Potential of Zero Charge-Based Capacitive Deionization

Abstract

The invention is a capacitive, aka electrostatic, deionization apparatus and method that solves the problem of short lifetime of conventional capacitive deionization (CDI) and of membrane capacitive deionization (MCDI) devices and methods by shifting the Potential of Zero Charge of electrode surfaces through surface modifications. Such electrode surface modifications provide very long lifetime capacitive deionization devices and methods.

1 . An electrostatic device in a structure comprising:
 at least one inlet, at least one outlet, at least one anode, at least one cathode, a switch operating to apply a short circuit or a user selectable DC constant voltage or constant current to at least one anode and to at least one cathode, and an ionic solution admitted through the at least one inlet and discharged through the at least one outlet, which ionic solution is deionized by contact with the at least one anode and the at least one cathode;   wherein a location of a potential of zero charge (E PZC ) of the at least one anode has been shifted by modification of a surface of the at least one anode to an increased E PZC  value.   
     
     
         2 . The electrostatic device of  claim 1 , wherein the modification of the at least one anode results from oxidation from heating in the presence of oxygen. 
     
     
         3 . The electrostatic device of  claim 1 , wherein the modification of the at least one anode results from exposure to acid. 
     
     
         4 . The electrostatic device of  claim 1 , wherein the modification of the at least one anode results from exposure to electrochemical oxidation. 
     
     
         5 . The electrostatic device of  claim 1 , wherein the modification of the at least one anode results from covalent attachment of functional groups that are negatively charged when in contact with the ionic solution and without voltage applied to the anode. 
     
     
         6 . The electrostatic device of  claim 1 , wherein the modification of the at least one anode results from covalent attachment of silica functional groups. 
     
     
         7 . The electrostatic device of  claim 1 , wherein the modification of the at least one anode results from covalent attachment of sulfonic acid groups. 
     
     
         8 . The electrostatic device of  claim 1 , wherein the modification of the at least one anode results from covalent attachment of any surface groups possessing net negative surface charges in aqueous solutions. 
     
     
         9 . An electrostatic device in a structure comprising:
 at least one inlet, at least one outlet, at least one anode, at least one cathode, a switch operating to apply a short circuit or a user selectable DC constant voltage or constant current to the at least one anode and to the at least one cathode, and an ionic solution admitted through the at least one inlet and discharged through the at least one outlet, which ionic solution is deionized by contact with the at least one anode and the at least one cathode;   wherein a location of a potential of zero charge (E PZC ) of the at least one cathode has been shifted by modification of a surface of the at least one cathode to a decreased E PZC  value.   
     
     
         10 . The electrostatic device of  claim 9 , wherein the modification of the at least one cathode results from reduction by exposure to a reducing treatment from heating in nitrogen, argon, or H 2 . 
     
     
         11 . The electrostatic device of  claim 9 , wherein the modification of the at least one cathode results from exposure to electrochemical reduction. 
     
     
         12 . The electrostatic device of  claim 9 , wherein the modification of the at least one cathode results from covalent attachment of functional groups that are positively charged when in contact with the ionic solution and without voltage applied to the at least one cathode. 
     
     
         13 . The electrostatic device of  claim 9 , wherein the modification of the at least one cathode results from covalent attachment of amine functional groups. 
     
     
         14 . The electrostatic device of  claim 9 , wherein the modification of the at least one cathode results from covalent attachment of alumina surface species. 
     
     
         15 . The electrostatic device of  claim 9 , wherein the modification of the at least one cathode results from reduction of specific carbon surface sites. 
     
     
         16 . The electrostatic device of  claim 15 , wherein the specific carbon surface sites are basal planes. 
     
     
         17 . A method of making an electrostatic device, the method comprising:
 forming a structure comprising at least one inlet, at least one outlet, at least one anode, at least one cathode, a switch operating to apply a short circuit or a user selectable DC constant voltage or constant current to the at least one anode and to the at least one cathode, and an ionic solution admitted through the at least one inlet and discharged through the at least one outlet, which the ionic solution is deionized by contact with the at least one anode and the at least one cathode;   wherein a location of a potential of zero charge (E PZC ) of the at least one anode has been shifted by modification of the anode surface to an increased E PZC  value; a E PZC  of the at least one cathode has been shifted by modification of a surface of the at least one cathode to a decreased E PZC  value; or a combination thereof.   
     
     
         18 . The method of  claim 17 , wherein the modification of the at least one anode results from a treatment selected from the group consisting of oxidation from heating in a presence of oxygen; exposure to acid; exposure to electrochemical oxidation; covalent attachment of functional groups that are negatively charged when in contact with the ionic solution and without voltage applied to the at least one anode; covalent attachment of silica functional groups; covalent attachment of sulfonic acid groups; and covalent attachment of any surface groups possessing net negative surface charges in aqueous solutions. 
     
     
         19 . The method of  claim 17 , wherein the modification of the at least one cathode results from a treatment selected from the group consisting of reduction by exposure to reducing treatment from heating in nitrogen, argon, or H 2 ; exposure to electrochemical reduction; covalent attachment of functional groups that are positively charged when in contact with the ionic solution and without voltage applied to the at least one cathode; covalent attachment of amine functional groups; covalent attachment of alumina surface species; and reduction of specific carbon surface sites. 
     
     
         20 . The method of  claim 19 , wherein the specific carbon surface sites are carbon basal planes.