WO2010146475A1

WO2010146475A1 - Supported catalysts

Info

Publication number: WO2010146475A1
Application number: PCT/IB2010/051372
Authority: WO
Inventors: Shan Ji; Sivakumar Pasupathi; Bernard Jan Bladergroen; Vladimir Mikhailovich Linkov; Xolelwa Ralam
Original assignee: University Of The Western Cape
Priority date: 2009-06-18
Filing date: 2010-03-30
Publication date: 2010-12-23
Also published as: ZA201106273B; EP2446494A1; EP2446494A4

Abstract

The invention discloses a supported catalyst for SPE electrolyser applications, in which the catalyst has a support carrying nanoparticles as oxygen evolution catalysts.

Description

SUPPORTED CATALYSTS

FIELD OF INVENTION

The present invention relates to supported catalysts.

More particularly, the present invention relates to supported catalysts for solid polymer electrolyte electrolysers.

BACKGROUND TO INVENTION

A SPE (solid polymer electrolyte) electrolyser system is potentially the best to produce hydrogen using renewable energy and is seen as the best electrolyser candidate for the sustainable energy future. In this hydrogen generation process, electricity is applied to the electrode assembly and water is split at the anode into oxygen and protons. The protons diffuse through the conducting membrane to the cathode to form hydrogen. When the electricity is provided by a renewable source of energy (e.g. wind or solar), then a truly zero emission hydrogen supply is achieved. SPE electrolysers can be coupled to distributed, intermittent renewable electricity sources (e.g. wind generators or solar panels), in order to store energy generated from excess electricity as hydrogen in gas cylinders or in metal hydrides. The stored hydrogen can later be used in hydrogen fuel cells to generate electricity at one's convenience (like a battery). In tie-grid configurations, electrolyser systems can also be used to store energy generated by electrical turbines during low demand periods or off peak times in order to reduce peak electricity costs. Finally, there is the possibility for hydrogen (mixed with air or oxygen) to be burnt directly as a fuel in cookers, heaters and welders/brazers.

The main disadvantage of SPE electrolysis at this point is that it is relatively expensive and the cost is compounded if a hydrogen compressor is added to a system to facilitate compact storage of hydrogen. The key is to make this technology cost effective by developing highly efficient catalysts and membranes and by integration of the hydrogen generation with hydrogen storage systems. Unsupported catalysts are employed so far due to the stability issues of the supports under the operating conditions of the electrolyser. IrO₂ and IrO₂-based dimensionally stable anodes have been promising candidates as electrocatalysts for SPE electrolysers, however, they account for a significant part of the electrolyser cost.

It is an object of the invention to suggest a supported catalyst, which will assist in overcoming these problems.

SUMMARY OF INVENTION

According to the invention, a supported catalyst for SPE electrolyser applications, includes

(a) nanoparticles as oxygen evolution catalysts; and

(b) a support for the nanoparticles as oxygen evolution catalysts.

Also according to the invention, a method to produce a supported catalyst for SPE electrolyser applications, which includes the steps of

(a) dispersing nanoparticles as oxygen evolution catalysts; and

(b) of supporting the nanoparticles as oxygen evolution catalysts.

The supported catalyst may be used as anode catalysts for SPE electrolysers.

The support for may include carbon nanotubes, carbon nanofibers, titanium nanotubes, titanium nanofibres and TiB₂.

The supported catalysts may be prepared by means of wet impregnation and autoclave methods.

The supported catalysts may be stable up to 1.7V vs standard hydrogen electrode for oxygen evolution reaction. The mass specific activities of the supported catalysts may be better than unsupported catalysts.

The stability of the support may be improved through pre-treatment by heating under controlled atmosphere or by acid treatment.

The stability and activity of the supported catalysts may be improved by tailoring their preparation conditions.

The nanoparticles may include be Pt, Ir, Ru, Ta, Sn or Pd or oxides of the same.

The nanoparticles may include binary mixtures of Pt, Ir, Ru, Ta, Sn and Pd and their oxides.

The supported catalyst may include highly dispersed and homogeneously distributed nanoparticles on the CNT support.

The supported catalyst may include highly dispersed IrO₂ nanoparticles over the CNT support.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described by way of example with reference to the accompanying schematic drawings.

In the drawings there is shown in :

Figure 1 : Stability of supported nanoparticles at various potentials;

Figure 2: Activity and stability of supported nanoparticles prepared at various PH;

Figure 3: Activity and stability of supported nanoparticles prepared at various temperatures; Figure 4: Mass specific activity of supported catalyst as compared to commercial unsupported catalyst; and

Figure 5: TEM image of CNT supported IrO2 nanoparticles.

DETAILED DESCRIPTION OF DRAWINGS

According to the invention, a supported catalyst includes supported nanoparticles as oxygen evolution catalysts for SPE electrolyser applications.

Carbon nanotubes (CNTs) were selected as supports due to their unique physical and thermal properties. The CNT supported catalysts were developed by impregnation reduction method and evaluated for its stability and performance as SPE electrolyser anodes. Also, the preparation conditions of the supported catalyst were optimized with respect to pH, reducing temperature and loading of Ir on the support. An alternative synthesis method was also investigated, where highly dispersed and homogeneously distributed nanoparticles on the CNT support was achieved.

The supported nanoparticles were characterised electrochemically using chronoamperometric technique to study the stability of catalysts under SPE electrolyser operating conditions i.e., up to 1.8V. CNT supported catalysts when electrochemically evaluated were found to be stable up to 1.7V, which is considered to be the maximum operating potential for the electrolyser to produce hydrogen at economically feasible power consumption.

The effect of synthesis conditions on the activity of catalysts was also studied. The studies revealed that the catalysts when synthesized at pH 6 exhibited greater stability and activity. The pH was then fixed at 6 and the reducing temperatures were varied from O⁰C to 8O⁰C and the catalyst activity and stability was evaluated. The catalysts prepared at 8O⁰C exhibited better activity and stability as compared to those prepared at lower temperatures. The mass specific activity of the prepared catalysts was then compared with that of commercial IrO₂ (Alfa Aesar) and was found to be better than twice the activity of the commercial catalyst. An alternative preparation method for the supported catalysts was also studied, where highly dispersed IrO₂ nanoparticles over the CNT support was achieved.

The supported catalysts were prepared using a wet impregnation method. The catalyst precursor solution was prepared beforehand by weighing a required amount of hexachloroiridic acid and dissolving it in hydrochloric acid. The required amount of carbon nanotubes were weighed into beaker and ultra pure water was added and the CNTs were ultrasonically dispersed for three minutes. Then, additional amount of ultrapure water was added and the solution was further ultrasonically dispersed for half an hour. The dispersion was then mechanically stirred and a catalyst precursor solution was added drop wise from a burette to form a heterogeneous mixture. The precursor solution was prepared by dissolving H₂IrCI₆-SH₂O in HCI. After two hours of stirring, the pH of the suspension was adjusted to the required value by adding NaOH and a required volume of formaldehyde was then gradually added drop wise into the mixture with a burette and was left to stir over night. The suspension was then filtered, washed copiously with water and dried in an oven for two hours at 8O⁰C. The catalysts were then oxidized in a furnace at 200⁰C to obtain IrO₂ZCNT and was then stored in an air tight container for characterization.

The electrochemical measurements were carried out with Eco-Chemie Autolab PGSTAT30 using a three electrode setup. A glassy carbon electrode coated with the catalyst layer was used as the working electrode. A platinum mesh was used as the counter electrode and Ag/ AgCI was used as the reference electrode. All the potentials referred herein are with respect to the standard hydrogen electrode.

The catalyst ink was prepared by ultrasonically blending the required amount of catalyst with ultra pure water and nafion (perflourinated ion-exchange resin) in the ultrasonic water bath for 3-20 minutes. The working electrode was prepared by pipetting the required amount of the catalyst ink onto the glassy carbon electrode and then dried in an oven at 8O⁰C.

Chronoamperometric analysis was employed to study the activity and stability of the catalysts at various potentials. Figure 1 shows the chronoamperometric graphs of CNT supported IrO₂ at different potentials ranging from 1.4 to 1.8V. The results show that no noticeable current was produced at 1.4V but above 1.4V the OER activity was noted, which increased with the potential. The figure also reveals that the oxidized CNT supported catalysts are stable up to 1.7V. Above 1.7V, the stability of the supported catalysts dropped significantly with time and finally dropped to zero within 1200 seconds of operation. The results indicate that the support is oxidised above 1.7V. It is evident from Figure 1 that the catalyst is stable up to 1.7V, which is a feasible operating voltage for electrolysis. This study proves that CNTs can be used as supports for SPE electrolyser anodes. The activity of supported catalysts can be further improved by tailoring their preparation conditions.

Example I

The method for preparing the catalysts was similar to the one described above but during the pH adjusting step, it was varied from 1.8 - 9. The catalysts were prepared at pH ranging from acidic to basic conditions and its effect on the catalyst activity was studied. Figure 2 shows the chronoamperometric graphs of catalysts prepared at various pH. The catalyst prepared at pH 6 showed the best performance.

Example II

The conditions for synthesizing the catalysts were the same as described earlier in the detailed description section. The pH was fixed at 6 and the temperature was varied from O⁰C to 8O⁰C while the reducing agent was added to the suspension. The activities of the catalysts are shown in Figure 3. The catalyst prepared at 8O⁰C was found have the highest activity of those tested. The mass specific activity of the supported catalyst was found to be twice better than that of the commercial unsupported catalyst (Figure 4).

Example III

The supported catalysts were prepared in a different way than described above. H₂IrCl6»6H2θ and sodium citrate (1-50: 1 weight ratio) were dissolved in ethylene glycol and stirred for 0.5-10 h to obtain the precursor solution. CNTs were added into H₂O/ethylene glycol mixture containing 1-90% H₂O and mechanically stirred for 30 min and were then dispersed in an ultrasonic bath for 30 min.

The H₂IrCU solution was added to the CNT slurry under stirring and then treated in an ultrasonic bath for 10-600 min. At the end of the reaction, the pH was adjusted within the range 6-14 using NaOH/ethylene glycol solution.

The CNT/ H₂IrCU mixture was then kept in an autoclave and heated at the temperature range from 100 to 200⁰C in a furnace and the temperature was programmed with a rate on increase of l-50°C/min.

The autoclave was cooled down slowly to room temperature and the pH adjusted to 1-7 using 10% HNO₃ in ethylene glycol. The sample was recovered by filtering, and washed with de-ionized water until all Cl^" ions are completely removed. The black precipitate was dried in oven at 6O⁰C for 1-8 h. After that, the sample was heated in the temperature range from 150-350⁰C to oxidize Ir. The TEM image of synthesized IrO₂/CNTs was shown in Figure 5.

The present invention thus relates to the development of supported IrO₂ nanoparticles as electrocatalysts for SPE electrolyser anodes. The support allows the dispersion of particles leading to a greater utilization of the active surfaces and thereby is expected to reduce the catalyst loading considerably.

Claims

PATENT CLAIMS

1. A supported catalyst for SPE electrolyser applications, which includes

(a) nanoparticles as oxygen evolution catalysts; and

(b) a support for the nanoparticles as oxygen evolution catalysts.

2. A supported catalyst as claimed in claim 1, which is adapted to be used as anode catalysts for SPE electrolysers.

3. A supported catalyst as claimed in claim 1 or claim 2, in which the support includes at least one component selected from the group consisting of carbon nanotubes, carbon nanofibers, titanium nanotubes, titanium nanofibres and TiB₂.

4. A supported catalyst as claimed in any one of the preceding claims, which is prepared by means of wet impregnation and autoclave methods.

5. A supported catalyst as claimed in any one of the preceding claims, which are stable up to 1.7V vs standard hydrogen electrode for oxygen evolution reaction.

6. A supported catalyst as claimed in any one of the preceding claims, in which the mass specific activities of the supported catalysts are better than unsupported catalysts.

7. A supported catalyst as claimed in any one of the preceding claims, in which the stability of the support is improved through pre-treatment by heating under controlled atmosphere or by acid treatment.

8. A supported catalyst as claimed in any one of the preceding claims, in which the stability and activity of the supported catalysts is improved by tailoring their preparation conditions.

9. A supported catalyst as claimed in any one of the preceding claims, in which the nanoparticles include at least one component selected from the group consisting of Pt, Ir, Ru, Ta, Sn, Pd and oxides of the same.

10. A supported catalyst as claimed in any one of the preceding claims, in which the nanoparticles include binary mixtures of Pt, Ir, Ru, Ta, Sn and Pd and their oxides.

11. A supported catalyst as claimed in any one of the preceding claims, which includes highly dispersed and homogeneously distributed nanoparticles on the CNT support.

12. A supported catalyst as claimed in any one of the preceding claims, which includes highly dispersed IrO₂ nanoparticles over the CNT support.

13. A method to produce a supported catalyst for SPE electrolyser applications, which includes the steps of

(a) dispersing nanoparticles as oxygen evolution catalysts; and

(b) of supporting the nanoparticles as oxygen evolution catalysts.

14. A method as claimed in claim 13, which includes the steps of wet impregnation and autoclave methods.

15. A method as claimed in claim 13 or claim 14, which includes the step of pre- treatment by heating under controlled atmosphere or by acid treatment.

16. A supported catalyst for SPE electrolyser applications substantially as hereinbefore described with reference to the accompanying drawings.

17. A method to produce a supported catalyst for SPE electrolyser applications substantially as hereinbefore described with reference to the accompanying drawings.