Hydrogen is the most abundant element on the earth's surface, where it is found primarily in water (H2O) and organic compounds. It is generally produced from hydrocarbons or water; and when burned as a fuel, or converted to electricity, it joins with oxygen to formagain water.

Basic chemical knowledge of Hydrogen is relevant for the understanding of the difference between Borohydride and Hydrogen as electron supplier.

Names of hydrogen species

General Term
Molecular(H2) Hydrogen molecular Dihydrogen
Atom(H0) Hydrogen atom Protium
Cation(H+) Hydrogen ion Proton
Anion(H-) Hydride ion Protide

  • Ho (one e-) -Can be found in metal hydride and H2. The catalyst of normal
    Fuel Cells (PEMFC) separates H2 into two Ho atoms.
  • H+ (no e-) -Can be found in a H2O molecule.
  • H- (two e-) -Can be found in protide compounds such as BH4-
  • H2 (one e-) -Molecular Hydrogen

THE 4-STATES OF HYDROGEN (Prepared by Prof. S. Suda)

  1. Hydrogen (H2: Diatomic hydrogen): PEMFC (PEFC) and electrolysis
    H2 (H-H) 
    2Ho (internally reversible)  2H+ + 2e- (irreversible)
    2H2 + O2 (internally reversible)
  2. Protium (Ho: Monatomic hydrogen): in metal hydrides for H-storage
    H2 2Ho (internally reversible)
  3. Proton (H+: Hydrogen ion): in PEMFC (PEFC)
    1/2H2 (H-H)  Ho (internally reversible)  H+ + e- (irreversible)
  4. Protide (H-: Hydride ion): ex. in MgH2, LiBH4, and NaBH4 for hydrogen storage and DBFC
    Ho + e-  H- (irreversible)
    2H- + 2e- (internally reversible)

Protide -- a high energy hydrogen species

Two electron attached on one hydrogen atom

In the complex ion of BH4-, hydrogen exists as a negative ion (protide: H-). The fact which needs attention is that a protide can release twice as much electron as a proton (normal PEMFC).

H-  H+ + 2e-

Protide is highly unstable and has a high energy level. In order to stabilize the Borohydride, it is resolved into the aqueous solution of sodium hydroxide or potassium hydride. After that it is very stable to use und easy to transport.

Through the catalystic reaction on the anode side of the Fuel Cell, the Borohydride is spited up and 8 electrons are separated from each Borohydride molecule.

BH-4 + 8OH-  BO-2 + 6H2O + 8e-

Borohydride (BH4-)4 Proton (H+) + 8e-

Properties of Borohydride as Hydrogen Storage Material

By A. Zuttel and L. Sclapbach, Nature p.353, Vol. 414 (2001)

This figure underlines the fact that sodium borohydride is a high hydrogen storage material. It is located in the group of highest hydrogen content, both weight basis and volume basis.

Major Problems of other hydrogen storage materials

Approx. 900atm is required to obtain same amount of H2 by compression. H2 compression is not suitable for mobile services by cylinder weight and safety consideration.

Several hundreds grams is required to store H2 as metal hydrides (practical hydrogen content of 2wt%). It is also disadvantage that continuous heat input is required under hydrogen generation.

Hydrogen from natural gas or methanol by reforming requires higher temperature for reaction. It causes longer start-up duration, delay of response, care for burn injury, etc. CO, byproduct of hydrogen, should be treated to avoid poisoning of PEFC electrode and intoxication. It is also disadvantage that continuous heat input is required under hydrogen generation.

In DMFC (Direct Methanol Fuel Cell), methanol fuel is used as low concentration to avoid crossover of methanol in fuel cell, leading lower energy density or recirculation of diluting water. Toxicity of methanol should be also considered.