VI) Cathodic Charge

May 6, 2009 - Department of Chemistry, George Washington University, Washington, DC 20052, and Department of Chemistry, University of Massachusetts, ...
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J. Phys. Chem. C 2009, 113, 9884–9891

Enhancement of Reversible Nonaqueous Fe(III/VI) Cathodic Charge Transfer Stuart Licht,*,† Yufei Wang,‡ and Gerald Gourdin‡ Department of Chemistry, George Washington UniVersity, Washington, DC 20052, and Department of Chemistry, UniVersity of Massachusetts, Boston, Massachusetts 02125 ReceiVed: March 6, 2009; ReVised Manuscript ReceiVed: April 22, 2009

The electrochemical preparation of Fe(III/VI) super-iron thin films cathodes on an extended conductive matrix significantly facilitates such film’s nonaqueous, reversible charge transfer. Fe(VI) salts can exhibit higher cathodic capacity and environmental advantages, and the films are of relevance toward the next generation charge storage chemistry for reversible cathodes. These films were electrochemically deposited on either smooth or platinized platinum by electrochemical reduction of Na2FeO4, which retains an intrinsic 3 e- cathodic charge storage capacity of 485 mAh g-1. The influence on cathode reversibility, capacity, and charge retention was probed as function of film deposition conditions (including the deposition potential and stirring rate, and the concentration of NaOH and K2FeO4, in the deposition electrolyte). The highest storage capacity films were formed from an 8.0 M NaOH electrolyte containing 50.0 mM of K2FeO4 and examined by FTIR. Those “super-iron” cathode films, which are up to 191 nm thick and electrodeposited on smooth platinum, sustained up to 300 reversible three-electron galvanostatic charge/discharge cycles to a high depth of discharge in 1 M LiPF6 in PC:DME electrolyte lithium cells. Thicker (greater than 500 nm thick) films deposited on smooth platinum evidence increasing passivity to Fe(VI) charge transfer and charge storage. Films deposited on platinized platinum overcome this passivity, permitting sustained nonaqueous charge/discharge cycling with a thicker (571 nm) Fe(VI) film cathode, and provide a first demonstration of significant Fe(VI)/lithium thinfilm secondary cell behavior. Introduction In recent years, the development of rechargeable thin-film batteries with a higher energy density has been of considerable interest. This interest has been driven by the increasing energy needs of portable electronics, vehicle, space and medical devices.1,2 Much of this interest has focused on thin-film lithium and lithium ion batteries. Lithium anode charge storage devices are generally limited by the cathode storage capacity. There is a critical need for higher charge storage, and the search for lithium and lithium-ion cell cathodes has generally focused on lithium insertion into lithiated derivatives of MnO2, V2O5, LiCoO2, and LiNiO2, which have storage capacities less than 300 mAh g-1.3 Fe(III) compounds have been explored both as cathode4 and (Li ion) anode5 materials in nonaqueous cells, however charge storage via higher valence iron compounds were not studied until recently.6-10 The known superiron-based cathodes (including those utilizing M2FeO4 and M′FeO4, where M ) Li+, Na+, K+, Cs+, and Ag+ and M′ ) Sr2+, Ba2+, and mixed cation salts) can be discharged, in accordance with the generalized reactions:

5 1 FeO42- + H2O + 3e- f Fe2O3 + 5OH-, 2 2 E ) 0.6 V vs SHE (1) or

FeO42- + 3H2O + 3e- f FeOOH + 5OH-, E ) 0.6 V vs SHE (2) The ferric discharge product tends to be amorphous. The specific Fe(III) formed and the precise value of the redox * To whom correspondence should be addressed. E-mail: [email protected]. † George Washington University. ‡ University of Massachusetts.

potential will vary with the depth of discharge, dehydration, the cation counterion, and the electrolyte. The theoretical storage capacity of such salts, including Li2FeO4 (601 mAh g-1), Na2FeO4 (485 mAh g-1), K2FeO4 (406 mAh g-1), Ag2FeO4 (399 mAh g-1), SrFeO4 (388 mAh g-1), and BaFeO4 (313 mAh g-1), is higher than the most widely used primary (MnO2 at 308 mAh g-1) or secondary-reversible (NiOOH at 290 mAh g-1) alkaline cathode materials. In nonaqueous lithium salt electrolytes, intercalation, rather than faradaic, processes tend to dominate cathodic charge storage. Interestingly, this tends not to be the case for nonaqueous Fe(VI) cathodes. In order to obtain a full picture of the electrochemical behavior of Fe(VI) salts in nonaqueous lithium cells, a wide variety of probes were employed including cyclic voltammetry and chronopotentiometry combined with X-ray photoelectron spectroscopy, X-ray diffraction, Mo¨ssbauer spectroscopy, atomic adsorption, atomic emission, in situ and ex situ atomic force microscopy imaging, and diffuse reflectance Fourier transform infrared spectroscopy. All of the results described converge to the conclusion that, MFe(VI)O4 compounds do not intercalate with lithium but rather undergo a quasi-reversible (faradaic) redox process that reduces the iron in the compounds from 6+ to 3+ states.11 We reported a class of electrochemical charge storage superiron/lithium primary batteries utilizing several Fe(VI) salts.7,9-17 The high energy and power capacity of these batteries under various conditions have been demonstrated.18-23 It has been identified that Fe(VI) salts (i) are insoluble in a wide variety of nonaqueous electrolytes, (ii) are not chemically reactive with the electrolyte, and (iii) can be discharged as cathodes in nonaqueous electrolyte.7 In contrast to the advantageous primary discharge characteristics of superiron batteries, the poor reversible charge transfer had been a challenge to be addressed. The formation of Fe2O3 has a detrimental effect on the reversible

10.1021/jp902157u CCC: $40.75  2009 American Chemical Society Published on Web 05/06/2009

Fe(III/VI) Cathodic Charge Transfer charge transfer due to conductivity constraints of ferric salts.18,21 The use of redox and electronic mediators, and overlayers, have further improved the extent of the primary Fe(VI) discharge but had not improved the reversible charge transfer.16,24,25,39,40 In our previous communications, we demonstrated that nanothin (3 nm) superiron films can sustain over 100 charge/ discharge cycles in aqueous alkaline,10 or nonaqueous,11 media. However, thicker films were not rechargeable due to the irreversible buildup of passivating (resistive) Fe(III) oxide that was formed during film reduction, and, for example, either a 100 nm or a 1000 nm superiron film had passivated after only 20 cycles or 2 cycles, respectively.10 Recently, we identified that an extended conductive matrix was able to facilitate a 100fold enhancement in charge storage for reversible Fe(III/VI) superiron thin films in alkaline aqueous solutions. A 100 nm Fe(VI) cathode, on the extended conductive matrixes, sustained 100-200 reversible three-electrode charge/discharge cycles, and a 19 nm thin-film cathode sustained 500 such cycles. With a metal hydride anode, full cell storage was probed, and a 250 nm superiron film cathode film sustained 40 charge/discharge cycles, and a 25 nm film was reversible throughout 300 cycles.26 The current work probes the first enhancement of the Fe(VI) conductive matrix in nonaqueous medium, with both fundamental charge transfer implications, as well as applied interest (to high capacity, rechargeable batteries). In this study we focus on Fe(VI) cathodic charge transfer in substantially thicker cathodes with increased charge-storage capabilities. This is the first demonstration of Fe(VI)/lithium thin-film cell secondary discharge/charge behavior. Fe(VI) salts exhibit higher cathodic capacity and environmental advantages, and the films are of relevance toward the next generation charge storage chemistry for reversible cathodes. Experimental Section Chemicals and Materials. K2FeO4 was synthesized as described previously.27 NaOH (Acros Organics, purum, pellets), H2PtCl6 (Alfa Aesar, metal basis 99.95%, 37.5% Pt), Na2PtCl6 (Alfa Aesar, metal basis 99.99%, 42.4% Pt), HClO4 (VWR, 0.1 N in water), propylene carbonate (PC) (Sigma-Aldrich, anhydrous, 99.7%), 1,2-dimethoxyethane (DME) (Sigma-Aldrich, anhydrous, 99.5%), and LiPF6 (Aldrich, battery grade) were purchased and used as received. Aqueous solutions were prepared from triply deionized water (Barnstead E-Pure). All nonaqueous solutions were dried over 4 Å molecular sieves for 3 days and then, transferred into an Ar-filled glovebox. Instruments. Electrodeposition investigations were carried out by using an AFCBP1 bipotentiostat (Pine Instrument Company), or an AUTOLAB PGSTAT30 potentiostat-galvanostat (EcoChemie). FTIR spectrum was measured by using a Nicolet 4700 Fourier transform infrared spectrophotometer (Thermo Electron Corporation). Electrode charge/discharge behavior was investigated by an Arbin Instruments BT4+ system. Substrate Pretreatment. The substrate pretreatment of the platinum used here, has been detailed in our prior study.26 In brief, platinum substrates were polished using aluminum oxide cloth (600 grit), etched in aqua regia (HCl/HNO3 (3:1)) for 10-20 min, sonicated in distilled water for 20 min, and then electrochemically cleaned by cycling between -0.2 and -1.5 V vs Ag/AgCl for 50 cycles at a scan rate of 500 mV s-1 in 1 M H2SO4. Platinization Methodologies. On pretreated platinum substrates, Pt was potentiostatically deposited in a three-electrode cell at 0.2 V vs Ag/AgCl from aqueous 0.1 M H2PtCl6. The

J. Phys. Chem. C, Vol. 113, No. 22, 2009 9885 working electrode was a Pt foil with an exposed geometrical area of 1 cm2, and circular Pt foil was used as the counter electrode. At the platinization potentials employed, electrodeposition substantially dominates, approaching 100% current efficiency in accord with26

PtCl62- + 4e- f Pt + 6Cl-; E(Pt(IV/II)) ) 0.76 V; E(Pt(II/0)) ) 0.68 V vs SHE (3) The absolute number of coulombs in the deposition process, Q(coulombs), is quantitatively measured and then, for convenience, converted to equivalents platinum in accord with eq 3. Preparation of Super-Iron Cathode Films. In the first section of the results, superiron films were electrodeposited from various alkaline K2FeO4/NaOH solutions at an applied potentiostatic voltage of 0.11 V vs Ag/AgCl in a voltammetric Plexiglas cell, and optimized as a function of solution composition. In the latter experiments reported herein, superiron films were consistently electrodeposited from 50 mM K2FeO4 dissolved in stirred (magnetic bar), 8.0 M NaOH in a voltammetric Plexiglas cell at a galvanostatic current of 10.0 mA. The working electrode was a 1 cm2 platinum disk or a 1 cm2 platinized platinum disk. The auxiliary electrode was a platinum rod, and the reference electrode was an Ag/AgCl/KCl (sat) encased in a 0.1 M NaNO3 jacket. Solutions were initially deaerated with nitrogen gas for a minimum of 5 min prior to the electrodeposition. A N2 gas atmosphere was maintained over the solution during the electrodeposition. The film electrode was carefully rinsed with 8.0 M NaOH solution, air-dried for 30 min, and then vacuum-dried for 2 days prior to use. During the optimization of the electrodeposition process, the surface of the working electrode was rinsed at each end of the experiment by an oxidative linear potential scan in 1.0 M HCl solution. Employed voltammetric Plexiglas cells were consistently, cleaned by soaking overnight in a solution of 10-2 M nitric acid, followed by DI water rinse, and results presented are the average of three replicate measurements. For Fe(III/VI) film formation, at the optimized deposition potential employed, the electrodeposition via the 3 electron reduction of Fe(VI) to Fe(III) substantially dominates, permitting the current efficiency to be assumed as 100%.26 Hence, the intrinsic capacity of the superiron films determined by integrating the current-time response curve, Q(C ) coulombs ) ampere seconds), provides a quantitative measure of the intrinsic charge capacity of the superiron films and, for convenience, a relative (not absolute) measure of the film thickness. The relative comparison of film thicknesses, T, is quantitative for all compared electrodes. One measure of this relative thickness is to use Fe2O3 as the principal ferric product, calculated in accord with eq 4 as

T ) [Q/3F](1/2FW)/area × d; where Fe2O3: FW ) 221.7 g mol-1, d ) 3.05 g cm-3 T ) 9.1 × 107 nm cm2 mol-1 × Q/(F × area(cm2)); F ) 96485 C mol-1 (4) For example as previously described,26 a 19 nm Fe(III/VI) films contains 69 nmol of Fe per cm2, and is capable of storing 20 mC of intrinsic capacity per cm2 (based on the observed 3 electrons of storage per Fe(III/VI) center). Similarly, thicker deposited 57, 191, and 573 nm Fe(III/VI) films used in this study have an intrinsic storage capacity respectively of 60, 202, and 605 mC per cm2. Cell Charge/Discharge Characterization. The thin-film cell with a lithium anode was prepared by using a conventional ECR 1620 coin cell. The coin cell was opened, the anode retained,

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Figure 1. Components of a coin cell configuration containing a Pt/ Fe(III/VI) electrode.

an appropriate electrolyte (1 M LiPF6 in PC: DME (1:1 by volume:volume)) was added and a new thin-film cathode was placed in the cell (Figure 1). Specifically, the new ECR 1620 anode was removed under argon in the glovebox, retaining the original separator over the lithium to avoid oxidation. The original electrolyte was removed by absorption contact with filter paper. A second ECR 1620 cell was opened by lathe to provide an undamaged cathode case, the internal cathode materials were removed, and the case was sonicated for 5 min in triply deionized water, followed by drying at 40 °C. The cell was assembled under a pure argon atmosphsere and crimped by a Carver laboratory Press. A total of three layers of epoxy were applied over the Teflon bands. Self-Discharge and Charge Retention. The self-discharge and charge retention tests were carried out at room temperature, 40, 45, 50, and 60 °C. The fully charged cells, which were stored over 7 days at the corresponding temperatures, were discharged separately. The ratios of charge/discharge capacity before and after the 7-day storage were measured. Results and Discussion Optimization of Fe(III/VI) Film Formation by Alkaline Electrodeposition on Smooth Platinum. The dissociation and decomposition of the M2FeO4 (M ) Li+, Na+, K+, Cs+, or Ag+) in alkaline solution can be expressed by eqs 5, 6 and 7.26 Ka, Kb, and Kc represent the stability constants respectively for

M2FeO4 ) MFeO4- + M+

(5)

MFeO4- ) FeO42- + M+

(6)

3 3 FeO42- + H2O ) FeOOH + O2 + 2OH2 4

(7)

Therefore, the total concentration of Fe and M can be expressed as eqs 8-10

[M2FeO4]Total ) [M2FeO4] + [MFeO4-] + [FeO42-] + [FeOOH] (8) [M]Total ) [M+]Free + [MFeO4-] + 2[M2FeO4]

(9)

[M]Total ) 2[M2FeO4]Total

(10)

In these studies, as the concentration of electrolyte, NaOH, is much higher than M2FeO4, we assume that the exchange reaction between Na+ and M+ is complete; therefore, eq 9 can be simplified as

[M]Total ≈ [M+]Free

(11)

Combined with eqs 8, 10, and 11, the concentration of possible electroactive species of MFeO4- and FeO42- can be expressed as eqs 12 and 13, respectively

Figure 2. Electrodeposition optimization on a smooth platinum electrode. Deposition conditions: applied potential, 110 mV versus Ag/ AgCl; deposition time, 10s; stirring rate, maximum, without disruptive turbulence: (a) the effect of NaOH concentration on charge storage in 50 mM K2FeO4 and (b), the effect of K2FeO4 concentration on charge storage in 8.0 M NaOH.

[FeO42-] ) [M2FeO4]Total 2[M2FeO4]Total 4[[M2FeO4]Total]2 1+ + + - 2 Kb KaKb [OH ] Kc

(12)

[MFeO4-] ) [M2FeO4]Total 2[M2FeO4]Total Kb KbKc + 1+ + 2[M2FeO4]Total Ka 2[M2FeO4]Total[OH ]

(13) Equations 12 and 13 indicate that whether MFeO4- or FeO42is the dominant species will depend on the total concentration of [M2FeO4] and [OH-]; however, the degree of dependence is quite different. Variation of [M2FeO4] or [OH-] would directly affect the morphology of Fe(III) film formation during electrochemical reduction, as a result, the charge storage and transfer properties of Fe(III/VI) thin-film could be altered. The solid state stability (stable to >99.9% year retention of the Fe(VI) valence state), and storage time, for K2FeO4 is much greater than for Na2FeO4, and hence it has been our chemical dissolution salt of choice.27 In this study, the effects on Fe(III/ VI) charge storage on smooth platinum were conducted by varying the concentrations of [K2FeO4] and [NaOH], the electrodeposition potential, and the magnetic stirring rate, respectively. Figure 2a presents the effect of the electrolyte, NaOH, concentration on charge storage in 50 mM K2FeO4. It is seen that the Fe(III/VI) charge storage capacity increased with increasing the concentration of NaOH from 1.0 to 8.0 M, while an increase beyond 8.0 M NaOH led to a decrease of Fe(III/ VI) charge storage. Increasing NaOH concentration (from 1.0 to 8.0 M) will inhibit the K2FeO4 solution phase decomposition process (eq 7), stabilizing the MFeO4- or FeO42- species (eqs 12 and 13), and as a result, the Fe(III/VI) charge storage is increased. In competition with this is the decrease in equivalent ionic conductivity of hydroxide with increasing concentration. For the NaOH electrolyte, this decrease is significant. For example at 18 °C the equivalent conduction of NOH solutions, decrease from 160 to 108 S cm2/equivalent, as concentration increases from 1 to 3 M, and the decrease is precipitous in more

Fe(III/VI) Cathodic Charge Transfer

Figure 3. FTIR diffuse reflectance spectrum (in absorbance mode) of (a) a Pt/Fe(III) electrode and (b) a Fe2O3 sample. The electrodepositional conditions of Pt/Fe(III) are the same as above with 50 mM K2FeO4 in 8.0 M NaOH. The Fe2O3 samples are freshly synthesized.

concentrated NaOH (decreasing to 69 S cm2/equivalent in 5 M NaOH). Consistent with the observed decrease in charge storage at 8 M NaOH, this decrease in mobility at higher concentrations appears to dominate over the stabilization enhancement. Therefore, in order to obtain a favorable charge storage, a compromise between decomposition and ion mobility needs to be considered. Figure 2b presents the concentration effect of K2FeO4 on charge storage in 8.0 M NaOH. It was found that the storage charge increased with increasing the concentration of K2FeO4 until a plateau was formed about 45 mM of K2FeO4, which approaches the saturation point of K2FeO4 in 8.0 M NaOH. In subsequent experiments in this study, 8.0 M of NaOH and 50.0 mM of K2FeO4 were used in the following experiments, except in noted special cases. In this concentrated alkaline environment, diffusion processes should be facilitated. Hence, variation of the (magnet bar) stirring rate was also probed, and generally, the higher the stirring rate, the greater the charge storage which can be obtained in the deposited Fe(III/VI) films; this is consistent with the expected compression of the diffusion layer with an increase in solution turbulence. Characterization of Fe(III) Thin-Film on Smooth Platinum. The 3-electron reduction of Fe(VI) can produce a variety of Fe(III) oxide and oxyhydroxide species such as (R, Fe2O3) and (R, γ, β, δ, FeOOH). Sodium, over potassium, species will dominate in the 8 M N+, 0.01 M K+ deposition solution, and a variety of cation-containing ferric salts such NaFeO2 are also possible in the film.28 Well-defined Fe2O3 particles gave three fundamental bands that ranged from 500 to 400 to 300 cm-1 respectively, and the bands shifted with varying size, shape, internal structure, and aggregation of particles.29-32 In our experiment, the surface morphologies of thin Fe(III) film on smooth platinum were examined by a Nicolet Nexus 670 Fourier transform infrared spectrophotometer. Figure 3a presents the FTIR diffuse reflectance spectra (in absorbance mode) of a 191 nm thin Fe(III) film which was freshly electrodeposited on a 1 cm2 platinum disk. A single peak at ∼429 cm-1, and a shoulder near 538 cm-1 are observed (farinfrared spectra less than 400 cm-1 were not examined due to the instrumental limitations). For comparison, nanocrystalline γ-Fe2O3 particles were synthesized according to the method developed by Massart et al.,32,33 and the FTIR absorption spectrum of this particle is shown in Figure 3b. Two adjacent peaks near 1500 cm-1 appear to be associated with residual free

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Figure 4. Discharge behavior at the 20th cycle of a 191 nm Fe(VI) film prepared on a Pt electrode with various deposition conditions. Films are deposited at a constant current of 10 mA for 10s from electrolytes containing various [K2FeO4] in 8.0 M (a) or 12.0 M NaOH (b). Subsequent nonaqueous galvanostatic cycling consists of charge at 0.02 mA cm-2 to 100% of the intrinsic 3e- Fe capacity, as limited by a maximum cut0ff voltage of 4.2 V, followed by discharge at 0.01 mA cm-2 to 90% DOD of this capacity as limited by a 1.5 V minimum voltage cutoff.

hydroxide.32,33 The strong similarity between our Fe(VI) electrodeposited film Fe(III) and the nanocrystalline γ-Fe2O3 particles indicates this as a principal component of the Fe(III) film. In ongoing investigations, we continue to probe the identity of the Fe(III) centers in the reduced form of the film Fe(III/VI) films. Fe(III/VI) Thin-Film Electrodeposition and Charge/ Discharge Properties. The Fe(III/VI) films were placed in a lithium cell with 1 M LiPF6 in PC:DME (1:1) electrolyte, and their quasi-reversibility was characterized as a function of hydroxide and Fe(VI) concentrations in the deposition solution. Specifically, a 191 nm Fe(III/VI) on smooth platinum was galvanostically deposited (at 10 mA, for 10s) in either 10, 50, or 80 mM K2FeO4. The film was placed in a lithium cell with 1 M LiPF6 in PC:DME (1:1) electrolyte and cycled through periodic galvanostatic charge/discharge cycles. Specifically, each cell was repeatedly subject to a 0.02 mA cm-2 charge, followed by a deep 0.01 mA cm-2 discharge. Figure 4 reports the voltage during the 20th discharge as a function of the intrinsic (100 mC, 3 electron) depth of discharge of these films. Figure 4, panels a and b, respectively present discharge cycle behavior for films prepared from either 8.0 or 12.0 M deposition solutions. It is evident in Figure 4 that the average discharge voltage and the depth of discharge are significantly higher for films deposited in 8 M, rather than 12 M, NaOH. Furthermore, in the preferred 8 M NaOH deposition solution, the average discharge voltage, and the depth of discharge are significantly higher for films deposited from 50 mM, rather than 10 or 80 mM K2FeO4 solutions. It is evident that the charge storage and transfer behavior of Fe(III/VI) thinfilm cells are significantly influenced by the electrochemical deposition conditions. In the 50.0 mM K2FeO4, 8.0 M of NaOH prepared film, the 20th discharge cycle commenced at 4.1 V and decayed to 3.1 V through an 80% depth of discharge, and no sharp decline of potential was observed within 20 cycles. This was substantially different than the discharge behavior of the Fe(III/VI) film prepared from the other deposition solutions, in which a high discharge voltage was only evident in the initial portion of the discharge, after which the potentials began to

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Figure 5. Electrochemical behavior of a 57-nm thin-film on a Pt electrode: (a) DOD (depth of discharge) % and SOC (state of charge) % vs cycle number and (b) typical potential profile of 57 nm electrodes upon galvanostatic cycling (current cycling conditions are as described in the Figure 4 legend).

Figure 6. Electrochemical behavior of a 191-nm thin-film on a Pt electrode: (a) DOD % and SOC % vs cycle number and (b) typical potential profile upon galvanostatic cycling (current cycling conditions are as described in the Figure 4 legend, but to an 80%, rather than 90% DOD, to retain a high voltage throughout the discharge).

sharply decline, and only 10% to 50% of DOD was obtained by the 20th galvanostatic discharge cycle. Fe(III/VI) Thin Film: Charge Transfer at Smooth Platinum. The charge/discharge cycling performances of 57 and 191 nm Fe(III/VI) thin film cells are presented in Figures 5a and 6a, respectively. The corresponding potential profiles of these thin film cells upon galvanostatic cycling are presented in Figures 5b and 6b. Specifically, each film cell was repeatedly subject to a 0.02 mA cm-2 galvanostatic charge, followed by a deep 0.01 mA cm-2 galvanostatic discharge (to 80% or 90%, as indicated in the figure legend, of the Fe(III/VI) film intrinsic 3e-capacity). As evident in Figures 5a and 6a, both 57 and 191 nm Fe(III/ VI) thin-film cells were able to sustain three-electron reversibility with almost no capacity fading during prolonged cycling. The 57 nm Fe(III/VI) film cell exhibited reversible behavior throughout more than 90 galvanostatic charge/discharge cycles with over 90% DOD. No deleterious effect was noted with cells overcharged to 120%, rather than 100% capacity (not shown in figure). During discharge of this cell, as see in Figure 5b, the voltage rapidly dropped from 4.0 to 3.65 V, followed by a relatively level potential (dropping from 3.65 to 3.5 V) throughout the remainder of the discharge. A thicker 191 nm Fe(III/VI) film cell also evidenced sustained charge-transfer reversibility at the same cathodic current density throughout

Licht et al.

Figure 7. Extended cycling charge-transfer behavior for a 191-nm Fe(III/VI) film on a platinized Pt electrode. (a) Electrode film potential during galvanostatic charge and (b) electrode film potential during galvanostatic discharge. Charge/discharge cycle numbers are indicated on the figure cycling (current cycling conditions are as described in the Figure 4 legend, but to an 83% DOD).

more than 300 galvanostatic charge/discharge cycles. Compared with the 57 nm thin films, the DOD of the 191 nm thin-film cells was lower, but remained high (∼80% deph of discharge) during cycling, and the polarization losses were higher, with the discharge potential dropping from 3.3 to 2.8 V during the discharge. In Figure 6a no capacity decay is observed during the course of over 300 charge/discharge cycles. A switching polarization is evident as a potential difference between the conclusion of the charging voltage and the onset of the discharge voltage in Figure 5b for the cycled 57 nm film. This difference is exacerbated with increase of film thickness, as evident for the 191 nm film in Figure 6b. This potential is related to the electronic transport, and contact, between the superiron film and the platinum current collector, as it is nearly eliminated with platinization (platinum black) coating on the smooth platinum prior to film deposition. Figure 7a summarizes the charge, and depth of discharge, behavior during repeated charge/discharge cycles of cells containing 191 nm Fe(III/VI) films deposited on a platinized platinum disk (films on highly platinized platinum are delineated in a latter section). The cell was charged to 100% of its theoretical capacity at a constant current density of 0.02 mA cm-2. As evident in Figure 7a, a sloping potential profile was observed, increasing from 3.1 to 4.1 V during the charging process. During the initial stage of charging, this charging potential increased only marginally (from 3.06 to 3.19 V) in the course of 300 charge/discharge cycles, also reaching a similar maximum end-point potential of ∼4.2 V throughout the 300 cycles. These results suggest a consistent discharge mechanism for each of these films throughout this high capacity anodic charge storage process. Figure 7b presents the discharge behavior in repeated charge/discharge cycles for the 191 nm Fe(III/VI) film. The film was discharged to 80% of its theoretical capacity at a constant current density of 0.01 mA cm-2. As evident in Figure 7b, there is a decreasing potential between 4 and 3 V during the cathodic process. The initial discharge potential is nearly the same during the first and 300th cycle (4.18 V compared to 4.19 V), and subsequent polarization losses were similar observed during these cycles. It is evident in Figure 7 that potential variations during both the charge and discharge processes are smooth; that is, there are no evident demarcations between the first, second, or third electron equivalents of oxidation or reduction during the film charge or discharge. In similar discharge conditions, Koltypin34

Fe(III/VI) Cathodic Charge Transfer

Figure 8. Self-discharge and charge retention measurement of a 191 nm Fe (VI/III) film after 7 days storage at 25, 40, 45, 50, or 60 °C (discharge conditions are as described in the Figure 7 legend).

found that the discharge capacity can be slightly higher than the charge capacity due to the reduction of electrolyte (propylene carbonate) when very low discharge potentials are permitted (1.5 V vs Li/Li+). In our experiments, the cutoff potential was normally over 2.0 V, and hence no evidence of electrolyte (PC) reduction was observed. Fe(III/VI) Thin Film: Charge Retention. These measurements address whether the film will undergo self-discharge when stored under open circuit conditions. Retention of stored charge at 22 °C over time was probed for the 191 nm Fe(III/VI) film cell at room temperature, and also at 40, 45, 50, or 60 °C. At room temperature, after 7 days storage, the average remained constant at ∼4.1 V both before and after storage, whereas the average OCV decreased to 4.02, 3.72, 3.62, and 3.45 V at 40, 45, 50, and 60 °C, respectively. In the next study it will be worthwhile to explore and distinguish whether this voltage decrease is due to electrolyte (decomposition) or cathode changes. Discharge potentials throughout the 80% depth of discharge are similar at room temperature, both before (Figure 7) and after (Figure 8) seven days of storage. As shown in Figure 8, the discharge capacity was still able to attain about 80% after 7 days storage at all the various storage temperatures. However, as with the OCV, at higher storage temperatures, the discharge potentials are lower following the storage period, and polarization losses increase with increasing storage temperature. Yet, even following a 50 °C storage, the final discharge potential at 80% DOD is greater than 3.0 V, although a distinct voltage plateau is observed in the discharge potential region between 3 and 3.2 V. High DOD, coupled with significant polarization, are consistent with either a significant, but not fully passivating, surface layer, or a decrease in electrolyte conductivity, which could occur during storage at higher temperatures. Passivation of Fe(III/VI) Charge Transfer on Smooth Platinum. The formation of passivating, irreversible Fe(III) centers is more likely in the case of thicker films Fe(III/VI), whereas thinner, electrodeposited films retain smaller grain size and improved, sustained Fe(III/VI) charge transfer.26 Charge/discharge cycling performance of a 573 nm Fe(III/ VI) thin film on smooth platinum is presented in Figure 9a. The corresponding potential profile of the first five galvanostatic cycles is presented in Figure 9b. Specifically, the film was repeatedly subject to a 0.02 mA cm-2 charge, followed by a deep 0.02 mA cm-2 discharge (to 80% of the Fe(III/VI) film theoretical, intrinsic 3e-capacity or to a 1.5 V discharge minimum). As evident in comparing Figures 5, 6, and 9, the

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Figure 9. Electrochemical behavior of a 573-nm thin-film on a Pt electrode: (a) DOD % and SOC % vs cycle number and (b) typical potential profile of 573 nm electrodes upon galvanostatic cycling. The cycling consists of charge at 0.02 mA cm-2, followed by discharge at 0.02 mA cm-2.

thinner 57 and 191 nm Fe(III/VI), both exhibit quasi-reversible behavior throughout repeated galvanostatic charge/discharge cycles, while the 573 nm film rapidly passivated (within the first cycle). In the initial cycle, at the start of this thicker film discharge, the discharge potential rapidly falls to less than 3 V, and by the third cycle, the discharge potential always remains below 3 V. After the first cycle, a minimum discharge voltage cutoff of 1.5 V severely limits the depth of discharge. While DOD is over 90% in the first cycle, It is only ∼50((5)% in cycles 7 through 25. These thick film limitations are alleviated, with facilitated thin film superiron charge transfer as a result of an expanded conductive matrix, as will be demonstrated and discussed in subsequent sections. Extension of Platinum Surface Morphology. The effective surface area of a variety of chemically or electrochemically treated electrodes was evaluated by integrating the area under the hydrogen desorption curve. A smooth platinum surface (prepared using the mechanical and chemical polishing techniques described in the Experimental Section) has an effective surface area of 2.5. A further, marginal increase of 8% in normalized electroactivity to 2.7 was observed for the electrochemically treated surface. We have additionally studied Pt electrodes with various degrees of Pt loading (electrodeposited platinum). As expected,26 a several order of magnitude increase in effective surface area can occur with platinization, with a substantial increase in observed current densities compared to that occurring at a smooth platinum surface. The effective surface area of (platinized) platinum increases with the degree of platinization. Approximately, a 1000-fold increase in electroactivity was obtained with a 10.4 mg cm-2 Pt deposit on Pt. A linear trend for the variation of this electroactivity with the amount of Pt deposited was observed in accordance with

normalized electroactivity ) 96 × (Pt loading, mg cm-2) + 2.7 (14) Platinization effects reported in the literature for the normalized hydrogen electroactivity on platinized platinum surfaces vary over a wide range.35-38 Greater levels of platinization lead to greater surface activity, but beyond a certain level these films will exhibit diminished mechanical and structural integrity. Discrepancies in normalized electroactivity can be attributed to surface morphologies obtained under different experimental conditions, as well as differences in the methodology of estimation. In this study, a surface enhancement of 250 at 2.6

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J. Phys. Chem. C, Vol. 113, No. 22, 2009

Figure 10. Electrochemical behavior of a 573-nm thin-film on a platinized Pt electrode: (a) DOD % and SOC % vs cycle number and (b) typical potential profile of 573 nm electrodes upon galvanostatic cycling (current cycling conditions are as described in the Figure 9 legend).

mg cm-2 of Pt load was measured through the relative area under the hydrogen desorption curve. Conductive Matrix: Platinized Platinum: Facilitated Fe(III/ VI) Charge Transfer. A substantial improvement in thicker film, nonaqueous charge transfer is observed when an extended conductive matrix was utilized as the film substrate. Figure 10a summarizes the depth of discharge in repeated charge/discharge cycles of a 573 nm superiron film formed at a platinum electrode loaded (platinized) with 2.6 mg cm-2 of Pt. As opposed to the results of the thick film deposited on smooth Pt in Figure 9, Figure 10 evidences significantly higher quasi-reversibility (around 55 discharge cycles), without the onset of significant passivation. Furthermore, compared to the 57 and 191 nm thickness films on smooth platinum presented in Figures 5 and 6, this film was able to sustain a higher discharge current (0.02 compared to 0.01 mA cm-2). As summarized in Figure 10b, the charging potential increases from 3 to 3.7 V during the first cycle anodic processes, and the discharge drops from 3.3 to 2 V during 80% depth of discharge (to 80% of the intrinsic 3e- cathodic capacity). As seen in Figure 10b, over the measured 55 cycles, the super-iron films retained and sustained these deep discharges with little observed variation in the charge/discharge profile. Also as seen in the figure, discharge capacity was lost in cycle 56, evidencing partial separation of the Fe(III/VI) film. Nevertheless, this study provides fundamental evidence that an appropriate Fe(III/VI) conductive lattice can significantly facilitate quasi-reversible charge transfer. Further studies to understand, and circumvent, the polarization of the Fe(III) state will lead to further gains in the practical, rechargeable superiron cathode thickness and surface area normalzed cathode capacity. Conclusions High-capacity Fe(III/VI) superiron films were electrochemically deposited on either smooth or platinized platinum by electrochemical reduction from solutions containing low concentrations of K2FeO4 in concentrated NaOH, to form a quasireversible Na2FeO4 films in nonaqueous media with an intrinsic 3e- cathode storage of 485 mAh g-1. The influence of the electrolyte concentration of NaOH, solute K2FeO4, as well as electrochemical conditions, such as electrodeposition potential and stirring rate on charge storage and transfer were studied during galvanostatic charge cycling. Results show that the charge storage and transfer behavior of Fe(III/VI) thin-film cells is

Licht et al. significantly influenced by the electrochemical deposition conditions and the thickness of the deposited film. In a lithium cell, with a 1 M LiPF6 PC:DME electrolyte, thin (