Polymer electrolyte fuel cell, method of manufacturing the same and inspection method therefor

Abstract

The durability of a polymer electrolyte fuel cell is very significantly improved by using a tightening pressure of about 2 to 4 kgf/cm 2 of area of electrode; or a tightening pressure of about 4 to 8 kgf/cm 2 of contact area between electrode and separator plate; or by selecting a value not exceeding about 1.5 mS/cm 2 for the short-circuit conductivity attributed to the DC resistance component in each unit cell; or by selecting a value not exceeding about 3 mA/cm 2 for the hydrogen leak current per area of electrode of each MEA. Further, in a method of manufacturing or an inspection method for a polymer electrolyte fuel cell stack, fuel cells having high durability can be efficiently manufactured by removing such MEAs or unit cells using such MEAs or such cell stacks having short-circuit conductivity values and/or hydrogen leak current values exceeding predetermined values, respectively.

We claim:  
     
         1 . A polymer electrolyte fuel cell comprising a cell stack having a plurality of unit cells tightened in a stacking direction of the stack, each unit cell comprising: an electrolyte membrane-electrode assembly comprising a hydrogen ion conductive polymer electrolyte membrane and first and second electrodes respectively placed on opposite major surfaces of the electrolyte membrane, each of the electrodes comprising a gas diffusion layer and a catalyst layer; a first electrically conductive separator plate contacting the first electrode and having a first gas flow channel for supplying and exhausting a fuel gas to and from the first electrode; and a second electrically conductive separator plate contacting the second electrode and having a second gas flow channel for supplying and exhausting an oxidant gas to and from the second electrode, wherein each of the electrodes is provided with a tightening pressure of about 2 to 4 kgf/cm 2  of area of each of the electrodes.  
     
     
         2 . A polymer electrolyte fuel cell comprising a cell stack having a plurality of unit cells tightened in a stacking direction of the stack, each unit cell comprising: an electrolyte membrane-electrode assembly comprising a hydrogen ion conductive polymer electrolyte membrane and first and second electrodes respectively placed on opposite major surfaces of the electrolyte membrane, each of the electrodes comprising a gas diffusion layer and a catalyst layer; a first electrically conductive separator plate contacting the first electrode and having a first gas flow channel for supplying and exhausting a fuel gas to and from the first electrode; and a second electrically conductive separator plate contacting the second electrode and having a second gas flow channel for supplying and exhausting an oxidant gas to and from the second electrode, wherein each of the electrodes is provided with a tightening pressure of about 4 to 8 kgf/cm 2  of contact area between each of the electrodes and its respective electrically conductive separator plate.  
     
     
         3 . The polymer electrolyte fuel cell according to  claim 2 , wherein each of the first and second gas flow channels has a groove width of about 0.8 mm to 1 mm, and a groove depth of about 0.3 to 1 mm.  
     
     
         4 . The polymer electrolyte fuel cell according to  claim 2 , wherein the contact area of each of the electrodes with its respective electrically conductive separator plate is equal to or greater than an area of each of the electrodes which is out of contact with its respective electrically conductive separator plate.  
     
     
         5 . A polymer electrolyte fuel cell comprising a cell stack having a plurality of unit cells tightened in a stacking direction of the stack, each unit cell comprising: an electrolyte membrane-electrode assembly comprising a hydrogen ion conductive polymer electrolyte membrane and first and second electrodes respectively placed on opposite major surfaces of the electrolyte membrane, each of the electrodes comprising a gas diffusion layer and a catalyst layer; a first electrically conductive separator plate contacting the first electrode and having a gas flow channel for supplying and exhausting a fuel gas to and from the first electrode; and a second electrically conductive separator plate contacting the second electrode and having a gas flow channel for supplying and exhausting an oxidant gas to and from the second electrode, wherein the electrolyte membrane-electrode assembly has a short-circuit conductivity of not greater than about 1.5 mS/cm 2 .  
     
     
         6 . A polymer electrolyte fuel cell comprising a cell stack having a plurality of unit cells tightened in a stacking direction of the stack, each unit cell comprising: an electrolyte membrane-electrode assembly comprising a hydrogen ion conductive polymer electrolyte membrane and first and second electrodes respectively placed on opposite major surfaces of the electrolyte membrane, each of the electrodes comprising a gas diffusion layer and a catalyst layer; a first electrically conductive separator plate contacting the first electrode and having a gas flow channel for supplying and exhausting a fuel gas to and from the first electrode; and a second electrically conductive separator plate contacting the second electrode and having a gas flow channel for supplying and exhausting an oxidant gas to and from the second electrode, wherein each of the unit cells comprising the electrolyte membrane-electrode assembly has a hydrogen leak current of not greater than about 3 mA/cm 2 .  
     
     
         7 . A method of manufacturing a polymer electrolyte fuel cell, comprising processes of: forming a plurality of unit cells; stacking the plurality of unit cells to form a cell stack; and tightening the cell stack in a stacking direction of the stack; the process of forming the plurality of unit cells comprising steps of: placing, on opposite major surfaces of a hydrogen ion conductive polymer electrolyte membrane, a first and a second electrode, each electrode comprising a gas diffusion layer and a catalyst layer, to form an electrolyte membrane-electrode assembly; placing a first electrically conductive separator plate contacting the first electrode and having a first gas flow channel for supplying and exhausting a fuel gas to and from the first electrode; and placing a second electrically conductive separator plate contacting the second electrode and having a second gas flow channel for supplying and exhausting an oxidant gas to and from the second electrode; and further comprising an inspection process comprising steps of: measuring a short-circuit conductivity of each electrolyte membrane-electrode assembly and/or measuring a hydrogen leak current of each unit cell; and removing such electrolyte membrane-electrode assemblies or unit cells or cell stack or cell stacks that have a short-circuit conductivity exceeding a predetermined short-circuit conductivity value or have a hydrogen leak current exceeding a predetermined hydrogen leak current value.  
     
     
         8 . The method according to  claim 7 , wherein the predetermined short-circuit conductivity value is about 1.5 mS/cm 2  and the predetermined hydrogen leak current value is about 3 mA/cm 2 .  
     
     
         9 . The method according to  claim 7 , wherein the short-circuit conductivity of each electrolyte membrane-electrode assembly is measured by: applying thereto a constant DC voltage to obtain a steady-state current, or applying a constant DC current to obtain a steady-state voltage; and converting the steady-state current or the steady-state voltage, by calculation, to yield the short-circuit conductivity.  
     
     
         10 . The method according to  claim 9 , wherein the constant DC voltage is not greater than about 0.5 V for each electrolyte membrane-electrode assembly.  
     
     
         11 . The method according to  claim 9 , wherein the constant DC current is not greater than about 5 mA/cm 2  of electrode area of each electrolyte membrane-electrode assembly.  
     
     
         12 . The method according to  claim 7 , wherein the step of measuring the short-circuit conductivity of each electrolyte membrane-electrode assembly is conducted by placing the first and second electrodes in a same atmosphere selected from the group consisting of an air atmosphere and an inert gas atmosphere.  
     
     
         13 . The method according to  claim 7 , wherein the step of measuring the hydrogen leak current of each unit cell is conducted by: supplying an inert gas to one of the electrodes and a fuel gas to another of the electrodes; applying to each electrolyte membrane-electrode assembly a constant DC voltage to obtain a steady-state current or a constant DC current to obtain a steady-state voltage; and converting, by calculation, a difference value obtained by subtracting a value corresponding to the short-circuit conductivity from a value calculated from the steady-state current or the steady-state voltage to yield the hydrogen leak current.  
     
     
         14 . The method according to  claim 7 , wherein the inspection process is performed before the process of forming the cell stack.  
     
     
         15 . The method according to  claim 7 , wherein the inspection process is performed after the process of forming the cell stack.  
     
     
         16 . A method of manufacturing a polymer electrolyte fuel cell, comprising processes of: forming a plurality of unit cells; stacking the plurality of unit cells to form a cell stack; and tightening the cell stack in a stacking direction of the stack; the process of forming the plurality of unit cells comprising steps of: placing, on opposite major surfaces of a hydrogen ion conductive polymer electrolyte membrane, a first and a second electrode, each electrode comprising a gas diffusion layer and a catalyst layer, to form an electrolyte membrane-electrode assembly; placing a first electrically conductive separator plate contacting the first electrode and having a first gas flow channel for supplying and exhausting a fuel gas to and from the first electrode; and placing a second electrically conductive separator plate contacting the second electrode and having a second gas flow channel for supplying and exhausting an oxidant gas to and from the second electrode; wherein the process of forming the plurality of unit cells further comprises, before the step of forming each electrolyte membrane-electrode assembly, a step of smoothing both major surfaces of each gas diffusion layer.  
     
     
         17 . An inspection method for a polymer electrolyte fuel cell comprising a cell stack having a plurality of unit cells tightened in a stacking direction of the stack, each unit cell comprising: an electrolyte membrane-electrode assembly comprising a hydrogen ion conductive polymer electrolyte membrane and first and second electrodes respectively placed on opposite major surfaces of the electrolyte membrane, each electrode comprising a gas diffusion layer and a catalyst layer; a first electrically conductive separator plate contacting the first electrode and having a first gas flow channel for supplying and exhausting a fuel gas to and from the first electrode; and a second electrically conductive separator plate contacting the second electrode and having a second gas flow channel for supplying and exhausting an oxidant gas to and from the second electrode; the inspection method comprising, before operation of electric power generation of the polymer electrolyte fuel cell, and before or after formation of the cell stack, steps of: measuring a short-circuit conductivity of each electrolyte membrane-electrode assembly and/or measuring a hydrogen leak current of each unit cell; and inspecting whether the measured short-circuit conductivity exceeds a predetermined short-circuit conductivity value, and/or whether the measured hydrogen leak current exceeds a predetermined hydrogen leak current value.  
     
     
         18 . The inspection method according to  claim 17 , wherein the predetermined short-circuit conductivity value is about 1.5 mS/cm 2  and the predetermined hydrogen leak current value is about 3 mA/cm 2 .  
     
     
         19 . The inspection method according to  claim 17 , wherein the short-circuit conductivity of each electrolyte membrane-electrode assembly is measured by: applying thereto a constant DC voltage not greater than about 0.5 V for each electrolyte membrane-electrode assembly to obtain a steady-state current, or applying a constant DC current not greater than about 5 mA/cm 2  of electrode area of each electrolyte membrane-electrode assembly to obtain a steady-state voltage; and converting the steady-state current or the steady-state voltage, by calculation, to yield the short-circuit conductivity.  
     
     
         20 . The inspection method according to  claim 17 , wherein the step of measuring the hydrogen leak current of each unit cell is conducted by: supplying an inert gas to one of the electrodes and a fuel gas to another of the electrodes; applying to each electrolyte membrane-electrode assembly a constant DC voltage to obtain a steady-state current or a constant DC current to obtain a steady-state voltage; and converting, by calculation, a difference value obtained by subtracting a value corresponding to the short-circuit conductivity from a value calculated from the steady-state current or the steady-state voltage to yield the hydrogen leak current.