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Electrochimica Acta 56 (2011) 5722–5730

Contents lists available at ScienceDirect

Electrochimica Acta

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nvestigation of the catalytic activity of LaBO3 (B = Ni, Co, Fe or Mn) prepared byhe microwave-assisted method for hydrogen evolution in acidic medium

hmed Galal ∗, Nada F. Atta, Shimaa M. Aliepartment of Chemistry, Faculty of Science, Cairo University, Al Gamaa Street, 12613 Giza, Egypt

r t i c l e i n f o

rticle history:eceived 2 November 2010eceived in revised form 1 April 2011ccepted 12 April 2011vailable online 27 April 2011

a b s t r a c t

LaBO3 (B = Ni, Co, Fe and Mn) were prepared by microwave-assisted citrate method. The electrocatalyticactivity toward hydrogen evolution reaction (HER) was investigated. XRD characterization showed thatpure perovskite crystals were indeed formed. SEM images showed that changing the type of the B-sitemetal ion affected the morphology of the prepared perovskites. The influence of the type of B-cationon the catalytic activity toward hydrogen evolution was studied and the order of the electrocatalytic

eywords:ERerovskitesicrowave synthesis

atalystIS

activity was LaFeO3 > LaCoO3 > LaNiO3 > LaMnO3, that was related to the calculated values of the activa-tion energy 51.61, 45.37, 41.15 and 55.05 kJ mol−1 for LaBO3 (B = Ni, Co, Fe and Mn), respectively. Thereaction order and the reaction mechanism for all the prepared perovskites were identified. In addi-tion, the effect of the partial substitution at the B-site in LaNi1 − xCoxO3 was also studied. It was foundthat among ternary perovskites, the catalytic activity of LaNiO3 decreased by increasing the fraction ofdoped-Co.

. Introduction

The hydrogen evolution reaction (HER) is an attractive reactionhat illustrates the importance of research in the field of renewablenergy. There is a significant technological interest in this reac-ion due to its important role in electrodeposition and corrosion of

etals in acids, in storage of energy via hydrogen production, ands the microscopic reverse of the hydrogen oxidation reaction inow-temperature fuel cells [1–3]. The electrocatalysis in the HER isne of the more important subjects in the field of electrochemistry.hree properties play an important role in selecting catalyticallyctive materials for hydrogen evolution: (a) an actual intrinsic elec-rocatalytic effect of the material, (b) a large active surface areaer unit volume ratio, both of which are directly related to theverpotential used to operate the electrolyzer at significant currentensities, and (c) catalyst stability [4]. Thus, from an electrochem-

cal point of view, the problem to be tackled in order to decreasehe cost of electrolytic hydrogen is the reduction of overpotentials.he desired decrease in overpotential can be achieved by choosingighly catalytically active electrode materials, or by increasing thective surface area of the electrode.

Transition metals are the best candidate for HER. The electrocat-lytic activity of transition metals for the HER can be enhanced byhe modification of the electronic structure of the electrode met-

∗ Corresponding author. Tel.: +20 02 35676561; fax: +20 02 35727556.E-mail addresses: galalh@sti.sci.eg, galalah1@yahoo.com (A. Galal).

013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2011.04.045

© 2011 Elsevier Ltd. All rights reserved.

als by alloying or by the use of some suitable preparation method.Perovskite-type oxides, have the general formula ABO3 (A: alkalineearth or lanthanide, responsible for the thermal resistance; B: tran-sition element, responsible for catalytic activity) when subjected toredox processes, perovskite-type oxides produce very small parti-cles, in the order of nanometers, with high metallic dispersion [5,6]thus, providing the best matrix for many transition metal catalysts.Many transitional metals, either in the state of element or oxide,possess catalytic activities for many oxidation and reduction reac-tions, which make perovskites containing the transition metal ionson the B-site to become strong candidates for studying as catalystsfor such reactions.

The microwave irradiation process (MIP), which is one ofthe novel processes evolved from microwave sintering, waswidely applied in inorganic/organic synthesis, food drying,microwave-induced catalysis and plasma chemistry. With its rapiddevelopment in recent decades, MIP has obtained a growing inter-est, especially in materials synthesis research. The advantages ofMIP have been summarized as below: (i) rapid reaction velocity;(ii) uniform heating; and (iii) clean and energy efficient. During thepast years, a lot of perovskites, such as GaAlO3, LaCrO3, etc., havebeen reported to be synthesized by MIP for their ferroelectricity,superconductivity, high-temperature ionic conductivity, or a vari-ety of magnetic ordering, etc. [7–9]. It was reported that smaller

grain size and more rapid lattice diffusion would be formed inmicrowave route than other wet chemical processes [10], whichmight enhance the lattice oxygen mobility in catalysis process.However, secondary phases were formed during the perovskite

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ynthesis by MIP [11] and the average particle size obtained was rel-tively higher than that obtained by other conventional synthesisethod such as sol–gel method.In this work, the microwave-assisted citrate method is intro-

uced for the first time to prepare LaBO3 (B = Ni, Co, Fe and Mn). Thisethod combined the advantages of both MIP and citrate-nitrateethod. As a single-phase perovskite of uniform particle size dis-

ribution, small average particle size (in the nanometer scales) andnhanced catalytic properties can be obtained. The influence of theype of B-cation on the catalytic activity for HER was studied byafel and electrochemical impedance (EIS) measurements. A com-lete kinetics study was carried out in which the reaction order,he activation energy and the reaction mechanism were identified.n addition, the effect of the partial substitution at the B-site inaNi1 − xCoxO3 was also studied.

. Materials and methods

.1. Chemicals

Lanthanum(III) nitrate hexahydrate (Sigma-Aldrich, puriss. p.a.,9%), nickel(II) nitrate hexahydrate (Bio Basic INC, 98–102%),obalt(II) nitrate (Belami Fine Chemicals PVT.LTD), iron(III) nitratePharma Chem, 99%), manganese(II) nitrate (Pharma Chem, 99%),queous ammonia solution 33%, citric acid 99% (Aldrich), nitriccid 70%, A.C.S. reagent (Aldrich), sulfuric acid 95–98% (Aldrich),raphite powder (Sigma-Aldrich, <20 �m, synthetic) and Paraffinil (Fluka) were used as received without further purification. Allolutions were prepared using double distilled water. All measure-ents were made in oxygen-free solution, which was achieved

y continuous purging of the cell electrolyte with nitrogen gas99.999% pure).

.2. Catalyst preparation

Stoichiometric measurements of La-nitrate and B-nitrate wereeighed, dissolved in distilled water and stirred for 5 min. To this

queous solution, a sufficient amount of citric acid was added sohat the molar ratio of citric acid to total metal ions is 1:1. The solu-ion was stirred well for uniform mixing. Ammonia was added todjust the pH of the solution at 6 (except for LaNiO3, the pH wasdjusted at 8). The solution was placed in a conventional microwaveven, with an operating power of 720 W, and the reaction was per-ormed under ambient air for 30 min. The microwave was operatedn 30 s cycles (20 s on, 10 s off). The precursor complex dehydratednd became more viscous with time producing a dark gel. The gelot ignited giving a voluminous fluffy powder. Heating the samplen the microwave was done in the cupboard for safety precautions.

ceramic nano-oxide was then obtained by calcinations at tem-erature = 900 ◦C in the muffle furnace (Thermolyne 6000).

.3. Electrochemical cell and equipments

A standard three-electrode, one compartment cell was used inll experiments. The counter electrode was a large surface arealatinum electrode. The reference electrode was a commerciallyvailable saturated silver/silver chloride electrode. The workinglectrode was a carbon paste electrode (CPE) (d = 0.63 cm), thenmodified CPE was prepared as follows: 0.125 g of reagent graderaphite powder was taken, washed with acetone, and dried whichas then mixed with 45 �L of paraffin oil. To modify the CPE, the

raphite powder was mixed with the modifier in a certain com-osition ratio. Both unmodified and modified carbon pastes wereacked into a Teflon holder that had been cut off at the end. Electri-al contact to the paste was established via a thin copper rod passed

ta 56 (2011) 5722–5730 5723

through the Teflon holder. The fresh surfaces were obtained by pol-ishing the electrodes on a clean paper until they showed a smoothand shiny appearance after every measurement.

All electrochemical measurements, the DC polarization and theelectro-chemical impedance spectroscopy (EIS), were carried outin 0.1 mol L−1 H2SO4 aqueous acid by using a Gamry-750 systemand a lock-in-amplifier that are connected to a personal computer.

DC polarization measurements of hydrogen evolution were car-ried out by first stabilization at open-circuit potential (OCP) until asteady-state OCP value was obtained (usually about 30 min.), thenconditioning the electrode at −0.2 V for 10 min, and at −0.3 V for5 min. Then a linear polarization measurement was made startingfrom −0.3 V to −0.6 V, at a scan rate of 1 mV s−1. The DC polarizationmeasurement was followed by a set of electrochemical impedancespectroscopy measurements at selected over-potentials.

The order of the reaction with respect to H+ was determinedat constant ionic strength of the solution by varying the H2SO4concentration between 0.05 and 0.5 mol L−1 (6 solutions), keepingthe ionic strength constant with Na2SO4. Only one DC polarizationmeasurement was taken at each H2SO4 concentration by the sameprocedure mentioned above.

The Scanning electron microscopy analysis was done by usingPhilips XL30. X-ray diffraction analysis was obtained using Shi-madzu XRD-700.

3. Results and discussion

3.1. XRD and surface characterization

LaBO3 (B = Ni, Co, Fe and Mn) were prepared by the microwaveassistant-citrate method at an operating microwave power of720 W and a microwave irradiation time of 30 min. All fourprepared perovskites, lanthanum nickelate, lanthanum cobaltite,lanthanum ferrite and lanthanum magnaite had high tolerancefactors, which were 0.866, 0.883, 0.838 and 0.875, respectively.The compositions prepared were characterized by X-ray powderdiffractograms. Fig. 1 shows the XRD of (A) LaNiO3, (B) LaCoO3,(C) LaFeO3 and (D) LaMnO3 prepared by microwave assistant-citrate method. XRD characterization showed that pure perovskitecrystals were indeed formed; the major diffraction peaks of the as-synthesized powders were matched with the theoretical ones. Theresults suggested successful incorporation of Ni3+, Co3+, Fe3+ andMn3+ at the La3+ cations sites confirming the formation of hexago-nal distorted rombohedral perovskite phase of LaNiO3 and LaCoO3,and orthorhombic perovskite phase of LaFeO3 and LaMnO3.

Some important structural parameters were calculated fromXRD data such as, particle size, lattice parameters, lattice vol-ume and theoretical density [12]. These parameters were listed inTable 1 and were found to be in good agreement with those for stan-dards. However, there was a deviation in the calculated (a) value forLaNiO3 and LaCoO3 from that of the standard which in turn causeda deviation in the values of the lattice volume and density. This maybe due to the distortion in the rhomobhedral phase.

The morphology of the prepared perovskites was studied bySEM. Fig. 2 shows the SEM images of (A) LaNiO3, (B) LaCoO3, (C)LaFeO3 and (D) LaMnO3 prepared by microwave assistant-citratemethod at a microwave operating power of 720 W and a microwaveirradiation time of 30 min. It can be seen that changing the type ofthe B-site metal ion affected the morphology of the prepared per-ovskites. A highly ordered and compact surface was observed incase of LaNiO3. While, in case of LaCoO3 and LaMnO3, the surface

was composed of agglomerations of nearly spherical grains, smallergrain size was observed for LaMnO3. A completely different mor-phology was observed for LaFeO3, where a porous surface consistedof framework of bone-like shape particles was shown.

5724 A. Galal et al. / Electrochimica Acta 56 (2011) 5722–5730

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ig. 1. XRD patterns of (A) LaNiO3, (B) LaCoO3, (C) LaFeO3 and (D) LaMnO3 preparere shown.

.2. Investigation of the catalytic activity toward HER by dc-Tafelinear polarization

The electrocatalytic activity of the prepared perovskites, LaBO3B = Ni, Co, Fe and Mn) toward HER was investigated by Tafel lin-ar polarization measurements. Fig. 3A shows a set of Tafel curves

able 1tructural parameters, calculated from XRD data, for standard LaBO3 (B = Ni, Co, Fe and0 min.

Crystal structure Average particlsize/nm

Standard LaNiO3 Hexagonal distorted rhombohedral

LaNiO3 sample Hexagonal distorted rhombohedral 19.8

Standard LaCoO3 Hexagonal distorted rombohedral

LaCoO3 sample Hexagonal distorted rombohedral85.7

Standard LaFeO3 Orthorhombic

LaFeO3 sample Orthorhombic 91.7

Standard LaMnO3 Orthorhombic

LaMnO3 sample Orthorhombic 30.8

the microwave-assisted citrate method at 720 W for 30 min. Miller indices (h, l, k)

recorded in 0.1 mol L−1 H2SO4 in the potential region of hydrogenevolution for CPEs modified with 10% (w/w%) LaBO3 (B = Ni, Co, Fe

or Mn). Fig. 3B shows the dependence of the cathodic current onthe tolerance factor. The existence of two Tafel regions (change inTafel slope) has already been reported in literature [13] for sim-ilar HER electrocatalytic materials, and a number of explanations

Mn) and those prepared by the microwave-assisted citrate method at 720 W for

e Latticeparameters (A)

Lattice volume(A3)

Theoreticaldensity (g/cm3)

a = 5.456 169.42 7.22c = 6.572

a = 4.112 96.31 12.70c = 6.577

a = 5.445 336.13 7.29c = 13.094

a = 4.086 189.77 12.91c = 13.124

a = 5.556 242.86 6.64b = 5.565c = 7.855

a = 5.559 243.83 6.63b = 5.579c = 7.862

a = 5.502 236.17 6.80b = 7.774c = 5.521

a = 5.517 235.35 6.82b = 7.769c = 5.492

A. Galal et al. / Electrochimica Acta 56 (2011) 5722–5730 5725

Fig. 2. SEM micrographs of (A) LaNiO3, (B) LaCoO3, (C) LaFeO3 and (D) LaMnO3 prepared by the microwave-assisted citrate method at 720 W for 30 min, with a magnificationof 35,000 times.

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ig. 3. (A) Linear Tafel polarization curves for the HER recorded on CPEs modified wicrowave-assisted citrate method at 720 W for 30 min in 0.1 mol L−1 H2SO4, scan

ave been given, a change in HER mechanism has been suggesteds one possible explanation, which can be attributed to the deple-ion of the d-electron density at the Fermi level of the perovskite

y adsorbed hydrogen [14] which remained partially uncompen-ated at lower overpotentials. Mass-transport limitations througharrow pores on the catalyst surface [15] or a decrease in the activeurface area [16–18] have also been suggested as possible reason

able 2ER kinetic parameters obtained by analysis of Linear Tafel polarization curves, togethicrowave-assisted citrate method at 720 W for 30 min.

Tolerance factor b (mV dec−1) jo (�A cm−2)

LaNiO3 0.866 336.3 −12.2LaCoO3 0.883 347.7 −40.8LaFeO3 0.838 232.4 −5.9LaMnO3 0.875 321.7 −3.3

% (w/w%) (—) LaNiO3, (. . .) LaCoO3, (—) LaFeO3 and (-..-..-) LaMnO3 prepared by the1 mV s−1 and (B) the dependence of the HER rate on the tolerance factor.

for the observed diffusion-like shape of the Tafel curves. By con-sidering the Tafelian region, the calculated values of Tafel slope,exchange current density, and transfer coefficient for the prepared

perovskites were listed in Table 2. According to the general HERmechanism in acidic media [19–21], these Tafel slope values indi-cated that the Volmer reaction step i.e. adsorption of hydrogenon the catalyst was the rate-determining step. It could also be

er with the tolerance factors for LaBO3 (B = Ni, Co, Fe and Mn) prepared by the

˛ j (�A cm−2) at −300 mV � (mV) at 0.5 mA/cm−2

0.18 −158.5 −3900.17 −348.9 −3190.25 −105.6 −3940.18 −41.9 >−600

5726 A. Galal et al. / Electrochimica Acta 56 (2011) 5722–5730

Fig. 4. (A) Linear Tafel polarization curves for the HER recorded on CPEs modified with 10% (w/w %) LaFeO3 prepared by the microwave-assisted citrate method at 720 W for30 min in x mol L−1 H2SO4; x = (—) 0.05, (. . .) 0.1, (—) 0.2, (-..-..-) 0.3, ( ) 0.4 and (-.-.-) 0.5 M at room temperature, scan rate = 1 mV s−1 and (B) typical plot of log j (A/cm2)at −0.6 V (saturated Ag/AgCl) vs. pH for the determination of the reaction order for LaFeO3.

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ig. 5. (A) Linear Tafel polarization curves for the HER recorded on CPE modified w0 min in 0.1 M H2SO4 at various temperatures; (—) 298, (. . .) 308, (—) 318, (-..-..-)ctivation energy values for LaFeO3.

oticed that the Tafel slope and transfer coefficient values deviatedrom the theoretical values 116 mV decade−1 and 0.5, respectively19–21]. This phenomenon has already been reported in the lit-rature [22] and has been explained as a characteristic feature forxide catalysts. Considering the exchange density values, presentedn Table 2, it can be concluded that the order of catalytic activ-ty is LaCoO3 > LaNiO3 > LaFeO3 > LaMnO3. Also, by considering bothhe current density values, measured at a fixed overpotential of

300 mV and the overpotential values, measured at current den-

ity of 0.5 mA cm−2 presented in Table 2, similar trend was obtainedompared to that obtained with the exchange current density, theame order of the electrocatalytic activity was observed. Thus, the

ig. 6. (A) Bode and (B) Nyquist plots showing An EIS response of CPE modified with 10%0 min in 0.1 M H2SO4 at overpotential −0.1 V, symbols are experimental and solid lines a

% (w/w %) LaFeO3 prepared by the microwave-assisted citrate method at 720 W ford (– –) 338 K; scan rate = 1 mV s−1 and (B) Arrhenius plot for the determination of

catalytic activity for LaBO3 (B = Ni, Co, Fe and Mn) prepared bythe microwave assistant-citrate method increased by increasingthe tolerance factor of the prepared perovskite, except in case ofLaMnO3. This could be explained on the basis that Mn metal itselfis not a catalyst for HER. In conclusion, the catalytic activity ofperovskites containing different transition metals in the B-sitesincreased by increasing the tolerance factor, provided that the tran-sition metal itself has a catalytic activity for HER. In other words, the

tolerance factor cannot be taken as the only factor controlling thecatalytic activity of LaBO3 perovskites. The nature of the B metal ionplays the main role, as will be shown later in the activation energycalculations section.

(w/w%) LaFeO3 prepared by the microwave-assisted citrate method at 720 W forre modeled data.

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.3. Order of reaction with respect to H+

The order of the reaction with respect to H+ was determined atonstant ionic strength of the solution by varying the H2SO4 con-entration keeping the ionic strength constant with Na2SO4. Onlyne DC polarization measurement was taken at each H2SO4 con-entration by the same procedure mentioned in the experimentalection. Fig. 4A shows a set of Tafel curves recorded in x mol L−1

2SO4 (x = 0.05, 0.1, 0.2, 0.3, 0.4, and 0.5 M) in the potential regionf hydrogen evolution for CPE modified with 10% (w/w%) LaFeO3,he same data for the other prepared perovskites are not displayed.ig. 4B represents the dependence of the cathodic current of HERn the H+ ion concentration. The reaction order values of LaBO3B = Ni, Co, Fe, and Mn) were 0.94, 0.74, 0.37 and 0.87 respectively.he fractional reaction order was expected for HER catalyzed byxide catalysts [22].

.4. Activation energy

In order to evaluate the temperature effect on the kinetics ofhe HER for the prepared catalysts, DC linear polarization (Tafel)

easurements were done for a wide temperature range, from98 K to 338 K. Fig. 5A shows a set of Tafel curves recorded onPE modified with 10% (w/w %) LaFeO3 at various temperatures.ig. 5B demonstrated that this increase was linear in a semi-ogarithmic plot, the Arrhenius equation. The activation energyalues for LaBO3 (B = Ni, Co, Fe and Mn) were 51.61, 45.37, 41.15nd 55.05 kJ mol−1, respectively. Thus, the order of the electrocat-lytic activity for HER based on the activation energy calculationsas LaFeO3 > LaCoO3 > LaNiO3 > LaMnO3. This trend agreed some-

ow with the trend obtained from Tafel measurements, whichas LaCoO3 > LaNiO3 > LaFeO3 > LaMnO3, with one difference that

aFeO3 has the superior catalytic activity among the preparederies. This can be explained as follows, by rising temperature,

able 3he electrical equivalent circuit parameters calculated from the NLLS analysis for (a) LaNitrate method at 720 W for 30 min.

� (V) Rs (� cm2) CPE1/F cm−2 m

(a)−0.025 6.50 59953.84 0.91−0.050 8.79 63951.94 0.92−0.075 9.14 65272.97 0.92−0.100 8.40 68873.08 0.92−0.125 10.39 72250.56 0.93−0.150 10.42 74698.08 0.93

(b)−0.025 16.35 38597.41 0.89−0.050 16.31 41620.38 0.90−0.075 15.70 43415.66 0.90−0.100 16.08 45541.87 0.91−0.125 16.16 48457.55 0.91−0.150 16.33 51222.85 0.92

(c)−0.025 8.52 136466.49 0.92−0.050 6.79 147493.98 0.93−0.075 7.36 154830.11 0.93−0.100 8.13 157187.92 0.93−0.125 3.92 161511.03 0.93−0.150 0.14 163595.43 0.94

(d)−0.025 17.85 52977.37 0.89−0.050 17.82 55404.20 0.91−0.075 17.69 56405.60 0.97−0.100 17.62 62531.78 0.98−0.125 21.64 64361.55 1.00−0.150 21.57 66400.73 1.02

Fig. 7. Electrical equivalent circuit used to explain the EIS response of the HER.

LaFeO3 catalyst activated and gave the highest catalytic activity.This agreed well with that mentioned in the literature that Fe hadthe highest catalytic activity for the HER compared to the most ofthe transition metals [23–25].

3.5. Electrochemical impedance spectroscopy

Fig. 6A and B shows examples of EIS spectra recorded on CPEmodified with 10% (w/w%) LaFeO3 at over-potential of −0.1 V.The data were presented in the form of both Nyquist and Bodeplots. The EIS spectra revealed the presence of two time con-stants. This was in agreement with EIS data obtained on otherHER electrocatalysts [21,26]. The EIS spectra revealed the pres-ence of two time constants. In order to derive a physical pictureof the electrode/electrolyte interface and the processes occurringat the electrode surface, experimental EIS data were modeled using

non-linear least-squares fit analysis (NLLS) software and electricalequivalent circuit. Fig. 6A and B show that a very good agree-ment between the experimental (symbols) and simulated (lines)data was obtained when the equivalent circuit shown in Fig. 7

iO3, (b) LaCoO3, (c) LaFeO3 and (d) LaMnO3 prepared by the microwave-assisted

R1 (� cm2) CPE2/F cm−2 n R2 (� cm2)

18538.73 15.16 0.04 57.6615425.45 18.90 0.07 20.84

9768.49 11.39 0.06 933.638722.98 11.80 0.06 920.407675.27 11.37 0.09 635.586184.04 11.34 0.09 697.96

15089.93 444.65 0.49 3.2812207.33 4427.99 0.49 3.59

8983.60 440.44 0.27 59.426422.44 475.49 0.35 50.725605.46 413.78 0.39 37.744675.94 475.41 0.48 35.41

81009.07 25.99 0.29 10.0958160.48 25.92 0.06 42.8047829.09 28.04 0.07 35.4035938.87 31.39 0.08 28.3330146.87 26.41 0.05 18.9420663.91 23.78 0.03 9.74

39388.34 13545.30 0.98 1008.5834225.70 9415.75 0.95 6406.7221586.33 25561.25 0.86 5538.8019377.08 21486.24 0.85 3801.5810815.28 25247.51 0.85 2947.89

9223.93 30678.32 0.86 1370.79

5728 A. Galal et al. / Electrochimica Acta 56 (2011) 5722–5730

Fig. 8. Nyquist plots showing EIS response of CPE modified with 10% (w/w %)LaFeO3 prepared by the microwave-assisted citrate method at 720 W for 30 mini−

wTptiwtocs%

Fig. 9. XRD patterns of (A) LaNi0.8Co0.2O3, (B) LaNi0.6Co0.4O3, (C) LaNi0.4Co0.6O3 and(D) LaNi0.2Co0.8O3 prepared by the microwave assistant-citrate method at 720 W for

F3

n 0.1 mol L−1 H2SO4 at various over-potentials, (�) −50, (©) −75, (�) −100, (�)125 and (�) −150 mV, symbols are experimental and solid lines are modeled data.

as used to describe the EIS response of the investigated catalyst.his model has been used to describe the response of the HER onorous electrodes [20,27,28]. It reflects the response of a HER sys-em characterized by two time constants, only one of them (CPE1)s related to the kinetics of the HER. This time constant changes

ith overpotential. The second time constant (CPE2) is related tohe porosity of the electrode surface, and does not change withver-potential. The dependence of each electrical equivalent cir-

uit parameter on applied over-potential was investigated. Fig. 8hows a set of EIS spectra recorded on CPE modified with 10% (w/w) LaFeO3 at various over-potentials. Table 3a–d shows the elec-

ig. 10. SEM micrographs of (A) LaNi0.8Co0.2O3, (B) LaNi0.6Co0.4O3, (C) LaNi0.4Co0.6O3 and0 min, with a magnification of 35,000 times.

30 min. Miller indices (h, l, k) are shown.

trical equivalent circuit parameters calculated from NLLS analysisfor LaBO3 (B = Ni, Co, Fe and Mn) prepared by microwave assistant-citrate method, respectively. With an increase in over-potential,CPE1 increased and R1 decreased. It can be concluded that the(CPE1-R1) is related to the HER charge-transfer kinetics, namelyto the response of double layer capacitance characterized by CPE1and HER charge transfer resistance characterized by R1. Contraryto the behavior of CPE1, the value of CPE2 was shown to be rela-

tively constant. At the same time, the value of R2 decreased. This isa typical behavior related to the porosity of the electrode surface.

(D) LaNi0.2Co0.8O3 prepared by the microwave-assisted citrate method at 720 W for

A. Galal et al. / Electrochimica Acta 56 (2011) 5722–5730 5729

Table 4Structural parameters, calculated from XRD data, for standard LaNi0.6Co0.4O3 and LaNi1 − xCoxO3, prepared by the microwave-assisted citrate method at 720 W for 30 min.

Average particle size (nm) Lattice parameters (A) Lattice volume (A3) Theoretical density (g/cm3)

Standard LaNi0.6Co0.4O3 a = 5.466 339.68 7.21c = 13.128

LaNi0.8Co0.2O3 sample 12.8 a = 4.103 192.18 12.74c = 13.182

LaNi0.6Co0.4O3 sample 10.2 a = 4.103 191.43 12.79c = 13.131

LaNi0.4Co0.6O3 sample 12.8 a = 4.097 191.89 12.7601333

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c = 13.2LaNi0.2Co0.8O3 sample 17.5 a = 4.13

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This EIS behavior is quite in consistence with the Tafel behav-or discussed previously. As from the Tafel measurements, thelow rate-determining step in the HER on the CPEs modified witherovskites was the adsorption of hydrogen (Volmer), while theesorption step was fast. Consequently, the absence of an EISydrogen adsorption time constant could be expected, as also con-rmed by the EIS measurements. This demonstrates that althoughIS and Tafel techniques are two quite different experimental tech-iques, results obtained by both techniques are comparable.

.6. The effect of the partial substitution at the B-site,aNi1 − xCoxO3, on the catalytic activity

A series of perovskite catalysts, LaNi1 − xCoxO3 (x = 0.2, 0.4, 0.6nd 0.8) was prepared by microwave assistant-citrate method at anperating microwave power of 720 W and a microwave irradiationime of 30 min. Fig. 9A–D shows the XRD patterns of LaNi1 − xCoxO3x = 0.2, 0.4, 0.6 and 0.8) prepared by microwave assistant-citrate

ethod respectively, the results illustrated successful formation ofhe ternary perovskite LaNi0.6Co0.4O3 at all the doping ratios. Thetructural parameters obtained from XRD data were calculated andisted in Table 4.

The morphology of the prepared perovskites was studied byEM. Fig. 10A–D shows the SEM images of LaNi1 − xCoxO3 (x = 0.2,.4, 0.6 and 0.8), respectively. For ternary perovskites in which< 0.5 (LaNi0.8Co0.2O3 and LaNi0.6Co0.4O3), phases were composedf agglomerations of nearly spherical grains, relatively smallerrains were observed for the latter. Further increase in the frac-ion of the doped-Co led to a highly porous network of bone-like

orphology as in case of LaNi0.4Co0.6O3 and a spongy rough surfaces in case of LaNi0.2Co0.8O3.

The prepared perovskites were tested as catalysts for theER. Fig. 11 shows the dependence of the cathodic current

ig. 11. A typical plot of log j (A cm−2) at −0.3 V (saturated Ag/AgCl) vs. the fractionf Co-doped.

191.32 12.79

on the faction of Co in Co-doped LaNiO3. The order of theelectrocatalytic activity was LaCoO3 > LaNi0.4Co0.6O3 > LaNiO3 >LaNi0.8Co0.2O3 > LaNi0.6Co0.4O3 > LaNi0.2Co0.8O3. From this order itcan be concluded that among ternary perovskites, the catalyticactivity of LaNiO3 decreased by increasing the fraction of doped-Co.However there was an exception in case of LaNi0.4Co0.6O3 whichwas more active than LaNiO3 and has the highest catalytic activ-ity among the prepared ternary perovskites. This perovskite hada unique morphology than the others. This morphology, affectsgreatly the activity of the catalyst, and it is the reason for its supe-rior catalytic activity. It is observed once before in case of LaFeO3,which also had the highest catalytic activity (the smallest activa-tion energy value) among the prepared LaBO3 (B = Ni, Co, Fe andMn) series, as mentioned before.

4. Conclusion

The B-site metals in perovskites play a great role on the activityof the whole catalysts toward hydrogen evolution process by form-ing the primary active sites. For LaBO3 (B = Ni, Co, Fe and Mn), theorder catalytic activity based on the values of exchange current den-sity, the current density values, measured at a fixed overpotential of−300 mV and the over-potential values, measured at current den-sity of 0.5 mA.cm−2 was LaCoO3 > LaNiO3 > LaFeO3 > LaMnO3. Thus,the catalytic activity increased by increasing the tolerance factorof the prepared perovskite, except in case of LaMnO3. This can beexplained on the basis that Mn metal itself is not a catalyst for HER.The order catalytic activity based on the values of activation energywas LaFeO3 > LaCoO3 > LaNiO3 > LaMnO3. One of the prepared per-ovskites; LaFeO3 has superior catalytic activity among the preparedseries was explained in terms of the rising temperature that impartsthe catalyst activation and acquired the highest catalytic activity.This agreed well with the literature which stated Fe had the high-est catalytic activity for the HER compared to most of the transitionmetals.

For Co-doped LaNiO3; the order of the electrocatalyticactivity was LaCoO3 > LaNi0.4Co0.6O3 > LaNiO3 > LaNi0.8Co0.2O3> LaNi0.6Co0.4O3 > LaNi0.2Co0.8O3. The catalytic activity of LaNiO3decreased by increasing the fraction of doped-Co with an exceptionin case of LaNi0.4Co0.6O3. This perovskite had a unique morphologycompared to the series prepared in this study. This morphologyinfluences greatly the activity of the catalyst and explains thesuperior catalytic activity obtained.

Acknowledgment

The authors would like to express their gratitude to Cairo Uni-versity through the office of the Vice President for Graduate Studiesand Research for the partial financial support.

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