Abstract:
An electrochemical cell assembly including a plurality of testing cells, a reference cell, and fluid connections between the testing cells and the reference cell. Each of the testing cells includes a working electrode, which is a rotating disk or ring-disk electrode, and a counter electrode. A chemical composition, whose intrinsic kinetic properties under defined mass-transfer are to be investigated, is deposited on the working electrode. The reference cell holds a reference electrode that serves as a common reference electrode for each of the testing cells. The assembly permits simultaneous testing of many chemical compositions under identical environment.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention pertains generally to electrochemical testing assemblies and, more particularly, toward multi-channel rotating disk or ring-disk electrode assemblies and associated testing procedures. 
     2. Description of Related Art 
     Generally, a three-electrode electrochemical cell includes a glass vessel holding a working electrode, with a counter electrode and a reference electrode in separate compartments. The electrodes are immersed in a testing solution, such as sulfuric acid, and a reference potential is applied between reference and working electrodes, whereas a current is established between the working and counter electrodes. This setup is used in basic research to investigate the kinetics and mechanisms of the electrode reaction occurring on the working electrode surface. 
     In some testing applications, a rotating disk electrode (RDE) or rotating ring-disk electrode (RRDE), which are hereinafter collectively referred to as the “RDE/RRDE”, is used as the working electrode in the three-electrode cell. RDE/RRDE are well known in the art and are commercially available from several sources, such as Princeton Applied Research of Oak Ridge, Tenn. 
     The RDE/RRDE is a specialized hydrodynamic electrode used in the study of the kinetics and mechanisms of electrode reactions for ensuring a known and controllable mass-transfer to the electrode. The mass-transfer is achieved by using a flat disc electrode that is rotated in the testing solution, resulting in a defined hydrodynamic boundary layer. Typically, the RDE/RRDE is coated with a chemical composition whose kinetic parameters in an electrochemical reaction are to be determined. More specifically, thin films of chemical compositions are applied to the electrode surface of the RDE/RRDE, and the inherent electron-transfer characteristics of the chemical compositions are determined in the testing procedure. Conventionally, such tests are very time consuming due to the electrochemical cleaning and gas purging processes, but yield valuable information of the intrinsic electrochemical properties and kinetics so as to warrant further study. 
     A conventional three-electrode cell testing apparatus  10  is illustrated in  FIGS. 1–2 , and is shown to include a glass vessel  12  having a generally central port  14  that receives the working electrode  16 , a first laterally disposed port  18  that receives a counter electrode  20 , and a second laterally disposed port  22  that receives a reference electrode  24 . The glass vessel  12  also typically includes a gas inlet port  26  for saturation of the testing solution  30  via a bubbling assembly  27  prior to a test procedure, as well as additional ports  28  for ventilation and gas flowing purposes. The glass vessel  12  holds the testing solution  30 , such as sulfuric acid, in which the working electrode  16  is submersed and with which the counter and reference electrodes  18 ,  22  in separate compartments communicate. 
     With reference to  FIGS. 6A–6B , the working electrode  16  is a RDE ( FIG. 6A ,  16 ) or a RRDE ( FIG. 6B ,  16 ′), which is conventionally formed as a disk  16   a  or ring-disk  16   a ′,  16   d ′ of electrode material, such as gold, glassy carbon, or platinum, that is imbedded in a rod of insulating material  16   b  ( 16   b ′), such as polytetrafluoroethylene (PTFE) or low expansion oxides. For testing purposes, the electrode material  16   a  ( 16   a ′) is coated (via a plasma deposition process, chemical vapor deposition process, powder ink or the like) with a chemical composition to be tested. The RDE/RRDE shaft  16   c , which is electrically connected to the electrode material  16   a  ( 16   a ′,  16   d ′), extends from the insulating material  16   b  ( 16   b ′) and is mechanically connected to a motor  32  that rotates the working electrode  16  ( 16 ′) at a stable, high speed (i.e., 100–8000 rpm), which leads to a well-defined solution flow pattern of mass transfer. In this regard, it is noted that maintenance of the rotary speed is important as this speed is directly related to flow pattern and laminar flow layer parameters at the electrode surface, and thereby affects the electron-transfer properties under investigation. In any event, the RDE/RRDE, which are collectively referred to herein as the working electrode  16 , is only used for a single testing procedure. 
     The reference electrode  24  has a well-known and stable equilibrium electrode potential, and provides a reference point against which the potential of the working electrode  16  is applied. Such reference electrodes are well known in the art and are readily commercially available from several sources, including Princeton Applied Research. Although the reference electrode  24  is received within the glass vessel  12 , the reference electrode is typically, and more specifically, disposed within a double bridge tube assembly  25  that is illustrated in  FIG. 7 . 
     The double bridge tube assembly, which is hereafter referred to as the reference electrode assembly  25 , includes the reference electrode  24  with the first bridge tube  24   a  and a second bridge tube  24   b  that protects the testing solution  30  from contamination by the reference electrode solution  30   a . Each of the bridge tubes  24   a ,  24   b  holds a solution  30   a ,  30   b , respectively, and includes a bridge, which are schematically illustrated and referred to as  24   a ′,  24   b ′, respectively. Normally the solution  30   b  in the second bridge tube  24   b  is the same as the testing solution  30 . The bridges  24   a ′,  24   b ′ are typically made from VYCOR frit that prevents contamination of the testing solution  30 , which could result from the leakage of the reference solution  30   a . Accordingly, the reference solution  30   a  surrounding the reference electrode  24  is doubly isolated from the testing solution  30  via the bridges  24   a ′,  24   b ′. Although the reference electrode  24  is reusable, and may be used for multiple testing procedures, it must be periodically tested to ensure that the electrode potential has not drifted over time. 
     Strictly speaking, there can be a small change in the potential of the reference electrode  24  depending on the electrolyte because of the presence of a liquid-junction potential. The liquid-junction potential is minimized by the use of high concentration solution, such as potassium chloride, as the solution  30   a  when the reference electrode  24  is a saturated calomel electrode. 
     The counter electrode  20  is used to make an electrical connection to the electrolyte or testing solution  30  (sulfuric acid) so that a current can be established between the working electrode  16  and the counter electrode  20 . The counter electrode  20  is usually made of inert materials (noble metals or carbon/graphite) to avoid its dissolution. Typically, the counter electrode  20  has high surface area and is disposed within its own bridge tube or chamber  20   a  that includes a frit bridge  20   b . The counter electrode bridge tube  20   a  is filled with a solution  30   c , which is preferably identical to the testing solution  30  used in the vessel  12  and the testing solution  30   b  used in the second bridge tube  24   b  of the reference electrode assembly  25 , while the solution  30   a  used in the first bridge tube  24   a  of the reference electrode assembly  25  may be different depending, in part, upon the particular reference electrode  24 . 
     Before a testing procedure in which the kinetics and mechanisms of electrode reaction will be investigated, the deposited material on the surface of the working electrode  16  needs to be cleaned. Therefore, the working electrode  16 , which is coated with a chemical composition whose properties are to be tested, is inserted into the testing solution  30  in the glass vessel  12 , and the reference electrode assembly  25  and counter electrode assembly are inserted into the glass vessel  12 . The testing solution  30  is saturated with a suitable gas, such as argon or nitrogen, via the bubbling assembly  27  to purge the testing solution  30 . Thereafter, the chemical composition on the working electrode is cleaned by the cyclic voltammetry in a desired potential region repeatedly. Thereafter, the solution is saturated with a required gas, such as oxygen or hydrogen, depending on the properties to be measured, through bubbling. Then, the RDE/RRDE is rotated at a stable, high speed by the motor  32 . By sweeping a potential between working electrode  16  and reference electrode  24 , a current is established between the counter electrode  20  and the working electrode  16  in the solution and is recorded. 
     While the aforementioned well-known testing apparatus and associated testing method has proven to be satisfactory and reliable, it suffers from several significant disadvantages. First, the testing procedure is relatively long (1–2 hours) and requires significant set-up in order to reliably reproduce the testing environment, which is vital to having reliable, repeatable results. Second, only one working electrode-mounted chemical composition may be tested during any given testing procedure. Third, the reference electrode may need to be calibrated between successive tests to account for drift of the reference potential, as may occur over time. 
     While these disadvantages are relatively minor when testing a small number of chemical compositions, they prove to be major disadvantages when testing thousands of compositions and wherein some of the thousands of compositions may need to be tested multiple times. Therefore, there exists a need in the art for an apparatus and method that permits multiple chemical compositions to be tested simultaneously in a three-electrode electrochemical cell. 
     SUMMARY OF THE INVENTION 
     The present invention is directed toward a method and apparatus that permits simultaneous testing of plurality chemical compositions in a three-electrode electrochemical cell assembly that employs multiple rotating disk/rotating ring disk electrodes as the working electrodes. 
     In accordance with the present invention, an electrochemical cell assembly includes a plurality of testing cells, a reference cell, and fluid connections between each of the testing cells and the reference cell. Each of the testing cells includes a working electrode, which is a rotating disk or ring-disk electrode, and a counter electrode, whereas the reference cell includes a reference electrode assembly. 
     In further accordance with the present invention, each of the working electrodes is connected to a rotator so as to be rotatably driven by the motor. A controller is provided to control the speed of rotation of the working electrode. When individual motors are used to rotate each working electrode, the controller is adapted to control the motors such that the rotational speed of the working electrodes may be controlled to be identical to one another. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and further features of the invention will be apparent with reference to the following description and drawings, wherein: 
         FIG. 1  is a perspective view of a conventional three electrode electrochemical cell; 
         FIG. 2  is a schematic view of the conventional cell of  FIG. 1 ; 
         FIG. 3  is a perspective view of an electrochemical cell according to the present invention; 
         FIG. 4  is a schematic partial elevational view of an electrochemical cell assembly according to the present invention; 
         FIG. 5  is a schematic plan view of the electrochemical cell assembly according to the present invention; 
         FIG. 6A  is a schematic cross-sectional bottom view of a conventional rotating disk electrode; 
         FIG. 6B  is a schematic bottom view of a conventional rotating ring disk electrode; 
         FIG. 7  is a schematic cross-sectional view of a conventional double bridge reference electrode assembly; 
         FIG. 8  schematically illustrates the assembly of  FIG. 5  with a magnetic or mechanical coupling to drive the working electrodes. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to  FIG. 3 , an electrochemical cell  100  according to the present invention (referred to hereinafter as the “testing cell”) is shown to include a glass vessel  102  having a generally central port  104  that receives a working electrode  106  and a laterally disposed port  108  that receives a counter electrode  110 . The testing cell  100  also typically includes a gas inlet port  112  for bubbling/saturation of the testing solution prior to a test procedure, as well as additional ports  114  for ventilation and gas flowing purposes. The testing cell  100 , as described to this point, is generally conventional. 
     The testing cell  100  also includes a port  116 , which preferably is at a level close to, or slightly below, the level of the working electrode  106 . The port  116  receives a pipe  118  that forms a passageway for communication with a reference cell  120 , which holds a reference electrode  124 . As will be apparent from the following discussion, the reference electrode  124  serves as a common reference electrode for a plurality of working electrodes  106  and, as such, is shared by a plurality of testing cells  100 . Accordingly, an individual reference electrode for each testing cell  100  is not necessary with the present invention. 
     With reference to  FIG. 4 , the electrochemical cell assembly  200  according to the present invention is shown to include a plurality or array of electrochemical cells (testing cells  100 , such as shown in  FIG. 3 ) and a single or common reference cell  120 . The reference cell  120  includes a double bridge assembly  125  that receives the reference electrode  124 . The double bridge assembly  125  is generally conventional, and includes an inner bridge tube  124   a  and an outer bridge tube  124   b . The inner bridge tube  124   a  is filled with a reference solution  130   a . The outer bridge tube  124   b  is also filled with a testing solution  130   b , which is identical to the testing solution  130  in the testing cell  100 . Each of the inner and outer bridge tubes  124   a ,  124   b  include a bridge  124   a ′,  124   b ′ (preferably formed from VYCOR frit) to isolate the interior of the bridge tubes  124   a ,  124   b  so as to avoid contaminating the testing solution  130  of the testing cell  100 . 
     Depending upon the type of reference electrode  124  used in the particular test, the reference solution  130   a  may be similar to the testing solution  130  (i.e., sulfuric acid) or may be different from the testing solution  130 . For example, when a Mercury-Mercurous Sulfate (MMS) reference electrode is used, the reference solution  130   a  is sulfuric acid of higher concentration than that of the testing solution  130 . On the other hand, when a Saturated Calomel (SCE) reference electrode is used, the reference solution  130   a  is potassium chloride, while the testing solution remains sulfuric acid, in which case the illustrated double bridge construction is necessary. 
     The reference cell  120  is in fluid communication with each of the testing cells  100  via the pipes  118 . Preferably, the reference electrode  124  is disposed in the double bridge assembly  125 , which is immersed in the testing solution  130  (sulfuric acid) contained within the testing cells  100 , the pipes  118 , and the remainder of the reference cell  120 . As noted before, the double bridge assembly  125  includes a pair of bridges  124   a ′,  124   b ′ or filters that fluidly isolate the reference solution  130   a  from the testing solution  130 , while permitting electrical connection or communication therebetween. It is assured that the distance between reference electrode  124  and each working electrode  106  is the same. 
     Although it is preferred to fluidly isolate the solution in reference electrode  124  in the double bridge assembly  125 , and thereby provide redundant isolation from the testing solution  130  by means of the bridges  124   a ′,  124   b ′, it is considered apparent that the outer bridge tube  124   b , and its associated bridge  124   b ′, could be disposed of and that this isolation function performed by a bridge  124   b ″ disposed within each of the pipes  118 , or at one end of the pipes  118 , as illustrated by dashed lines in  FIG. 4 . Further, while the pipes  118  are preferred, it is contemplated that these pipes could be replaced with siphon-type fluid connections  118 ′, which are preferred by some researchers. 
     It is important to note that with a common reference electrode according to the present invention, an array of testing cells  100  can be employed. While the array depicted in  FIG. 5  includes six testing cells, this is only for purposes of clarity and brevity. Rather, it is contemplated that the array may consist of 8, 16, 64, 96, 128 or any number of testing cells  100  that may be physically disposed around the reference cell  120 . For example, the testing cells  100  may occupy several concentric rings or rows surrounding the reference cell  100 . It is further noted that the testing cells  100  may be disposed vertically above and below the reference cell  120 . 
     Each of the testing cells  100  has a working electrode  106  and a counter electrode  110 . Preferably, the counter electrode  110  is disposed on the side of the working electrode  106  opposite to the connection of the pipe  118  with the testing cell  100 , as illustrated in  FIGS. 4–5 . 
     The counter electrode  110  is conventional in design, and is disposed within a bridge tube or compartment  110   a  that includes a frit bridge  110   b.    
     The working electrode  106  is a conventional RDE/RRDE having an electrode material  106   a  imbedded in an inert insulating body  106   b . A metal shaft  106   c  extends from the body  106   b  and electrically connects the electrode material  106   a  to a controller/analyzer  144 , discussed hereinafter. A chemical composition, whose electron transfer characteristics are to be examined, is coated, via known deposition techniques, on the outer surface of the electrode material  106   a . The working electrode shaft  106   c  is secured to a rotator  140  that drives the working electrode  106  at a stable, verifiable rotational speed, such as between about 100–8000 rpm or more. The rotator  140  of the array of testing cells  100  may be a motor or may be a device that is magnetically or mechanically driven by a master motor, as described hereinafter. 
     In use, the reference electrode  124  is disposed within the reference cell  120 , and the reference cell  120  is connected to each of the testing cells  100  via a pipe  118 , as illustrated. A counter electrode  110  and working electrode  106  are inserted into each testing cell  100 , and the working electrode  106  is rotatably secured to its associated rotator  140 . In the embodiment illustrated in  FIG. 4 , the rotator  140  is a motor that is controlled by a motor controller  142 . Preferably, a multi-channel potentiostat  144  (such as sold as a Potentiostat/Galvanostat by Princeton Applied Research and as a MultiStat by Solartron Analytical of Houston, Tex.) is used to apply the desired potential to the electrodes during the testing procedure, and to record the current of the test in real time. Naturally, the motor controller  142  and the potentiostat  144  may be integrated into a single device; typically a computer based multi-channel control system. 
     During the testing procedure a reference potential is established in the assembly  200  via the common reference electrode  124 , a current is generated through the testing solution between the counter electrode  110  and the working electrode  106  within each testing cell  100  while the working electrode  106  is rotated at a desired speed by the associated rotator  140 . Through sweeping the potential, current density of the electrochemical reaction on the working electrode surface can be measured, which offers valuable information about the kinetics of the reaction. 
     Due to the electrical connection with the testing solution, the single reference electrode  124  is common to each of the testing cells  100 , greatly reducing the costs, set-up work, and time associated with each testing procedure. Moreover, the multiple tests simultaneously conducted will inherently have identical testing environments, which leads to more consistent results. 
     While the present invention has been described with particularity herein, it is considered apparent that numerous modifications, rearrangements, and substitutions of parts may be resorted to without departing from the scope and spirit of the present invention. For example, instead of providing individual motors for each working electrode, it is contemplated that a single motor may be used to more reliably and accurately drive each working electrode. This alternative is schematically illustrated in  FIG. 8 , wherein a single motor  140   a  (i.e., master motor) is linked to the rotators  140  by a coupling  141   a ,  141   b . The coupling  141   a ,  141   b  may be mechanical (i.e., gears, toothed drive belts, etc.) or may be magnetic. With this arrangement, only one motor  140   a  is required and the rotators  140  and associated working electrodes will be reliably and consistently driven at identical rotational speeds. Accordingly, control over the electrochemical cell assembly  200  is greatly simplified. 
     Accordingly, the present invention is not to be limited by the currently preferred embodiments described herein, but rather is only to be defined by the claims appended hereto.