Catalytic nickel oxide sheet, method for obtaining it and use thereof

Abstract

The present invention relates to an enhanced catalytic nickel oxide sheet having an organic part which includes non-stoichiometric nickel oxides dispersed in an organic matrix, wherein the catalytic sheet is supported on a substrate. The invention also relates to a method for obtaining the catalytic film and to its uses as an electrode in electrocatalysis of water or in photocatalysis.

Claims

1. A nickel oxide catalytic film, wherein the catalytic film is provided with: an inorganic part, and an organic part, wherein the inorganic part comprises a non-stoichiometric nickel oxide in oxidation states of Ni (II) and Ni (III), wherein the non-stoichiometric nickel oxide is crystalline, and wherein the organic part is present between 10% and 30% wt. of the total weight of the film and the non-stoichiometric nickel oxide is dispersed therein, so that the organic part is a support for the inorganic part, and wherein the organic part is composed of at least one organic compound selected from the group of an alkoxide, acetate, and amine, and wherein the catalytic film is supported on a substrate.

2. The catalytic film according to claim 1, wherein the catalytic film has a thickness of between 20 and 600 nm.

3. The catalytic film according to claim 1, wherein the catalytic film is one layer.

4. The catalytic film according to claim 1, wherein the catalytic film supported on a substrate is an electrode.

5. The catalytic film of claim 1, wherein the catalytic film is a photocatalytic electrode.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For a better understanding of what has been said, some drawings are attached in which, schematically and only by way of non-limiting example, a practical case of realization is represented.

(2) FIG. 1 shows a graph of UV-Visible absorbance of a catalytic film obtained according to Example 1 for a 0.9M Ni(AcO).sub.2 solution in methoxyethanol for different aging times in step i)-c) and constant temperature of 50 C.

(3) FIG. 2 shows a graph of the UV-Visible transmission spectrum of a catalytic film obtained according to Example 2 for different curing temperatures in step iii)-e).

(4) FIG. 3 shows a graph of the X-ray diffraction spectrum of a catalytic film obtained according to Example 2 for different curing temperatures in step iii)-e).

(5) FIG. 4 shows transmission electron microscope (TEM) images of a catalytic film obtained according to Example 2 and cured, in step iii)-e), at a temperature of 100 C. compared to a cure at a temperature of 500 C.

(6) FIG. 5 shows a graph of the water hydrolysis employing the catalytic film obtained according to Example 2 of the invention with cure temperature of 100 C.

(7) FIG. 6 shows a bar diagram of the overpotential (V) of prior art oxides in 1M NaOH at 10 mA cm.sup.2 described by McCrory et al in Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction in J. Am. Chem. Soc. 2013, 135, 16977-16987.

DETAILED DESCRIPTION OF THE INVENTION

(8) Preferred embodiments for carrying out the present invention are described below.

Example 1

(9) Initially, the NiOx precursor solution was prepared. A 0.9 M solution of nickel acetate tetrahydrate (2.2 g) in methoxyethanol (V=10 mL) was prepared to which 0.04 mL of MEA was added. The mixture was stirred by dissolving a portion of Ni(AcO).sub.2. The mixture was then heated in a thermostatic bath at 30-70 C. for 5-60 min. After 5 min all Ni(AcO).sub.2 was dissolved. The aging step was followed by UV-VIS spectroscopy (see FIG. 1).

(10) FIG. 1 shows a narrow band of absorbance in UV at 397 nm, and another wide band in visible at 670 nm with a shoulder at 754 nm. After 10 minutes of aging, the bands became more intense and shifted slightly towards IR 400, 679 and 755 nm, respectively. As the reaction time increased, the wavelength of the bands were unchanged, but they increased slightly in intensity up to 60 min, a time at which the reaction was considered complete since no change in intensity was observed up to 180 min. After 180 min, it was observed that the solution was no longer crystalline transparent due to the formation of a translucent turquoise gel. This change was attributed to the hydrolysis and polycondensation of nickel complexes resulting in the formation of acetate oxy-hydroxides and nickel methoxyethoxide (Ni(OH).sub.2, NiOOH) with sizes above 100 nm and gelling of the undesirable solution.

Example 2

(11) From the data extracted from the absorbance spectra of Example 1, a Ni(AcO).sub.2 solution of 0.45 M aged for 60 min at 70 C. was employed as NiOx precursor solution. A thin film of NiOx was continued to be deposited on a glass substrate by spincoating at a speed of 2,000 rpm for 20 s. FIG. 2 shows the transmittance curves for different curing temperatures performed over a 20 min time period at that temperature.

(12) The formation of the NiOx layers was followed by UV-Vis spectroscopy (see FIG. 2). The presence of non-stoichiometric NiOx was confirmed by absorption in the visible between 900 and 350 nm. The decrease in transmittance from 350 nm was due to the fact that glass is not transparent to UV. It should be noted that stoichiometric NiO (Ni(II)) is a broadband semiconductor that does not absorb light in the visible spectrum, so the radiation absorption was due to the part of Ni being present in the form of Ni(III) that it does absorb in the visible. As the cure temperature increased from 50 C. to 500 C. (see FIG. 2), the absorption in the visible increased due to the formation of more Ni(III); as the temperature increased, the evaporation of the solvent took place as well as the decomposition of the acetates and the methoxyethoxides and MEA, generating NiOx with greater relative amount of Ni(III). An inflection point was observed at curing temperatures above 200 C., where the transmittance at 550 nm went from 91% at 50 C. to 80% at 200 C. From 200 C. the change was less significant.

(13) Trials

(14) In order to determine the crystallinity of the non-stoichiometric nickel oxide formed in the catalytic film, an X-ray diffraction test was performed for different curing temperatures (see FIG. 3).

(15) It was observed that at the different curing temperatures of 50 C. to 500 C., NiOx showed no diffraction peaks, even at temperatures up to 500 C. All observed peaks belonged to silicon, which is the substrate used to take the measurements. The absence of characteristic NiO, NiOOH, or Ni(OH).sub.2 peaks confirmed that NiOx films were formed by nanometric crystalline domains, i.e., very small-sized nanocrystals.

(16) To determine the presence of an organic part after the curing step, images were taken with a transmission electron microscope (TEM) (see FIG. 4). In FIG. 4 the differences obtained between a cured film, for example, at 100 C. with respect to another cured film at 500 C., from a solution with the same composition are shown. With the 500 C. film a very compact material was obtained with about 25 nanometers of thickness. Surprisingly, the film with a cure at 100 C. was obtained with a thickness of 100 nanometers with separation between the grains. Contrary to what might be expected, at lower temperatures the material showed better catalytic properties and adequate stability.

(17) A test of the catalytic properties of the catalytic film obtained was then performed. The most representative measure of catalytic activity was the overpotential needed to reach current densities of 10 mA/cm.sup.2. The overpotential is defined as the excess energy that has to be applied for the reaction to occur, that is, the activation energy. In general, all chemical reactions have an activation energy. Catalysts reduce said activation energy. In electrochemical terms, the activation energy can in some way be equated to the overpotential. Therefore, we proceeded to check the overpotential necessary to perform the electrolysis of water using an electrode formed by a sheet of nickel with the catalytic film. The overpotentials obtained were of the order of 0.29 V (290 mV) (see FIG. 5) which demonstrated improvement of the overpotentials measured with reference materials such as Ir and Ru oxides (see FIG. 6), where specifically the IrOx (non-stoichiometric iridium oxide) shows a overpotential of 0.33 V. Finally, the small peak around 100 mV (FIG. 5) evidenced the formation of Ni(III) oxide and, therefore, the passage of a part of Ni(II) from the Ni(II) state to Ni(III).