WO2013011146A2

WO2013011146A2 - Method of synthesis of electrocatalytically active porous carbon material for oxygen reduction in low-temperature fuel cells

Info

Publication number: WO2013011146A2
Application number: PCT/EP2012/064363
Authority: WO
Inventors: Jaan Leis; Mati Arulepp; Maike KÄÄRIK; Anti Perkson
Original assignee: OÜ Skeleton Technologies
Priority date: 2011-07-21
Filing date: 2012-07-21
Publication date: 2013-01-24
Also published as: WO2013011146A3

Abstract

The method for the synthesis of nitrogen-enriched carbon material where for producing the nitrogen enriched carbon first the metal atoms (M) are removed from nitrogen enriched carbide MCaNb and thereafter the surface of carbon is deactivated in reducing atmosphere having in result a porous material with a carbon-nitrogen skeleton. In result is obtained the nitrogen-enriched carbon material where the nitrogen atoms are substituting a part of carbon atoms in the crystal lattice of carbide enabling to shift the oxygen reduction potential to positive potential values compared to non-enriched carbon material thus allowing the electrocatalytic reduction of oxygen in low-temperature fuel cells or metal-air batteries.

Description

Method of synthesis of electrocatalytically active porous carbon material for oxygen reduction in low-temperature fuel cells

Technical Field

The present invention relates to the synthesis of nitrogen-enriched carbon materials. The invention also relates to the application of these materials for the electrochemical reduction of oxygen.

Background Art

Electro-catalytic reduction of oxygen is one of the key issues for a low-temperature fuel cell, but also finds application in metal-air batteries and biosensors. More specifically, for the efficient oxygen reduction it is crucial to have an electrode with efficient electrocatalytic properties. So far the best catalysts are noble metals such as platinum and it`s complexes, which are very expensive and therefore not applicable in large-scale industrial production. To replace the noble metal catalysts many types of carbon materials are studied for the electrocatalytic reduction of oxygen, among these pyrolytic graphite, highly oriented pyrolytic graphite, glassy carbon, boron doped diamond, reticulated vitreous carbon, carbon nanotubes and many others. Catalytic behaviour of carbons can be enhanced by the chemical doping with nitrogen. Treatment of carbon with ammonia and cyanides significantly improves the catalytic activity especially in alkaline solutions. It is found that pyridinic and graphitic nitrogen has great impact to the catalytic behaviour of carbon electrodes in acidic and alkaline solutions.

Use of metal carbonitrides for manufacturing of the carbon materials of the different pore size distribution is known where during the manufacture of said materials the carbon material porosity is regulated with different amount of nitrogen which is removed from the carbonitride crystals during carbonisation process.

Disclosure of Invention

This invention defines the novel class of carbon materials for the electrochemical reduction of oxygen. This invention also describes a method for making of such nitrogen-enriched porous carbon materials. The invention is partly based on the method of halogenisation of carbides to carbon, however, with a distinction that during halogenisation is formed a chemically stable carbon skeleton, which contains the nitrogen atoms integrated in carbon skeleton. A mass balance of carbon formation from carbide is based on a general chemical equation:

M_xC + xy/2X₂ → C + xMX_y

where M is a metal, X is a halogen and x and y are the stoichiometric constants.

The most popular halogenisation method is a treatment of carbide with halogen gas at high temperature, whereby the chlorine atoms react with metal atoms in crystal lattice of carbide and the chlorides produced, are removed by the inert carrier gas or the excess of chlorine. Usually, porous carbons are derived from metal carbides at temperatures below 1200 °C. At higher temperatures drastically increases the tendency to form multilayered graphitic lamellas that leads to the more ordered structures and formation of turbostratic graphite.

From the other side, the electrochemical reduction of oxygen is the more effective, the more the active surface area is present in carbon. Porosity and a size of the pores in carbide-derived carbons noticeably depend on the temperature of chlorination but also on the crystal structure and chemical composition of the precursor carbide. As a rule, the larger a number of metal atoms relative to carbon atoms, the larger the average pore size obtained in carbon. At the same time, the density of pores and a surface area in the unit volume is the largest in the case of stoichiometric MC carbides, of course, if the temperature of chlorination does not exceed the temperature of self-organizing the graphene layers – approximately 800 °C. At temperatures below 800-900 °C normally is formed a high-surface area microporous carbon with a dominating pore size of 7-8 Å. Because of the high rate of homogeneity these small micropores are hardly accessed by whatever molecules, especially in liquid phase, and therefore such material is not suitable as catalyst or even more so – as substrate for catalyst.

Prior art describes the enlarging of the micropores by applying the post-oxydation treatment to carbon. Such treatment inevitably modifies the carbon surface with the oxygen-bearing functional groups – as a result, the reducing properties of carbon become worse. Reducing properties of carbon can be improved by doping with the nitrogen atoms. The simplest way is to treat the carbon material with ammonia at high temperature. In this way it is possible to post-treat the carbide-derived carbon directly after chlorination procedure, but also after the oxidation of carbon if applied. A drawback of the ammonia-treatment is a surface-functionalisation of carbon with NH₂groups, which increase the chemical instability of the material and decrease it’s life when used as a catalyst. Amino groups also have rather poor catalytic activity in acidic solutions.

Present invention describes a method, which enables to make simultaneously the microstructure desired, while avoiding the formation of micropores with restricted access, and to enrich the carbonaceous material with catalytically active nitrogen atoms, among which the C-N=C, C-N-C bonds, incorporated in condensed carbon hexagons, predominate the C-NH₂ groups.

One of the goals to be solved with this invention is to make the nitrogen-doped carbon material possessing excellent oxygen reducing behaviour, whereby the method of making does not incorporate any kind of post-treatment of carbon material with ammonia or other nitrogen containing reagents.

One possibility to achieve the goal is at first to enrich metal carbide by nitrogen atoms in the way that nitrogen atoms substitute a part of carbon atoms in the crystal lattice of carbide. Such material is also called as metal carbonitride – for example titanium carbonitride, TiC_aN_b, which in variable carbon to nitrogen atomic ratio (a/b) can be purchased for example from H.C Starck. TiCN with varied atomic ratio can be made by thermal treatment of non-stoichiometric titanium carbide in dry nitrogen atmosphere or by reacting metal with elemental carbon in dry nitrogen atmosphere as described in

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5314656

B UNIV CALIFORNIA [US] 19940524

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Such material is also possible to make via carbothermal reduction of titanium oxide, titanium chloride, and titanium hydride in nitrogen atmosphere at temperatures 1000-1600 °C as in the case of titanium oxide is described by the following equation:

This method for example is considered in ANIMESH, Jha, et al, Formation of titanium carbonitride phases via the reduction of TiO2 with carbon in the presence of nitrogen, Journal of Material Science, 15011999, 34, 2, 307-322Possibility to vary the N/C ratio in carbonitride is very important as the nitrogen content partly determines the nitrogen content in the nitrogen-doped carbon material derived from this carbonitride. Also porosity and pore size distribution in carbon is directly influenced by the nitrogen content in precursor carbonitride, especially when higher temperatures are used to extract metal from carbonitride.

Next step toward nitrogen doped carbon is to remove the metal atoms from carbonitride by reacting with the halogen, for example in chlorine flow. When the halogen is chlorine, the chemical reaction can be described by the following equation:

MC_aN_b + z/2 Cl₂ → C + MCl_z + CN_x + (b-x)/2 N₂

where M is a metal (e.g. Ti) and a, b, z and x are the stoichiometric constants, whereby x depends on the reaction conditions, e.g. reaction temperature. Depending on the temperature of chlorination the result will be a porous material with a carbon-nitrogen skeleton. Important issue in achieving the goals of invention is selection of chlorination temperature. From one side, the higher temperature favours formation of the larger average pore size that is desirable for ensuring good access to the pores and catalytically active surface of nitrogen-doped carbon. From the other side, the higher chlorination temperature increases tendency of removing nitrogen atoms from carbon in the form of N₂ that may noticeably decreases catalytic activity of the material. Therefore, according to this invention a relevant control over carbonitride elemental content (C/N ratio) and chlorination temperature of carbonitride together can provide a desired porosity and nitrogen content in the nitrogen-doped carbon material, which can be used for the electrocatalytic reduction of oxygen in low-temperature fuel cells or metal-air batteries.

Final step toward high-quality nitrogen enriched carbon is the post-treatment (by deactivating the surface of carbon in reducing atmosphere) of nitrogen-enriched carbon material, made by chlorinating the nitrogen enriched carbide, which aim is to deactivate the surface of carbon by bonding the uncoupled valence electrons, which otherwise reduce the electrochemical stability of carbon if such material, for instance, is used in electrochemical devices such as low-temperature fuel cells, metal-air batteries or electrochemical capacitors. The other purpose of post-treatment is to remove the residues of chlorine, chemically or physically absorbed in carbon that also may destabilise the electrochemical performance of carbon. Different reducing reagents used for creating reducing atmosphere can be used for that purpose (for example ammonia, hydrogen etc), however, the best is to treat carbon in hydrogen flow at temperature above 500 °C that is sufficient to remove chemically bound chlorine. The post-treatment temperature, however, should not exceed 900-1000 °C that may decompose the N-C bonds and remove the nitrogen from porous carbon-nitrogen skeleton.

Brief Description of Drawings

Fig. 1 is a graph representing polarisation curves for oxygen reduction of different carbon materials in O₂-saturated 0.5M H₂SO₄ solution (the RDE voltammetry curves for O₂ reduction of modified GC electrodes (noted on figure) in O₂ saturated 0.5M H₂SO₄ electrolyte at ω = 1900 rpm, v=10mVs^-1),

Fig. 2 is a graph representing polarisation curves for oxygen reduction of different carbon materials in O₂-saturated 0.1M KOH solution (the RDE voltammetry curves for O₂ reduction of modified GC electrodes (noted on figure) in O₂ saturated 0.1 M KOH electrolyte at ω = 1900 rpm, v=10mVs^-1).

Best Mode for Carrying Out the Invention

A following example describes the synthesis procedure for making the nitrogen-enriched porous carbon from carbonitrides.

Titanium carbonitride (C/N - 50/50, H.C. Starck, Lot. 75624, 150 g) powder was placed in the horizontal tubular quartz furnace and was treated with chlorine gas (AGA, 2.8) for 4 hours at 800 °C. A flow-rate of chlorine was 2.5 l/min. By-produced TiCl₄ was removed in the flow of excess chlorine and neutralised in alkali solution. During heat-up and cooling the reactor was purged with argon. Reaction product was additionally post-treated in hydrogen flow at 800 °C during 2.5 hours, the purpose was to deactivate the carbon surface and to remove all the chlorine residues absorbed in the nitrogen enriched carbon material. Characteristics of the product are presented in Table 4.

A porous structure of carbon materials was characterized by different benzene and nitrogen adsorption methods. The low-temperature nitrogen adsorption experiments were done by using the Gemini 2375 device (Micromeritics). Specific surface of carbon samples was calculated according to the Brunauer-Emmet-Teller (BET) theory below relative nitrogen pressure (P/P₀) of 0.2. A total pore volume was calculated at relative nitrogen pressure (P/P₀) of 0.95. Pore size distribution and the fractions of different pore sizes (V_micro and V_meso) were determined according to the method of Barrett-Joyner-Halenda (BJH), considering that pores are slit-type pores and micropores are pores with a size below 2nm and mesopores are pores larger than 2nm.

The adsorption of benzene was measured for carbon samples placed in the benzene vapour at room temperature and normal pressure and weighed periodically with 10-seconds interval until the maximum weight of the carbon sample was reached. By using the maximum weight value, the volume of micropores was calculated according to the equation:

W_s = (m₂ – m₁) / m₁ · d_C6H6 [cm³g^-1]

where m₁ and m₂ are the initial and final weights of the carbon sample, respectively, and d_C6H6 is a density of benzene at room temperature.

Specific surface areas (S_BET) according to the BET theory, micropore volumes according to benzene (W_s) and total pore volumes according to nitrogen (V_tot) of inventive examples are presented in Tables 1-3.

TABLE 1 is a table showing the effect of nitrogen content in precursor carbide (carbonitride) on the porosity of carbon derived from this carbide (carbonitride)

Table 1

Carbon	Carbide	T_react[°C]	W_s[cm³/g]	S_BET[m²/g]	V_tot[cm³/g]	V_micro[cm³/g]	V_meso[cm³/g]
comparison 1	TiC	800	0.61	1467	0.70	0.61	0.09
example 1	TiC0.5N0.5	800	0.66	639	0.40	0.20	0.20
example 2	TiC0.7N0.3	800	0.74	1267	0.64	0.45	0.19

TABLE 2 is a table showing the effect of chlorination temperature on the porosity of carbon derived from TiC0.5N0.5

Table 2

Carbon	Carbide	T_react[°C]	S_BET[m²/g]	V_tot[cm³/g]	V_micro[cm³/g]	V_meso[cm³/g]	V_micro/V_meso
example 1a	TiC0.5N0.5	300	109	0.05	0.04	0.01	4.0
example 1b	TiC0.5N0.5	400	411	0.22	0.14	0.08	1.8
example 1c	TiC0.5N0.5	500	763	0.40	0.28	0.12	2.3
example 1d	TiC0.5N0.5	600	494	0.27	0.17	0.10	1.7
example 1e	TiC0.5N0.5	700	596	0.36	0.20	0.16	1.3
example 1f	TiC0.5N0.5	800	639	0.40	0.20	0.20	1.0
example 1g	TiC0.5N0.5	900	752	0.52	0.20	0.32	0.6
example 1h	TiC0.5N0.5	1000	630	n/a	n/a	n/a	n/a
example 1i	TiC0.5N0.5	1100	653	0.45	0.18	0.27	0.7

TABLE 3 is a table showing the effect of chlorination temperature on the porosity of carbon derived from TiC0.7N0.3

Table 3

Carbon	Carbide	T_react[°C]	S_BET[m²/g]	V_tot [cm³/g]	V_micro [cm³/g]	V_meso [cm³/g]	V_micro/V_meso
example 2a	TiC0.7N0.3	300	24	0.01	~0	~0	n/a
example 2b	TiC0.7N0.3	400	555	0.19	0.18	0.01	18
example 2c	TiC0.7N0.3	500	392	0.19	0.16	0.03	5.3
example 2d	TiC0.7N0.3	600	540	0.26	0.22	0.04	5.5
example 2e	TiC0.7N0.3	700	939	0.46	0.34	0.12	2.8
example 2f	TiC0.7N0.3	800	1267	0.64	0.45	0.19	2.4
example 2g	TiC0.7N0.3	900	1330	0.74	0.40	0.34	1.2
example 2h	TiC0.7N0.3	1000	1060	0.66	0.26	0.40	0.7
example 2i	TiC0.7N0.3	1100	1058	0.67	0.27	0.40	0.7

Procedure for electrochemical evaluation.

Coatings of catalytically active carbons, listed in Table 4, on the glassy carbon (GC) electrode surface were made from aqueous carbon suspensions containing 0.5% nafion (or 0.06% PTFE). The coatings were prepared by placing of corresponding carbon suspension (20µL), made by the aid of ultrasonic agitation (30min), onto the surface of GC and allowing the solvent to evaporate at 80°C.

The electrochemical tests with modified GC electrodes were carried out in both 0.5M H₂SO₄ and 0.1M KOH solutions, whereas the CDC-Nafion was used for acidic solution and CDC-PTFE for the alkaline solution. For oxygen reduction processes the solution was saturated with O₂-gas.

The EDI101 rotating disc device equipped with CTV101 speed control units (Radiometer) was used for the rotating disc electrode (RDE) experiments. The electrode rotation rate was varied between 360 and 4600 rpm. A saturated calomel electrode was employed as a reference electrode and the Pt-wire as a counter electrode was used. The potential was applied with Autolab potentiostat AUT128N and the experiments were controlled with General Purpose Electrochemical System software.

TABLE 4 is a table showing the properties of carbons used in the experiments of electrocatalytic oxygen reduction

Table 4

Carbon	origin^f	T_react[°C]	W_s [cm³/g]	S_BET[m²/g]	V_tot [cm³/g]
Comparison 2	Vulcan XC72R	-	n/a	241	n/a
^aexample 3	TiCN (H2)	800	0.29	683	0.86
^bComparison 3	SiC (H2)	1000	0.49	1083	0.54
^cComparison 4	SiC (NH3)	1000	0.49	1104	0.55
^dComparison 5	Al4C3 (H2)	1000	0.44	613	0.81
^eComparison 6	Al4C3 (NH3)	1000	0.44	532	0.92

^a – 3747, ^b – 3712, ^c – 3710, ^d – 3604, ^e – 3787, ^f – H2 or NH3 in brackets shows the reagent used to post-treat the carbon powder

Example 3 and Comparisons 3-6 are carbon materials made by Skeleton Technologies OÜ, Comparison 2 is an activated carbon Vulcan XC72R from Cabot Corp.

The background subtracted RDE voltammetry curves of oxygen reduction on GC electrode modified with different CDCs and Vulcan XC72R are shown on Fig 1 (the RDE voltammetry curves for O₂ reduction of modified GC electrodes (noted on figure) in O₂ saturated 0.5M H₂SO₄ electrolyte at ω = 1900 rpm, v=10mVs^-1), and Fig 2 (the RDE voltammetry curves for O₂ reduction of modified GC electrodes (noted on figure) in O₂ saturated 0.1 M KOH electrolyte at ω = 1900 rpm, v=10mVs^-1). The RDE measurements conducted at potential scan rate of 10mV s^-1 clearly confirm the enhanced electrocatalytic behaviour of CDC from carbonitride (example 3 (3747)) compared to nitrogen unmodified or post-modified carbons in both acidic and alkaline solutions.

The nitrogen-enriched carbon material synthesised according to present method allows to enhance the working voltage and power density of the low-temperature fuel cells or metal-air batteries whereas the oxygen reduction potential of said nitrogen-enriched carbon material in acidic electrolyte is shifted by at least 0.3V and in alkaline electrolyte by at least 0.1V to positive potential values compared to non-enriched carbon material. Thus providing possibilities to reduce the manufacturing cost of the fuel cells compared to Pd (Palladium)/Pt (Platinum) catalysed fuel cells.

Claims

The method for the synthesis of nitrogen-enriched carbon material where for producing the nitrogen enriched carbon 1) the metal atoms (M) are removed from nitrogen enriched carbide MC_aN_b where M is a metal and a, b are the stoichiometric constants, and thereafter the surface of carbon is deactivated in reducing atmosphere having in result a porous material with a carbon-nitrogen skeleton.
A method for the synthesis of nitrogen-enriched carbon material according to claim 1, characterised in that the carbide is enriched by nitrogen atoms in the way that nitrogen atoms substitute a part of carbon atoms in the crystal lattice of carbide.
A method for the synthesis of nitrogen-enriched carbon material according to claim 1, characterised in that the nitrogen-enriched titanium carbide is treated with chlorine gas at the temperature between 300 °C to 1100 °C and is subsequently post-treated with reducing agent such as hydrogen at a temperature between 500 °C to 1000 °C.
A method for the synthesis of nitrogen-enriched carbon material according to claim 1 characterised in that the elemental content of nitrogen-enriched titanium carbide, noted as C/N ratio, and chlorination temperature together are controlled to provide a desired porosity and nitrogen content in the nitrogen-enriched carbon material.
The nitrogen-enriched carbon material synthesised according to claims 1-4 where oxygen reduction potential of said nitrogen-enriched carbon material in acidic electrolyte is shifted by at least 0,3V to positive potential values compared to non-enriched carbon material.
The nitrogen-enriched carbon material synthesised according to claims 1-4 where oxygen reduction potential of said nitrogen-enriched carbon material in alkaline electrolyte is shifted by at least 0,1V to positive potential values compared to non-enriched carbon material.
A use of the nitrogen-enriched carbon material according to claims 5-6 for the electrocatalytic reduction of oxygen in low-temperature fuel cells or metal-air batteries.