WO2017081669A1

WO2017081669A1 - Combinatorial screening array

Info

Publication number: WO2017081669A1
Application number: PCT/IB2016/056830
Authority: WO
Inventors: Roelof Jacobus KRIEK
Original assignee: North West University
Current assignee: North West University
Priority date: 2015-11-13
Filing date: 2016-11-13
Publication date: 2017-05-18
Anticipated expiration: 2018-05-13

Abstract

The present invention relates to an array of combinatorial screening electrodes and to an electrochemical system for combinatorial screening of different electrocatalysts incorporating such an array. The array 12 of combinatorial screening electrodes 14 are arranged on a surface of a substrate 16, with each electrode 14 having a respective electrode surface area. The substrate 16 has inner and outer zones (18, 20), with the inner zone 18 having an inner zone surface area and the outer zone 20 surrounding the inner zone 18. All electrodes 14 forming part of the array 12 are arranged within the outer zone 20. The inner zone surface area is furthermore at least 5 times larger than the electrode surface area of the largest electrode14 in the array 12.

Description

COMBINATORIAL SCREENING ARRAY

FIELD OF THE INVENTION

The present invention relates to an array of combinatorial screening electrodes and to an electrochemical system for combinatorial screening of different e!ectrocatalysts incorporating such an array.

BACKGROUND TO THE INVENTION

The use of electrochemical methods in carrying out reactions is well known in the art, and includes use of said methods in electrodeposition and catalytic reactions. Various approaches have accordingly been developed for carrying out and screening electrochemical processes. For example, US patent 6,756,109 describes a means of electrodeposition and screening of diverse materials onto an array of individually addressable electrodes in a combinatorial fashion.

The approach of combinatorial screening of electrodeposited electrocatalysts in a multiple screening electrode array has significant advantages in terms of rapidly depositing and screening a number of potential materials within a single reactor.

Known prior art arrays of screening electrodes (as shown in figure 1 ) comprise a circular substrate having a "main dimension", with an array of screening electrodes arranged centrally on the substrate. A reference electrode is disposed above, and in close proximity to, the substrate. The distance between the reference electrode and the substrate is less than 1/3 of the main dimension of the substrate.

However, the known prior art array produces significant experimental errors and unreliable results in practice.

In this specification, the phrase "main dimension" will be understood to refer to: the diameter of a circular substrate, or in the case of a substrate of square configuration, a side length of the square substrate. The phrase "electrical path length" refers to the distance between a point on the reference electrode closest to the substrate, and the surface of a specific screening electrode forming part of the array. The phrase "relative difference" will be understood to refer to the difference between two values, divided by one of the two values.

OBJECT OF THE INVENTION

Accordingly it is an object of the present invention to provide an array of combinatorial screening electrodes and an electrochemical system for combinatorial screening of different electrocataiysts with which the applicant believes the aforementioned disadvantages may at least be alleviated or which would provide useful alternatives to known arrays and systems incorporating same.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided an array of combinatorial screening electrodes arranged on a surface of a substrate, each electrode having a respective electrode surface area, the substrate having inner and outer zones, the inner zone having an inner zone surface area and the outer zone surrounding the inner zone. All of the electrodes forming part of the array are arranged within the outer zone. The inner zone surface area is at least five (5) times larger than the electrode surface area of the largest electrode in the array.

The inner zone surface area may preferably be at least sixteen (16) times larger than the electrode surface area of the largest electrode in the array. More preferably the inner zone surface area may be at least sixty (60) times larger than the electrode surface area of the largest electrode in the array. Typically, the inner zone surface area may be between sixty (60) and eighty (80) times larger than the electrode surface area of the largest electrode in the array.

The array may comprise a multiple of eight (8) screening electrodes. Preferably the array comprises sixty-four (64) screening electrodes. All of the screening electrodes may be of similar shape and dimension, and may, for example, be square-shaped. Alternatively, the electrodes may be circular. In some embodiments, the surface areas of the screening electrodes may all be equal. The array may comprise a number of screening electrodes so that a ratio of the surface area of the substrate to the combined surface area of all the screening electrodes falls within a predetermined range.

The array may comprise a plurality of imaginary closed figures concentrically arranged about a centre point of the substrate. All of the screening electrodes may be located on one of the imaginary closed figures in equi-spaced relation. The imaginary closed figures may comprise concentric imaginary squares or circles.

Each of the screening electrodes may be provided in electrical contact with an electrical contact located in an outer peri heral region of the substrate.

According to a second aspect of the invention there is provided an electrochemical system for combinatorial screening of different electrocatalysts, the system comprising:

a chamber defining an electrolytic cell;

an array of combinatorial screening electrodes, each electrode having a respective electrode surface area, the array arranged on a substrate within the chamber, the substrate having a main dimension; and

a reference electrode extending in the chamber towards the substrate, the reference electrode terminating a distance h from the substrate, the configuration being at least one of a) wherein the distance h is between one third and two thirds of the magnitude of the main dimension of the substrate; and b) wherein the substrate has inner and outer zones, the inner zone having an inner zone surface area and the outer zone surrounding the inner zone, wherein all electrodes forming part of the array are arranged within the outer zone, and wherein the inner zone surface area is at least five (5) times larger than the electrode surface area of the largest electrode in the array.

It will be appreciated that by increasing the distance h, the relative difference between the electrical path length between the reference electrode and the inner-most electrode of the array and the electrical path length between the reference electrode and the outer-most electrode of the array, may be reduced, which may have advantageous effects on the accuracy of the results obtained by the system. Increasing the distance h however increases the electrical resistance between the reference electrode and the screening electrodes of the array. Distance h is thus constrained by practical considerations.

The electrolytic cell may further comprise a perforated disc-shaped counter electrode located in spaced relation relative to the array and the reference electrode may extend through a centre hole in the counter electrode. The perforated counter electrode may ensure that there is a constant distance between each combinatorial screening electrode and the counter electrode.

The electrochemical system may further comprise a digital controller and screening device. The number of electrodes constituting the array may depend on a principal series of numbers used in digital operations of the digital controller and screening device, to represent a bit; the series being in the form of eight (8) and multiples of eight (8).

BRIEF DESCRIPTION OF THE FIGURES

The invention will now be described further, by way of example only, with reference to accompanying figures wherein:

figure 1 is a schematic representation in plan view of an array of screening electrodes according to known prior art; electrodes and electrical contacts being represented by squares and rectangles respectively;

figure 2 is a perspective partially cut-away view of an example embodiment of an electrochemical system for combinatorial screening of different electrocatalysts;

figures 3 to 5 are schematic representations in plan view of different example embodiments of arrays of combinatorial screening electrodes on a surface of a substrate; and

Figure 6 is a side view of a substrate and reference electrode used in an example embodiment of an electrochemical system for combinatorial screening of different electrocatalysts.

DETAILED DESCRIPTION WITH REFERENCE TO THE FIGURES

Referring to figure 2, an example embodiment of an electrochemical system for combinatorial screening of different electrocatalysts is generally designated by reference numeral 10. Figures 3 to 5 show different example embodiments of an array 12 of combinatorial screening electrodes 14 (which may also be referred to as working electrodes) arranged on a surface of a substrate 16. Each e!ectrode 14 has a respective electrode surface area. The substrate 16 has an inner zone 18 and an outer zone 20. The inner zone 18 has an inner zone surface area and the outer zone 20 surrounds the inner zone 18. An outer periphery of the inner zone 18 is indicated by reference numeral 22. Ali electrodes 14 forming part of the array 12 are arranged within the outer zone 20. The inner zone surface area is at ieast 5 times larger than the electrode surface area of the largest electrode 14 in the array 12.

The inner zone surface area is preferably at Ieast 16 times larger, more preferably at Ieast 60 times larger, and typically between 60 and 80 times larger than the electrode surface area of the largest electrode 14 in the array 12.

The array 12 typically comprises a multiple of eight (8) electrodes 14. The number of electrodes 14 that constitutes the array 12 ranges between eight (8) and one hundred and forty four (144). Preferably (and as shown in the examples of figures 3 to 5), the array 12 comprises sixty four (64) screening electrodes 14.

In the example embodiments shown, the screening electrodes 14 are square- shaped and equally sized, and therefore have equal electrode surface areas. By way of example, the number of electrodes 14 used in the array 12 may be selected based on the electrode surface areas and the surface area of the substrate 16. In cases where relatively small substrates 16, or relatively large electrodes 14 are used, the array 12 may comprise fewer than 64 electrodes 14, whereas in cases where relatively large substrates 16, or relatively small electrodes 14 are used, the array 12 may comprise more than 64 electrodes 14. The number of electrodes 14 may therefore be selected so that the ratio of the surface area of the substrate 12 to the combined surface area of all the screening electrodes 14 always falls within a predetermined range.

The array 12 comprises a plurality of imaginary closed figures concentrically arranged about a centre point (not shown) of the substrate 16. Each screening electrode 14 is located on one of the imaginary closed figures. In the examples provided, the imaginary closed figures are square. Inner-most electrodes 14.1 are all located on the inner imaginary square 24, while outer-most electrodes

14.2 are located on the outer imaginary square 26. Electrodes 14 located intermediate the inner-most and outer-most electrodes (14.1 , 14.2), are located on a first intermediate imaginary square 28 (as in the case of the example of figure 3) or on both a first and second intermediate imaginary squares (28, 30) (as in the case of the examples of figures 4 and 5). The electrodes are equi- spaced on the imaginary squares.

Each electrode 14 is individually connected to a respective electrical contact 32, which is located in an outer peripheral region 33 of the substrate 16. The array 12 of screening electrodes 14 arranged on the substrate 16 is used in the electrochemical system 10 for screening different electrocatalysts. The layout of the electrochemical system 10 is shown in figure 2. The system 10 comprises a chamber 34 providing an electrolytic cell. The chamber 34 is defined by a cylindrical glass tube 36, of which the opposite ends are closed by end caps 38.1 and 38.2 respectively. The end caps (38.1 , 38.2) are kept in position by four connecting pillars 40, and secured to the pillars 40 by wing nuts 42. The cylindrical glass tube 36 defines a combined electrolyte inlet and drain 44. End cap 38.1 is provided with a gas vent 46.

System 10 further comprises a counter electrode 48 intermediate the end plates (38.1 , 38.2). The substrate 16 on which the array 12 is arranged is located within the tube 36, towards the end cap 38.2.

A central reference electrode 50 extends into the tube 36 through end cap 38.1 and through and beyond counter electrode 48, along a central axis of tube 36. The reference electrode 50 is electrically isolated from the end cap 38.1. The reference electrode terminates a vertical distance h above the substrate 16 and is perpendicular to the centre point of the substrate 16.

Counter electrode 48 is used to monitor and control current between combinatorial array 12 and counter electrode 48. System 10 is further provided with a multi-channel potentiostat (not shown) connected to the contacts 32 via a plurality of connectors 56 and pins 58.

Given the limitations of the substrate size and the spacing of individual electrodes 14, the total number of electrodes 14 may by increased by increasing the number of electrodes 14 located on each imaginary square (especially by adding outer-most electrodes 14.2 to the outer imaginary square 26) or by adding further intermediate imaginary squares. Preferably, 64 electrodes 14 are used and arranged on the substrate 16 as depicted by Figures 3 to 5.

Figure 6 shows a side view of the substrate 16 and the reference electrode 50. The inner imaginary square 24, (and therefore an innermost electrode 14.1 ) coincides with the inner reference lines 52, while outer imaginary square 26, (and therefore an outermost electrode 14.2) coincides with the outer reference lines 54. A first electrical path length M. therefore extends between an innermost electrode 14.1 and a point on the reference electrode closest to the substrate, while a second electrical path length I2 extends between an outermost electrode 14.2 and the point on the reference electrode closest to the substrate.

The applicant has found that the experimental errors and unreliable results caused by the prior art systems stem from the relative difference between the first and second electrical path lengths (h_, and I2). Thus, the closer the relative difference between the first and second electrical path lengths (\± and 12) is to 0, the smaller the experimental errors and unreliable results caused by the system will be. Therefore, the experimental errors and the extent of the unreliable results as aforementioned can be reduced by configuring the system 10, or arranging the array 12 so that the ratio of the first electrical path length M to the second electrical path length J2 is as close as possible to 1. By increasing the size of the inner zone 18 according to the examples of the current invention provided in figures 3 to 5 (thereby increasing the distance xj., which is the distance between the centre point of the substrate and the inner reference lines 52), the first electrical path length M. is thus increased. If the distance x2, which is the distance between the inner and outer reference lines (52, 54) (and therefore the distance between the innermost and outermost electrodes (14.1 , 14.2)) is kept constant, the relative difference between the two electrical path lengths (M , I2) decreases, so that the ratio of the two electrical path lengths (Jl, |2), is closer to 1 than what is achievable by the known prior art systems and arrays.

The effect that the relative difference in electrical path lengths has on the accuracy of the system can be attributed to the electrical resistance between the reference electrode 50 and the screening electrodes 14 being a function of the electrical path length between the reference electrode 50 and the respective electrode.

Array 12 according to the invention is particularly suitable for use in combinatorial screening electrocatalysts to establish their respective efficiencies in electrochemical reactions in aqueous electrolyte solutions.

In use, system 10 is assembled as depicted in figure 2 and a deposition head used to coat each of the electrodes 14 with a preselected metal or mixture of metals (electrocatalysts) to be screened and tested in a manner known in the art and substantially as described in US patent 6,756,109. Preferably the electrocatalyst is deposited on the electrodes 14 by means of a sputtering technique, as known in the art. In this instance evaporative sputtering is used, with a combinatorial approach being adopted for rapid development and screening of electrocatalyst compositions.

In use, sputtering is followed by calibration of the device in order to reduce deposition rate/power curves for all electrocatalysts being deposited. Sputtering is carried out under a low-pressure argon atmosphere. Following calibration, deposition is carried out individually for each respective electrode 14 in array 16.

Chamber 34 is filled with an appropriate electrolytic solution. Contacts 32 are connected to a 64-channel potentiostat (not shown). Electrochemical reactions are carried out within chamber 34, with the results being recorded for each electrode 14 in array 12. This allows for a number of different electrocatalysts deposited on the electrodes 14 to be screened and tested for the same reaction, allowing combinatorial screening of a number of different electrode constituents and coatings (forming the electrocatalysts) at the same time. The known potential difference across electrodes 14 allows measurement of the relative current densities of different electrocatalysts in comparison to the counter electrode 48.

It was found that the accuracy of measurements taken when using system 10 is substantially improved relative to measurements taken when using the prior art system and array.

Claims

Claims:

1 . An array of combinatorial screening electrodes arranged on a surface of a substrate, each electrode having a respective electrode surface area, the substrate having inner and outer zones, the inner zone having an inner zone surface area and the outer zone surrounding the inner zone, wherein all electrodes forming part of the array are arranged within the outer zone, and wherein the inner zone surface area is at least five (5) times larger than the electrode surface area of the largest electrode in the array.

2. The array of combinatorial screening electrodes according claim 1 , wherein the inner zone surface area is at least sixty (60) times larger than the electrode surface area of the largest electrode in the array.

3. The array of combinatorial screening electrodes according claim 1 , wherein the array comprises a multiple of eight (8) screening electrodes.

4. The array of combinatorial screening electrodes according to any one of the preceding claims, wherein the array comprises sixty four (64) screening electrodes.

5. The array of combinatorial screening electrodes according to any one of the preceding claims, wherein all of the respective electrode surface areas are equal and wherein the screening electrodes are square-shaped.

6. The array of combinatorial screening electrodes according to claim 1 , wherein the number of screening electrodes constituting the array is selected so that a ratio of the surface area of the substrate to the combined surface area of all the screening electrodes falls within a predetermined range.

7. The array of combinatorial screening electrodes according to any one of the preceding claims, wherein the array comprises a plurality of imaginary closed figures concentrically arranged about a centre point of the substrate, and wherein each screening electrode is located on one of the imaginary closed figures.

8. The array of combinatorial screening electrodes according to claim 7, wherein the screening electrodes are located on the imaginary closed figures in equi-spaced relation.

9. The array of combinatorial screening electrodes according to any one of claims 7 and 8, wherein the concentric imaginary closed figures comprise imaginary squares.

10. The array of combinatorial screening electrodes according any one of the preceding claims, wherein each of the screening electrodes are provided in electrical contact with an electrical contact located in an outer peripheral region of the substrate.

1 1 . An electrochemical system for combinatorial screening of different electrocatalysts, the system comprising:

- a chamber defining an electrolytic cell;

- an array of combinatorial screening electrodes, each electrode having a respective electrode surface area, the array arranged on a substrate within the chamber, the substrate having a main dimension; and

- a reference electrode extending in the chamber towards the substrate, the reference electrode terminating a distance h from the substrate, the configuration being at least one of a) wherein the distance h is between one third and two thirds of the magnitude of the main dimension of the substrate; and b) wherein the substrate has inner and outer zones, the inner zone having an inner zone surface area and the outer zone surrounding the inner zone, wherein all electrodes forming part of the array are arranged within the outer zone, and wherein the inner zone surface area is at least five (5) times larger than the electrode surface area of the largest electrode in the array.

12. The electrochemical system according claim 1 1 , comprising a perforated disc-shaped counter electrode which is located in spaced relation relative to the array and wherein the reference electrode extends through an aperture in the counter electrode.

13. The electrochemical system according to any one of claims 1 1 and 12, wherein all of the respective electrode surface areas are equal and wherein the screening electrodes are square-shaped.