Gas evolution in electrolysis

Abstract

An electrochemical half-cell operates to form a gas at a solid surface which may be an electrode. The electrolyte liquid comprises an additive, which is a high molecular weight flexible linear polymer or a viscoelastic linear surfactant. A flow path through the half-cell is configured to compel flow of liquid through the half-cell to make a succession of changes of direction. The electrolyte liquid is pumped through the half-cell at a rate which is sufficient that the additive and flow path configuration put the flowing electrolyte in a state of elastic turbulence which causes bubbles of gas to detach from the surface on which they are formed while they are still small, freeing the surface area for further reaction. The half-cell may be part of an electrolyser making hydrogen and oxygen from water.

1 . A system comprising an electrochemical half-cell wherein the electrochemical half-cell comprises:
 a liquid capable of undergoing an electrochemical reaction to form a gas at one or more solid surfaces within the half-cell;   a structure defining liquid flow paths to carry flow of the liquid towards contact with the one or more solid surfaces; and   at least one pump for propelling the liquid along the flow paths;   wherein:   the liquid is a solution containing a solute enabling the liquid to display elastic turbulence, and   the flow paths are configured to compel changes in the direction of liquid flow, to cause elastic turbulence within flow of the liquid in contact with the one or more solid surfaces where gas is formed.   
     
     
         2 . The system of  claim 1 , wherein the one or more solid surfaces are part of a porous layer, and liquid flow paths extend through the porous layer. 
     
     
         3 . The system of  claim 1 , wherein the structure defining liquid flow paths is formed of a solid material and comprises obstructions which compel changes in the direction of liquid flow. 
     
     
         4 . The system of  claim 1 , wherein the one or more solid surfaces are on an electrode connected to a supply of electric current. 
     
     
         5 . The system of  claim 1 , wherein the one or more solid surfaces have a reaction catalyst thereon. 
     
     
         6 . The system of  claim 1 , wherein the solute which enables the liquid to display elastic turbulence is a polymer comprising at least 5,000 monomer units in one or more linear polymer chains each comprising at least 1,000 monomer units, connected one to the next by a single covalent bond so that one monomer unit can rotate relative to adjoining monomer units. 
     
     
         7 . The system of  claim 6 , wherein the polymer is a polyacrylamide or hydrolyzed polyacrylamide. 
     
     
         8 . The system of  claim 1 , wherein the solute which enables the liquid to display elastic turbulence is a viscoelastic surfactant which forms wormlike micelles. 
     
     
         9 . An electrolyser for production of hydrogen and oxygen from water by electrochemical reaction comprising:
 a half-cell as defined in  claim 1 , wherein electrochemical reaction in the half-cell converts hydroxyl ions to oxygen.   
     
     
         10 . The electrolyser of  claim 9 , wherein the electrolyser is a proton exchange membrane (PEM) electrolyser comprising:
 one or more pairs of half-cells in which the liquids are aqueous solutions and acidic, each pair of half-cells having a proton exchange membrane between them, inhibiting flow of gas between the half-cells while allowing passage of hydrogen ions through the membrane from one half-cell to the other.   
     
     
         11 . The electrolyser of  claim 9 , comprising one or more pairs of half-cells in which the liquids are aqueous solutions and alkaline, each pair of half-cells having a separator between them, inhibiting flow of gas between the half-cells while allowing passage of hydroxyl ions through the separator from one half-cell to the other. 
     
     
         12 . A process for production of gas from an aqueous solution by reaction at one or more solid surfaces, the process comprising:
 flowing an aqueous liquid through an electrochemical half-cell wherein the electrochemical half-cell comprises one or more solid surfaces and liquid flow paths leading to contact with the one or more solid surfaces, the liquid comprising a dissolved constituent capable of undergoing an electrochemical reaction to form a gas at the one or more solid surfaces, and   applying electrical potential to the half-cell and thereby causing formation of gas bubbles at the one or more solid surfaces, wherein:   the liquid comprises a solute enabling the liquid to display elastic turbulence;   the flow paths leading to contact with the one or more solid surfaces cause changes in the direction of flow of the liquid; and   pumping the liquid along the flow paths so that the liquid is in a state of elastic turbulence while it is in contact with the one or more solid surfaces where gas is formed.   
     
     
         13 . The process of  claim 12 , wherein the one or more solid surfaces are surfaces on porous material and the flow paths extend through the porous material. 
     
     
         14 . The process of  claim 12 , wherein the solute which enables the liquid to display elastic turbulence has a molecular weight of at least 106 Daltons and comprises at least 5,000 monomer units in one or more linear polymer chains each comprising at least 1,000 monomer units, connected one to the next by a single covalent bond so that one monomer unit can rotate relative to adjoining monomer units. 
     
     
         15 . The process of  claim 12 , wherein the solute which enables the liquid to display elastic turbulence is a viscoelastic surfactant which forms wormlike micelles. 
     
     
         16 . A process for the production of hydrogen and oxygen by electrolysis of aqueous liquids in first and second electrochemical half-cells with a separator between them inhibiting mixing of gases formed in the half-cells, wherein each half-cell comprises one or more solid surfaces and liquid flow paths leading to contact with the one or more solid surfaces the process comprising:
 flowing a first aqueous solution along the flow path of the first electrochemical half-cell, the solution comprising a dissolved constituent capable of undergoing an electrochemical reaction to form oxygen at the one or more solid surfaces of the first half-cell;   flowing a second aqueous solution along the flow path of the second electrochemical half-cell, the second solution comprising a dissolved constituent capable of undergoing an electrochemical reaction to form hydrogen at the one or more solid surfaces of the second half-cell; and   applying electrical potentials to the half-cells and thereby causing the formation of oxygen and hydrogen at the one or more solid surfaces of the first and second half-cells respectively;   wherein, for at least one of the half-cells, the liquid comprises a solute enabling the liquid to display elastic turbulence;   the flow path in contact with the one or more solid surfaces causes changes in the direction of flow of the liquid, and   pumping the liquid along the flow paths with the liquid being in a condition of elastic turbulence while it is in contact with the one or more solid surfaces.   
     
     
         17 . The process of  claim 16 , wherein the one or more solid surfaces comprises a porous material and the flow paths extend through the porous material. 
     
     
         18 . The process of  claim 16 , wherein the solute which enables the liquid to display elastic turbulence has a molecular weight of at least 106 Daltons and comprises at least 5,000 monomer units in one or more linear polymer chains each comprising at least 1,000 monomer units, connected one to the next by a single covalent bond so that one monomer unit can rotate relative to adjoining monomer units. 
     
     
         19 . The process of  claim 16 , wherein the solute which enables the liquid to display elastic turbulence is a viscoelastic surfactant which forms wormlike micelles. 
     
     
         20 . The process of  claim 16 , wherein:
 the liquid of the first half-cell, in which oxygen is formed, comprises a solute enabling the liquid to display elastic turbulence;   the flow path in contact with the one or more solid surfaces of the first half-cell causes changes in the direction of flow of the liquid, and   pumping the liquid along the flow paths into contact with the one or more solid surfaces of the first half-cell with the liquid in a condition of elastic turbulence while it is in contact with the one or more solid surfaces of the first half-cell.