07) 4144–4148www.elsevier.com/locate/matlet

Materials Letters 61 (20

Proton conduction in La0.9Sr0.1Ga0.8Mg0.2O3−α ceramic prepared viamicroemulsion method and its application in ammonia synthesis at

atmospheric pressure

Feng Zhang a, Qing Yang a, Bo Pan a, Rui Xu a, Hongtao Wang b, Guilin Ma a,⁎

a Key Lab. of Organic Synthesis of Jiangsu Province, School of Chemistry and Chemical Engineering, Suzhou University, Suzhou, 215123, PR Chinab Department of Chemistry, Fuyang Normal College, Fuyang, Anhui, 236041, PR China

Received 28 July 2006; accepted 16 January 2007Available online 26 January 2007

Abstract

The popular electrolyte of La0.9Sr0.1Ga0.8Mg0.2O3−α was prepared via microemulsion method by using (NH4)2CO3–NH4OH as the co-precipitation reagent, followed by calcination and sinteration in air. The sintered ceramic was characterized by scanning electron microscopy(SEM) and powder X-ray diffraction (XRD) methods. The conduction behaviors in La0.9Sr0.1Ga0.8Mg0.2O3−α were investigated by using theelectrochemical methods including gas concentration cells and electrochemical hydrogen permeation. The results indicate that the charge carriersin La0.9Sr0.1Ga0.8Mg0.2O3−α are protons under hydrogen atmosphere. Ammonia was synthesized at atmospheric pressure successfully by usingLa0.9Sr0.1Ga0.8Mg0.2O3−α as solid electrolyte for the first time.© 2007 Elsevier B.V. All rights reserved.

Keywords: Ammonia synthesis; Microemulsion; LaGaO3; Proton conduction

1. Introduction

Solid electrolytes are very important functional materials fortheir potential applications in sensors, solid oxides fuel cell(SOFC), membrane reactors and other high temperatureelectrochemical devices [1,2]. Yttria-stabilized zirconia (YSZ),as the traditional electrolyte of SOFC, is extensively investi-gated, but the high operation temperature is its drawback. In thecourse of searching new and more compatible electrolyte forSOFC, Ishihara [3] found that Sr and Mg doped LaGaO3-basedoxides may be the promising electrolytes for SOFC atintermediate temperature for their higher oxide ionic conduc-tivity than YSZ at the same temperature. And since then, anumber of papers on LaGaO3-based ceramics were reported.

The traditional preparation for LaGaO3-based electrolyte issolid state reaction, but the gallate prepared by this methodusually contains a second phase [4]. As a consequence, wetchemical synthesis methods for preparing LaGaO3-based

⁎ Corresponding author. Tel.: +86 512 65880326.E-mail address: 32uumagl@suda.edu.cn (G. Ma).

0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2007.01.060

electrolyte had been developed one after another by researchers.The wet chemical methods include Sol–Gel, hydrothermalreaction, the Pechini method, co-precipitation and so on [5–7].All these wet processes are an attractive preparation route forLaGaO3-based electrolyte because they can lower the sinteringtemperature and provide high pure powders.

The development of nanotechnology has opened up a newsight in materials science. The great changes in physicochem-ical properties (such as magnetic, optic, and electrochemicalproperties) may happen when the particle sizes reduces, evenwithout any compositional change [8]. The wet chemicalmethods mentioned above had been used for the synthesis ofLaGaO3 ceramic, but another new method, microemulsionmethod, which can provide a more relatively simple andpowerful route for controlling the size and uniformity ofparticles, has not been applied to the synthesis of LaGaO3-based electrolyte so far. In this study, the well-crystallized andsingle-phase ceramic was obtained via the microemulsionmethod.

In the past decade, the investigations on the conductionbehaviors of LaGaO3-based electrolyte focused on its oxide

4145F. Zhang et al. / Materials Letters 61 (2007) 4144–4148

ionic conduction, but there is no investigation about the protonconduction in LaGaO3-based electrolyte except in Good-enough's research [9]. In 1994, Goodenough reported that theconductivities of La0.9Sr0.1Ga0.8Mg0.2O3−αwere the same in dryair, ambient air and water-saturated (at room temperature) air, sothere was no evidence of proton conduction in Sr and Mg dopedLaGaO3. Since then, the proton conduction in Sr and Mg dopedLaGaO3 has been overlooked for many years. In our recentstudies, we found a superior proton conduction in LaGaO3-based electrolyte [10].

One of the very important functions of high temperatureproton conductors is its application in ammonia synthesis atatmospheric pressure. The traditional ammonia synthesismethod is the Haber process, which needs high temperatures(450–600 °C) and especially high pressures (150–300 bar).Ammonia synthesis at atmospheric pressure became possible byusing proton electrolyte since it was synthesized successfully atatmospheric pressure by Marnellos and Stoukides [2].

In this study, we investigated the proton conduction behaviorin La0.9Sr0.1Ga0.8Mg0.2O3−α (LSGM) prepared via microemul-sion method and applied it to ammonia synthesis at atmosphericpressure.

2. Experimental

2.1. Preparation of La0.9Sr0.1Ga0.8Mg0.2O3−α ceramic

All the reagents and solvents are analytical-grade. Themicroemulsion, A, containing La3+, Sr2+, Ga3+, Mg2+ and PEGwas prepared as follows. The required amounts of La(NO3)3,Sr(NO3)2, Ga(NO3)3 and Mg(NO3)2 reagents were firstdissolved by stirring in 50 ml distilled water, and 40 ml ofcyclohexane and 15 ml of dehydrate alcohol were added. ThenPEG as a dispersing reagent was added, and its concentrationin the microemulsion was 0.05 g/ml. The microemulsion, B,was prepared in a similar way. Cyclohexane 40 ml anddehydrate alcohol 15 ml were added into the solution of(NH4)2CO3–NH4OH, which serves as a co-precipitationreagent, and then PEG was added and its concentration inthe microemulsion B was 0.05 g/ml. Subsequently, themicroemulsion B was dripped into microemulsion A understirring at 50 °C, resulting in the production of the white co-precipitate precursor. The precursor was filtered and dried byan infrared lamp.

The as-prepared precursor was calcined at 1100 °C for 4 h inair. The obtained oxides were ground with anhydrous ethanolusing a planetary ball mill machine and dried by infrared lamp,followed by sieving (100 mesh). The obtained ultrafine powderwas pressed into pellets (diameter 20 mm, thickness 2 mm) by ahydrostatic pressure of 2.5×108 Pa, and sintered at 1400 °C for6 h in air. The resulting ceramic was made into a thin disk toserve as the electrolyte for the electrochemical determinations.

2.2. Characterization

SEM (S-570, HITACHI) was used to characterize themorphology of La0.9Sr0.1Ga0.8Mg0.2O3−α ceramic, and the

structure characterization was determined at room temperatureby powder X-ray diffraction analysis (Rigaku D/MAX-IIIC X-ray diffractometer).

2.3. Electrochemical measurement

To estimate the contribution of proton to the conduction, ahydrogen concentration cell was constructed as follows byusing the ceramic sample as an electrolyte diaphragm and itselectromotive forces were measured. Pure hydrogen at 1 atmand a mixture of hydrogen and argon were applied as theelectrode gases.

H2; PtjLa0:9Sr0:1Ga0:8Mg0:2O3−αjPt;H2–ArðpH2Þ:

To verify the proton conduction in the ceramic sampledirectly, the electrochemical hydrogen permeation (hydrogenpumping) through the specimen was examined by sendingdirect current to the electrolytic cell:

ðþÞH2; PtjLa0:9Sr0:1Ga0:8Mg0:2O3−αjPt;Arð−Þ:Pure hydrogen at 1 atm was supplied to the anode chamber

and dry argon was passed through the cathode chamber to carrythe generated gas at the cathode to a hydrogen detector(Shanghai Gainforce SG33A), where the concentration of thegenerated hydrogen gas was detected and the hydrogenevolution rate v in the standard state was calculated by usingthe following equation:

v ¼ 273:15dVArdxð273:15þ TÞdS

where VAr is the flow rate of the carrier gas (Ar), x is theconcentration of the hydrogen gas generated in the mixed gas ofhydrogen and argon, T is the surrounding temperature and S isthe area of the cathode. The theoretical rate was obtained basedon Faraday's law, assuming that the charge carriers in thespecimen are only protons.

For the electrochemical measurements, porous platinumelectrodes were used onto faces of the sample, and experimentaltemperature range was 600–1000 °C.

2.4. Ammonia synthesis at atmospheric pressure

The details on equipment and experimental conditions forammonia synthesis in this study were similar to Refs. [2,11].The electrolytic cell for ammonia synthesis was constructedas:

ðþÞH2;Ag–PdjLa0:9Sr0:1Ga0:8Mg0:2O3−αjAg–Pd;N2ð−Þ:

The Ag–Pd pastes (weight percent: 20% Pd) were used aselectrodes and catalysts and the surface area of each electrodewas 0.95 cm2. Pure hydrogen gas at 1 atm was introduced to theanode chamber and dry pure N2 gas (dried by P2O5) wasintroduced to the cathode chamber. The flow rate of the twogases was 30 ml/min. A direct current was sent to the

Fig. 1. Powder X-ray diffraction patterns of LSGM.

Fig. 3. EMFs of the hydrogen concentration cell: H2, Pt |LSGM|Pt, H2–Ar(pH2

).

4146 F. Zhang et al. / Materials Letters 61 (2007) 4144–4148

electrolytic cell. Ammonia was synthesized in the cathode at550 °C, and the following reactions occurred:

3H2→6Hþ þ 6e− ð1ÞN2 þ 6Hþ þ 6e−→2NH3 ð2Þ

The generated NH3 was absorbed by 10 ml of diluted sulfuricacid with initial pH value of 3.5 for 20 min, and then theconcentration of NH4

+ in this solution was analyzed by usingNessrel's reagent and spectrophotometry. The blank test wasalso performed under open circuit condition.

3. Results and discussion

3.1. Powder X-ray diffraction

Fig. 1 shows the powder X-ray diffraction patterns of LSGM, whichwere calcined at 1100 °C and sintered at 1400 °C in air. The ceramicsample sintered at 1400 °C showed a single phase of perovskite. Themicroemulsion method used in the present study shows its superiorityin preparing single phase of LSGM compared with the traditionalmethod. The lattice parameters are a=0.5512 nm, b=0.5520 nm,c=0.7827 nm.

Fig. 2. SEM image (surface) of LSGM sintered at 1400 °C.

3.2. Morphology

Fig. 2 shows the SEM image of LSGM sintered at 1400 °C. It isclear from Fig. 2 that the ceramic sample is well-crystallized and verycompact, and the grain was about 4 μm in size. The relative density ofthe sample was 95.2%.

3.3. Electrochemical properties

To investigate the conduction behavior in LSGM, the electromotiveforces of the hydrogen concentration cell using LSGM as electrolyte

Fig. 4. Hydrogen evolution rate as a function of current density at 900 °C. Dottedline shows the theoretical value.

Fig. 5. Rate of NH3 formation at 550 °C as a function of current density.

4147F. Zhang et al. / Materials Letters 61 (2007) 4144–4148

were measured. Fig. 3 shows the EMF response of the cell to hydrogenpartial pressure, pH2

. The solid symbols in Fig. 3 stand for the observedEMFs. As shown in this figure, each observed value Eobs was close tothe corresponding theoretical value Ecal, which can be calculated fromthe Nernst's equation:

Ecal ¼ RT2F

lnpH2 ðIÞpH2 ðIIÞ :

The ionic transport number, ti, which was determined from Eobs/Ecal (= ti), was unity at temperatures from 600 to 1000 °C in hydrogenatmosphere, indicating that the sample was a pure ionic conductor inhydrogen atmosphere.

To verify the proton conduction in LSGM directly, an electro-chemical hydrogen permeation experiment was performed. If thesample is a proton conductor, and when a direct current is sent to theelectrolytic cell, H2 will become protons by losing electrons on theanode surface, which permeate through the electrolyte to the cathode,where protons obtain electrons and turn into H2. The hydrogen gasgenerated in the cathode chamber was detected by a hydrogen detectorand the hydrogen evolution rate was calculated. The theoretical rate canbe calculated from Faraday's equation, assuming that the chargecarriers in the sample are only protons:

Vth ¼ 60� 22:4� I2� F � S

where I, F and S stands for the current, Faraday's constant and the areaof the electrode, respectively.

The typical result is shown in Fig. 4. As shown in this figure, whenthe current density was less than 12 mA cm−2, the hydrogen evolutionrate is close to the theoretical rate, indicating that the charge carriersunder hydrogen atmosphere in LSGMwere protons. In other words, theionic transport numbers calculated from Fig. 3 represent proton. Whenthe current density was more than 12 mA cm−2, there were somedeviations from the theoretical values. This may be relevant to theelectrode polarization and a limiting proton current due to lowhydrogen concentration at the interface between electrode material (Pt)and electrolyte [12].

3.4. Synthesis of ammonia at atmospheric pressure

The ammonia produced in the cathode was absorbed by 10 ml ofdiluted sulfuric acid with pH value of 3.5 for 20 min. When Nessler'sreagent was added to the solution, it became an orange-yellow solutionimmediately, and the precipitation appeared gradually after being inplace for 1 h. This phenomenon indicated that NH4

+ is present in thissolution. It is clear that the ammonia was synthesized successfully atatmospheric pressure by using LSGM as electrolyte. On the other hand,the ammonia synthesis confirmed that LSGM has proton conductionunder hydrogen atmosphere.

The rate of ammonia formation at 550 °C was measured throughspectrophotometry method and the result is shown in Fig. 5. The protontransport number of LSGM was unity, so the ratio I/2F is equal to theelectrochemical molar flux and the theoretical amount of the NH3

formation should be 2/3× I/F [2], the efficiency of NH3 formationcalculated from the ratio of experimental rate of NH3 to the theoreticalrate is above 70% when the current was less than 1 mA. And themaximum rate of ammonia formation in the cathode chamber was2.37×10−9 mol s−1 cm−2. The reasons of the low rate of ammoniaformation and efficiency may be due to several factors. One of the

important reasons may be due to the very small surface area ofelectrode and catalyst [11,13]. The flow rate of H2 and N2, theactivation of catalyst, electrode polarization and the thickness ofelectrolyte may all result in the decrease of the rate of ammoniaformation. Besides, the rate of ammonia decomposition in the cathodechamber may be also an important reason. Consequently, to enhancethe efficiency of ammonia formation, we should increase the surfacearea of electrode and catalyst, prepare a thin-film electrolyte and find anew and more efficient catalyst for ammonia formation. The conditionsof ammonia synthesis still need to be investigated and optimized.

4. Conclusion

A single phase of LSGM ceramic was obtained via themicroemulsion method. The proton conduction in LSGM hasbeen neglected up to now. But in the present study, the result ofelectrochemical hydrogen permeation directly confirmed thatthe charge carriers in LSGM under hydrogen atmosphere areprotons. Ammonia was synthesized successfully at atmosphericpressure by using LSGM as electrolyte, and the rate of ammoniaformation in the cathode chamber was 2.37×10−9 mol s−1

cm−2. Although the rate of ammonia formation was not veryhigh under the experimental conditions, it pointed out adirection of ammonia synthesis by using proton conductorlike LSGM as electrolyte. On the other hand, the ammoniaformation at atmospheric pressure using LSGM as electrolytefurther confirmed the proton conduction in LSGM underhydrogen atmosphere.

Acknowledgement

The authors would like to thank the Chinese Natural ScienceFoundation (No. 20171034) for its financial support.

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