unique flow of the steams in a microchannel allows a parallel laminar flow without mixing with each other. We investigated the cell performance with different iron-based soluble salts as oxidants. The best performance was achieved by FeCl3 as the oxidant, achieving the highest power density of 192.71 mW cm-3 and the largest limiting current density of 883.33 mA cm-3 (normalized to the volume of the electrode). And we optimized the performance of the FeCl3-based microfluidic fuel cell adjusting the flow rates, HCl concentrations and FeCl3 concentrations. The optimal flow rate was 20 mL h-1. Although the maximum power density of the cell with 4 M HCl and 0.5 M FeCl3 solution reached high values of 212.21 mW cm-3 and 250.42 mW cm-3, respectively, both cell performance were under mass transfer control at high current densities. Keywords: microfluidic fuel cells, iron salts, flow rate, HCl concentration, FeCl3 concentration

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1. INTRODUCTION Fuel cells have gained an increasing attention as a family of power sources capable of generating electricity from the oxidation of fuels and reduction of oxidants.1 As a subgroup of fuel cells, microfluidic fuel cells (MFCs) with small volumes and lightweights are especially favorable power sources for portable electronics and microelectronics.2,3 The most outstanding feature of MFCs is their membrane-free configurations. “Membrane-free” means no physical barriers, such as proton exchange membranes, the costly but indispensable components for most traditional fuel cells. These membrane-free fuel cells were first demonstrated by Ferrigno et al.2 Such configurations which significantly reduce the manufacturing cost as well as eliminate a series of issues associated with the membrane, such as membrane degradation and consequent electrolyte cross-over, are also reported to be beneficial for enhancing power density.4 The membrane-free configurations are enabled by the laminar flow of anolytes (containing fuels) and catholytes (containing oxidants). Both electrolytes contain supporting electrolyte to sustain high conductivity for ionic transport. And the flow rates of two streams are carefully adjusted to ensure that both electrolytes can flow in parallel stably without disturbing and mixing with each other. The power density of MFCs is generally limited by the reaction kinetics on two electrodes,5 particularly on cathode surface where reduction occurs.6 While formic acid,7,8 methanol,9 hydrogen,10 hydrogen peroxide11 and vanadium redox species12,13 have been 3



demonstrated as possible fuels, oxygen gas remains a typically chosen oxidant. Yet, the small solubility (2-4 mM) and diffusivity (about 2 × 10-5 cm2 s-1) in supporting electrolytes of oxygen gas largely hinders the achievement of high power density.8 Therefore, substituting oxygen with other electrolyte-soluble oxidants (most are water-soluble ones since aqueous electrolytes are almost exclusively used), such as hydrogen peroxide (H2O2),14 sodium perchlorate (NaClO4),15 potassium permanganate (KMnO4)16 and vanadium redox species,17,18 is emerging as a viable strategy to improve the power density of MFCs. The enhancement is rooted from the fast reduction kinetics of the substituted oxidants. However, the above listed oxidants are not ideal. H2O2 and NaClO4 generate a large amount of oxygen gas and chlorine gas, respectively, when being reduced. The evolved bubbles can alter the hydrodynamic condition of electrolytes and tend to mix fuels and oxidants that lead to small function of MFCs. The reducing product of KMnO4 is water-insoluble MnO2. MnO2 precipitation can thus clog the fine microfluidic flow channels with diameters ranging from 1 µm to 1000 µm and block electrodes from contacting electrolytes. Vanadium species can be readily reduced without the aid of reduction catalysts, but most of the water soluble vanadium species can pose severe environmental concerns and health hazards due to their toxicity. Tight sealing and packaging are usually necessary and their operation must be carefully monitored, which inevitably increase the operation cost. Recently, water-soluble redox couples employing Fe3+/Fe2+ redox pair have been used in redox flow cells.19, 20 The iron-based redox couple has several advantages over other 4


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oxidants including fast reduction kinetics due to its single-electron reaction pathway, large water solubility, no gas yielded during reduction (only valence change of iron cations), inexpensiveness and environmentally benignity. Unfortunately, as far as we know, a Fe3+/Fe2+redox pair adopted as the oxidant of MFCs is not yet reported. And furthermore, a systematic study elucidating the relationship between iron salt typeand performance of MFCs, yet a meaningful associated with a MFC engineering is not seen. In this paper, we aim to find out by examining the interplay between the compositions of iron salts and power density of MFCs. Specifically, we chose the four typical iron salts, i.e., ferric chloride (FeCl3), ferric sulfate (Fe2(SO4)3), ferric nitrate (Fe(NO3)3) and ammonium ferric sulfate (NH4Fe(SO4)2). Iron citrate and iron phosphate are ruled out because of their extremely low water solubility. Potassium ferricyanide is excluded as it produces an acutely toxic hydrocyanic gas when contacting protons. On the anode side, we choose formic acid serves as the fuel given its large overall theoretical open circuit potential and maximum efficiency.21 The working mechanism of our MFC is thus based on a cathode redox reaction with the half reactions at 298 K shown below: Anode: (oxidation of fuels) HCOOH → CO2 + 2H+ +2e- E0=-0.198 V vs.SHE Cathode: (reduction of oxidants) 2Fe3+(aq) + 2e-→2Fe2+(aq) E0=+0.77 V vs.SHE 5



2. EXPERIMENTAL SECTION 2.1. Preparation of Electrolytes The anolyte is composed of formic acid with concentration of 1 M as the fuel and 1 M sulfuric acid as the supporting electrolyte. In the experiment investigating the effects of iron salt compositions on the cell performance, the molarity of Fe3+ of all catholytes is fixed to be 0.2 M. And the catholyte varies with several iron salts as the oxidants (oxidant/supporting electrolyte): 0.1 M Fe2(SO4)3/1 M H2SO4, 0.2 M FeCl3/2 M HCl, 0.2 M Fe(NO3)3/2 M HNO3, 0.2 M NH4Fe(SO4)2/1 M H2SO4. All the chemicals are of analytical grade and the solvent is distilled water. All catholytes are put in air for 24 hours to ensure iron salts are fully hydrolyzed. Both the anolyte and catholyte are sent into the inlets of the cell by a dual-channel syringe pump (LSP02-1B, Baoding Longer Precision Pump Co., Ltd, China). The flow rates of the electrolytes are varied ranging from 5 mL h-1 to 50 mL h-1. 2.2. Preparation of Electrodes The two electrodes are two pieces of identical porous carbon electrodes with a dimension of 1.5 mm (wide) × 15 mm (long) × 0.2 mm (thick) that are cut from a sheet of commercial carbon paper (HCP020N, ShanghaiHesen Ltd. Co., China). Prior to use, the two carbon strips are treated consequently by sonication in ethyl alcohol, dipping in isopropanol and triple rinsing with deionized water to eliminate their impurities on the surface and enhance their hydrophilicities. The pre-treated carbon strips are directly used 6


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as the cathode. For the anode, the pre-treated carbon strip is loaded with Pd particles as catalysts to facilitate the oxidation of HCOOH. The Pd catalyst is plated onto the carbon electrode surface via a facile electro-deposition method. Briefly, a piece of pre-treated carbon strip is immersed in a batch of Pd plating solution (1.0 wt.% PdCl2 in 1 M HCl solution) and a constant potential of 0.0 V vs.Ag/AgCl is applied.15, 22 The deposition is halted until a catalyst mass loading of 5 mg cm-2 is obtained. A piece of titanium foil with a thickness of 0.02 mm is attached to each electrode to transport electrons out and into the electrodes. 2.3. Physicochemical characterization The surface morphologies of the porous carbon paper electrode with and without Pd catalyst are shown in SEM images, which were gained by a SU8010 SEM (Hitachi) at an acceleration of 5 kV. Before the SEM observation, both the samples were sprayed with a thin gold layer. The crystal structure of the Pd catalyst electrodeposited on the carbon paper was characterized by X-ray diffraction (XRD) measurement on a D/max-2500PC (Japan) equipment using Cu-Kα radiation (λ=0.15406 nm) at room temperature. The 2θ regions between 10° and 90° were recorded at a scan rate of 6 mV min-1. 2.4. Configuration of microfluidic fuel cells The structure of our microfluidic fuel cell is schematically depicted in Figure 1. The cell is assembled with two identical 8 mm-thick pieces of polymethyl methacrylate 7



(PMMA) plates. The upper plate has four penetrated holes with a diameter of 2 mm, functioning as the reactants inlets and outlets. These holes are fabricated by the laser engraving machine. The lower PMMA plate contains two 1.5 mm (wide) × 15 mm (long) × 0.2 mm (thick) chambers for placing electrodes and a 1.0 mm (wide) × 10 mm (long) × 0.1 mm (thick) chamber to accommodate electrolyte. Due to 5 mm long carbon paper as a current collector, the active carbon paper electrode is 10 mm in length, 1.0 mm in width and 0.2 mm in height. The microfluidic channels are all made by using standard soft lithograhy techniques.23 The entire microfluidic fuel cell is assembled by six 3 mm screw bolts (not shown).

Figure 1. Exploded view of the microfluidic fuel cell design and assembly.

2.5. Electrochemical Characterizations The electrochemical performance of the microfluidic fuel cells with different electrolyte compositions is evaluated by a Zennium electrochemical workstation (Zahner, Germany). Cyclic voltammetry (CV) is carried out via a three-electrode configuration, with the aforementioned carbon strip as the working electrode, a piece of platinum sheet (10 mm×

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50 mm) as the counter electrode and an Ag/AgCl electrode [0.198V vs. standard hydrogen electrode (SHE)] as the reference electrode. The CV curves are collected in a potential window from 0.0 V to 1.2 V at a scan rate of 10 mV s-1. The polarization curves of the cells are obtained by the stepwise decreasing potential of the fuel cell from open circuit voltage to 0.0 V with an interval of 0.1 V, and meanwhile recording the current at each steady state usually within 180s after the potential is decreased. To gauge the cathode potential, an Ag/AgCl reference electrode (0.198V vs. SHE) is placed near the outlet of catholyte. The anode potentials are calculated by subtracting the measured cathode potentials from the corresponding cell voltages. For the flow-through electrode, areal power densities and current densities are normalized either to the cross-sectional area of the electrode

24, 25

or to the projected

area.2, 26 Volumetric power densities and current densities are normalized to the volume of the electrode.27, 28 Specifically, the projected area, the cross-section area and the volume of the electrode are 0.15 cm2, 0.02 cm2 and 0.003 cm3, respectively. respectively. To avoid ambiguity in performance comparison, the volume current density and power density are adopted to provide meaningful perspectives and advantages. All the measurements are conducted at temperature of 293 ± 2 K.

3. RESULTS AND DISCUSSION 3.1. Physicochemical characterization of Electrodes and catalysts The surface morphologies of the carbon paper without and with Pd catalyst are shown 9



in Figure S1-S2. The SEM images of carbon paper (Figure S1) clearly present that carbon paper is made of carbon fibers in a staggered arrangement and many macro-pores. And from the SEM images of Pd catalyst (Figure S2), it is obvious that the Pd catalysts electrodeposited on the carbon paper show a highly dendritic structure with different dimensions of the branches. The X-ray diffraction patterns of the Pd catalysts are depicted in Figure S3. From the diffraction peaks of XRD patterns of Pd catalysts, the Pd particles electrodeposited on the carbon paper show a single crystallographic structure (JCPDS, Card NO. 88-2335). The average size of the Pd catalysts is 17.2 nm in this experiment according to the Pd (2 2 0) facet using the Debye-Scherrer equation22, 29 after calculation. The analysis of the XRD patters and calculation of the average size of Pd particles were shown in supporting material. 3.2. Redox activity of different iron salts We performed CV to compare the redox activities of all the chosen iron salts on the carbon paper in aqueous electrolytes containing corresponding salts. The collected CV curves are shown in Figure 2.

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Figure 2. The CV curves collected for different iron salts with a scan rate of 10 mV s-1.

For all salts, a pair of redox peaks centered at ~ 0.7 V vs. Ag/AgCl is clearly presented. The anodic peak at ~ 0.8 V vs. Ag/AgCl corresponds to Fe2+-e-→Fe3+ while the cathodic peak at ~ 0.6 V vs. Ag/AgCl is ascribed to Fe3++e-→Fe2+ (other redox peaks may come from the oxygen containing functional groups, supported by XPS characterization (Figure S4-S5 and Stable 1)). The observation indicates that all iron salts are possible redox candidates. As seen from Figure 2, the CV curves of Fe2(SO4)3 and NH4Fe(SO4)2 is almost identical, suggesting that the electrochemical behavior of the two iron salts are similar. The reduction peak current of Fe2(SO4)3 and NH4Fe (SO4)2 is -5.57 mA and -5.48 mA, respectively. A closer inspection of the cathodic peaks reveal that both FeCl3 and Fe(NO3)3 exhibit much pronounced peak current compare to the other two iron salts. And the peak reduction current of FeCl3 and Fe(NO3)3 at ~0.6 V is -6.21 mA and -7.56 mA, respectively. From the CV test, we are confident to claim that the redox activity, and particularly the reduction activity, of FeCl3 and Fe(NO3)3 is much higher than the other two salts. High reduction activity is a prerequisite for achieving high performance for MFCs. 11



3.3. Cell performance with the different iron salts We next assembled MFCs with different iron salt aqueous solutions as the catholytes with the anolyte fixed to be 1 M HCOOH aqueous solution. Figure 3 shows the polarization curves along with the power density curves at different current densities. The limiting current densities and peak power densities of MFCs with different iron salt catholytes are listed in Table 1.

Figure 3. Power density curves (a), polarization curve (b) and electrode potentials curves (c) of MFCs with different iron salt catholytes at a flow rate of 20 mL h-1.

Among all the catholytes, the MFC with FeCl3 as the oxidant displays the highest power density of 192.71 mW cm-3 and the largest limiting current of 883.33 mA cm-3. The maximum power density is approximately 1.77 times higher than that of Fe2(SO4)3, 1.65 times higher than that of NH4Fe (SO4)2 and 1.94 times higher than that of Fe (NO3)3. To better understand the excellent performance of the FeCl3-based MFC, the potentials of both cathode and anode are measured and plotted in Figure 3(b). It is clear that the cathode potentials of the FeCl3 solution at different current densities remain always the highest. The reason of the superior performance of FeCl3-based MFC to other MFCs can 12


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be attributed to the complexation reactions between Fe3+ and Cl- ions. The complex ions such as FeCl4- and FeCl22+ could promote the reduction of Fe3+ ions via an ion-bridge mechanism.30 Table 1. The limiting current and peak power densities of MFCs with varied iron salt catholytes MFC

limiting current density (mA cm-3)

peak power density (mW cm-3)

FeCl3-based

883.33

192.71

Fe2(SO4)3-based

627.54

69.63

NH4Fe (SO4)2-based

720.0

72.72

Fe(NO3)3-based

173.34

65.45

It is also equally important to understand why the Fe(NO3)3-based MFC, though Fe(NO3)3 displayed comparable reduction activity as FeCl3, failed to present outstanding performance (the lowest peak power density of 65.45mW cm-3 and the least maximum current density 173.34 mA cm-3). As shown in Figure 3, while the cathode potential decreases comparably slow to the cathode potential of FeCl3-based MFC, the anode potential of the Fe(NO3)3-based MFC experiences a rapid rise from -149 mV to +438 mV. The observation suggests that the cell performance of Fe(NO3)3-based MFC is mainly limited by its anode performance. The phenomenon is expected to be due to the strong oxidation property of HNO3 that can directly react with aldehyde group (-CHO) of HCOOH, thus reducing the magnitude of the output electricity and rapidly increasing the anode potential. Another information revealed by Figure 3 is the rapid decay of cathode potential of the Fe2(SO4)3-based MFC and the NH4Fe(SO4)2-based MFC. It indicates that the 13



performance of the two MFCs is restricted by the cathode performance. This conclusion is consistent with the results revealed by CV measurements that the two iron salts are not as reducible as FeCl3 and Fe(NO3)3. Now we have identified the optimal performance is achieved by the FeCl3-based MFC. In the following three sections, we will focus on tuning its performance by changing the three parameters of flow rates, FeCl3 concentration and HCl concentration. 3.4. Influence of flow rates on the performance of the FeCl3-based MFC Reactant flow rate plays an important role in determining the performance of MFCs as it is closely related to the mass transfer rate and whether a steady and laminar flow can be obtained. The polarization and power density curves collected under different flow rates, ranging from 5 mL h-1 to 50 mL h-1 are illustrated in Figure 4. The limiting current densities and peak power densities of MFCs with different flow rates are listed in Table 2.

Figure 4. Power density curves (a), polarization curve (b) and electrode potentials curves (c) of the FeCl3-based MFC collected at the different flow rates.

Notably, both the limiting current density and maximum power density first increase when increasing flow rate from 5 mL h-1 to 20 mL h-1, and then decrease when flow rate 14


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surpasses 20 mL h-1. The optimal performance is achieved at 20 mL h-1, with a highest limiting current density and power density of 883.33 mA cm-3 and 192.71 mW cm-3, respectively. Clearly, there is a trade-off relationship between flow rate and performance. When flow rate is not too fast (i.e., less than or equal to 20 mL h-1 in this case), the mass transfer can be facilitated by increasing flow rate as well as reducing the mixing region in the middle of the flow channel. Both factors contribute positively to improve the reaction kinetics of the redox reactions on two electrodes. However, when the flow rate further increases, hydrodynamic instability occurs and disturbs the stable liquid-liquid interface of the laminar flow between the catholyte and anolyte, leading to convective mixing. As a consequence, the fuel and oxidant readily react with each other in the bulk of the electrolyte and few electrons can be extracted by the outer electric circuit. The reduced number of external electrons results in the reduced current density as well as power density. Table 2. The limiting current and peak power densities of FeCl3-based MFCs with varied flow rates flow rate (mL h-1)



5

206.24

75.05

10

426.67

121.53

20

883.33

192.71

30

701.78

145.06

50

463.33

115.60

3.5. Influence of HCl concentrations on the cell performance HCl in catholyte serves as a supporting electrolyte to provide enough protons and 15



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sustain high conductivity of the catholyte. The power density, polarization and potential curves with varied HCl concentrations from 0.5 M to 4.0 M have also been researched (note: the concentration of FeCl3 and flow rate is fixed to be 0.2 M and 20 mL h-1, respectively). The limiting current densities and peak power densities of MFCs with different HCl concentrations are listed in Table 3.

Figure 5. Power density curves (a), polarization curve (b) and electrode potentials curves (c) of the FeCl3-based MFC collected at the different HCl concentrations. Table 3. The limiting current and peak power densities of FeCl3-based MFCs with different HCl concentrations HCl concentration



0.5 M

505.37

99.01

1.0 M

666.05

112.26

2.0 M

883.33

192.71

4.0 M

746.67

204.30

As displayed in Figure 5(a), the maximal power density of the cell increases with increasing the HCl concentration. The highest power density of the cell at 4 M HCl is 212.21 mW cm-3, about 2.36 times higher than the cell with 0.5 M HCl (89.81 mW cm-3). The enhanced power density is correlated to the reduced cell resistance given that high 16


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HCl concentration contribute to large solution conductivity. Figure 5(b) compares the polarization curves of cells with different HCl concentrations. The limiting current density first increases with increasing HCl concentration to 2.0 M, and then decreases when concentration of HCl exceeds 2.0 M (883.33 mA cm-3 at 2.0 M and 746.67 mA cm-3 at 4.0 M). The decrement in limiting current density at elevated HCl concentration could be a result of Pd catalyst poisoning,31, 32 where Cl- could possibly diffuse through the liquid/liquid interface to anode electrode, occupying some active sites and reducing the activity of Pd catalyst. The 4.0 M HCl cell exhibits mass transfer control as evidenced from the reduced current density at high current densities. It is worth noting that although the maximal power density of the cell with 4 M HCl is the highest, its open cell voltage is the lowest (753 mV) among all cells. The reduced open circuit potential can also be explained by the Pd catalyst poisoning by Cl- at elevated HCl concentrations. 3.6. Influence of FeCl3 concentrations on the cell performance FeCl3 as an oxidant in this experiment plays an important role determining the cell performance. The influence of FeCl3 concentration ranging from 0.1 M to 0.5 M (while remaining HCl concentration to be 2 M) on the cell performance are investigated and the results are demonstrated in Figure 6. The limiting current densities and peak power densities of MFCs with different HCl concentrations are listed in Table 4. As seen from Figure 6(a), the largest power density of the cells increases with 17



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increasing the FeCl3 concentration, specifically from 82.96 mW cm-3 at 0.1 M FeCl3 to 250.42 mW cm-3 at 0.5 M FeCl3 (by almost two times enhancement). On one hand, higher FeCl3 concentrations produce more Fe3+ ions, and result in stronger oxidation ability of the catholyte. On the other hand, higher FeCl3 concentrations afford more ions to the catholyte and improve the conductivity of the catholyte and decrease the cathode resistance.

Figure 6. Power density curves (a), polarization curve (b) and electrode potentials curves (c) of the FeCl3-based MFC collected at the different FeCl3 concentrations. Table 4. The limiting current and peak power densities of FeCl3-based MFCs with varied FeCl3 concentrations FeCl3 concentration



0.1 M

491.43

82.96

0.2 M

883.33

192.71

0.3 M

971.87

200.60

0.5 M

715.44

250.42

While the maximal power density of the cell with 0.5 M FeCl3 solution is largest, its limiting current density first increases with increasing the FeCl3 concentration up to 0.3 M FeCl3 and decreases at 0.5 M FeCl3. The limiting current density is 491.43 mA cm-3 at 18


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0.1 M FeCl3 solution, 883.33 mA cm-3 at 0.2 M FeCl3 solution, 971.87 mA cm-3 at 0.3 M FeCl3 solution, and 674.30 mA cm-3 at 0.5 M FeCl3 solution. As seen from Figure 6(b), the power density of the cells with the low and intermediate current densities (< 844.6 mA cm-3) under 0.5 M FeCl3 are the largest due to the largest oxidation concentration and least catholyte resistance, but the cell performance becomes controlled by mass transfer at high current densities. It is mainly attributed to the Cl- poisoning effect on the catalytic performance of Pd and the unbalanced amount between fuel/catalyst and Fe3+ ions.

4. CONCLUSIONS In this manuscript we have presented a systematic study on the interplay between the iron-based catholyte and the MFC performance. Four different iron salts, namely FeCl3, Fe2(SO4)3, Fe(NO3)3 and NH4Fe(SO4)2 are chosen as the oxidant candidates for this study. The CV tests of the four iron salts on their reducing ability shows that FeCl3 and Fe(NO3)3 have the highest activity among all the salts. Electrochemical measurements involving MFCs with the different catholytes confirms that the FeCl3-based MFC exhibits the best performance with the highest power density of 192.71 mW cm-3 and the largest limiting current of 883.33 mA cm-3. The reasons accounting for the different performance are discussed by gauging the potential of cathode and anode individually. In addition, we have also investigated the influence of the flow rates and the concentrations of FeCl3 and 19



HCl on the performance of the FeCl3-based MFC. The results showed that at a flow rate of 20 mL h-1, the cell performance was optimal. Under the condition of 4 M HCl or 0.5 M FeCl3, the maximal power density of the cell is largest, while its limiting current density is not largest.

SUPPORTING MATERIAL A supporting material file associated with this article is available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION Corresponding Author * Tel./Fax: +86 379 64230418. E-mail: [email protected].

Funding Sources This work is supported by the National Natural Science Foundation of China (Grant Nos.51506046) , by the Key Laboratory of Low-grade Energy Utilization Technologies and Systems (Chongqing University, Grant Nos. LLEUTS-201602) and by the Doctoral Scientific Research Foundation of Henan University of Science and Technology (Nos. 13480033).

Conflicts of interest There are no conflicts to declare. REFERENCES (1) Dyer, C. K. Fuel Cells for Portable Applications. J. Power Sources. 2002, 3, 8-9. 20


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Flow

Cell

in

Sodium

Acetate

Solution.

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For Table of Contents

Figure 1. Power density curves of microfluidic fuel cells with different iron salt catholytes at a flow rate of 20 mL h-1.


Investigating the Composition of Iron Salts on the Performance of

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