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9CHANGING THE PHASE COMPOSITION AND CATALYTIC PROPERTIES OF THE FE/N/C CATALYST BY ELECTROCHEMICAL TREATMENT
Nadirov R.K.
al-Farabi Kazakh national university, C.Sc., senior lecturer
Sabirov Ye.A.
al-Farabi Kazakh national university, master student
ABSTRACT
The results of treatment of Fe/N/C catalysts by anodic and cathodic polarization were presented. It was found that electrochemical treatment of the starting material changes its phase composition and catalytic activity toward oxygen reduction reaction. For the first time, a method of increasing the activity of catalysts by electrochemical treatment was proposed.
Keywords: Fe/N/C, catalyst, electrochemical treatment.
Due to their high environmental and great theoretical value of the conversion coefficient of chemical energy into electrical energy (about 80%), fuel cells hold significant promise [1,2]. Low-temperature fuel cells with proton exchange membrane attract the most attention. Hydrogen and atmospheric oxygen are the fuel for this type of cells. The main obstacle to the widespread use of fuel cells is the high cost of the catalysts, that is electrodes (anodes and cathodes) which contain the noble metals, most often platinum. In this regard, there are searching for new catalysts materials whose characteristics are not inferior to catalysts based on noble metals, worldwide. Among such materials, nitrogen pyrolyzed transition metal complexes deposited on a carbon support (Fe/N/C) are considered to be the most promising [3-5]. However, the activity of these catalysts for the oxygen reduction reactions is not sufficient for commercial use of fuel cells based on Fe/N/C [6-8].
In this paper, the method of electrochemical activation is proposed to increase the activity of Fe/N/C catalysts for the oxygen electrochemical reduction reactions.
The samples of Fe/N/C catalyst were synthesized according to known technique [9]. Then, the samples obtained were electrochemically treated to change their phase composition, and thereby, to change their catalytic properties toward oxygen reduction reaction (ORR).
To determine the conditions of the electrolysis under galvanostatic and potentiostatic modes, polarization curves on synthesized Fe/N /C materials in appropriate aqueous and non-aqueous solutions were obtained. We have found that there are two waves one peak on the anodic branch of the cyclic voltammograms (CV) obtained in aqueous solutions of 0.1M H2SO4 at a potential scanning rate of 5 mV/s. The first wave (E=-0,2 V; hereinafter all potential values are given relative to silver chloride reference electrode Ag/AgCl) corresponds to the oxidation of the metallic iron of the original catalyst to Fe2+; a small peak, following this wave, is related to the oxidation of Fe2+ to Fe3+. The second wave
observed in the voltammogram is associated with the restructuring of the material surface during anodic polarization. Two waves on cathodic branch correspond can be associated with the recovery process of Fe3+ to Fe2+ and further recovery to metallic iron.
It should be noted that the CV obtained on the same catalyst, are not well reproduced after two or three measurements. This is due, apparently, with a change in the material surface after an electrochemical polarization.
Changing the composition of the solution in a borate buffer solution (pH = 6.3) change the shape of the polarization curves. Two peaks can be observed on the anodic curve; the first one with maximum at E = -155 mV can be associated with metallic iron (E0 =-0,44 V); the second one with maximum at E = -200 mV is due to dissolution of cementite:
FesC - 6e = 3Fe2+ + C, E0 = -0,472 V
Anodic potentiostatic curves obtained in aqueous 0.1 M H2SO4 are presented in Fig. 1.
It can be clearly defined that increasing of the value of potential entails the increasing the initial current; this current at E=+1,100 V is determined to be 600 mV and dramatically increase with increasing duration of the polarization.
Processes of dissolving metallic iron, cementite and graphite carbon particles presented in the original carbon black, contribute to the described current growth:
C + 2H2O = C02+4H+ + 4e
It is known that the above-mentioned reaction is characterized by the following values of the standard electrode potential:
E0 = 0,207 - 0,059pH+0,0148lgPCO2
40
duration, ir in
Figure 1 - Anodic potentiostatic curves obtained in aqueous 0.1 MH2SO4
Our calculations show that the electrochemical reaction of carbon dissolution can take place in the aforementioned conditions.
Gas emission process is visually observed on the electrode at E = +1,100 V; this fact is related primarily, with the decomposition of water to produce oxygen; besides, carbon dioxide formation also makes some contribution to the gas emission process.
It should be mentioned that ponentiastatic curves, like CVs, are not well reproduced. This fact can be explained by the heterogeneity of the starting material's surface which is enhanced by the electrochemical treatment process.
Cathodic potentiostatic curves obtained in aqueous solutions of 0.1 M H2SO4 are characterized by the wave of hydrogen releasing at E = -0,8 V.
When carrying out the process in the presence of 0.1M FeSO4 at E = -0.5 V, the current of Fe2 + discharging to metallic iron is observed.
It is known that carbides, including iron carbides, have an increased activity relative to hydrogen. Furthermore, the hydrogen released during cathodic polarization of the sample may be reacted with the oxides, included in the original catalyst. Partial hydrogen adsorption by the carbon material surface is also possible. Thus, the cathodic polarization of Fe/N/C catalyst changes its composition, which should certainly lead to changes in the catalyst properties toward oxygen reduction reaction.
The anodic polarization of the initial sample does not lead to the formation of new phases in the material, however, is changing, and in some cases substantially, the phase relationship in the Fe/N /C catalyst.
Table 1 provides information on changing the quantitative phase ratio in the anodic treated catalyst (potentiostatic mode), depending on the operating mode (electrolysis mode).
Table 1
Phase Content (%) in the catalyst samples, depending on the processing conditions in potentiostatic mode
Phase Number of sample
1 2 3 4 5 6 7
FeS 19 19 21 22 22 23 23
FeO 6 6 7 7 9 9 9
Fe2Û3 10 10 10 12 11 12 12
FeSi 4 4 5 6 6 6 7
Fe3C 29 28 25 22 18 11 8
Fe3N 19 20 20 21 23 23 25
a-Fe 8 8 10 10 11 12 14
The conditions of samples treatment: 1: without treatment; 2: E= -0,3 V, 30 min; 3: E= -0,3 V; 30 min; 4: E= -0,1 V, 30 min; 5: E = -0,1 V, 60 min; 6: E = +0,1 V, 30 min; 7: E = +0,1 V, 60 min.
XRF analysis results confirm assumptions about the nature of the processes made at the analysis of electrochemical measurements. The anodic treatment of the
starting material promotes partial dissolution of a-Fe, Fe3C, and carbon particles, resulting in a reduction of the cementite in the processed material. The content of a-Fe, on the contrary, increases, due to the release of free iron in the decay of the cementite.
Conclusions
Thus, the possibility of changing the composition (and hence the properties) of Fe/N/C catalysts by electrochemical treatment is demonstrated for the first time. Anodic polarization leads to partial dissolution of a-Fe, Fe3C and carbon particles. Carrying out the electrolysis in the presence of an iron salt leads to formation of the metallic iron, thereby increasing its content in the treated sample.
References
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РАЗРАБОТКА РАЦИОНАЛЬНОГО СПОСОБА СИНТЕЗА МОЛИБДАТА СВИНЦА В ВЫСОКОДИСПЕРСТНОМ СОСТОЯНИЙ В РАСПЛАВАХ СИСТЕМЫ N a2M о O2O7 - Pb3O4
Шурдумов Г.К.
Кабардино-Балкарский государственный университет им. Х.М. Бербекова, г. Нальчик, доктор химических наук, профессор Черкесов З.А.
Кабардино-Балкарский государственный университет им. Х.М. Бербекова, г. Нальчик, кандидат
химических наук, доцент Кандурова Э.Ф.
Пшигаушева А.Г.
Кабардино-Балкарский государственный университет им. Х.М. Бербекова, г. Нальчик, магистранты
DEVELOPMENT OF RATIONAL METHOD FOR THE SYNTHESIS OF LEAD MOLYBDATE IN VYSOKODISPERSNOM STATES IN MELTS OF THE SYSTEM Nа2MоO2O7 - PbsO4
Shurdumov G. K., Kabardino-Balkar state University. after H. M. Berbekov, Nalchik, doctor of chemical Sciences, рrofessor
Cherkesov Z.A., Kabardino-Balkar state University. after H. M. Berbekov, Nalchik, candidate of chemical Sciences, docent
Kandurova E.F.
Pchigausheva A.G., Kabardino-Balkar state University. after H. M. Berbekov, Nalchik, candidate of chemical Sciences, undergraduates
АННОТАЦИЯ
В работе представлен материал по термодинамическому анализу взаимодействия димолибдата натрия с ортолюмбатом свинца, а также данные по разработке рационального способа синтеза PbMoO4 в расплавах этой системы, отличающийся безотходностью, высокими производительностью и выходом целевого продукта марки «х.ч.» в нанокристаллическом состоянии. Она содержит также материал по идентификации синтезированного продукта современными методами исследования.
ABSTRACT
The paper presents material on the thermodynamic analysis of the interaction dimolybdate sodium icolumbia lead, as well as data on the development of a rational method for the synthesis of PbMoO4 in the melts of this
