Patent Description:
An ideal process for hydrogen production is to use water as a hydrogen source together with electricity or sunlight for conversion. Accordingly, photo/electrocatalytic water splitting has attracted attention as one of the most promising hydrogen production processes. Nevertheless, the development of highly active photo/electrocatalysts for water splitting was hampered for a long time because of the difficulty of working with the thermodynamically uphill reaction. Unfortunately, catalysts, especially noble metals such as Platinum, are needed to speed the hydrogen evolution reaction (HER) kinetics, which limits the widespread application of this technology.

Another research focus is on improving the existing materials by either engineering suitable structures, increasing the surface area e.g. by nano-structuring, doping, straining, or edging, or using co-catalysts to increase a given material's efficiency. Current strategies for the catalyst design focus on increasing the number and activity of local catalytic sites, such as the edge sites of molybdenum disulfides (MoS<NUM>) in the Hydrogen Evolution Reaction. However, the electrocatalysts obtained from aforementioned strategies are often highly complex with uncertain exposed crystal surface and high density of defects, which bring more problems such as the difficulties in controlling active sites. In addition, the improvement of electrocatalytic activity by increasing the density of active sites with these strategies is very limited.

<CIT> discloses a mixed oxide that has a delafossite type of crystal lattice structure for the conversion of synthesis gas to linear alcohols and aldehydes. The mixed oxide can be represented by the formula: CuxMaFebO2x, wherein M is selected from the group consisting of Cr and Al and a, and b is equal to or almost equal to x.

<CIT> provides a hydrothermal treatment to NaAlO<NUM> and Ag<NUM>O to obtain delafossite-type AgAlO<NUM> material. The material can be used as catalyst for burning carbon without corroding a supporting honeycomb structure.

<CIT>provides a precious metal elements free compound for ammonia (NH<NUM>) and carbon oxide (CO) oxidation. The compound comprises an (earth)alkali component (Mg, Ba, or K), a transition metal (V, Cr, Mo, Mn, Fe, Co, Ni, Cu, Zn, Ag, or combinations thereof), and a metal oxide support.

<CIT> discloses a photocatalyst including an element selected from Cu, Al, Ti, Ga, Cd, Zn, W, Fe, Sn, Si, In or any combination thereof. Exemplary catalyst compounds include CuAlO<NUM>, TiO<NUM>, CuO, Cu<NUM>O, NiO, GaAs, GaP, CdSe, ZnO, WO<NUM>, Fe<NUM>O<NUM>, SnO<NUM>, SiC, CuGaO<NUM>, and CuInO<NUM> or any combination thereof. It can convert water into hydrogen and oxygen, and more particularly convert water into hydrogen and oxygen using sunlight.

<NPL>, reported the synthesis of delafossite CuAlO<NUM>. It can be used as photocatalyst for hydrogen evolution. But it only works at alkaline media and no photoactivity was observed below pH <NUM>.

<NPL> discloses delafossite CuAlO<NUM> nanoparticles with electrocatalytic activity toward oxygen and hydrogen evolution reactions.

<NPL> reported the synthesis of polycrystalline CuGaO<NUM> pellets with a delafossite structure from Ga<NUM>O<NUM> and CuO by high temperature solid-state synthesis. With the synthesized CuGaO<NUM> pellets as a photo electrode, the photoelectrochemical water reduction to molecular hydrogen was demonstrated at a higher potential than the flat-band potential under UV light illumination.

<CIT> describes a solution by separating an oxygen evolution photocatalyst and a hydrogen evolution photocatalyst by an electrically conductive separator layer.

<CIT> discloses the use of wide-spectrum excitation of noble metal core/semiconductor shell hybrid nanoparticles for unassisted photocatalytic splitting of water. The metal core/semiconductor shell composite nanoparticles comprise a noble metal (e.g. Au, Ag, Pt, Pd, or noble metal alloy) core which is coated with a wide-bandgap semiconductor photocatalyst (e.g. TiO<NUM>, ZnS, Nb<NUM>O<NUM>) transparent to optical excitation in the visible and near-infrared (NIR) spectral ranges, consistent with plasmon absorption bands of the metal core.

<CIT> relates to a photocatalyst for generating hydrogen from water using visible light irradiation comprising nanocrystalline cobalt (II) oxide nanoparticles.

It was an object of the present invention to provide a photo/electrochemical cell comprising an electrode for hydrogen reduction comprising more affordable and more efficient catalysts for the Hydrogen Evolution Reaction (IIER). It was a specific object of the invention to increase the intrinsic electrochemical activities by optimizing the conductivity and carrier mobility of the catalysts for HER. This would facilitate the electron transfer between the catalysts surface and the adsorbates, which in turn would speed-up the reaction kinetics of the HER. At the same time the catalysts should have high catalytic activity for HER, so that they can be used for large scale hydrogen production. It would be preferable to provide catalysts which come as close as possible to the most efficient catalysts such as e.g. Pt which only needs an overpotential of <NUM> mV to deliver a current density of <NUM> mA cm-<NUM>. Moreover, the cost for such catalysts should be lower than that of known catalysts, specifically lower that pure Platinum. Additionally, the catalysts should exhibit a high chemical stability and durability to allow for a stable and extended lifetime of the photo/electrochemical cells, so that they can produce hydrogen continuously for days or even months.

The present invention provides a photo/electrochemical cell comprising an electrode for hydrogen reduction comprising compounds of delafossite oxides of the formula ABOx(I), In formula (I).

The catalysts can be used directly as an electrode for hydrogen reduction in the photo/electrochemical cell. The catalysts show remarkable performance in acidic media, with low overpotentials, and excellent stability over time. The catalysts according to the present invention work at a much lower overpotential than pure Pt at large current densities.

The present invention is directed to a photo/electrochemical cell comprising an electrode for hydrogen reduction comprising compounds of delafossite structure of the formula (I).

Most preferred are the delafossite oxide compounds PdCoO<NUM> and PtCoO<NUM>. PdCoO<NUM> and PtCoO<NUM> have a layered structure and crystallize in a rhombohedral space group, which is built by an alternating stacking of [A] layers and [BO<NUM>] slabs along the c-axis. Most delafossite oxides are insulators, but some, e.g. PdCoO<NUM>, PdCrO<NUM>, or PtCoO<NUM>, are good metals. The in-plane conductivities at room temperature are only about <NUM>µΩ cm, which is even higher than that of pure metals such as Pd, Cu, and Au. However, their carrier density is approximately <NUM> *<NUM><NUM> cm-<NUM>. This is a factor of three lower than that of a 3d transition metal. This results in a long mean free path length of up to <NUM>, which is the longest of any known large carrier density metal. Considering that the "B"-elements are much cheaper than Platinum, and oxygen is free, the cost of catalysts can be decreased by as much as <NUM>%. Yet, the HER activity is even higher than that of pure Platinum at high working current densities of > <NUM> mA cm-<NUM>.

The delafossite oxide catalysts used according to the present invention can be manufactured e.g. by mixing an "A"-halide such as a chloride, bromide or iodide with a "B"-oxide preferably by co-grinding the two compounds, preferably in an inert gas (e.g. N<NUM>, Ar) atmosphere for <NUM> to <NUM> minutes. The mixed powder is then heated to <NUM> - <NUM>, preferably <NUM> - <NUM>, most preferred to about <NUM> for <NUM> - <NUM> hours, preferably <NUM> - <NUM> hours, most preferred for about <NUM> hours and then cooled down to <NUM> - <NUM>, preferably <NUM> - <NUM>, most preferred to <NUM> - <NUM> below the highest heating temperature at a cooling rate of <NUM> - <NUM>/hour, preferably <NUM> - <NUM>/hour, most preferred about <NUM>/hour and then kept at this temperature for <NUM> - <NUM> hours, preferably <NUM> - <NUM> hours, most preferred about <NUM> hours. Finally, the composition is cooled to room temperature at a rate of <NUM> - <NUM>/hour, preferably <NUM> - <NUM>/hour, most preferred at about <NUM>/hour. This reaction is preferably performed in a sealed tube, e.g. a quartz tube, preferably under reduced pressure of between <NUM> - <NUM> and <NUM> - <NUM> Pa.

The electrocatalysts used in the present invention exhibits a very low resistivity at room temperature, which is in the range of <NUM> - <NUM>µΩ cm in the temperature range of <NUM> - <NUM>. This facilitates easy electron transfer between the catalyst and electrolyte. Moreover, the present electrocatalysts show higher activity under acidic conditions than Pt foil. The overpotential to deliver a current density of <NUM> mA/cm<NUM> is only <NUM> mV. The Tafel slope is as low as <NUM> mV/dec. in acidic (pH = <NUM>) medium. All these values are lower than that of Pt foil (<NUM> mV @<NUM> mA/cm<NUM> with Tafel slope of <NUM> mV/dec. ), and even comparable with nano Pt/C catalysts (<NUM> mV @<NUM> mA/cm<NUM> with Tafel slope of <NUM> mV/dec. The exchange current density is determined to be <NUM> mA/cm<NUM>, which is higher than that of Pt/C catalyst with a value of <NUM> mA/cm<NUM>. The high chemical stability and electrochemical activity of the electrocatalyst do not change even after the compounds have been exposed to air for <NUM> months.

The electrocatalyst used in the present invention consists of a compound with delafossite structure of the ABOx, wherein x is > <NUM> and ≤ <NUM>, A is independently selected from a transition metal of IUPAC groups <NUM> and <NUM>, and B is independently selected from a transition metal of.

IUPAC group <NUM>, <NUM>, <NUM> or <NUM>. For example, the ABOx, compound can be grown on a conductive substrate such as Ni foam, carbon cloth, or can be mixed with graphene to increase the mobility and conductivity. However, it has surprisingly been found that the present ABOx, compounds can be used in single crystal form directly as a working electrode. In this case the electrode has a size of about <NUM> to <NUM> × <NUM> to <NUM> × <NUM> to <NUM><NUM>, preferably about <NUM> × <NUM> × <NUM><NUM>. The single crystal electrode can then be attached to a wire, e.g. Cu wire, e.g. with silver paint.

The invention is explained in more detail below with reference to examples.

Powders of reagent-grade PdCl<NUM> (<NUM>+% purity; Alfa Aesar) and CoO (<NUM>% purity; Alfa Aesar) were ground together for about one hour under an inert gas atmosphere. The mixed powder was then sealed in a quartz tube under a vacuum of <NUM> × <NUM>-<NUM> Pa. The sealed quartz tube was heated in a vertical furnace to <NUM> for <NUM> hours and cooled down to <NUM> at a rate of <NUM>/hour and kept at this temperature for <NUM> hours. Finally, the furnace was cooled from <NUM> to room temperature at a rate of <NUM>/hour.

X-ray powder diffraction patterns were obtained from a D8 Advance X-ray diffractometer (Bruker, AXS) using Cu Kα, radiation. The microstructure of the samples was examined by scanning electron microscope (SEM, FEI Quanta <NUM> F) with capabilities for energy dispersive X-ray spectroscopy (EDX). Transport measurements were performed using standard four-probe ac techniques in 4He cryostats (Quantum Design).

HER catalytic measurements were performed on the Autolab PGSTAT302N with impedance module electrochemistry workstation with a conventional three electrode cell configuration. A Ag/AgCl (<NUM> KCl) electrode was used as the reference electrode, and a graphite rod was used as the counter electrode. The electrolyte was <NUM> H<NUM>SO<NUM> solution and purified by Ar before use. Linear sweep voltammograms were recorded using a PdCoO<NUM> single crystal electrode with a scan rate of <NUM> mV/S. Stability tests were carried out at a current density of <NUM> mA / cm<NUM> in the initial test for <NUM> hours. To check the chemical stability of the catalyst, the same measurement was repeated again after exposing the crystal to air for two weeks. All potentials were referenced to a reverse hydrogen electrode (RHE).

PdCoO<NUM> single crystals were synthesized and separated from unreacted CoO and from CoCl<NUM> powder by cleaning the product with boiling alcohol. <FIG> shows the crystal structure of PdCoO<NUM>. It is constructed from two-dimensional layers with the edge-linked CoO<NUM> octahedra connected by O-Pd-O dumbbells. The Co atoms in the octahedral site of PdCoO<NUM> are in a nonmagnetic low-spin state. It is rhombohedral with space group R -<NUM> (space group no.

<FIG> shows the SEM image of a typical PdCoO<NUM> single crystal. The crystal has a flat surface with metallic luster. The sharp steps can be seen clearly from <FIG> on the crystal surface, indicating the lateral growth and the layered structure. This means that the exposed flat surface is the a-b plane and constructed alone the c-axis.

<FIG> shows the EDS spectra of the investigated crystal. The chemical composition was determined to be Pt, Co and O with stoichiometry ratio of close to <NUM>:<NUM>:<NUM>, further indicating the high purity of the synthesized crystal.

<FIG> shows the in-plane temperature dependence of electrical resistivity of the single crystal. The resistivity decreases with decreasing temperature over the whole temperature range, suggesting the metallic behavior. The resistivity at room temperature is only <NUM>µΩ cm, which is lower than all the reported oxide metals. The residual resistivity ratio ρ300K/ ρ15K is typically <NUM> to <NUM>, indicating the high purity of the sample.

<FIG> shows the three electrode electrocatalysis system. The PdCoO<NUM> single crystal was attached on the Cu wire with silver paint and used as working electrode directly. <NUM> H<NUM>SO<NUM> solution was used as electrolyte and purified with Ar gas for <NUM> minutes before the measurement. The surface area of the single crystal was determined to be about <NUM><NUM>.

The HER polarization curves for Cu wire, commercial Pt/C, and PdCoO<NUM> electrocatalysts are shown in <FIG>. It can be seen that Cu wire is not active in the measurement potential scale. The commercial Pt/C needs an overpotential of <NUM> mV to deliver a current density of <NUM> mA/cm<NUM>. The value for the PdCoO<NUM> single crystal is only <NUM> mV. More interestingly, the overpotential to deliver a bigger current density is much lower for PdCoO<NUM>. <FIG> shows the polarization curve for PdCoO<NUM> in a larger potential window. It only needs an overpotential of <NUM> mV to achieve a current density of <NUM> mA/cm<NUM>.

Tafel slope and exchange current density were obtained by fitting the experiment data with the Butler-Volmer equation (<FIG>). The Tafel slope of the Pt/C and PdCoO<NUM> are <NUM> and <NUM> mV/dec. , respectively. The exchange current density was <NUM> mA/cm<NUM> for PdCoO<NUM>, which is also higher than that of Pt/C (<NUM> mA/cm<NUM>). These experimental data unequivocally demonstrate that the PdCoO<NUM> is a highly active HER catalyst.

Claim 1:
Photo/electrochemical cell comprising an electrode for hydrogen reduction comprising a compound of the formula (I)

        ABOx     (I)

wherein
x is > <NUM> and ≤ <NUM>,
A is independently selected from a transition metal of IUPAC groups <NUM> and <NUM>, and
B is independently selected from a transition metal of IUPAC group <NUM>, <NUM>, <NUM> or <NUM>.