Fuel   Cell   Science:

Horizon Fuel Cell Technologies of Singapore

Senior chemistry in Australia needs to strengthen the electrochemical nature of matter in teaching.The example below highlights classical chemistry teaching in Australia. When students select a lead acid battery and "one other type" they should select the obvious, fuel cell type. The lead acid battery generating current via its galvanic elements of participating electrodes, becomes challenged by an electrolysis(NaClaq) reaction using platinum electrodes generating current when the power supply is stopped. The galvanic parts are preserved as the chlorine gas and hydrogen gas are the electrodes of difference. Action taking place on the surface of the (Pt) electrodes at room temperature. It is a battery generating current and voltage albeit observed by a multimeter. It is reverse electrolysis, the fuel cell effect. It is also the experiment in concept early scientists used when they discovered such and called it the "Gas battery". Students are so close to this so such a small next "step" is imperative. In several years students can select a specific strand with a link to a TAFE college, and study H2 and fuel cell science/engineering education.Here they would learn about all fuel cell types and virtually have a basic qualification "101" when they left high school.

Electrolysis

Basics:

1[ An electrolyte is a substance in which ions are present, usually in water. This can be an acid, base or salt solution. Ions are charged particles in the solution of water(aqueous). The process is dissociation.The water molecule's polarity is responsible for this process. Metals and hydrogen form positive ions, non-metals form negative ions.

The following shows examples.

Substance in solution Name:  Positive ions (cations)   Negative ions (anions)

H2SO4 sulphuric acid   H+, H+ SO42-

NaOH caustic soda Na+ OH-   NaCl common salt Na+ Cl-   ]

2[ For electrolysis one must have two similar electrodes into the solution. They are connected to the poles of a source of direct current (a battery)]. 3[The positively charged ions (cations) migrate to the minus pole(cathode)]. 4[The negatively charged ions (anions) move to the plus pole (anode)]. 5[On the electrodes the ions are discharged and are then present as neutral particles (atoms or molecules)]. 6[In this form they react with the chemical environment and precipitate on the electrode, escape upward as a gas or are involved in secondary reactions].

* A simple example of electrolysis : Use thin platinum electrodes and multimeter

Common salt (NaCl), dissolved in water, dissociates to sodium ions and chloride ions (Na+ and Cl-). Cl- migrates to the positive electrode (anode), is discharged (give up an electron) and forms gas molecules (Cl2), which rise to the surface as small bubbles. One can smell the chlorine gas (as in a swimming pool with chlorinated water). Na+, on the other hand, migrates to the negative electrode (cathode) and is discharged there (takes up an electron). Sodium, a highly reactive metal, is unstable in water and is immediately converted in a secondary reaction to sodium hydroxide (NaOH) . For this to happen, an OH- ion must be come from the water (H2O), leaving an H+. The H+ ions join to form hydrogen molecules, which rise at the cathode as small bubbles. The reaction products of the electrolysis of common salt solution are hence chlorine gas (Cl2) and hydrogen gas (H2)

8[Anode reactions:

Electron release (oxidation) Cl- =Cl + 1 e-

Formation of chlorine (gas) molecule 2 Cl to Cl2

Overall reaction 2 Cl- =2 e- + Cl2

Cathode reactions:

Electron acceptance (reduction) Na+ + 1 e- = Na

Secondary reaction: formation of caustic soda Na + H2O =NaOH + H

Formation of hydrogen (gas) molecule 2 H = H2

Overall reaction 2 Na+ + 2 e- = 2 NaOH + H]

The galvanic parts consists of :

ANODE \  ELECTROLYTE  \ CATHODE.

9[These elements allow chemical energy to be converted into electrical energy.The best-known case is when anode and cathode are of different metals, e.g. copper (Cu) and zinc (Zn).The base metal, in this case Zn, corrodes: positively charged Zn2+ ions go into solution, leaving electrons at the Zn electrode (which becomes a minus pole). When Zn2+ ions precipitate on the Cu electrode, they need 2 electrons for discharging. Hence there occurs a deficiency of electrons on the Cu electrode (which becomes a plus pole). Such an electrical imbalance must be corrected. The electrons would prefer to migrate in a direct path from the minus pole through the electrolyte to the plus pole, but an electrolyte conducts only ions and not electrons, i.e. is an insulator for the latter. These must therefore make the detour around an external circuit (and perform work for us) before the charges can be equalised. The instant [exothermic] corrosion reaction of the zinc thus provides us with electrical energy. All the batteries we use in portable electronic appliances work in accordance with this principle].

Gas battery / galvanic element:

Refer to above electrolysis example and chemical reactions " 8 ".Now stop power supply ,and attach the multimeter. Set multimeter to correct scale "20".Observe voltage .

How does the gas battery work?

10[We do not have different electrode materials and a voltage should not appear. This argument would be correct, but there were gas bubbles left on the (Pt)electrodes as a result of the electrolysis.] 11[In our example of a salt solution, we effectively have chlorine gas and hydrogen gas electrodes, i.e. different electrodes. Here we see a further important point]:12[ The chemistry takes place not in the electrode but exclusively at its surface.

Gas battery / reverse electrolysis:

*13[On the gas-covered electrodes and in the electrolyte solution, the reverse reaction to electrolysis takes place]. As a result of the electrolysis, hydrogen and chlorine gas are present. These recombine to form hydrochloric acid (HCl), a synthesis. 14[This reaction can only take place when platinum electrodes are used. Without the catalytic effect of the platinum surface, this reaction cannot proceed (exothermally) at room temperature. The scientific term here is heterogeneous catalysis].

* Fuel cell reaction:

15[This is  the reverse reaction to electrolysis, the production of hydrochloric acid. In the following, the overall reaction, partial reactions and standard potentials E0 of the redox pairs H2/H+ and Cl2/Cl-  are given.

Overall reaction H2 + Cl2 =2 HCl

Electron acceptance (reduction) Cl2 + 2 e- = 2 Cl- E0 (Cl2/Cl-) = 1.36 Volts

Electron release (oxidation) H2 – 2 e- = 2 H+ E0 (H2/H+) = 0 Volt (by definition)  ]


Advanced classes:

Electrolysis of Methanol:

An aqueous solution of methanol is circulated past the anode, where methanol and water undergo the reaction

CH3OH + H2O --> CO2 + 6H+ + 6e

The hydrogen ions pass through the membrane to the cathode, where they are reduced to hydrogen molecules in the reaction

6H+ + 6e --> 3H2

Thus, the net reaction in the cell is

CH3OH --> CO2 + 3H2

with carbon dioxide liberated on the anode side and hydrogen liberated on the cathode side. Because the membrane is not totally impermeable by water and methanol, traces of these substances pass through along with the protons. However, the water and methanol can easily be removed from the hydrogen stream by use of a molecular sieve, as is routinely done to remove traces of water and oxygen from hydrogen streams produced in water electrolysers.

If the solid-electrolyte membrane in the cell is made of Nafion™ (or equivalent) perfluorosulfonic acid-based proton-conducting polymer, then the cell can be operated in the temperature range from 5 to 120 °C. The concentration of methanol in the aqueous solution can range from 0.1 to 8 molar. The membrane is the electrolyte, and it is not necessary to acidify the solution to make it electrically conductive.

The theoretical operating potential of the cell is 0.02 V, though in practice, a useful amount of electrolysis is not achieved until the potential is raised to 0.3 V. In contrast, the potential needed to electrolyze water is more than 1.4 V, even in the most efficient electrolysers.The potential needed to obtain a given current density in electrolysis of methanol is more than 1 V below the potential needed to obtain the same current density in electrolysis of water. The electrical power consumed in electrolysis is reduced proportionately.


Sodium Borohydride / hydrogen peroxide: Cell

A liquid solution of sodium borohydride is introduced to the anode of the fuel cell and a solution of hydrogen peroxide is introduced to the cathode. The fuel cell reactions are as follows:

Anode: NaBH4 + 8Na+ + 8OH- to NaBO2 + 6H2O + 8Na+ + 8e-;

Eo= -1.24 V3

Cathode: 8Na+ + 8e- + 4H2O2  to  8Na+ + 8OH-; Eo= 0.95 V

Cell: NaBH4 + 4H2O2 to NaBO2 + 6H2O; Vcell= 2.19 V

The polarization characteristics of a sodium borohydride/hydrogen peroxide fuel cell with a 25-cm2 active area operating at 40 oC with 5, 10 and 15% hydrogen peroxide concentrations is shown in Figure 1. The cell voltages at 240-mA/cm2 are 0.42, 0.49, and 0.56 V at 5, 10, and 15% concentration of hydrogen peroxide, respectively. A 70 mA/cm2 improvement in current density can be achieved at a cell voltage of 0.5 V by increasing the hydrogen peroxide concentration from 5 to 15%. A plot of the power densities as a function of applied current density for a cell operating at 40 oC with 5, 10 and 15% hydrogen peroxide concentrations is shown in Figure 2. The peak power density for the cell operating at 5, 10, and 15% hydrogen peroxide are 105, 118, and 139-mW/cm2, respectively. These power densities are on the order of three times greater than the observed power densities of the methanol/hydrogen peroxide fuel cell under similar operating conditions.

Figure 1. Current-Voltage characterization of a Sodium borohydride/hydrogen peroxide MEA as a function of hydrogen peroxide concentration. Cell operating at 40 oC, 10% sodium borohydride solution.

Figure 2. Power density as density for a sodium borohydride/ MEA operating at various hydrogen concentrations. Cell operating borohydride solution


Technology  -   Reversible PEMFC (educational purposes)


HOME