(5j) Acid and Alkaline Hydrogen-Bromine Fuel Cell Systems for Electrical Energy Storage

Comparison of Acid and Alkaline Hydrogen-Bromine Fuel
Cell Systems

Trung Van Nguyen^a*, Venkata Yarlagadda^a,
Guangyu Lin^b, Guoming Weng^c, Vanessa Li^c, and
Kwong-Yu Chan^c

^aDepartment
of Chemical & Petroleum Engineering

The University of Kansas

Lawrence, KS, USA

^bTVN
Systems, Inc.

Lawrence, KS, USA

^cDepartment
of Chemistry

The University of Hong Kong

Hong Kong SAR, China

^*Corresponding Author: cptvn@ku.edu

Abstract

The hydrogen bromine (H₂-Br₂) fuel
cell system is an attractive system for electrical energy storage because of
its high round-trip conversion efficiency, high power density capability, and anticipated
low costs.

The hydrogen-bromine fuel cell system can be operated
in the acid or alkaline modes. The charge and discharge electrode reactions in
an acid H₂-Br₂ fuel cell system are as follows:

Bromine
Electrode:

Br_{2 (aq)} + 2e- ↔
2Br^-_(aq), E^o
= +1.09 V

Hydrogen
Electrode:

H_{2 (g)} ↔ 2H⁺ _(aq) + 2e-, E^o
= +0.0 V

The H⁺ ions migrate from the
hydrogen side across a proton conducting membrane to the bromine side during
discharge to combine with the Br^- ions to form hydrobromic acid.

Overall
Reaction:

H_{2 (g)} + Br_{2 (aq)} ↔ 2HBr _(aq),
E^o = +1.09 V

The charge and discharge electrode reactions
in an alkaline H₂-Br₂ fuel cell system are as follows:

Bromine
Electrode:

Br_{2 (aq)} + 2e- ↔
2Br^- _(aq), E^o
= +1.09 V

Hydrogen
Electrode:

H_{2 (g)} + 2OH^-
_(aq) ↔ 2H₂O + 2e-, E^o = +0.83 V

The cations (e.g., K⁺), associated
with the OH^- ions, migrate from the hydrogen electrode across a
cation (K⁺) conducting membrane to the bromine side and combine with
the Br^- ions to form KBr as shown in the overall reaction.

Overall
Reaction:

H_{2 (g)} + Br_2
(aq) + 2KOH _(aq) ↔ 2H₂O + 2KBr _(aq),    E^o = +1.92 V

Based on the reactions shown above the
alkaline system offers higher cell voltage, which is an advantage because of
potentially higher power output. However, the hydrogen reactions in this system
are two-phase reactions involving gaseous hydrogen and liquid-phase hydroxide
ion reactants and will require more complex electrode structure and fuel cell
design. The other advantages of this system include the fact that non-noble
catalysts can be used for the hydrogen actions and lower corrosiveness.

This presentation will discuss the advantages and
disadvantages of the alkaline and acid H₂-Br₂ fuel cell
systems and compare the discharge and charge performance of both systems.

Acknowledgements

This work was funded in part by the National
Science Foundation through grant number EFRI-1038234 and the Research Grants
Council of Hong Kong through a General Research Fund (GRF HKU 700210P). A
Visiting Professorship to Trung Nguyen was provided by the Initiative on Clean
Energy and Environment (ICEE), University of Hong Kong.