ELECTROCATALYSTS FOR HYDROGEN EVOLUTION REACTIONS (HER) WITH DELAFOSSITE OXIDES ABO2

Abstract

The present invention refers to material comprising a compound of the formula ABOx wherein x is >1.5 and ≤2.5, A is independently selected from a transition metal of IUPAC groups 10 and 11, and B is independently selected from a transition metal of IUPAC group 6, 7, 8 or 9 or a main group element of IUPAC group 13, as highly active catalyst for hydrogen evolution reaction (HER).

Claims

1. A hydrogen evolution reaction (HER) catalytic material comprising a compound of the formula (I):
ABOx  (I) wherein x is >1.5 and ≤2.5, A is independently selected from a transition metal of IUPAC groups 10 and 11, and B is independently selected from a transition metal of IUPAC group 6, 7, 8 or 9 or a main group element of IUPAC group 13.

2. The catalytic material according to claim 1, wherein x is 2, and A is independently selected from: Pt, Pd, or Ag, and B is independently selected from: Co, Al, Cr, Fe, In, Nd or Rh.

3. The catalytic material according to claim 1, wherein the compound is PdCoO.sub.2 or PtCoO.sub.2.

4. A process for the manufacture of a compound according to claim 1, having the formula (I):
ABOx  (I) wherein x is >1.5 and ≤2.5, A is independently selected from a transition metal of IUPAC groups 10 and 11, and B is independently selected from a transition metal of IUPAC group 6, 7, 8 or 9 or a main group element of IUPAC group 13; said process comprising mixing an “A”-halide with a “B”-oxide and heating the mixture to 600-1000° C.

5. An electrode in a photo/electrochemical cell comprising a compound according to formula (I):
ABOx  (I) wherein x is >1.5 and ≤2.5, A is independently selected from a transition metal of IUPAC groups 10 and 11, and B is independently selected from a transition metal of IUPAC group 6, 7, 8 or 9 or a main group element of IUPAC group 13.

6. The electrode as claimed in claim 5, wherein the electrode is a hydrogen reduction electrode.

7. A process for producing hydrogen comprising reducing hydrogen in a photo/electrochemical cell having an electrode comprising a HER catalytic material as claimed in claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] FIG. 1: Crystal structure of PdCoO.sub.2.

[0017] FIG. 2a: SEM image of PdCoO.sub.2 single crystal.

[0018] FIG. 2b: Magnified SEM image of PdCoO.sub.2 single crystal.

[0019] FIG. 3: EDS analysis of the single crystal.

[0020] FIG. 4: Temperature dependence of resistivity of PdCoO.sub.2 single crystal.

[0021] FIG. 5: Picture of the electrocatalysis system.

[0022] FIG. 6: HER polarization curves of Cu wire, 20% Pt/C, and PdCoO.sub.2 single crystal.

[0023] FIG. 7: Polarization curve in a large applied potential window.

[0024] FIG. 8: Tafel plots of 20% Pt/C, and PdCoO.sub.2 single crystal.

[0025] FIG. 9: Current-time (I-t) chronoamperometric response of PdCoO.sub.2 electrocatalyst at an overpotential of 50 mV in the initial test, and 40 mV after exposed in air for two weeks.

DETAILED DESCRIPTION OF THE INVENTION

[0026] The present invention is directed to HER catalytic material comprising compounds of delafossite structure of the formula (I)

ABOx  (I)

wherein
x is >1.5 and ≤2.5,
A is independently selected from a transition metal of IUPAC groups 10 and 11, and
B is independently selected from a transition metal of IUPAC group 6, 7, 8 or 9 or a main group element of IUPAC group 13.

[0027] In a preferred embodiment

X is 2, and

[0028] A is independently selected from: Pt, Pd, or Ag, and
B is independently selected from: Co, Al, Cr, Fe, In, Nd or Rh.

[0029] Most preferred are the delafossite oxide compounds PdCoO.sub.2 and PtCoO.sub.2. PdCoO.sub.2 and PtCoO.sub.2 have a layered structure and crystallize in a rhombohedral space group, which is built by an alternating stacking of [A] layers and [BO.sub.2] slabs along the c-axis. Most delafossite oxides are insulators, but some, e.g. PdCoO.sub.2, PdCrO.sub.2, or PtCoO.sub.2, are good metals. The in-plane conductivities at room temperature are only about 3 μΩ cm, which is even higher than that of pure metals such as Pd, Cu, and Au. However, their carrier density is approximately 1.6*10.sup.22 cm.sup.−3. 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 0.6 nm, 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 75%. Yet, the HER activity is even higher than that of pure Platinum at high working current densities of >10 mA cm.sup.−2.

[0030] The delafossite oxide catalysts 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.sub.2, Ar) atmosphere for 10 to 60 minutes. The mixed powder is then heated to 600-1000° C., preferably 700-900° C., most preferred to about 800° C. for 3-8 hours, preferably 4-7 hours, most preferred for about 5 hours and then cooled down to 20-100° C., preferably 40-80° C., most preferred to 50-70° C. below the highest heating temperature at a cooling rate of 5-10° C./hour, preferably 6-8° C./hour, most preferred about 7° C./hour and then kept at this temperature for 10-50 hours, preferably 20-40 hours, most preferred about 30 hours. Finally, the composition is cooled to room temperature at a rate of 70-120° C./hour, preferably 80-100° C./hour, most preferred at about 90° C./hour. This reaction is preferably performed in a sealed tube, e.g. a quartz tube, preferably under reduced pressure of between 10-3 and 10-4 Pa.

[0031] General Properties

[0032] The electrocatalysts of the present invention exhibits a very low resistivity at room temperature, which is in the range of 0.05-3 μΩ cm in the temperature range of 5-300 K. 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 10 mA/cm.sup.2 is only 33 mV. The Tafel slope is as low as 30 mV/dec. in acidic (pH=0) medium. All these values are lower than that of Pt foil (71 mV @10 mA/cm.sup.2 with Tafel slope of 74 mV/dec.), and even comparable with nano Pt/C catalysts (28 mV @10 mA/cm.sup.2 with Tafel slope of 34 mV/dec.). The exchange current density is determined to be 0.795 mA/cm.sup.2, which is higher than that of Pt/C catalyst with a value of 0.518 mA/cm.sup.2. The high chemical stability and electrochemical activity of the electrocatalyst do not change even after the compounds have been exposed to air for 3 months.

[0033] The electrocatalyst of the present invention consists of a compound with delafossite structure of the ABOx, wherein x is >1.5 and ≤2.5, A is independently selected from a transition metal of IUPAC groups 10 and 11, and B is independently selected from a transition metal of IUPAC group 6, 7, 8 or 9 or a main group element of IUPAC group 13. 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 0.5 to 1.5×0.5 to 3.0×0.05 to 1.0 mm.sup.3, preferably about 1×2×0.1 mm.sup.3. The single crystal electrode can then be attached to a wire, e.g. Cu wire, e.g. with silver paint.

Example

[0034] The invention is explained in more detail below with reference to examples.

[0035] Powders of reagent-grade PdCl.sub.2 (99.99+% purity; Alfa Aesar) and CoO (99.995% 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 5×10.sup.−4 Pa. The sealed quartz tube was heated in a vertical furnace to 800° C. for 5 hours and cooled down to 740° C. at a rate of 7.5° C./hour and kept at this temperature for 30 hours. Finally, the furnace was cooled from 740° C. to room temperature at a rate of 90° C./hour.

[0036] Analytical Methods

[0037] 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 200 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).

[0038] HER catalytic measurements were performed on the Autolab PGSTAT302N with impedance module electrochemistry workstation with a conventional three electrode cell configuration. A Ag/AgCl (3 M KCl) electrode was used as the reference electrode, and a graphite rod was used as the counter electrode. The electrolyte was 0.5 M H.sub.2SO.sub.4 solution and purified by Ar before use. Linear sweep voltammograms were recorded using a PdCoO.sub.2 single crystal electrode with a scan rate of 1 mV/S. Stability tests were carried out at a current density of 50 mA/cm.sup.2 in the initial test for 24 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).

[0039] Composition and Structure

[0040] PdCoO.sub.2 single crystals were synthesized and separated from unreacted CoO and from CoCl.sub.2 powder by cleaning the product with boiling alcohol. FIG. 1 shows the crystal structure of PdCoO.sub.2. It is constructed from two-dimensional layers with the edge-linked CoO.sub.6 octahedra connected by O—Pd—O dumbbells. The Co atoms in the octahedral site of PdCoO.sub.2 are in a nonmagnetic low-spin state. It is rhombohedral with space group R−3 m (space group no. 166).

[0041] FIG. 2a shows the SEM image of a typical PdCoO.sub.2 single crystal. The crystal has a flat surface with metallic luster. The sharp steps can be seen clearly from FIG. 2b 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.

[0042] FIG. 3 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 1:1:2, further indicating the high purity of the synthesized crystal.

[0043] Physical Transport Properties

[0044] FIG. 4 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 2.1 μΩ cm, which is lower than all the reported oxide metals. The residual resistivity ratio ρ300K/ρ15K is typically 50 to 60, indicating the high purity of the sample.

[0045] Electrocatalytic Activity Assessment

[0046] FIG. 5 shows the three electrode electrocatalysis system. The PdCoO.sub.2 single crystal was attached on the Cu wire with silver paint and used as working electrode directly. 0.5 M H.sub.2SO.sub.4 solution was used as electrolyte and purified with Ar gas for 30 minutes before the measurement. The surface area of the single crystal was determined to be about 0.008 cm.sup.2.

[0047] The HER polarization curves for Cu wire, commercial Pt/C, and PdCoO.sub.2 electrocatalysts are shown in FIG. 6. It can be seen that Cu wire is not active in the measurement potential scale. The commercial Pt/C needs an overpotential of 30 mV to deliver a current density of 10 mA/cm.sup.2. The value for the PdCoO.sub.2 single crystal is only 33 mV. More interestingly, the overpotential to deliver a bigger current density is much lower for PdCoO.sub.2. FIG. 7 shows the polarization curve for PdCoO.sub.2 in a larger potential window. It only needs an overpotential of 112 mV to achieve a current density of 600 mA/cm.sup.2.

[0048] Tafel slope and exchange current density were obtained by fitting the experiment data with the Butler-Volmer equation (FIG. 8). The Tafel slope of the Pt/C and PdCoO.sub.2 are 32 and 30 mV/dec., respectively. The exchange current density was 0.795 mA/cm.sup.2 for PdCoO.sub.2, which is also higher than that of Pt/C (0.68 mA/cm.sup.2). These experimental data unequivocally demonstrate that the PdCoO.sub.2 is a highly active HER catalyst.

[0049] The long-term chemical and electrochemical stability were tested and shown in FIG. 9. With an overpotential of 50 mV, the catalysts deliver a current density of about 50 mA/cm.sup.2 and kept stable for 24 hours. The single crystal was exposed to air for 2 weeks and the same measurement was repeated. The electrode could work stably in 48 h with overpotential of 50 mV. This indicates the excellent stability of the single crystalline catalyst.