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Electrochemistry Communications 9 (2007) 371–377

Preliminary investigation on the physicochemical propertiesof calcium ferrate(VI)

Zhihua Xu a,b, Jianming Wang a,*, Haibo Shao a, Zheng Tang a, Jianqing Zhang a,c

a Department of Chemistry, Zhejiang University, Hangzhou 310027, PR Chinab College of Chemistry and Applied Chemistry, Huanggang Normal University, Huanggang 438000, PR China

c Chinese State Key Laboratory for Corrosion and Protection, Shenyang 110015, PR China

Received 24 July 2006; received in revised form 6 September 2006; accepted 19 September 2006Available online 1 November 2006

Abstract

Calcium ferrate(VI) powders were synthesized from potassium ferrate(VI), and characterized by titration analysis, elemental analyzer,SEM, XRD, IR, TG and DSC. The results showed that the synthesized sample mainly consists of calcium ferrate(VI), and calcium fer-rate(VI) may exist as CaFeO4 Æ 2H2O with a highest obtained purity of 74.9%. The relatively higher Fe(III) impurity and crystalloid watermight be responsible for the poor stability of the calcium ferrate(VI) sample. The results of galvanostatic discharge experiments indicatedthat the calcium ferrate (VI) sample displays better intrinsic rate discharge capability and larger discharge capacity at lower temperatures(615 �C).� 2006 Elsevier B.V. All rights reserved.

Keywords: Calcium ferrate(VI); Purity; Stability; Discharge performance

1. Introduction

Ferrate(VI) is the high-oxidation-state compound ofiron which possesses a strong oxidizing power, relativelyhigh redox potential and environmentally friendly reduc-tion product. Due to its advantages, ferrate(VI) has beenused as an alternative to common oxidants, such as toxicchlorine and chromate, etc. in organic synthesis and wastewater treatment [1,2]. Recently, the ‘super-iron’ batterieshave been reported to have a higher capacity and energyadvantage compared to conventional alkaline batteries[3–6]. As summarized in Eq. (1), ferrate(VI) salts undergoa three-electron reduction at relatively higher potentials.

FeO2�4 þ 5=2H2Oþ 3e� ! 1=2Fe2O3 þ 5OH�

E ¼ 0:55 � 0:75 Vðvs: SHEÞ ½7�ð1Þ

1388-2481/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.elecom.2006.09.015

* Corresponding author. Tel.: +86 571 87951513; fax: +86 57187951895.

E-mail address: wjm@cmsce.zju.edu.cn (J. Wang).

In order to improve the electrochemical charge transferand storage of the ferrate(VI) cathode, Licht and co-work-ers have studied the effects of electrode additives [8,9] andnonaqueous electrolyte [10] on ferrate(VI) cathodes. Theyhave also investigated Mn and Ag mediation of ferrate(VI)cathode to facilitate ferrate(VI) charge transfer, and theactivation of ferrate(VI) salt, K2FeO4, yielded a substantialimprovement in storage capacity [11,12]. Within the pastseveral years an increasing number of investigations inthe detailed preparation, characterization and physico-chemical performance of some ferrate(VI) salts, such asSrFeO4, BaFeO4, K2Sr(FeO4)2, Na2FeO4, Rb2FeO4 andCs2FeO4, have appeared in the literatures [4–6,13–16],and the products of these preparations were investigatedto provide a high-energy electrochemical discharge in theferrate(VI) batteries. Especially, studies show that BaFeO4

has good discharge performance at higher current com-pared to K2FeO4 cathode [4–6]. But the instability andenvironmental impact restrict BaFeO4 to be used as cath-ode in a large scale [17]. Recently, Ag2FeO4, with unusual5 electron intrinsic capacity was also presented, however,

372 Z. Xu et al. / Electrochemistry Communications 9 (2007) 371–377

the impurity and instability impair it as a promising super-iron battery cathode [18]. The attempts to seek alternativeferrate(VI) salts with high intrinsic storage capacity, suchas CaFeO4 and MgFeO4, etc. will be of encouraging.

According to the method in Ref. [3], the theoreticalthree-electron discharge capacity of CaFeO4 can reach ashigh as 503 mAh/g. Prior earlier reports of calcium fer-rate(VI) synthesis, such as by exchange reactions betweenbarium ferrate or potassium ferrate [19], have been difficultto reproduce [20]. In this paper, calcium ferrate(VI) is syn-thesized and characterized, and the investigation of thismaterial as a ferrate(VI) cathode is reported for the firsttime.

2. Experimental

2.1. Calcium ferrate(VI) synthesis

In the synthesis of calcium ferrate(VI), 100 ml chilledsaturated Ca(NO3)2 and Ca(OH)2 solution was filteredand cooled to near 0 �C (using ice bath). 1.5 g K2FeO4,which was prepared through the ex situ electrosynthesis[21] and had a purity of 98.7%, was added into the cooledsolution. With continuous stirring, about 2 g CaCl2 wasadded to the above solution. After 15 min of stirring, thesolution was chilled to �5 to �3 �C for about 2–3 h. Thenthe solution with the purple precipitate was filtered by asintered glass filter and washed sequentially with the fol-lowing organic solvents for removing adsorbent waterand other salts: two times of cyclohexane, seven times ofisopropanol, and again two times of cyclohexane, andfinally, three times of diethylether. The resultant purpleproduct, calcium ferrate(VI), was dried for about 7 h in avacuum desiccator containing P2O5 as desiccant (at 2–3mbar) at 15 �C. After dried, the calcium ferrate(VI) powderwas kept in a desiccator using P2O5 as desiccant.

2.2. Calcium ferrate(VI) characterization

The maximal purity of CaFeO4 determined by modifiedchromite method [7,22] was 61.1%. Total Fe content wasdetermined by the Zimmermann–Reinhardt method [23],using the oxidation/reduction titrations with potassiumpermanganate determination, and the detailed analyticalprocedure was previously described in Ref. [24]. Theamount of Ca2+ was determined by a EDTA titrationmethod [25], and K+ was determined by potassium tetra-pheylboron weight method [26].

The contents of H, C and N elements in the sampleswere determined using an elemental analyzer (Carlo Erba1110). Thermogravimetric (TG) analysis and DSC werecarried out using NETZSCHSTA 409 PG/PC thermal ana-lyzer heating from 30 to 650 �C (10 �C/min) in the N2

atmosphere. FTIR spectrum was measured by a NicoletNexus 670 Fourier transform infrared spectrophotometerin a conventional KBr pellet, as described in Ref. [7], usingBaSO4 as a Fe(VI) FTIR standard. XRD measurement was

performed using Rigaku D/Max 2550 X-ray diffractome-ter, with Cu Ka radiation at 40 kV and 300 mA, scan rate8� (2h)/min. The morphologies of samples were observedusing a SEM with a SIRION microscopy (FEI, USA).

Electrochemical tests were carried out in a three-elec-trode glass cell. Ferrate(VI) electrode was prepared as fol-lows: 90 mg potassium ferrate(VI) or 70 mg calciumferrate(VI) (the mass of the corresponding sample is70 mg/purity), 16 wt% acetylene black and one or twodrops of saturated KOH solution were mixed thoroughly,then the mixture was incorporated into two pieces of foamnickel (2 cm · 2 cm) uniformly, which could maximize thesurface area of ferrate(VI) cathode to improve the chargetransfer. And then the two pieces of foam nickel containingferrate(VI) salts were pressed together at the pressure of8 MPa. The counter electrode was a platinum foil andthe reference electrode a Hg/HgO electrode. The dischargeexperiments were carried out using an Arbin BT-2000 sys-tem. It should be noted that the specific discharge capaci-ties of the two ferrate(VI) salts were calculated accordingto the weights of pure K2FeO4 and CaFeO4, respectively.

All reagents used were analytical grade and the solutionwas prepared with deionized water. The purities of calciumferrate(VI) and potassium ferrate(VI) used were 58.4% and98.2%, respectively, except for special statement.

3. Results and discussion

3.1. Composition and structure of calcium ferrate(VI)

The morphologies of calcium ferrate(VI) and electrosyn-thesized K2FeO4 are shown in Fig. 1. Just like K2FeO4

powders (Fig. 1b) [21], calcium ferrate(VI) powders(Fig. 1a) also display thin flake shape. However, it can beseen that CaFeO4 has much smaller particle size and worsecrystallinity than K2FeO4.

Fig. 2 compares the X-ray powder diffraction (XRD)spectra for calcium ferrate(VI) with different purities andK2FeO4. It can be found from Fig. 2 that the characteristicpeaks of the two ferrates are completely different. Theobtained XRD spectrum of K2FeO4 is in good agreementwith the previously results [21,27], which is an orthorhom-bic crystal system (Pnma). The featured diffraction peaks ofK2FeO4 and some possible compounds such as Ca(NO3)2,iron oxide and CaFeO3, etc. can not be evidently found inthe XRD spectrum of calcium ferrate(VI). The XRD pat-tern of the calcium ferrate(VI) with lower purity containsthe same peaks as that of the sample with higher purity,but with a lower intensity. However, the XRD spectrumof calcium ferrate(VI) decomposed in the dessicator atroom temperature displays no characteristic absorptionpeaks. This indicates that the reduction products of cal-cium ferrate(VI) and the impurities are amorphous, whichmay be the reason why the featured diffraction peaks of thepresumably impurities such as Ca(NO3)2 and iron oxide,etc. could not be obviously observed in the XRD spectrum.That is to say, the featured absorption peaks in the XRD

Fig. 1. SEM photos of (a) CaFeO4 and (b) K2FeO4.

Z. Xu et al. / Electrochemistry Communications 9 (2007) 371–377 373

spectrum should originate from the characteristic of cal-cium ferrate(VI). Certainly, at present the structural fea-tures of calcium ferrate(VI) based on its XRD spectra arenot clear, which will be further clarified in later work.

Fig. 3 displays the infrared spectra for calcium fer-rate(VI) and potassium ferrate(VI). It can be seen thatthe two FTIR spectra are readily distinguishable. The threepeaks between 1250 and 1000 cm�1 in the two FTIR spec-tra result from the adsorption of BaSO4 standard. TheFTIR spectrum of K2FeO4 is similar to those reported byother authors [13]. A main peak about 807 cm�1 and ashoulder peak about 781 cm�1 in the K2FeO4 spectrumare attributed to the stretching frequencies of four equiva-lent, symmetric Fe–O bonds. The infrared spectrum of cal-cium ferrate(VI) has not been previously reported. Thespectrum shows a strong and broad absorption band cen-tered around 3500 cm�1 as well as two strong absorptionbands at about 1657 and 1623 cm�1. These bands areattributed to the stretching and bending vibrations ofwater. The water might be crystalloid or adsorbent water.The band at 1386 cm�1 may be due to the absorption ofNO�3 , which may be introduced into calcium ferrate(VI)during the preparation. The strong absorption peak at800 cm�1 and two slight shoulder peaks between 880 and

10 20 30 40 50 60 70 80

CaFeO4 decomposition

CaFeO4 Purity = 29%

CaFeO4 Purity = 58.4%

K2FeO

4

inte

nsit

y

2θ/deg.

Fig. 2. X-ray powder diffraction spectra of CaFeO4 and K2FeO4.

810 cm�1 are expected to the characteristic peaks ofFeO2�

4 , which will be confirmed by the latter results. It isnoted that the characteristic peaks of calcium ferrate(VI)have similar shapes with those of BaFeO4 [15], althoughthe positions and relative intensities of the peaks are some-what different.

Fig. 4 presents the quantitative FTIR analyses of cal-cium ferrate(VI) with different treatments [7,15,18] andK2FeO4 with various amounts. The two shoulder peaksbetween 880 and 810 cm�1 can be clearly observed in theFTIR spectrum of calcium ferrate(VI) sample (purity51.2%) with a relatively narrow wavenumber range (400–1600 cm�1). The absorption peaks of calcium ferrate(VI)and K2FeO4 between 650 and 950 cm�1 decrease with thedecrease in the amount of the FeO2�

4 species, comparedwith the normalized absorption of BaSO4 at 1080 cm�1.It can be seen from Fig. 4a that after the prepared calciumferrate(VI) sample has been stored for 5 or 9 days in a des-iccator at 25 �C, the relatively intensities of the peak about800 cm�1 and the two shoulder peaks between 880 and810 cm�1 obviously reduce, which may result from thedecease in the purities of the samples due to decomposition.

4000 3500 3000 2500 2000 1500 1000 500

1657

cm

-1

1623

cm

-1 1386

cm

-1

780

cm-1

807

cm-1Calium ferrate (VI)

Abs

orpt

ion

Wavenumbers / cm-1

Potassium ferrate (VI)

800

cm-1

Fig. 3. FTIR spectra for calcium ferrate(VI) and K2FeO4. The absorptionpeaks (1050–1150 cm�1 and 770–580 cm�1) of BaSO4 used as a Fe(VI)FTIR standard were measured for comparison.

1600 1400 1200 1000 800 600 4000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6a) 10 mg BaSO4

e) decomposition product of (c) in the infrared lamp

d) 10 mg BaSO4 + 19 mg CaFeO

4sample ( after stored 9 days

at 25 °C; purity 18.9%)

c) 10 mg BaSO4 + 19 mg CaFeO

4sample( stored for 5 days at 25 °C; purity 34.3%)

a

b) 10 mg BaSO4 + 19 mg fresh CaFeO

4( purity 51.2%)

e

d

cb

Abs

orba

nce

Abs

orba

nce

Wavenumber / cm-1

Wavenumber / cm-1

1600 1400 1200 1000 800 600 4000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

g) 10 mg BaSO4 + 6.5 mg pure K2FeO

4

f) 10 mg BaSO4 + 3.6 mg pure K2FeO

4

a) 10 mg BaSO4

h) 10 mg BaSO4 + 9.7 mg pure K2FeO

4

hg

f

a

a

b

Fig. 4. FTIR spectra of calcium ferrate(VI) with different treatments (a)and the corresponding K2FeO4 (b).

Table 1The elemental composition of calcium ferrate(VI) sample with a purity of58.4%

Determined by titration analysis (molarratio)

Determined byelemental analyzer(molar ratio)

Fe(VI) Total Fe Ca2+ K+ H N C

1 1.10 1.17 0.08 8.20 0.35 0

50 100 150 200 250 300 350 4006065707580

859095

100

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

DSC

/ uV

/mg

TG

/ %

80.4 °C

50 100 150 200 250 300 350 400

80

85

90

95

100

-0.5

0.0

0.5

1.0

1.5

TG

/ %

Temperature / °C

Temperature / °C

294.7 C °

DSC

/ uV

/mg

a

b

Fig. 5. TG and DSC curves of (a) calcium ferrate(VI) and (b) potassiumferrate(VI).

374 Z. Xu et al. / Electrochemistry Communications 9 (2007) 371–377

By contrast, Fig. 4b shows that as the amount of K2FeO4

in the KBr pellet decreases, the intensities of the main peakabout 807 cm�1 and the shoulder peak about 781 cm�1

lower, which is consistent with the variation trend of thepeak about 800 cm�1 and the two shoulder peaks between880 and 810 cm�1 in Fig. 4a. After the calcium ferrate(VI)sample has been heated in the infrared lamp in the air (thecolor of the sample turns from purple to brown), the peakat 800 cm�1 and the two shoulder peaks between 880 and810 cm�1 disappear, and a new absorption about870 cm�1 which might originate from the decompositionproducts occurs. It can be concluded from the above resultsthat the peak at 800 cm�1 and the two shoulder peaksbetween 880 and 810 cm�1 in Figs. 3 and 4 may be thecharacteristic absorption peaks of calcium ferrate(VI).

Table 1 lists the molar ratio of the elements determinedby titration analysis and Elemental Analyser. The molarratio of potassium and calcium in the product is found tobe less than 7%, which indicates that the conversion from

potassium iron to calcium iron salt is almost complete.The molar ratios of calcium and total Fe relative to Fe(VI)is 1.17 and 1.10, respectively, which are greater than theexpected molar ratio of 1:1. Considering the molar ratioof Fe(VI), calcium and nitrogen being 1:1.17:0.35, it is rea-sonable to believe that the impurity of calcium is calciumnitrate, consistent with the IR result. The excess iron mightbe lower valence iron compounds such as Fe2O3 orFe(OH)3, etc. [18]. The reasons why the above impuritiesare not found in the XRD spectrum of the fresh calciumferrate sample may be their relative lower contents andamorphous state. Table 1 also shows that the molar ratioof H to Fe(VI) is 8.20:1, much higher than the amount ofthe reasonable adsorbent water.

Fig. 5 depicts the TG and DSC curves of calcium fer-rate(VI) and potassium ferrate(VI). It can be observed thatthe decomposition of calcium ferrate(VI) occurs at about80.4 �C, much lower than those of potassium ferrate(VI)(294.7 �C) and BaFeO4 (254.8 �C) [17]. This implies thatthe calcium ferrate(VI) sample has poor thermal stability,which might result from its structural feature and certainamount of crystalloid water. The TG curve in Fig. 5ashows that the loss of weight at 120 and 400 �C is about24% and 33%, respectively. According to the results oftitration analysis, element determination analysis, XRDand IR spectra, calcium ferrate(VI) may exist as

Z. Xu et al. / Electrochemistry Communications 9 (2007) 371–377 375

CaFeO4 Æ xH2O. When the heating temperature is higherthan the decomposition temperature, reaction (2) occurs,and reaction (3) appears with further increasing heatingtemperature. If x is assumed to be 2, the losses of weightaccording to reaction (2) and both reactions (2) and (3)are 21.4% and 30.7%, respectively. Considering certainamount of adsorbent water, the calculation results maybe thought to be consistent with the experimental resultsobtained from the TG curve. Therefore, calcium fer-rate(VI) may exist as CaFeO4 Æ 2H2O, and its correspond-ing maximal obtained purity is 74.9%. It should benoticed that the gradual increase of the weight loss between100 and 400 �C may mainly be due to the occurrence ofreaction (3).

2CaFeO4 � xH2O! Fe2O3 þ 1:5O2 þ 2CaðOHÞ2þ ð2x� 2ÞH2O ð2Þ

CaðOHÞ2 ! CaOþH2O ð3Þ

3.2. Stability and electrochemical performance of calcium

ferrate(VI)

Fig. 6 shows the stability of calcium ferrate(VI) sampleswith various purities at different temperatures. It can beseen that the purity of calcium ferrate(VI) sample graduallydecreases with the prolongation of conserved time at all thetemperatures tested, and the decomposition rate increaseswith the rise of temperature. It can be calculated fromthe data in Fig. 6 that the average decomposition rates ofthe samples with 58.4% and 40.2% purities at 0 �C are1.52%/day and 2.39%/day, respectively, and the corre-sponding data at 25 �C are 3.03%/day and 3.81%/day. Thisindicates that the CaFeO4 sample with higher purity hasrelatively better stability. It was also found during theexperiments that the calcium ferrate(VI) samples with dif-ferent purities completely turned from purple to brown

0 1 2 3 4 5 6 720

30

40

50

60

70

Purity = 40.2% 0 ° C 25 °C

CaF

eO4

puri

ty b

y ch

rom

ite a

naly

sis

%

Time / days

Purity = 58.4% 0 ° C 15 ° C 25 ° C

Fig. 6. Stability of solid CaFeO4 samples with various purities at differenttemperatures.

overnight at 55 �C in the dessicator (not shown inFig. 6). The poor stability of calcium ferrate(VI) samplemay originate from its relatively higher Fe (III) impurity[18], crystalloid and/or absorbent water, and the strongerpolarization effect of Ca2+ ion on FeO2�

4 [28].The discharge curves of calcium ferrate(VI) samples

with different purities are presented in Fig. 7. It can be seenthat the electrochemical performance calcium ferrate(VI)samples enhances with the increase of its purity, and thecalcium ferrate(VI) with 58.4% purity displays much largerdischarge capacity and much higher discharge potentialthan the two samples with relatively low purities. TheCaFeO4 sample with low purity has higher content of Fe(III) impurity with very high resistivity [16], hence its utili-zation efficiency can be greatly reduced due to worse elec-tronic conductivity. This may be mainly responsible forthe deterioration in the electrochemical performance ofthe CaFeO4 sample with relatively low purity. In addition,larger decomposition rates of the samples with low puritiesin the electrolyte are also detrimental to its electrochemicalperformance. It should be noticed that the results in Figs. 6and 7 imply the stability and electrochemical performanceof CaFeO4 can be greatly enhanced by improving itspurity.

Fig. 8 displays the discharge curves of calcium fer-rate(VI) and potassium ferrate(VI) electrodes at variouscurrents at 15 �C. For the same ferrate(VI) sample, theelectrode presents lower discharge potential and smallerdischarge capacity at larger discharge current, which ismainly due to the rise of electrode polarization withincreasing discharge current. The calcium ferrate(VI) elec-trode displays smaller discharge capacity than potassiumferrate(VI) electrode at smaller discharge current (31 mA/g). However, it presents not only much higher dischargepotential but also much larger discharge capacity thanK2FeO4 electrode at relatively larger discharge current

0 50 100 150 200 250 300 350

-0.4

-0.2

0.0

0.2

0.4

0.6

E (

V v

s.H

gO/H

g)

Discharge capacity (mAh/g)

Purity=29% Purity=37% Purity=58.4%

Fig. 7. Discharge curves of CaFeO4 with different purities at a rate of100 mA/g at 15 �C.

0 50 100 150 200 250 300 350

-0.4

-0.2

0.0

0.2

0.4

0.6K

2FeO

4

31 mA/g 100 mA/g

CaFeO4

31 mA/g 100 mA/g

E (

V v

s.H

gO/H

g)

Discharge capacity (mAh/g)

Fig. 8. Discharge curves of CaFeO4 and K2FeO4 at 15 �C at differentdischarge currents.

376 Z. Xu et al. / Electrochemistry Communications 9 (2007) 371–377

(100 mA/g). This indicates calcium ferrate(VI), just likeBaFeO4, has much better intrinsic rate discharge capabil-ity, although a relatively larger amount of Fe (III) impurityexists in the sample. It should be noted that K2FeO4 with asmall AgO additive exhibits high current discharge perfor-mance comparable to that of BaFeO4 [12]. From anotherpoint of view, larger discharge current means less dischargeduration, and this may result in less calcium ferrate(VI)loss due to decomposition in the electrolyte, which mighthave certain contributions to better discharge performanceof calcium ferrate(VI) electrode at relatively larger dis-charge current.

The discharge curves of calcium ferrate(VI) and potas-sium ferrate(VI) at a rate of 100 mA/g at various tempera-tures are illustrated in Fig. 9. It can be seen that thetemperature characteristic of calcium ferrate(VI) electrode

0 50 100 150 200 250 300 350 400

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

E (

V v

s.H

gO/H

g)

Discharge capacity (mAh/g)

5 °C 15 °C 25 °C

a

Fig. 9. Discharge curves of (a) CaFeO4 and (b) K

is greatly different from that of potassium ferrate(VI) elec-trode. The K2FeO4 electrode presents more positive dis-charge potentials and larger discharge capacities at highertemperatures. Whereas both the discharge potential anddischarge capacity of calcium ferrate(VI) electrode decreasewith the increase of temperature, although its open-circuitpotential and initial discharge potential are elevated athigher temperatures. Under present experimental condi-tions temperature may have two contrary influences onthe discharge performance of ferrate(VI) electrodes. Onthe one hand, ferrate(VI) salts have higher reaction activityat higher temperatures, and this can enhance the dischargeperformance of ferrate(VI) electrodes by decreasing elec-trode polarization. On the other hand, higher temperatureleads to larger decomposition rate of ferrate(VI) salts,which will undoubtedly deteriorate the electrochemical per-formance of ferrate(VI) electrodes. For calcium ferrate(VI)sample, temperature may have larger impacts on the latterthan the former, so its discharge performance declines withthe increase of temperature. Since K2FeO4 has better sta-bility, in our experimental conditions (5 �C–25 �C) temper-ature may produce less effect on its decomposition rate,and then the discharge performance of K2FeO4 electrodeis obviously improved with the rise of temperature. It isalso noted from Fig. 9 that calcium ferrate(VI) electrodedisplays much larger discharge capacity than K2FeO4 elec-trode at both 15 and 5 �C.

The preparation and physicochemical properties of cal-cium ferrate(VI) powders have been probed. From prepa-ration point of view, the purity (58.4%) of CaFeO4

(71.5% for CaFeO4 Æ 2H2O) may be somewhat low, but avariety of alternative syntheses including adjusting pH,changing the concentrations of reactants, and improvingfiltration and washing procedures, etc. did not improvethe related purity. It can be seen from the above results thatthe present purity of the synthesized calcium ferrate(VI)has been sufficient for the investigation of its physicochem-

Discharge capacity (mAh/g)

0 50 100 150 200 250 300

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

5°C 15°C 25 °C

E (

V v

s.H

gO/H

g)

b

2FeO4 at 100 mA/g at different temperatures.

Z. Xu et al. / Electrochemistry Communications 9 (2007) 371–377 377

ical properties. The calcium ferrate(VI) sample displaysmuch better intrinsic rate discharge capability and largerdischarge capacity at relatively lower temperatures(615 �C), although a relatively larger amount of Fe (III)impurities with very low conductivity exist in this material.Nevertheless, the above results indicate that relatively lar-ger impurity content has resulted in poor stability anddeclined discharge performance of calcium ferrate(VI) sam-ple with intrinsic larger capacity and better rate capability.The efforts of increasing the purity of calcium ferrate(VI)by using improved preparation methods will be of greatinterest.

Acknowledgements

This work was supported by National Natural ScienceFoundation of China (Approval No. 50172041). Theauthors also gratefully acknowledge the financial supportof Chinese State Key Laboratory for Corrosion andProtection.

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