Chemically Regenerative Redox Fuel Cells Using ... - ACS Publications

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Chemically Regenerative Redox Fuel Cells Using Iron Redox Couples as a Liquid Catalyst with Cocatalysts Sang-Beom Han,† Da-Hee Kwak,† Hyun Suk Park,† In-Ae Choi,† Jin-Young Park,† Kyeng-Bae Ma,† Ji-Eun Won,† Do-Hyoung Kim,† Si-Jin Kim,† Min-Cheol Kim,† and Kyung-Won Park*,† †

Department of Chemical Engineering, Soongsil University, Seoul 156743, Republic of Korea S Supporting Information *

ABSTRACT: Chemically regenerative redox fuel cells (CRRFCs) using liquid catalysts as an alternative to solidstate cathode catalysts have been intensively studied. Here, we studied Fe2+/Fe3+ as a liquid catalyst with Fe-macrocycles as a cocatalyst in CRRFCs. The Fe2+-oxidation rate was enhanced in the presence of Fe-Phthanolocyanine. The single cell having the cathode supplied by the liquid catalyst with Fe-Pc showed a maximum power density of ∼249 mW cm−2.

KEYWORDS: liquid catalyst, chemically regenerative, redox couple, polymer electrolyte, fuel cells, cocatalyst

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CRRFCs are Fe2+/Fe3+, V4+/V5+, and Br−/Br2.19−22 In this study, we proposed an iron redox couple (Fe3+/Fe2+) as a liquid catalyst having high electrochemical activity, reactivity with oxygen, and solubility in acid electrolytes. The electrochemical oxidation reaction of H2 as a fuel occurs on the solid-state Pt catalyst at the anode (Equation 1). On the other hand, the electrochemical reduction reaction of Fe3+ to Fe2+ occurs on the carbon electrode (Equation 2). The reduced Fe ions can be chemically regenerated/oxidized into Fe3+ via O2 (Equation 3) with Fe-macrocycles as cocatalysts for the oxidation (Figure 1). Thus, the overall reaction of the CRRFC using iron redox couple of Fe3+/Fe2+ as a liquid catalyst is Equation 4, exhibiting the reversible electrochemical potential of 0.77 V. Anode reaction:

INTRODUCTION In polymer electrolyte membrane fuel cells (PEMFCs), precious metals such as Pt have been utilized as both anode and cathode catalysts.1−3 Especially, since the electrochemical reduction reaction at the cathode is relatively sluggish compared to the oxidation reaction at the anode, the loading amount of catalyst at the cathode is 3−4 times higher than that at the anode.4−6 To overcome the essential problem of the commercialization of PEMFC, solid-state nonprecious metal (NPM) cathode catalysts for oxygen reduction reaction (ORR) have been consistently studied for over 50 years.7−10 The solidstate NPM catalysts should have an excellent electronic conductivity and stability in strong acid medium and under high potential of ∼1 V. Until now, the heteroatom doped carbon nanostructures as a promising candidate for ORR catalysts have been intensively proposed because of their particular electrocatalytic properties, electrochemical stability, and high electronic conductivity.3,11 However, despite their improved electrocatalytic activity in electrochemical half cells, NPM catalysts still exhibit low cell performance and deteriorated durability due to the structural limitations through the practical single-cell measurements.12−17 Recently, chemically regenerative redox fuel cells (CRRFCs) using liquid catalysts as an alternative of solid-state cathode catalysts including Pt and NPM have been intensively studied. In particular, Vorotyntsev and co-workers introduced various candidates as liquid catalysts for CRRFCs.18 The redox couples as a liquid catalyst for CRRFCs need to have the following characteristics: (1) high solubility in acid solutions, (2) electrochemical activity in the range of 0.7 and 1.2 V, (3) chemical regeneration via O2. The representative candidates for © XXXX American Chemical Society

(Pt/C)

2H 2 ⎯⎯⎯⎯⎯⎯→ 4H+ + 4e−

E o = 0V

(1)

Cathode reaction on carbon felt: 4Fe3 + + 4e− → 4Fe2 +

E o = 0.77V

(2)

Chemical regeneration reaction: 4Fe2 + + O2 + 4H+ → 4Fe3 + + 2H 2O ΔGo = −42.28 kcal mol−1

(3)

Overall reaction (unit cell + regeneration reactor): Received: May 18, 2016 Revised: July 13, 2016

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DOI: 10.1021/acscatal.6b01388 ACS Catal. 2016, 6, 5302−5306

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ACS Catalysis

using a 0.1 μm cellulose filter. The filtered solution was diluted using 1 M H2SO4. The UV−vis absorbance of the diluted solution was measured in the range of 200−500 nm using the Jasco V-650 spectrophotometer. Unit Cell Measurement. The unit cell measurement was performed using a fuel cell station (CNLPEM005-01, CNL Energy Co.). 40 wt % Pt catalyst on Vulcan XC-72R (0.1 mgPt cm−2, E-TEK) as an anode was coated on a gas diffusion electrode. The membrane-electrode-assembly was fabricated using a hot-pressing method with the anode of 5 cm2 and Nafion 212 (thickness: 25 μm) as a solid-state electrolyte at 110 °C and 50 bar for 2 min. The commercial carbon felt (SQ 10001, CeTech Co. Ltd.) with a thickness of 6.5 mm was utilized as a cathode. The unit cell measurement was carried out at 80 °C by supplying H2 (RH 100%) as a fuel to the anode with a flow rate of 50 mL min−1 and oxidized 0.5 M FeSO4 + 1 M H2SO4 solution to the cathode with a flow rate of 5 mL min−1.

Figure 1. Schematic representation of chemically regnerative redox fuel cells using Fe redox couple as a liquid catalyst with Fe-macrocycles as cocatalysts. (1) Electrochemical reduction reaction of Fe3+ into Fe2+ on the carbon electrode. (2) Chemical regeneration of reduced Fe ions to Fe3+ via O2 with Fe-macrocycles as cocatalysts. +

2+ 3+ − (Fe /Fe )

O2 + 4H + 4e ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ H 2O

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RESULTS AND DISCUSSION For CRRFCs, liquid catalysts need to meet the following requirements:25,26 (1) electrochemical reactions of liquid catalysts occur on nonprecious catalysts such as carbon materials, (2) the corresponding electrochemical reduction reaction is highly active, that is, high current density at high reduction potentials (eq2), and (3) the electrochemically reduced liquid catalysts are chemically oxidized via an oxidant such as O2 (eq 3). Herein, we proposed the iron redox couple (Fe2+/Fe3+) as a liquid catalyst for CRRFCs. To characterize the electrocatalytic properties of the liquid catalyst, the electrochemical reaction on a glassy carbon electrode in 0.25 M Fe2(SO4)3 + 1 M H2SO4 at 25 °C was evaluated using a conventional electrochemical three-electrode system, as shown in Figure S1. The cyclic voltammogram (CV) of an iron redox couple in the solution of Fe2(SO4)3 + H2SO4 (Figure S1a) reveals the electrochemical reduction of Fe3+ at ∼0.7 V vs NHE on the carbon electrode and the subsequent electrochemical oxidation of Fe2+ into Fe3+. The linear sweep voltammograms (LSVs) of electrochemical reduction of Fe3+ into Fe2+ were obtained with varying rotating speed of the glassy carbon electrode (Figure S1b). With increasing rotating speed from 100 to 900 rpm, the reduction current densities on the electrode gradually increased. However, at over 900 rpm, the current did not show profound increment, demonstrating that the rate-determining-step is the surface reaction rather than the mass-transfer dominant polarization. To determine the activation energy of the electrochemical reduction of Fe3+ to Fe2+, the LSVs were obtained with varying reaction temperature in 0.25 M Fe2(SO4)3 + 1 M H2SO4 (Figure 2a). With increasing temperature from 25 to 56 °C, the reduction current densities on the electrode gradually increased. As shown in Figure 2b, the Arrhenius plot of the reduction reaction was obtained using the reduction current densities at 0.6 V. In the plot, the slope (i.e., the activation energy of Fe3+ reduction on the carbon electrode) was determined to be ∼10.35 kcal mol−1 K−1, compared to the activation energy of ∼40 kcal mol−1 K−1 in typical oxygen reduction reaction on a pure Pt catalyst.27−29 Moreover, the slopes from the Tafel plots of the Fe3+ reduction obtained from Figure 2a were 123, 130, 139, and 151 mV dec−1, measured at 25, 34, 44, and 56 °C, respectively (Figure S2). In general, it has been reported that the slope of the Tafel plot for oneelectron reaction is ∼120 mV dec−1 at 25 °C.30−33 The slope in

(4)

In the CRRFC using an iron redox couple of Fe3+/Fe2+ as a liquid catalyst (eq 3), the regeneration/oxidation of Fe2+ into Fe3+ has been known to be slow and has been conducted via microbiological reaction or chemical process by O2.23,24 However, the conversion rate of Fe2+ into Fe3+ is still low and should be improved for the commercialization of CRRFCs. In this study, in order to improve the conversion rate of Fe2+ into Fe3+, we proposed the chemical regeneration through O2 bubbling in the presence of Fe-macrocycles as cocatalysts (Figure 1).

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EXPERIMENTAL SECTION Electrochemical Measurements of Fe3+ Reduction Rate and Activation Energy. The electrochemical properties of Fe2+/Fe3+ as a liquid catalyst were evaluated in a typical electrochemical cell using potentiostate/galvanostat instrument (Eco Chemie, AUTOLAB, PGSTAT 101). The three-electrode electrochemical system consists of glassy carbon (diameter: 3 mm), Pt wire, and Ag/AgCl(3 M KCl) as the working, counter, and reference electrodes, respectively. The electrochemical reactions of Fe2+/Fe3+ on the glassy carbon electrode without solid-state catalysts were measured. Cyclic voltammograms (CVs) and linear sweep voltammograms (LSVs) were obtained in 0.25 M Fe2(SO4)3 + 1 M H2SO4 at a scan rate of 50 and 5 mV s−1, respectively. The reduction current densities of Fe3+ into Fe2+ were evaluated with different electrode rotating speeds. To obtain the activation energy of Fe3+-reduction, the reduction current densities of Fe3+ into Fe2+ were evaluated in the range of reaction temperature from 25 to 56 °C. All potentials were converted into NHE. Fe2+-Oxidation (Regeneration) Measurement Using UV−vis Absorbance. To measure Fe2+-oxidation into Fe3+, the solution of 0.5 M FeSO4 + 1 M H2SO4 (200 mL) was supplied to the regeneration reactor (Figure. S5) and then maintained at 80 °C for 5 h under O2 bubbling. To increase the contact area of gas/liquid, oxygen bubbling was performed using the glass filter below the regeneration reactor. 0.25 mM Fe-Pc and Fe-TMPP as cocatalysts for Fe2+-oxidation were added to the solution. During the Fe2+-oxidation into Fe3+, the oxidized solution (2 mL) was collected every 30 min. The impurities in the collected oxidized solution were removed 5303

DOI: 10.1021/acscatal.6b01388 ACS Catal. 2016, 6, 5302−5306

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Figure 3. (a) Concentration ratio of Fe3+ to Fe-iontotal measured from the intensity at 303 nm in Figure S3 as a function of reaction time. (b) Reaction rate of Fe2+-oxidation vs reaction time plotted by differentiating the plots of concentration ratio vs reaction time in panel a.

Figure 2. (a) LSVs measured with varying reaction temperature in 0.25 M Fe2(SO4)3 + 1 M H2SO4. (b) Arrhenius plot (ln (j, current density) vs 1/T (absolute temperature)) of the Fe3+-reduction reaction obtained from the reduction current densities at 0.6 V.

the plot increases with increasing reaction temperature with the following equation:34−36 2.303RT | η| = a + log|i| nαF

intensity at 303 nm in Figure S3 as a function of reaction time. The reaction rate of Fe2+ oxidation can be determined by differentiating the plots of concentration ratio vs reaction time (Figure 3a), as shown in Figure 3b. In the absence of Fe2+ oxidation catalysts (noncatalysts) and presence of Fe-TMPP, ∼30% Fe ions were oxidized into Fe3+ within 2 h, after which the oxidation rate dropped rapidly. It is likely that the relatively low conversion rate in the presence of Fe-TMPP is ascribed to a low degree of dispersion of Fe-TMPP in an aqueous solution such as 0.5 M H2SO4, resulting in the lack of effective catalysts to react with oxygen species in the solution. Especially, in the presence of Fe-Pc, the Fe2+ oxidation rate significantly increased, and the ∼93% Fe ions were then oxidized into Fe3+ after 5 h (i.e., the relatively high conversion rate of 93% for the Fe-Pc compared to 33% and 37% for the noncatalysts and Fe-TMPP, respectively). Mukerjee et al. proposed the most well-known ORR mechanism for Fe-macrocycle compounds, and Karlin et al. demonstrated, similar to our study, the ORR mechanism for Fe-porphyrin structure in acidic electrolyte.37−39 In this study, we proposed the mechanism of the chemical oxidation of Fe2+ as a liquid catalyst via O2 in the presence of Fe-Pc (Figure 4). The Fe2+ as a central metal ion in the Femacrocycles can adsorb oxygen, thus oxidizing Fe2+ in Fe-Pc into Fe3+ (Figure 4a). The adsorbed oxygen species oxidizes Fe2+ in the solution into Fe3+, reducing Fe3+ in Fe-Pc to Fe2+ (Figure 4b,c) and producing H2O (Figure 4d). Especially, the relatively low conversion rate in the presence of Fe-TMPP is ascribed to a low degree of dispersion of Fe-TMPP in an aqueous solution such as 0.5 M H2SO4, resulting in a lack of effective catalysts to react with oxygen species in the solution.

(5)

where η, R, T, n, α, F, and i are the overpotential, gas constant, number of electron, electron transfer coefficient (or symmetry factor), Faraday constant, and current density, respectively. In the CRRFC using the iron redox couple (Fe2+/Fe3+) as a liquid catalyst, the Fe2+ oxidation (i.e., chemical regeneration reaction of Fe3+ from the reduced Fe2+) is crucial in the complete overall reaction of the CRRFC. To evaluate the reaction rate of Fe2+ oxidation via O2 (eq 3), Fe2+ in 0.5 M FeSO4 + 1 M H2SO4 was chemically oxidized at 80 °C by means of continuous O2 bubbling. In the solution, if the concentration of H2SO4 solution is lower than 0.5 M of FeSO4, the pH can be rapidly changed during the oxidation of Fe2+ to Fe3+. Accordingly, the concentration of H2SO4 solution was twice higher than that of FeSO4. It has been reported that the Fe2+ oxidation is very slow under an acid atmosphere in the absence of oxidation catalysts. Thus, in order to increase the oxidation rate, 5,10,15,20-Tetrakis (4-methoxy phenyl)-21H, 23H-porphine iron chloride (Fe-TMPP), and iron phthalocyanine (Fe-Pc) were used as a catalyst for Fe2+ oxidation. The UV−vis absorbance spectra and photographs of the solutions of 0.5 M FeSO4 + 1 M H2SO4 after O2 bubbling for different reaction times were obtained as shown in Figure S3. With increasing reaction time, the relative intensity of the absorbance spectrum increases and the color of the solutions changes from cyan (Fe 2+ ) to yellow (Fe 3+ ). Figure 3a shows the concentration ratio of Fe3+ to Fe-ion total measured from the 5304

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measured using the reduction reaction of the liquid catalyst on the carbon felt as a replacement of O2 as an oxidant. The liquid catalysts without the oxidation catalysts and with Fe-TMPP exhibited large ohmic loss and dominant mass transfer polarization, due to the deficient supply of the liquid catalyst caused by the low conversion rate of 33%−37%, with maximum power densities of ∼65.0 and ∼76.8 mW cm−2, respectively, which is similar to the reported value.18,40 On the other hand, the liquid catalyst with Fe-Pc showed a dominant ohmic polarization without activation and mass transfer losses and much higher maximum power density of ∼249 mW cm−2 compared to those of the liquid catalysts without the oxidation catalysts and with Fe-Pc. Furthermore, it was reported that the typical cell using Fe2+/Fe3+ redox couples showed ∼170 mW cm−2.40 The improved single cell performance using the liquid catalyst with Fe-Pc is attributed to the sufficient supply of the oxidized liquid catalyst caused by the high Fe2+-oxidation rate of over 90% in the presence of Fe-Pc. Furthermore, as shown in Figure S6, the cell using the liquid catalyst with Fe-Pc exhibited a similar performance to a standard PEMFC using the Fe/N4 electrode as a nonprecious cathode catalyst. However, for highperformance CRRFCs comparable to electrochemical power sources, the development of highly efficient liquid catalysts with higher Eo remains for further works. In the CRRFC using an iron redox couple of Fe3+/Fe2+ as a liquid catalyst (Equation 3), the regeneration/oxidation of Fe2+ into Fe3+ has been known to be slow and has been conducted via microbiological reaction or chemical process by O2.23,24 However, the conversion rate of Fe2+ into Fe3+ is still low and should be improved for the commercialization of CRRFCs. In this study, in order to improve the conversion rate of Fe2+ into Fe3+, we proposed the chemical regeneration through O2 bubbling in the presence of Fe-macrocycles as cocatalysts (Figure 1).

Figure 4. Mechanism of chemical oxidation of Fe2+ as a liquid catalyst into Fe3+ via O2 in the presence of Fe-Pc.

The CVs and LSVs in the Fe3+-containing solutions oxidized with Fe2+ via O2 for 5 h were obtained as shown in Figure S4. The cathodic peaks appearing at ∼0.25 V in Figure S4a are related to Fe3+ reduction to Fe2+. The reduction current density in the Fe3+-containing solution oxidized in the presence of FePc was highest due to an improved conversion rate by Fe-Pc. In the LSVs (Figure S4b), the solution oxidized in the presence of Fe-Pc exhibited the maximum reduction current density of ∼10 mA cm−2, which is similar to that in the Fe3+ solution of 0.25 M Fe2(SO4)3 + 1 M H2SO4 (previously observed in Figure S1), resulting from the high Fe2+-oxidation rate of over 90% in the presence of Fe-Pc. To characterize the utilization of CRRFC with Fe2+/Fe3+ as a liquid catalyst, the unit cell measurement was carried out at 80 °C by supplying H2 as a fuel at the anode with a flow rate of 50 mL min−1 and an oxidized 0.5 M FeSO4 + 1 M H2SO4 solution at the cathode with a flow rate of 5 mL min−1. During twoelectron pathway of oxygen reduction reaction in the PEMFC, Fe2+ could produce hydrogen peroxide and might deteriorate the properties of Nafion membrane due to oxygen radical caused by Fenton reaction. Thus, in this study, Fe2+ supplied in the low pH solution could reionize rather than adsorb the Nafion. Figure 5 shows the polarization curves (current density vs voltage (I−V) and current density vs power density (I−P))

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CONCLUSIONS We characterized the fast reduction of Fe3+ into Fe2+ on the carbon electrode due to the low activation energy of the Fe3+ reduction process. The reduced Fe2+ could be chemically regenerated by oxidizing via O2 in the presence of Fe-Pc, exhibiting a relatively high conversion rate of ∼93%. The single cell, having the cathode supplied by the liquid catalyst with FePc, showed a maximum power density of ∼249 mW cm−2. The high cell performance using the liquid catalyst with Fe-Pc was attributed to the sufficient supply of the oxidized liquid catalyst caused by the high Fe2+-oxidation rate in the presence of Fe-Pc.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b01388. Additional UV−vis absorbance spectra and electrochemical data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Figure 5. Polarization curves measured using the reduction reaction of the liquid catalyst on the carbon felt at cathode. The liquid catalysts were regenerated via O2 without and with Fe-TMPP and Fe-Pc as cocatalysts. The unit cell measurement was carried out at 80 °C by supplying H2 as a fuel at the anode with a flow rate of 50 mL min−1 and oxidized 0.5 M FeSO4 + 1 M H2SO4 solution at the cathode with a flow rate of 5 mL min−1.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Energy Technology R&D Program of the Korea Institute of Energy Technology 5305

DOI: 10.1021/acscatal.6b01388 ACS Catal. 2016, 6, 5302−5306

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ACS Catalysis Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20138520030800 and No. 20153030031670).

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DOI: 10.1021/acscatal.6b01388 ACS Catal. 2016, 6, 5302−5306