RU2422950C1 - Method of making silicion-based electrodes of fuel cells coated with nano-catalysts - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: method involves formation of a metal primer on the surface of electrodes, particularly from solutions of gold-containing precursors via chemical deposition, depositing solutions of platinum-containing precursors and then thermal decomposition with photon annealing. The solutions of platinum-containing precursors used can be ammonium hexachloroplatinate solution.

EFFECT: high efficiency of electrodes of fuel cells, characterised by high rate of cathode reaction on electrodes, characterised by high values of open-circuit voltage.

5 cl, 1 dwg, 7 ex

Description

The invention relates to hydrogen energy and catalytic chemistry and can be used in chemical current sources, in particular in portable fuel cells with solid polymer or liquid electrolytes.

Electrodes of a fuel cell with nanocatalysts are one of the key components that largely determine productivity, efficiency, service life of a fuel cell, and its cost, since platinum is used mainly as a catalyst, which has the highest efficiency compared to other metals. The catalyst, being in the environment of fuel (hydrogen on the anode electrode) or oxidizer (oxygen on the oxygen electrode) must also simultaneously provide electrical contact with the current-conducting electrode and “proton” contact with the proton-conducting electrolyte. To increase the specific activity of the catalyst and reduce its consumption, a prerequisite is to obtain a nanoscale catalyst with a high specific surface.

In conventional solid polymer fuel cells, carbon is used as an electrode, and platinum group elements located on a carbon carrier are used as a catalyst.

A known method of preparing such an electrode (RF patent No. 2324538, IPC B01J 37/04, publ. 05/20/2008), comprising applying to the surface of the electrodes platinum-containing catalysts located on a carbon carrier. In this case, the fraction of nanoparticles fixed on the surface of the carbon carrier is at least 99%. To ensure reliability, a mixture of tin oxides and hydroxides (0.5-20 wt.%) May be present on the surface of the carrier.

The disadvantages of the proposed method is the presence in the structure of the "carrier" - autonomous carbon particles, ranging in size from 20-100 nanometers, the contact of which with the electrode is achieved by mechanical clamping of the array of carriers to the electrode. However, the clamping contact, being randomly determined, does not provide the required reliability and stability. In addition, such designs cannot be used to miniaturize silicon-based fuel cells.

A known method of preparing electrodes with nanocatalysts for fuel cells on silicon electrodes. Most of the proposals come down to the formation of catalysts on a silicon surface by vacuum deposition or CVD methods (Nechitailov A.A. et al. Catalytic properties of composite layers of amorphous carbon-platinum in fuel cells. Physics and Technology of Semiconductors, 2008, Volume 42, no. 10, p. 1273-1278).

However, the formation of a catalyst by these methods on the inner surface of pores even with a relatively low aspect ratio is associated with insurmountable technical problems.

A known method of manufacturing electrodes of fuel cells based on silicon with nanocatalysts deposited on their surface, in which composite porous layers with catalyst clusters embedded in the porous matrix are formed by vacuum deposition on the surface of silicon or porous silicon (A. Nechitailov et al. Catalytic properties of composite layers Amorphous Carbon-Platinum in Fuel Cells, Physics and Technology of Semiconductors, 2008, Volume 42, Issue 10, pp. 1273-1278) In addition to the function of the catalytic layer, these porous layers can simultaneously perform the function of a gas diffusion layer. The advantages of the proposed method is the possibility of forming a gas diffusion, catalytic layer with a thickness of several units to 150 nanometers.

The disadvantages of the proposed method are the impossibility of forming continuous layers both on the surface of pores with a diameter of more than 150 nanometers and on the inner surface of macropores and, as a result, the inefficiency of applying the method for electrodes based on macroporous silicon.

A known method of manufacturing electrodes of fuel cells (Yeltsin OS and others. The catalytic properties of platinum in fuel cells. Letters in ZhTF, 2008, volume 34, issue 16, pp. 36-40), in which a platinum catalyst is applied directly to Electrodes made of silicon or macroporous silicon. Precipitation is carried out from platinum-containing precursors by chemical reduction of platinum directly with silicon. The chemical reduction process is carried out at temperatures from -10 ° C to + 26 ° C. The highest catalytic activity was obtained at a negative temperature (-10 ° C).

The disadvantage of this method is the low reliability of the electrodes of the fuel cells due to the low stability of the catalyst in aggressive conditions of functioning of the fuel cells. After several tens of on-off cycles of fuel cells with such an electrocatalyst, or several hours of finding the electrodes with the catalyst in a 0.5-1 M H ₂ SO ₄ solution, the catalytic activity drops almost twice.

A known method of manufacturing electrodes of fuel cells based on silicon with nanocatalysts deposited on their surface (patent US 7,198,864 B2, IPC H01M 2/14, publ. 2005-04-28), adopted as a prototype, including the formation on the surface of porous silicon electrodes of metal primers, with the subsequent application of platinum-containing precursors on them and their drying. The formation of the primer was carried out by a multi-stage process. An aqueous solution of [Pt (NH ₃ ) ₄ ] (OH) ₂ -xH ₂ O with a pH of 8.5 was taken as platinum-containing catalyst precursors. At first, the silicon wafer was kept in solution for some time. As a result, a platinum-containing complex is deposited on the silicon surface. Residues of unreacted radicals were removed in cold water. Due to the low stability of the complex, it is necessary immediately after washing to carry out the thermal reduction of platinum by slowly heating the samples from 20 to 400 ° C.

However, these requirements both complicate the process and tighten the requirements for the precursors used, requiring exact observance of the pH of the solutions.

The present invention solves the problem of simplifying the technology while increasing the efficiency of the fuel cell electrodes.

The problem is solved by the method of manufacture of silicon-based fuel cell electrodes with nanocatalysts deposited on their surface, including the formation of a metal primer on the surface of the electrodes, the application of solutions of platinum-containing precursors on it and their subsequent thermal decomposition. The novelty of the invention lies in the fact that thermal decomposition is carried out by photon annealing.

It is known the use of thermal annealing for the decomposition of platinum salts (Nekrasov BV Inorganic chemistry. "Chemistry", 1973, volume 2, p. 413, 419). The use of photon annealing for the thermal decomposition of salts is unknown. We unexpectedly found that its use in the decomposition of solutions of platinum-containing precursors on the surface of a metal primer deposited on electrodes of fuel cells based on silicon or porous silicon leads to an increase in the efficiency of the cathodic reaction on the electrodes.

Photon annealing can be carried out by irradiation with powerful incoherent radiation of halogen lamps. The main part of the emission spectrum lies in the intrinsic absorption region of silicon (λ≤1.1 μm), which determines the high heating efficiency of the samples. In this case, the heating rate of the samples can reach (150-200) ^o / sec and depends on the design features of the photon annealing unit. At these rates of temperature rise, the desired temperature is reached in a few seconds. In this case, the catalyst recovery process takes place in a time that does not exceed 1 minute.

A study of the state of the art has shown that cases of using photon annealing for thermal reduction of catalysts in the manufacture of catalysts on fuel cell electrodes are unknown.

In this application, the term primer means a material used for preliminary application to the surface before forming a layer of nanocatalyst.

The term precursor refers to a chemical substance, an initial component, or a participant in intermediate reactions during the synthesis of a substance.

Standard electrodes of silicon can be used as fuel cell electrodes, for example, KDB7.5 or KEF7.5 brand plates, as well as porous silicon membranes or layers of porous silicon.

As solutions of platinum-containing precursors, a solution of ammonium hexachloroplatinate can be used.

Optimal for the present set of features is to obtain a primer from solutions of gold-containing precursors by chemical deposition.

To increase the reliability of the electrodes of the fuel cells after the formation of the metal primer, photon annealing is performed.

In hydrogen-oxygen fuel cells, regardless of the type of proton conductor used, the speed of the cathodic process determines the magnitude of the open circuit voltage of the half cell of the fuel cell (U _o ). All other things being equal, the higher U _o value corresponds to a higher cathodic reaction rate on the catalyst and, consequently, greater electrode efficiency. Theoretically, at room temperature, the value of U _o can reach a value of 1.23 B. Almost the maximum values of U _o are in the range of 0.75-0.95 B.

The highest electrocatalytic activity of the electrode, characterized by the largest value (U _o ), is achieved by the optimal choice of power and time of photon annealing. Moreover, an increase in power corresponds to a decrease in the time of photon annealing.

The technical result of the invention is the increased efficiency of the fuel cell electrodes, characterized by a high cathodic reaction rate on the electrodes, characterized by high values of the open cell voltage of the half cell of the fuel cell 0.82 B≤U _o ≥0.95 B. In addition, the use of photon annealing allows to reduce the temperature effect on the structure of the catalyst and, thereby, reduce the likelihood of enlarging the size of platinum clusters, which leads to an increase in the effective surface of the electrocatal platinum, which ultimately increases the efficiency of the electrodes.

The drawing shows a graph of the dependence of the voltage of the open circuit U _o from the operating time of the half-cell of the fuel cell.

Used techniques.

To form the primer, a gold-containing precursor is taken, which is a mixture of solution A and solution B in a volume ratio of 1:24, which are prepared according to the following recipe.

Solution A is prepared on the basis of: per 100 ml of aqua regia, concentrated nitric acid and concentrated hydrochloric acid (in a volume ratio of 1: 3), 0.240 g of pure metallic gold is added. The solution is heated to about 80 ° C and held until the final dissolution of gold. Then the solution is cooled to room temperature.

Solution B is prepared as follows: add 250 g of ammonium fluoride to 250 ml of distilled water, mix and pour 20 ml of concentrated hydrochloric acid after complete dissolution.

A comparative assessment of the effectiveness is carried out by measuring the open circuit voltage U _o in the half cell of the fuel cell. The half cell of the fuel cell is part of a standard two-electrode electrochemical cell with separated spaces of the working electrode and the reference electrode. As a reference electrode, a standard hydrogen electrode (SHE) is used. Measurements of the voltage U _o are carried out at room temperature, in a 1M solution of H ₂ SO ₄ , which is used as an electrolyte. Measurements are carried out in the established mode, which occurs after 20-30 minutes.

Photon annealing is carried out at a facility with a maximum irradiation power of 45 W cm ^-2 .

The following examples confirm, but do not exhaust, the invention.

Example 1

KDB7.5 standard silicon wafers are placed in a gold-containing primer solution (composition solution: A: B = 1: 24) for 10-15 minutes. After removing the plates from the solution and drying in air at a temperature not exceeding 80 ° C, a platinum-containing precursor of the composition is applied to the surface: 0.5 ml of an aqueous solution of (NH ₄ ) ₂ PtCl ₆ (3 g of platinum per liter of solution) and 3.5 ml of isopropyl alcohol. The sample is dried in air at room temperature. For complete chemical reduction of platinum and the formation of nanosized catalyst particles, photon annealing is carried out at a power of 27 W cm ^-2 and annealing time of 39 sec. The efficiency of the cathode electrode was characterized by a voltage value U _o = 0.855 volts:

- at a power of 27 W cm ^-2 and annealing time of 42-45 sec, the efficiency of the cathode electrode was characterized by a voltage value U _o = 0.83 volts;

- at a power of 27 W cm ^-2 and annealing time of 37-38 sec, the efficiency of the cathode electrode was characterized by a voltage value U _o = 0.82 volts.

Thus, the highest electrode efficiency was obtained with a photon annealing power of 27 W cm ^-2 and an annealing time of 39-41 sec.

Example 2

The same as in example 1, only porous films on silicon were used as the electrode material, and photon annealing was carried out at a power of 37 W cm ^–2 and annealing time of 7–9 sec. The efficiency of the cathode electrode was characterized by a voltage value U _o = 0.86 volts:

- at a power of 37 W · cm ^-2 and annealing time of 4-6 seconds, the efficiency of the cathode electrode was characterized by a voltage value U _o = 0.69 volts;

- at a power of 37 W · cm ^-2 and annealing time of 10-12 sec, the efficiency of the cathode electrode was characterized by a voltage value U _o = 0.82 volts.

Thus, the highest electrode efficiency was obtained with a photon annealing power of 37 W cm ^–2 and annealing time of 7–9 sec.

Example 3

The same as in example 1, only electrodes from porous membranes were used as the electrode material, and photon annealing was performed at a power of 36 W cm ^–2 and annealing time of 8–9 sec. The efficiency of the cathode electrode was characterized by a voltage value U _o = 0.89 volts:

- at a power of 37 W · cm ^-2 and annealing time of 6-7 sec, the efficiency of the cathode electrode was characterized by a voltage value U _o = 0.84 volts;

- at a power of 37 W · cm ^-2 and annealing time of 11-12 sec, the efficiency of the cathode electrode was characterized by a voltage value U _o = 0.845 volts.

Thus, the highest electrode efficiency was obtained with a photon annealing power of 37 W cm ^–2 and an annealing time of 8–9 sec.

Example 4. The same as in example 1, only as a platinum-containing precursor was used the composition of 0.5 ml of an aqueous solution of hexochloroplatinic acid H ₂ PtCl ₆ (3 g of platinum per liter of solution) and 3.5 ml of ethyl alcohol. The efficiency of the cathode electrode was characterized by a voltage value U _o = 0.92 volts.

Example 5. The same as in example 2, only after drying the primer in air, carry out photon annealing of the electrodes. At a power of 27-37 W cm ^-2 and a time of 6-10 sec, the catalytic efficiency of the electrodes is determined by the subsequent modes of catalyst formation. So when conducting photon annealing at a power of 37 W · cm ^-2 and annealing time of 7 seconds, the efficiency of the cathode electrode was characterized by the voltage U _o = 0.86 volts.

The reliability of the fuel cell electrodes after the formation of the metal primer using photon annealing is confirmed by the stability of the values of U _o for a long time of operation of the electrodes in an aggressive environment (0.5-1M solution of H ₂ SO ₄ ), which is shown in the graph of the dependence of the value of U _o on time the operation of the half-cell of the fuel cell (curve 2 in the drawing) with the above parameters. For comparison, curve 1 in the drawing shows a sharp decrease in the value of U _o characteristic of porous silicon electrodes, without the proposed primer treatment.

Example 6. For comparison, the manufacture of electrodes was carried out according to the prototype method. The efficiency of the cathode electrode was characterized by a voltage value U _o = 0.79 volts.

As can be seen from the above examples, the proposed method provides ease of manufacture of electrodes of fuel cells with high efficiency.

Claims (5)

1. A method of manufacturing electrodes of fuel cells based on silicon with nanocatalysts deposited on their surface, comprising forming a metal primer on the surface of the electrodes, applying platinum-containing precursors to it and their subsequent thermal decomposition, characterized in that the thermal decomposition is carried out by photon annealing. 2. The method according to claim 1, characterized in that as the electrodes of the fuel cells using silicon wafers, porous membranes or layers of porous silicon. 3. The method according to claim 1, characterized in that as solutions of platinum-containing precursors use solutions of ammonium hexachloroplatinate. 4. The method according to claim 1, characterized in that the metal primer is formed from solutions of a gold-containing compound by chemical deposition. 5. The method according to claim 4, characterized in that to increase the reliability of the electrodes of the fuel cells after the formation of the metal primer conduct photon annealing.

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