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(E) Dissolved Fuel Alkaline Fuel Cell 

1.                  L. X. Yang and B. Z. Jang, “Dissolved Fuel Alkaline Fuel Cell,” US Patent Pending, 10/962,555, Oct. 12, 2004. 

  As illustrated in FIG.8, a prior-art DF-AFC 10 consists of a fuel anode 14, an air cathode 12, and a mixture 16 of electrolyte and fuel that separates the two electrodes.  The electrolyte comprises an alkaline solution (e.g., KOH) with a fuel (such as sodium borohydride, NaBH₄) dissolved in it.  The fuel anode carries an electro-catalyst (e.g., platinum, Pt) to promote the following anode reaction:

            Anode: NaBH₄ + 8OH^- → NaBO₂ + 6H₂O + 8e^-                                        (1)

The water molecules generated at the anode go into the electrolyte solution, but a portion of water is then used at the cathode. The electrons generated at the anode travel to the cathode side of the fuel cell by passing through an external load that connects the anode and the cathode.  Air or oxygen is supplied to the cathode where the electro-reduction of oxygen occurs, resulting in the following chemical reaction:        

            Cathode: 2O₂ + 4H₂O + 8e^- → 8 OH^-                                                              (2)

Although the fuel is also fully in contact with the cathode, this has not caused any major detrimental effect because the cathode catalyst is not platinum. The overall fuel cell reaction is given by:

            Overall:  NaBH₄ + 2O₂ → NaBO₂ + 2H₂O                                                                  (3)

It is of great technological interest to note that eight (8) electrons are generated per fuel molecule consumed.  Further, thermodynamic calculations indicate that the theoretical open circuit voltage (OCV) of sch a cell is approximately 1.64 V, which is significantly higher than that achievable by a hydrogen fuel cell (typically 1.2 V).  These two features indicate that DF-AFC based on alkali metal borohydride such as NaBH₄ potentially have an exceptionally high power density. 

            However, the catalyst (e.g., Pt) that promotes the direct borohydride oxidation of Eq. (1) also tends to promote the hydrolysis reaction:

            Side Reaction:  NaBH₄ + 2H₂O → NaBO₂ + 4H₂                                                        (4)

This side reaction, if not properly controlled, could result in a significant voltage reduction and/or power loss.  However, we have discovered that the 4 H₂ molecules produced, if captured or constrained by the surface pores of a highly porous anode layer, may be oxidized immediately to produce 8 H⁺ and 8 electrons via the following reaction:

            Reaction of Constrained H₂: 4H₂  → 8H⁺ + 8e^- ; OCV = 1.2 V                                    (5)

Although a lower voltage of 1.2 V is generated, the eight electrons may be recovered if the anode structure is properly designed and the side reaction, Eq.(4) does not proceed too quickly.  It is also known that if the concentration of NaBH₄ in the electrolyte is low and the electrolyte concentration is high, the side reaction, Eq.(4), is significantly slowed down. 

             The present invention provides a dissolved-fuel alkaline fuel cell that comprises four components: a) a fuel anode; b) a first oxygen cathode; c) an electrolyte in ionic contact with the anode and the first cathode, wherein the electrolyte comprises an alkaline solution and a first fuel dissolved in the alkaline solution; and d) a fuel reservoir comprising a solid fuel in physical contact with or in feeding relation to the alkaline solution. The first fuel and/or the solid fuel may be selected from the group consisting of NaBH₄, KBH₄, LiAlH₄, KH, NaH, LiBH₄, NaAlH₄, (CH₃)₃NHBH₃, NaCNBH₃, CaH₂, LiH, Na₂S₂O₃, Na₂HPO₃, Na₂HPO₂, K₂S₂O₃, K₂HPO₃, K₂HPO₂, NaCOOH and KCOOH.  However, NaBH₄ and KBH₄ are the best choices for serving as a fuel.  The fuel reservoir can readily replenish a fuel into the electrolyte-fuel mixture or solution to ensure that the fuel cell continuously generates electrical current without an interruption or a voltage spike. The present fuel cell is simple in design, inexpensive to make, capable of providing a relatively high output voltage, and an exceptionally long service life.