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CHEMICAL SCIENCES
Enhancing the activity of Fe/N/C catalysts toward oxygen reduction
reaction by cathodic treatment 1 2 Nadirov R. , Sabirov Ye. (Republic of Kazakhstan)
Повышение активности катализаторов Fe/N/C реакции
восстановления кислорода катодной обработкой Надиров Р. К.1, Сабиров Е. А.2 (Республика Казахстан)
1Надиров Рашид Казимович /Nadirov Rashid - кандидат химических наук;
2Сабиров Ерлан Амирбекович /Sabirov Yerlan - магистрант, кафедра общей и неорганической химии,
Казахский национальный университет им. аль-Фараби, г. Алматы, Республика Казахстан
Abstract: in this paper, applying two-stage cathodic polarization for increasing the activity of Fe/N/C catalysts have been suggested. The resulting catalyst, obtained on the base of poly-m-phenylenediamine have been used as an object for further investigations. The sample of Fe/N/C catalyst, obtained by electrochemical treatment, characterized by a higher activity towards oxygen reduction reaction.
Аннотация: в статье предложено использование двухстадийной катодной поляризации для повышения активности катализаторов Fe/N/C. Катализатор, полученный на основе поли-м-фенилендиамина, использован в качестве объекта дальнейших исследований. Образец катализатора, полученный электрохимической обработкой, характеризуется повышенной активностью в отношении реакции восстановления кислорода.
Keywords: Fe/N/C catalyst, cathodic treatment, oxygen reduction reaction.
Ключевые слова: катализатор Fe/N/C, катодная обработка, реакция
восстановления кислорода.
Introduction
In 1989 Jager and co-workers demonstrated the possibility of obtaining a Co/N/C catalyst, capable of oxygen reducing in an acidic medium, by heat treatment of cobalt acetate, adsorbed on a carbon basis, in the presence of polyacrylonitrile as a precursor of nitrogen [1]. Since then, a number of research groups have significantly contributed to developing methods of producing such type of catalytic materials [2-4]. The results of those works are summarized in a recent review [5].
In 2014, Chinese scientists reported a new catalyst Fe/N/C on the basis of poly-m-phenilenidiamin [8]. The catalyst is stable in acidic medium and shows high catalytic activity toward oxygen reduction reaction and of H2O2 oxidation in comparison to previously known catalysts. However, the activity of catalyst obtained towards ORR is still not high sufficient for use in fuel cells. To enhance the activity of the carbon surface of the catalyst, various physical and chemical methods are used, characterized by disadvantages such as low environmental friendliness, complexity, and time-consuming. It was of interest to investigate the influence of electrochemical processing of Fe/N/C catalysts synthesized on the catalytic activity of the treated samples toward ORR.
Methods
The experiment consisted of three steps: synthesis of Fe/N/C catalyst on the basis of m-phenylenediamine by using the known procedure [8]; cathodic polarization of the catalyst synthesized; testing treated catalyst toward ORR.
For the preparation the working electrode, 10 mg of the catalyst was dispersed in 1 ml of water, 1 ml of ethanol and 100 д! of Nafion ion exchange resin for 1 hour until a
homogeneous mass formed. 50 ^l of the material obtained were applied to the end of a titanium rod coated with a plastic sheath for insulation. Calculated amount of the catalyst on the electrode was 246 g /cm2. The titanium rod used as a counter electrode. The cell containing Ag/AgCl reference electrode was separated from the working cell by using a salt bridge. Working electrode polarization was performed using a potentiostat-galvanostat Elins. After cathodic processing, the working electrode was washed with distilled water and subjected to testing toward ORR, by cathodic polarization in 0.1 M H2SO4 in a voltage range from 1.0 to 0.2 V at a scan rate of 10 mV/s.
Results and Discussion
By using XRD, phase composition of the treated samples was determined as, %: FeS -19; FeCl3 - 5; FeO - 6; Fe2O3 - 10; FeSi - 4; Fe3C - 29; Fe3N - 19; Fe - 8.
It can be seen that the main phases of the sample in the catalyst are iron and its compounds. Iron chloride (III) is the precursor for the catalyst synthesis and its presence in a sample is explained by the insufficient degree of decomposition of chloride. Iron sulfide formed by the reaction of metallic iron (precursor) with either ammonium persulfate, or elemental sulfur, which is formed from ammonium persulfate. The formation of iron oxides, apparently, due access of air to the reaction mixture during the heat treatment. The oxygen diffuses well in a porous medium; this explains the relatively high total content of iron oxides in the heat-treated sample. Iron silicide is formed by the interaction of iron with silicon, which is impurity of precursors. Both iron carbide and nitride are formed by reaction of metallic iron by heating with the appropriate precursors of carbon and nitrogen.
The cyclic voltammogram (Fig. 1) recorded on the sample synthesized in 0.1 M H2SO4, has a form that is typical for Fe/N/C catalysts.
Two peaks, the one is at 0.65 V and the second is at 0.70 V, caused be the reversible reaction occurred:
O2 + 4H+ + 4e = 2H2O
As noted in the recent review of Fe/N/C catalysts, the active sites in them are the iron nitrides, carbides, and sulfides [7]. It was logical to assume that these increasing the proportion of this compounds in the catalyst will increase its catalytic activity toward ORR.
This problem can be solved by electrochemical treatment of the catalyst in aqueous solution.
Scan rate - 10 mV*s-1
0
1,2
Potential, V (RHE)
Fig. 1. Cyclic voltammogram recorded on the Fe/N/C catalyst in 0.1 MH2SO4
Known data on iron nitrides show these compounds may be synthesized by high-temperature treatment including electrolysis of melts. Iron carbide may not also be produced by electrolysis of solution. The possible way of increasing the content of iron carbides and nitrides in the sample may be a reduction of iron oxides by hydrogen, generated on the cathode, and further formation of iron sulfides under conditions of electrochemical polarization of electrode.
After cathodic polarization of the sample in 0.5 M NaCl + 0.5 M HCl solution for 30 min. at E = -0.8 V (potentiostatic mode) , the content of iron was found to be reduced; phase composition was determined as: FeS - 20%; FeCl3 - 4%; FeO - 1%; FeSi - 4%; Fe3C - 30%; Fe3N -19%; a-Fe - 12%. Cathodic polarization of the sample in 0,025 M Na2S +
0.005.M NaCl solution for 40 min at current density 0.7 mA/cm2 (galvano static mode) changes the phase composition of the sample: FeS - 22%; FeCl3 - 1%; FeO - 1%; FeSi -4%; Fe3C - 30%; Fe3N -20%; Fe3S4 - 19%; a-Fe - 3%. Reducing content of iron chloride (III) in the sample can be related to its partial dissolution during treatment.
As it follows from the curve, cathodic treatment of the sample results in an increase of current density of oxygen reduction reaction (3.0 vs. 2.4 mA/cm2) at E = -0.8 V. Activities of both initial catalyst and electrochemically (cathodic polarization) treated one, expressed in «current/ mass of the sample» ratio, were determined as 9.8 h 12.2 A/g, respectively. This fact indicates an increase in the catalytic activity of the original Fe/N/C on the basis of poly-m-phenylenediamine against ORR of 25% in its two-stage processing of the cathode.
This shows a 25% increase of catalytic activity of original Fe/N/C catalyst toward oxygen reduction reaction after two-stage cathodic processing. Conclusions
By varying the conditions of synthesis (nature and ratio of the starting reactants, temperature, etc.), it can be possible to increase the activity of original Fe/N/C catalyst. Electrochemical treatment can serve as a suitable method to achieve this goal.
References
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