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Home , Potassium ferrocyanide

17-3 The Effects of /Potassium and their Derivatives on the Performance of Solid-State Supercapacitor Xiangyang Zhou, Xiaoyao Qiao, Chen Zhang, and Yuchen Wang Department of Mechanical and Aerospace Engineering, University of Miami 1251 Memorial Drive, Coral Gables, Florida, USA, FL33124 [email protected] / 1-305-284-3287 Azzam N. Mansour, Gordon H. Waller, and Curtis A. Martin NSWCCD, 9500 MacArthur Boulevard, West Bethesda, Maryland, USA MD20817 [email protected] / 301-227-4451

Abstract: In previous studies, we have demonstrated that function of mediators, including increasing capacitance of the addition of metal mediators or species supercapacitor and promoting the ionic conductivity of the into polymer electrolytes can effectively increase the electrolyte, have been observed and analyzed. specific energy and power of a mediator containing As a mediator, derivatives or analogues supercapacitor relative to a supercapacitor without a (PBAs) have fast reversible redox kinetics and high specific mediator. The benefits of combining mediators into the capacitance. However, compared to other redox mediators, composite electrode, which contains powder and PBAs have unique advantages in practical applications: polymer electrolyte, are the introduction of pseudo- capacitance, promotion of ionic and electronic 1. PBAs are insoluble in most solvents. For other redox conductivities, and serving as additional sources for mediators, such as K3Fe(CN)6/K4Fe(CN)6 and NaI/I2, electric double layer capacitance. In order to further which are soluble in an aqueous electrolyte, an expensive enhance the performance of the mediator supercapacitor selective membrane must be employed to prevent the and to elucidate the charge/discharge mechanisms, a mediator-crossover issue for non- solid-state SC. systematic study was conducted. In this report, we will 2. PBAs containing multi-valent transition metals, such as present the research progress in three aspects: 1) synthesis K2MnFe(CN)6, could further improve the capacitance of of mediators in the form of KMFe(CN)6 (M=transition K3Fe(CN)6 and K4Fe(CN)6, because Mn can have oxidation metal); 2) structural characterization of the synthesized states of 2+, 3+, and 4+ under normal potential range. compounds using X-ray Photoelectron Spectroscopy (XPS) 3. For EDLCs and pseudocapacitors, the are consumed and X-ray diffraction (XRD); 3) experimental validation of to form the EDL or complete redox reactions. However, time dependent charge and discharge process. The full cell PBAs can provide extra ions to the electrolyte to maintain the measurements revealed that the specific energy of concentration, as shown in the following redox reaction: supercapacitors with mediators where M is Mn, Fe, Co, 2+ 2+ 3+ 3+ - + increased by more than 40% relative to that of K2Mn Fe (CN)6 → Mn Fe (CN)6 + 2e + 2K supercapacitors containing K3Fe(CN)6/K4Fe(CN)6. The objectives of the present research work are as follows. 1. Develop reliable and repeatable chemical synthesis Keywords: supercapacitor; potassium ferrocyanide, procedure to produce series of transition metal based PBAs. ; mediator; solid-state; XPS; XRD. 2. Characterize the synthesized PBAs using XPS and XRD Introduction to verify the structures and compositions of the PBAs. A mediator is a redox material that undergoes fast and 3. Fabricate mediator supercapacitors using the PBAs to reversible reactions. Examples of mediator pairs are NaI/I2, evaluate the performance of the mediator supercapacitors K4Fe(CN)6/K3Fe(CN)6. Our research group proposed a new as a function of mediators. concept of mediator supercapacitor (SC), which is 4. Study the effect of the mediators as ion providers during composed of large amount of activated carbon and small charge/discharge. amount of mediator [ 1 , 2 ]. This type of SC can be considered as an EDLC combined with a pseudocapacitor, Experimental Approach or a hybrid SC. The Faradaic reactions can increase the Sample Preparation: The K Fe(CN) and K Fe(CN) capacitance of the electrodes, replenish the number of free 4 6 3 6 mediators were obtained from Sigma-Aldrich. An attempt ions in the electrolyte, and minimize reduction of ionic was made to prepare a number of K Fe(CN) and conductivity during the charge/discharge processes. 3 6 K4Fe(CN)6 analogues using the following procedures: In previous work by Zhou’s group, a couple of (1) Dissolve 0.01 mole of transition metal (Mn2+, Co2+, mediators/redox species were reported to be effective in Ni2+, Cu2+, Zn2+) nitrate in 100 mL of deionized water with enhancing the performance of supercapacitors. The magnetic stirring at room temperature to get Solution 1.

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(2) Dissolve 0.01 mole K3Fe(CN)6 or K4Fe(CN)6 in 100 mL the stoichiometry of the cyanide group. Considering the of deionized water with magnetic stirring at room semi-quantitative nature of XPS composition analysis, the temperature to get Solution 2. potassium concentrations of 11.0 and 11.4 for (3) Add these two solutions dropwise into 100 mL of K2MnFe(CN)6 and K2CoFe(CN)6, respectively, are in deionized water with magnetic stirring at room general agreement with its nominal value of 12.5. temperature. Precipitate forms immediately but stirring However, the potassium concentrations of 6.3 and 5.5 for continues for another 2 hours. K2CuFe(CN)6 and K2ZnFe(CN)6 are well below nominal value indicating that these two mediators will not form (4) The precipitate is centrifuged at 6500 rpm, washed with pure phases with the indicated nominal stoichiometry deionized water for three times, and dried in -filled using the synthesis procedure described here. glovebox at 50°C for 1 day. (5) The raw product is grounded by hands with a mortar Table 1. Summary of XPS composition of various mediators. and pestle and kept in a nitrogen-filled glovebox. Sample C N O K Fe M % The general chemical reactions are shown as follow: K4Fe(CN)6 40.7 22.8 9.8 22.5 4.2 N/A

K3Fe(CN)6 + M(NO3)2→ KMFe(CN)6 + 2KNO3 K3Fe(CN)6 43.8 26.5 6.6 18.5 4.7 N/A K Fe(CN) + M(NO ) → K MFe(CN) + 2KNO K2MnFe(CN)6 39.0 30.7 5.9 11.0 5.8 7.6 4 6 3 2 2 6 3 a b K2CoFe(CN)6 39.1 31.7 4.1 11.4 5.5 8.3 K4Fe(CN)6 + Fe(NO3)3→ KFeFe(CN)6 + 3KNO3 K2CuFe(CN)6 43.3 33.4 5.6 6.3 6.10 5.3 K ZnFe(CN) 44.2 29.7 6.4 5.5 4.88 9.4 XPS and XRD Measurements: XPS spectra were collected 2 6 using Physical Electronics VersaProbe II Scanning XPS KMnFe(CN)6 44.0 29.4 7.9 5.6 5.2 8.0 KFeFe(CN)6 46.1 27.2 13.3 0.5 12.9 N/A Microprobe using monochromatic Al Kα (1486 eV) X-ray a b source with a focused beam size of 200 microns rastered KCoFe(CN)6 42.3 34.2 6.3 0.6 4.9 11.7 over and area of 1000 x 200 microns. The structure and KCuFe(CN)6 46.8 31.2 9.0 0.4 6.1 6.5 phase purity of mediators were investigated by powder X- KZnFe(CN)6 51.7 28.5 7.0 0.4 4.0 8.4 ray diffraction using the Bruker D8 Advance a The Fe 2p3/2 concentration is distorted due to interference Diffractometer with Cu Kα X-rays (λ = 1.5404 Å). b with the Co Auger lines. The Co 2p3/2 concentration is Supercapacitor Fabrication: The electrodes were prepared distorted due to interference with the Fe Auger line. using the following steps: 1) adding 0.05 g polyvinylidene The potassium concentration of 5.6 is roughly close to the fluoride (PVDF) into 3 mL N-methyl-2-pyrrolidone solvent nominal value of 6.7 for KMFe(CN)6 where M is Mn but (NMP) and stirring at 80°C until PVDF is totally dissolved is reduced significantly below 1 when M is Fe, Co, Cu, or (this process may take 3 hours), 2) adding 0.1 g of Zn. Hence, the samples with potassium concentrations mediator, 0.9 g of activated carbon (AC), and 0.05 g of below 1 do not support the formation of significant conductive carbon to the previous solution and stirring for quantities of KMFe(CN)6. The concentrations of Fe are in 2 hours to form consistent slurry, 3) coating the composite qualitative agreement with nominal value for all slurry onto a 50 cm2 carbon paper with a drawdown tool, mediators. The Mn, Cu and Zn concentrations are and 4) The coated electrode is then dried in a glovebox at somewhat close to the nominal value. Note that the Fe 80°C for 24 hours. The same procedure was used to concentration for KFeFe(CN)6 is close to the nominal prepare an electrode without mediators and used as a value of 13.3. To be noted, oxygen was found to be reference for comparison purposes. Two aluminum foils present in all mediators with concentrations in the range were used as current collectors, a PVDF with 1 M of of 4.2-8.6 % except in the case of KFeFe(CN)6 where the tetraethylammonium tetrafluoroborate based polymer concentration is highest at 13 %. The presence of oxygen electrolyte membrane similar to those described in Refs. 1 was also confirmed on the basis of XRD indicating that and 2 were used as the separator. The SC cell was oxygen was not only present as surface contaminant but assembled in a glove box with nitrogen using a vacuum also in the bulk of the sample most likely in the form of sealing machine. The specific energy was evaluated by adsorbed and structurally bonded water. Based on XPS normalizing the measured data with the total mass of the results, it is more appropriate to describe the electrode including active carbon, conductive graphite, stoichiometry of the mediators as KxMyFe(CN)6•zH2O PVDF binder, and mediator. The cyclic voltammetry and where x, y, and z depend on the transition metal galvanostatic charge/discharge (GCD) testing was component as proposed previously [3]. For simplicity, conducted using Gamry model 3000 electrochemical hereafter, we refer to the mediators with the desired measurement system. stoichiometry throughout the paper.

Quantitative deconvolution of the Fe 2p3/2 spectra (Figure Results 1) revealed they consist of either one component or two XPS: Based on XPS composition results (Table 1), the components corresponding to Fe with oxidation state of 2+ surface region consists mostly of C and N in accord with

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or a mixture of 2+ and 3+, respectively. The component KCoFe(CN)6•0.5H2O & Co1.5Fe(CN)6•6H2O and with biding energy near 708.8 eV is assigned to the KCuFe(CN)6•7H2O & Cu1.5Fe(CN)6•5H2O, and 2+ presence of Fe and is present in all mediators [4]. The Zn1.5Fe(CN)6•4.7H2O & ZnFe(CN)5(NO). Some of the second component with binding energy near 711 eV is XRD assigned phases are consistent with the higher 3+ concentration of the metal component (Mn, Co, Cu, Zn) assigned to Fe and is present in KMFe(CN)6 (M: Fe, Cu relative to that of Fe as shown in Table 1. To be noted, the and Zn). The Mn 2p3/2 region (Figure 2) was deconvoluted into three components with binding energies near 641.7, XRD pattern of KFeFe(CN)6 is similar to those of 643.4, and 646.4 eV. The first two components correspond K2MFe(CN)6 (M: Mn, Co, or Cu) in Figure 3 despite that to the presence of Mn species with oxidation states of 2+ the potassium concentration based on XPS is well below and 3+/4+ [ 5 ], respectively. The third component the value based on nominal stoichiometry. Again, it is corresponds to a shake-up structure. The chemistry of Mn appropriate to describe the stoichiometry of the mediators as K M Fe(CN) •zH O in agreement with XPS results appears to be similar in both KMnFe(CN)6 and x y 6 2 K2MnFe(CN)6 with the majority of Mn (~75%) present in 120000 the 2+ oxidation state. 3.2 Fe 2p3/2 90000

2.4 KZnFe(CN)6 60000 K2ZnFe(CN)6 K2ZnFe(CN)6 KCuFe(CN)6 1.6 Intensityu.)(a. K CuFe(CN) K2CuFe(CN)6 30000 2 6 K CoFe(CN) KCoFe(CN)6 2 6 K2CoFe(CN)6 K2MnFe(CN)6

Normalized Intensity Normalized 0.8 KFeFe(CN)6 0 KMnFe(CN)6 10 20 30 40 50 60 70 80 K2MnFe(CN)6 2-Theta (degrees) 0.0 718 715 712 709 706 Figure 3. XRD pattern of K2MFe(CN)6 (M: Mn, Co, Cu, Zn) Binding Energy (eV) Mediators. 96000 Figure 1. Fe 2p3/2 XPS region of mediators with different transition metals.

1.5 72000

Fe L3M45M45 Mn 2p3/2 1.2 KZnFe(CN)6 Mn 2p1/2 48000 KCuFe(CN)6 0.9 KMnFe(CN)6 Intensityu.) (a. 24000 KFeFe(CN)6 KMnFe(CN) 0.6 6 KMnFe(CN)6 Normalized Intensity Normalized 0.3 0 10 20 30 40 50 60 70 80 K2MnFe(CN)6 2-Theta (degrees) 0.0 660 654 648 642 636 Figure 4. XRD patterns of KMFe(CN)6 (M: Mn, Fe, Co, Cu, Zn) mediators. Binding Energy (eV) The cyclic voltammetry curves of the SCs were generated Figure 2. Mn 2p XPS region of K2MnFe(CN)6 and using a Gamry electrochemical testing system at a scan rate KMnFe(CN)6 mediators. of 10 mV s-1. Clearly the specific currents for the mediator

SCs (Figure 5) are much greater than that of the active XRD: The XRD data for the K2MFe(CN)6 (M: Mn, Co, carbon based SC. Small humps appearing at about 1.6 V Cu) (Figure 3), are closely related to the orthorhombic and 1.3 V are indicative of redox reactions. GCD curves lattice assigned to K2MFe(CN)6•2H2O, cubic lattice (Figure 6) clearly indicate that the specific capacitance and assigned to K2CoFe(CN)6 and K2CuFe(CN)6. In the case energy for the PB mediator SC are much greater than those of Zn mediator (Figure 3), the XRD data are closely of the reference SC. The specific energy values at the 0.5 related to the rhombohedral lattice assigned to A/g rate are tabulated in Table 2. The maximum value was KZn1.5Fe(CN)6-xH2O (x = 2.5 and/or 4.5) [6]. achieved with the Mn mediator SC. It was anticipated that

XRD data for KMFe(CN)6 (Figure 4) can be tentatively redox reactions for Mn can lead to multiple valence assigned to Mn1.5Fe(CN)6•2H2O & KMn(CN)6, changes, such as from 2+ to 3+ and 4+.

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Table 2. Specific energy values for mediator and activated carbon based SCs. SC Specific Anode Cathode Energy mediator mediator Wh·kg-1 Reference 16 Activated Active carbon carbon Ferrocyanide 30 K4Fe(CN)6 K3Fe(CN)6 Mn Mediator 44 K2MnFe(CN)6 KMnFe(CN)6 Fe Mediator 42 KFeFe(CN)6 KFeFe(CN)6 Co Mediator 39 K2CoFe(CN)6 KCoFe(CN)6 Cu Mediator 31 K2CuFe(CN)6 KCuFe(CN)6 Zn Mediator 23 K2ZnFe(CN)6 KZnFe(CN)6 Figure 5. Cyclic voltammetry of activated carbon based SC and mediator containing SCs as indicated in the legend. Conclusions 1. Transition metal based Prussian Blue analogues were synthesized. The PB analogues were characterized using XPS and XRD to acquire the elemental compositions, oxidation states and structural information. The stoichiometry of the synthesized material has the genral formula KxMyFe(CN)6•zH2O. 2. Supercapacitors were fabricated uisng the mediators. The Figure 6. Galvanostatic charge/discharge of activated specific capacitance and energy of the Mn and PB mediator SC is much greater than that of the active carbon based SC carbon (left) SC and KFeFe(CN)6 mediator SC (right). The curves from right to left: 10, 20, 40, 80, and 160 mA. and greater than the K3Fe(CN)6/K4Fe(CN)6 mediator SC. 3. Ion release from the mediators into the electrolyte mitigates loss of ions in the electrolyte during the charging cycle.

Reference

1. Y. Yin, J. Zhou, A.N. Mansour, and X. Zhou, “Effect of NaI/I2 mediators on properties of PEO/LiAlO 2 based all- solid-state supercapacitors,” J. Power Sources. Vol. 196 , pp. 5997–6002, 2011. 2. A.N. Mansour, J. Zhou, and X. Zhou, “X-ray absorption Figure 7. GCD results of three SCs with 0.1 M electrolyte spectroscopic study of sodium iodide and iodine mediators in concentration at charge/discharge current of 0.089A g-1. a solid-state supercapacitor,” J. Power Sources. Vol. 245, pp. 270–276, 2014. In order to evaluate the effect of ion release from 3. S. J. Gerber and E. Erasmus, Electronic Effects of Metal mediators, GCD curves were evaluated in an electrolyte Hexacyanoferrates: An XPS and FTIR study, Mater. Chem., with low concentrations of ions. One set of results are Vol. 203, pp. 73-81, 2018. shown in Figure 7. It is clear that at a low concentration of 4. Y. Zhang, Y. Wen, Y. Liu, D. Li, J. Li, “Functionalization of 0.1 M of tetraethylammonium tetrafluoroborate and total single-walled carbon nanotubes with Prussian blue,” amount of electrolyte of 0.1 mL, the Mn mediator SC Electrochem. Commun., Vol. 6, pp. 1180-1184, 2004. achieves the maximum specific capacitance and energy. 5. W.-J. Li, S. L. Chou, J.-Z.Wang, J.-L.Wang, Q.-F. Gu, H.-K. The specific capacity of the KFeFe(CN)6 mediator SC is Liu, S.-X. Dou, “Multifunctional conducing polymercoated similar to that of the activated carbon based SC. The Na1+xMnFe(CN)6 cathode forsodium-ion batteries withsuperiorperformance via a facileandone- explanation is that the KFeFe(CN)6 mediators contain only 0.5 atomic % of K+ whereas K MnFe(CN) contains 11.0 stepchemistryapproach,” Nano Energy, Vol. 13, pp. 200-207, 2 6 2015. atomic % of K+. During the charging cycle, the oxidation of the mediators at the positive electrodes releases potassium 6. E. Garnier, P. Gravereau, A. Hardy, “Zeolitic : ions. These released ions compensate loss of ions in the the structure of Na2Zn3[Fe(CN)6]2.xH2O,” Acta Crystallogr. electrolyte due to formation of electrical double layers. This Sect. B. Vol. 38, pp. 1401-1405, 1982. deficiency of ions is clearly shown by the transition from linear dependence of the cell voltage on time to exponential dependence.

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