Article pubs.acs.org/IECR
Enhanced Electricity Generation from Electrochemical Oxidation of FeII in an Air−Cathode Fuel Cell Amended with Chelating Anions Lin-Feng Zhai,† Zhong-Hua Tong,‡ Min Sun,*,† Wei Song,† Shan Jin,† and Hideki Harada§ †
Department of Chemical Engineering, Hefei University of Technology, Hefei 230009, China Department of Chemistry, University of Science & Technology of China, Hefei 230026, China § Department of Civil Engineering, Tohoku University, Sendai 980-8579, Japan ‡
ABSTRACT: The air−cathode fuel cell approach is promising for ferrous (FeII) ion removal from acid mine drainage because iron and electricity are simultaneously recovered in the treating process. Here we show that electricity generation from FeII can be enhanced by amending chelating anions to facilitate FeII oxidation at the anode of the fuel cell. A series of FeII-fed fuel cells were operated with various chelating anions, including carboxylate, phosphate, and borate ligands. The average power densities of these fuel cells varied over a wide range from 0.08 ± 0.5 to 107.85 ± 1.50 mW m−2. Citrate-amended fuel cells operated at pH 8−9 and carbonate-amended fuel cells operated at pH 6−8 exhibited greater charge-recovery efficiencies than others, which ranged from 93.5% to 96.1%. The redox potential of an anodic solution and redox activity of FeII were two important factors affecting the electrooxidation of FeII in fuel cells. species such as Fe(CO3)22−.12 Salicylate and ethylenediaminetetraacetic acid (EDTA) dramatically increased the FeII oxidation rate due to the formation of stable FeIII complexes, while phthalate and ethylene glycol bis(2-aminoethyl ether)N,N,N',N'-tetraacetic acid were found to form strong FeII complexes that inhibited FeII oxidation.13,14 These results suggest that electrochemical oxidation of FeII might be influenced by chelating anions as well. Chelating anions, such as carbonate, citrate, and phosphate, are widely spread in natural water. Specifically, carboxylate ligands are usually encountered in AMD.15,16 Therefore, interactions between FeII and naturally occurring chelating anions should be taken into consideration when the fuel cell is used to treat AMD. However, so far very little information is available on the electrooxidation behavior of FeII in the presence of chelating anions. In order to develop a virtually practical fuel-cell technology for AMD treatment, the present work thus aims to elucidate the influence of inorganic/organic chelating anions of the electrooxidation of FeII in a fuel-cell system. A series of air−cathode fuel cells were operated with various chelating anions amended in the anodic solution, including carboxylate (carbonate, EDTA, citrate), phosphate, and borate ligands. The performance of the fuel cell was evaluated in terms of the FeII oxidation rate, current density, and energy-recovery efficiency. The results from this study would provide a comprehensive understanding of the FeII-fed fuel cell as well as the electrochemical characteristics of iron(II) ligand complexes.
1. INTRODUCTION Acid mine drainage (AMD) contains a large amount of dissolved ferrous irons (FeII) resulting from the oxidation of pyrite. Because ferrous ions not only decrease the pH but also cause turbidity of natural water, AMD should be deferrized before it is discharged into the environment.1 The FeII ions can be removed from AMD by passive treatment with abandoned discharges or active treatment with regulated discharges.2−4 A typical AMD treatment process involves alkaline neutralization, air oxidation, and metal precipitation, producing a chemical sludge containing large amounts of ferric (FeIII) precipitates.5,6 To limit the AMD sludge discharge and offset the cost for AMD treatment, the recovery of iron from AMD sludge has been extensively studied.4,7,8 However, the complex operational requirements related to the separation of iron from other metals have made it difficult to achieve effective iron recovery.9,10 Thus, AMD treatment remains economically expensive and is at poor efficiency in resource recovery so far. An alternative approach based on the air−cathode fuel cell was recently developed for FeII removal from AMD, which can simultaneously recover both iron and electricity.10,11 In such a fuel cell, utilization of an air−cathode results in spontaneous oxidation of FeII to FeIII at the anode, concomitant with electricity generation. Trials on synthetic AMD treatment demonstrated that FeII was completely oxidized to the FeIII state in the fuel cell, with electricity generation at high energyrecovery efficiency of up to nearly 100%.10,11 This fuel-cell technology would be cost-effective if the produced Fe III compounds can be successfully harvested, while electricity generation is an additional advantage. Natural chelating ions play an important role in regulating the speciation of metals in the environment, thus affecting the oxidation rates of metals. Previous studies have shown that chelating anions have a profound effect on the FeII oxidation kinetics in aerobic systems.12−14 Carbonate was observed to accelerate FeII oxidation via the formation of kinetically active © 2013 American Chemical Society
Received: Revised: Accepted: Published: 2234
November 21, 2012 December 31, 2012 January 22, 2013 January 22, 2013 dx.doi.org/10.1021/ie3032185 | Ind. Eng. Chem. Res. 2013, 52, 2234−2240
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average power densities, FeII conversions, and Coulombic efficiencies (CEs) were calculated. The average power density was calculated using the measured cell voltage over 12 h of fuelcell operation.11 A fuel cell was operated as the control without anion addition at each pH value. All experiments were conducted in duplicate at an ambient temperature of 25 °C. 2.4. Analytical Methods. The FeII concentration was determined by potassium dichromate titration with sodium diphenylamine sulfonate as an indicator.19 Before measurement, samples were acidified with 5 M HCl to ensure that FeII was totally dissolved in solution. The solution pH was measured using a pH meter (Mettler-Toledo, Switzerland). In the process of fuel-cell operation, the voltage across the 1 kΩ external resistor was recorded at 10 min intervals using a data acquisition system (USB2801, ATD Co., China). The current and power were calculated using Ohm’s law, and the current and power densities were then normalized by the projected surface areas of two sides of the anode (10 cm2). CE was calculated as previously described,18 based on the assumption that 1 mol of electrons is produced from oxidation of 1 mol of FeII. The various FeII complexes were electrochemically characterized by a redox potential−pH curve and cyclic voltammetry (CV) on a CHI 660D electrochemical workstation (CH Instruments Inc., USA), with glassy carbon (3 mm diameter) as the working electrode, platinum as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. The electrolyte solution contained 3.5 mM FeII, 50 mM chelating anion, and 200 mM NaCl. In order to obtain the redox potential−pH curve, the solution pH was first adjusted to 4.0 and then gradually increased by NaOH titration. The corresponding redox potential during the pH increase was continuously recorded. CV was scanned between −0.8 and +0.8 at 0.1 V s−1, with an initial scan from low to high potential, followed by a reversed one. All of the tested samples were purged with nitrogen for 30 min prior to electrochemical measurements, and a nitrogen atmosphere was maintained over the solution during the measurements.
2. MATERIAL AND METHODS 2.1. Chemicals. The 0.14 M FeII stock solution was prepared by dissolving ferrous sulfate (FeSO4·7H2O, Benchmark) in distilled water, acidified with HCl to pH 2.0. Sodium bicarbonate (NaHCO3), sodium tetraborate (Na2B4O7), sodium citrate (C 6 H 5 Na 3 O 7 ·2H 2 O), edetate disodium (C10H14N2Na2O8·2H2O), and a mixture of Na2HPO4 and NaH2PO4 were used as the chelating compounds. 2.2. Fuel-Cell Design and Operation. The glass-made single-chamber fuel cell was employed in the experiment,17,18 with an anodic volume of 175 mL (Figure 1). A 2 × 2.5 cm2
Figure 1. Laboratory-scale prototype of the fuel cell.
non-wet-proofed carbon cloth (GEFC Co., China) was located in the chamber as an anode. The cathode was a 2 × 2 cm2 carbon paper (090S, wet-proofed, Toray Industries Inc., Japan) with a 0.05 mg cm−2 platinum catalyst coating on one side. The coated side of the cathode was positioned facing the protonexchange membrane (GEFC-10N, GEFC Co., China), and the uncoated side was directly exposed to air. Titanium wires (1 mm in diameter) were used to connect both electrodes and the external load of a 1 kΩ resistor. The anodic chamber was filled with a solution that contained 200 mM NaCl as the electrolyte and 50 mM chelating anions and sparged with N2 for 30 min to remove dissolved oxygen. The FeII stock solution (3.75 mL) was then added to the anodic chamber in an anaerobic glovebox, and the solution pH was adjusted with 1 M HCl or NaOH. The total liquid volume in the anodic chamber was 150 mL, and the initial concentration of FeII was 3.5 mM. The solution pH varied in the range of 5.0−9.0 according to the experiment design. The fuel cell was left in an open-circuit mode until a constant opencircuit voltage was obtained and then was started by connecting the circuit with a 1 kΩ resistor. 2.3. Batch FeII Oxidation Experiments in the Fuel Cell. Two sets of batch tests were performed to determine the effect of an individual chelating anion on the power output of the fuel cell. The first set of batch tests was conducted to observe the evolution of circuit currents in the fuel cells. The batch test was ended when the cell voltage dropped below 1 mV, and the final pH of the solution and the FeII concentration were measured. The total number of charges produced in the cycle was calculated by integrating the current over time. In the second set of batch tests, the fuel cells were operated for 12 h and the
3. RESULTS 3.1. Performance of the Fuel Cells Amended with Different Chelating Anions. Figure 2 illustrates electricity generation in the fuel cells as a function of the chelating anion and solution pH. The current densities of all of the fuel cells shared a similar evolution tendency. Upon connection of the circuit, instant electricity generation was observed, followed by an immediate drop in the current density, which then gradually fell to null over the next several hours. Such a current decay could be attributed to the continuously reduced Fe II concentration over the batch-mode operation.11 Results show that the initial level of the current density as well as its decay rate was strongly dependent upon the chelating anions. The fuel cells supplied with EDTA, carbonate, and citrate generally obtained higher initial current densities than those supplied with borate or phosphate. Electricity generation was maintained for relatively longer time in citrate-amended fuel cells and shorter time in phosphate-amended ones. The total charges produced in different reactors were calculated and are shown in Table 1. Theoretically, total charges of 50.65 C could be recovered from the oxidation of 150 mL of 3.5 M FeII in the fuel-cell system. However, all of the reactors produced less total charges compared with the theoretical value because of incomplete oxidation of FeII and FeII consumption by diffused 2235
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1.50 mW m−2 in carbonate-amended fuel cells operated at pH 8. The power density increased with the pH for citrateamended reactors, whereas the opposite tendency occurred in phosphate-amended ones. For EDTA-amended fuel cells, lower power densities were observed at neutral pH, which was, however, found to facilitate power production for carbonateand borate-amended ones. The FeII conversions and CEs were also strongly correlated with the chelating anions, with FeII conversions varying from 2.67 ± 1.72% to 36.34 ± 1.36% and CEs from 4.9 ± 1.7% to 92.1 ± 6.7%, respectively. Generally, higher power density corresponds to faster FeII oxidation and more effective energy recovery. Taking both the power output and energy-recovery efficiency into consideration, the performance of FeII-fed fuel cells was enhanced by the chelating anions in the order carbonate > EDTA > citrate > borate > phosphate. 3.2. Effectiveness of the Buffering Capacity for pH Control. Figure 4 compares the initial and final pH values of the anodic solution in a batch cycle. The carbonate exhibited excellent buffering capacity relative to the experimental pH range. For EDTA-buffered reactors, the solution pH showed very small changes except at pH 5.0, where a relatively larger decrease in the pH was observed. Citrate demonstrated sufficient buffer capacity in an acidic solution, whereas for borate, the pH was well controlled in an alkaline solution. The phosphate solution could be buffered only under neutral conditions. Notice that the solution pH showed a decreasing tendency in all of the fuel cells, no matter which anion was amended. Such a pH decrease could be attributed to the release of H+ from ferrous/ferric iron hydrolysis over the fed-batch cycle.22 3.3. Electrochemical Characterization of the Iron(II) Ligand Complexes. In order to gain insight into the role of chelating anions in the electrooxidation of FeII, different iron(II) ligand complexes were electrochemically characterized with a redox potential−pH curve and CV. As shown in Figure 5, an increase of the solution pH results in a more negative redox potential, thus facilitating FeII oxidation. As the pH increases from 5.0 to 9.0, the redox potential of an iron(II) carbonate solution drops most significantly from −50 to −800 mV. For the FeIIEDTA solution, the redox potential remains constant at ca. −200 mV in the range of pH 5.0−7.0. A similar platform is also observed for the redox potential−pH curve of an iron(II) citrate solution, which is located at pH 6.0−7.8. Figure 6 shows the cyclic voltammograms of a glassy carbon electrode in a 200 mM NaCl solution containing 3.5 mM FeII and 50 mM chelating anion. One couple of redox peaks attributed to the redox couple of FeII/FeIII is clearly observed on the CV graphs. Useful information regarding the redox activity of FeII can be provided by investigating the intensity and position of the FeII oxidation peak. The redox activity of
Figure 2. Evolution of the current density of FeII-fed fuel cells amended with different chelating anions in the batch test.
oxygen.20,21 As shown in Table 1, during the whole batch cycle, FeII was only partly oxidized in some fuel cells. Notably, in phosphate-amended fuel cells at pH 7−9, more than 80% of FeII remained unoxidized at the end of one cycle, resulting in very few charges being recovered. Moreover, a single-chamber fuel-cell architecture was employed to facilitate operation in this work. Oxygen diffused through the cation-exchange membrane might oxidize FeII and thus cause a considerable portion of the charge loss.20,21 Nevertheless, citrate-amended fuel cells operated at pH 8−9 and carbonate-amended fuel cells operated at pH 6−8 exhibited a good performance for the energy recovery, evidenced by great charge-recovery efficiency ranging from 93.5% to 96.1%. In order to fully evaluate the fuel-cell performance, the average power densities, FeII conversions, and CEs were obtained over the initial 12 h of fuel-cell operation (Figure 3). Note that the control reactors without anion addition failed to produce electricity. The power output of the fuel cell was well induced by the amended chelating anions. As shown in Figure 3, the average power densities obtained in these anion-amended fuel cells varied over a wide range from 0.08 ± 0.5 mW m−2 in phosphate-amended fuel cells operated at pH 8 to 107.85 ±
Table 1. FeII Conversions and Total Charges Recovered in One Fed-Batch Cycle for the Fuel Cells Amended with Different Chelating Anions FeII conversion (%)
total charge recovered (C)
chelating anion
pH 5
pH 6
pH 7
pH 8
pH 9
pH 5
pH 6
pH 7
pH 8
pH 9
EDTA citrate carbonate phosphate borate
99.99 98.82 38.93 36.79 13.74
100.00 98.83 99.61 31.89 8.03
100.00 98.69 99.73 18.58 85.50
100.00 98.56 99.13 17.09 100.00
100.00 98.86 85.52 13.30 39.58
28.76 17.08 6.14 6.52 0.34
26.89 18.37 47.39 4.05 0.63
11.29 35.16 48.51 1.09 19.04
26.34 48.68 47.70 0.41 47.13
27.02 48.62 22.70 0.38 14.57
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Figure 3. Average power densities, FeII conversions, and CEs of FeII-fed fuel cells amended with different chelating anions.
Figure 4. Initial versus final pH values of the anodic solution in a batch cycle. Figure 5. Typical potential versus pH curves of an FeII solution in the presence of different anions.
FeII is generally depressed by an increase of the pH except in a borate solution, where the highest redox activity of FeII is observed at pH 9.0. In a carbonate solution at pH 6, the strong FeII oxidation peak located at −350 mV suggests that FeII can be easily oxidized under this condition. This is in good agreement with the high energy output of corresponding carbonate-amended fuel cells. The oxidation peaks of FeII obtained in a phosphate solution are one magnitude weaker than those in other chelating solutions, indicating a lower redox activity of FeII. Therefore, phosphate-amended fuel cells exhibited a poorer performance.
4. DISCUSSION Chelating anions present in solution can form complexes with both FeII and FeIII, and the oxidation of chelated FeII results in the formation of a corresponding FeIII complex. On the one hand, the FeII oxidation rate is regulated by the stability of the FeII complex, which controls the release of free FeII from the complex. Delayed Fe II oxidation occurs when a low concentration of free FeII is available for oxidation.13 On the other hand, FeII oxidation is affected by the stability of the FeIII 2237
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Figure 6. Cyclic voltammograms of a glassy carbon electrode in a 200 mM NaCl solution containing 3.5 mM FeII and 50 mM chelating anions.
complex. Enhanced FeII oxidation can be obtained through conversion of the FeII complex to a more stable FeIII complex. In the control fuel cells without chelating anion addition, FeII oxidation was dramatically hindered by the kinetically inert species FeCl+ and FeSO4,12 resulting in failure of electricity generation. Carboxylate anions are known to facilitate FeII oxidation through the formation of FeII complexes, which react faster than the aquo complexes.23 Therefore, the fuel cells amended with carobxylate anions, including carbonate, EDTA, and citrate, exhibited favorable energy output. Carbonate is an important ligand in all natural waters and may form a variety of complexes with FeII. A previous study demonstrated that electricity generation in an AMD-fed fuel cell can be effectively enhanced by increasing the carbonate concentration.11 Similarly, in this work, we observed accelerated electrooxidation of FeII in carbonate-amended fuel cells, which might be attributed to the formation of kinetically reactive FeOH and FeCO3 species.12,24−26 At pH 5.0, the concentrations of active FeII species were low enough to be negligible, whereas the inert hydrated ferrous ions dominated the solution, resulting in limited FeII oxidation.12,26 As the pH increased above 6.0, the concentrations of FeOH and FeCO3 species rose steeply,22,25 therefore dramatically accelerating FeII oxidation. What should be noted is that the electrooxidation of FeII in a carbonate solution was controlled by both the redox potential of the solution and the redox activity of iron(II) carbonate complexes. Increasing the solution pH facilitated FeII oxidation by lowering the redox potential of the solution. Conversely, the redox activity of iron(II) carbonate complexes was attenuated at high pH, as evidenced by the weakened FeII oxidation peak in the cyclic voltammograms (Figure 6). As a result, carbonate-amended fuel cells showed the best performance when operated at a neutral pH of 7. As an artificial chelating agent, the bicarboxylate ligand EDTA has a high capacity to form complexes with both FeII
and FeIII. EDTA is known to facilitate FeII oxidation by transforming the FeIIEDTA complex to a more stable FeIIIEDTA complex.13 Thus, the performance of a FeII-fed fuel cell was appreciably enhanced by EDTA addition. In this system, the redox potential of the anodic solution remained constant from pH 5 to 7, while the redox activity of FeII gradually decreased, thus leading to an attenuated fuel-cell performance. As the pH increased from 7 to 9, an enhanced fuel-cell performance was observed, despite of the weakened redox activity of FeII. Such an observation might be ascribed to the greatly decreased redox potential of the solution. Therefore, the poorest performance of EDTA-amended fuel cells was given at pH 7. The tricarboxylate ligand citrate has been proven to play a role in FeII oxidation in aerobic systems, although the underlying affecting mechanism remains ambiguous.27,28 Theis and Singer reported that citrate could accelerate FeII oxidation by O2 at least for the initial several minutes.27 However, in another study, a retarding effect of citrate on FeII oxidation was observed, which was ever ascribed to the high stability of iron(II) monocitrate complexes.28 In fact, citrate can chelate FeII to form kinetically active dicitrate complexes other than monomeric species.29 Moreover, iron(III) citrate complexes are far more stable than iron(II) citrate complexes.30 Therefore, FeII oxidation should be improved by citrate addition, as demonstrated in this work. The performance of citrate-amended fuel cells was greatly enhanced by an increase of the pH, probably due to the more negative redox potential of the solution at high pH values. Phosphate constituents have a pronounced capacity to chelate FeII, and iron(II) phosphate complexes, such as [Fe(H2PO4)]+ and [FeHPO4], are more redox-reactive than aquo complexes.31,32 However, in this work, phosphateamended fuel cells presented noticeably lower current densities than those amended with other chelating anions. Such a 2238
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The electrooxidation of FeII in a fuel cell was influenced by both the redox potential of the anodic solution and the redox activity of FeII. On the one hand, the redox potential of the solution generally decreased with an increase of the pH, thus favoring the electrooxidation of FeII. On the other hand, the redox activity of FeII was decreased by an increase of the pH except in a borate solution, where the highest redox activity of FeII was observed at pH 9. Therefore, the optimal pH for the fuel-cell operation was quite different among different chelating anions.
phenomenon may be ascribed to the low solubility of iron(II) phosphate complexes, which hindered the electrooxidation of FeII by limiting the release of free FeII.13 As shown in Table 1, no more than 40% FeII could be oxidized in phosphateamended fuel cells, thereby causing low power output. Because the iron(II) phosphate complexes preferably precipitated at high pH, phosphate-amended fuel cells were better operated under acidic conditions. In a borate solution, dissociated ion B(OH)4− can react with FeII to form iron(II) borate complexes such as FeB(OH)4+.33 The solution pH exhibited a significant influence on the performance of borate-amended fuel cells. From pH 5 to 8, the total number of recovered charges increased by more than 2 orders of magnitude from 0.34 to 47.17 C. Because of the more negative redox potential of the solution and the strengthened redox activity of FeII at high pH, borate-amended fuel cells were preferably operated under alkaline conditions. The results presented in this work clearly show that an enhanced performance of a FeII-fed fuel cell can be achieved by selecting suitable anions to facilitate the electrooxidation of FeII at the anode. Among the five chelating anions, carbonate exhibits a more significant enhancement to the electrooxidation of FeII. This result is consistent with previous studies that have applied carbonate as the main constituent of the anodic solution in AMD-treated fuel cells.10,11 However, in a carbonate solution, the product FeIII easily forms precipitates, which adhere on the electrode and on the wall of the reactor, thus bringing difficulties to the fuel-cell operation and subsequent iron-recovery process. Because iron precipitates do not occur in EDTA and citrate solutions in practical AMD treatment, carbonate and EDTA/citrate can be combined in practical AMD treatment to avoid iron precipitation. Considering the sufficient buffering ability of carbonate and EDTA solutions, a mixture of carbonate and EDTA appears to be the best chelating agent in AMD-treated fuel cells for a favorable cell performance and a fluent cell operation. Notably, the chelated FeIII recovered from AMD shows great potential in the liquid iron redox process for the production of elemental sulfur (S0) from hydrogen sulfide (H2S) waste gas.34 In the liquid iron redox process, FeIII is presented as an oxidizing agent to convert H2S to S0, and chelating agents such as EDTA are essential to hold FeIII in solution. The chelated FeIII recovered from AMD treatment may be an economical iron source for the liquid iron redox process.21 Of course, further research will be needed on many aspects of AMD-treated fuel cell technology before it can be successfully commercialized.
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AUTHOR INFORMATION
Corresponding Author
*Tel: +086-551-2901450. Fax: +086-551-2901450. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the NSFC (Grant 51008108) and the NSFC−RGC Joint Project (Grant 21021140001) for partial support of this work.
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REFERENCES
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5. CONCLUSIONS An air−cathode fuel-cell system was constructed to produce electricity from the electrooxidation of FeII. The chelating anions, including carboxylate, phosphate, and borate ligands, demonstrated various degrees of accelerating effect on the electrooxidation of FeII. By taking both the power output and energy-recovery efficiency into consideration, the performance of FeII-fed fuel cells was enhanced by chelating anions in the order carbonate > EDTA > citrate > borate > phosphate. The fuel-cell performance was a strong function of the solution pH. Electricity generation increased with the pH for citrate-amended reactors, whereas the opposite tendency occurred in phosphate-amended ones. For EDTA-amended reactors, lower power densities were observed at neutral pH, which was, however, found to facilitate the power production for carbonate- and borate-amended ones. 2239
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dx.doi.org/10.1021/ie3032185 | Ind. Eng. Chem. Res. 2013, 52, 2234−2240
