Coupling Chemical and Electrical Energy with Half-Reactions

Conceptual Exploration of Battery and Fuel Cell Technology

Collin SchmidtChemical Engineering Undergraduate

What is a half-reaction?

𝐹𝑒2𝑂3+2𝐴𝑙→2𝐹𝑒+𝐴𝑙2𝑂3

Chemists’ understanding of half-reactions came from studying oxidation states of metals.

Consider the Thermite Rxn from General Chemistry, train track welding and bored science teachers:

This reaction is highly exothermic. The energy produced results from oxygen transferring from Iron to Aluminum.

Why does the oxygen transfer?

The energetics of this reaction can be studied by heating each metal separately with a supply of oxygen. This reveals the oxidation reaction for each metal:

2𝐹𝑒+32𝑂2→𝐹𝑒2𝑂3

2 𝐴𝑙+32𝑂2→𝐴𝑙2𝑂3

Measuring the energy changes for each of these reactions reveals that the aluminum oxidation is much more favorable. So much so that aluminum can strip the oxygen out of rust, driving the iron oxidation reaction in reverse. The net reaction is known as the Thermite Reaction.

−1(2𝐹𝑒+32𝑂2→𝐹𝑒2𝑂3)

2 𝐴𝑙+32𝑂2→𝐴𝑙2𝑂3

𝐹𝑒2𝑂3+2𝐴𝑙→2𝐹𝑒+𝐴𝑙2𝑂3

What is a half-reaction? (cont.)2𝐹𝑒+

32𝑂2→𝐹𝑒2𝑂3 2 𝐴𝑙+

32𝑂2→𝐴𝑙2𝑂3

In each of these reactions, the Oxidation State of the metal and the oxygen changes. If we apply conservation of charges to these changes, it reveals new reactions that could sum up to the Oxidation Reactions:

𝐹𝑒3+¿+3𝑒−→𝐹𝑒(𝑠 )¿ 𝐴𝑙3+¿+3𝑒−→ 𝐴𝑙(𝑠 )¿

32𝑂2+6 𝑒

−→𝑂2−3❑ 3

2𝑂2+6 𝑒

−→𝑂2−3❑

From this analysis, chemists theorized that on the molecular level, electrons must flow from one species to another.

The reaction step showing the combination of a metal species with a number of electrons is known as a Half-Reaction, since it requires a sister reaction to supply the electrons it consumes.

Half-Reactions to ElectricityBatteries and Fuel Cells operate by separating charged molecules from electrons at the mircoscopic level, and accumulate macroscopic flows of electrons and ions.

• The electrons are ions are separated by physical characteristics which determine their diffusion rate through other matter.• Electrons move rapidly through conducting materials.• Ions diffuse through solutions, but not through solid metals.

• The energy available for this process is determined by the Net Reaction that occurs.

Electron

Ion

Conducts through Metal

Diffuses through Solution

Transport Process of Ions Determines Battery Design

Why isn’t there a Thermite Battery? The energy change for the reaction is extremely large, wouldn’t it make a strong battery?

𝐴𝑙2𝑂3

𝐴𝑙(𝑠)

• Aluminum Oxide nearly the same grain size as elemental aluminum.

• Aluminum Oxide is extremely stable. Burning magnesium is one of the few convenient ways to penetrate it.

• Thermite battery would have to work at extremely high temperatures, or require an extremely powerful catalyst.

Acid/Base Solutions as Electrolytes

• Aqueous solutions of Acids/Bases can be used to transfer ions from one metal to another.

• Diffusion and electron extraction/donation occurs at room temperature.

• Materials and relatively cheap and well understood.

Storage Battery

• Metal Electrodes as site of oxidation/reduction.

• Electrolyte capable of oxidation/reduction.

• Circuit path for electrons.

• Diffusion path for ions.

• In the past, these could be recharged by replacing electrolyte.

Ions

Electrons

Modern Alkaline Battery

𝑍𝑛(𝑠 ) ,𝑝𝑜𝑤𝑑𝑒𝑟𝑒𝑑𝑃 𝑜𝑟𝑜𝑢𝑠 𝐼𝑜𝑛𝑇𝑟𝑎𝑛𝑠𝑓𝑒𝑟𝑖𝑛𝑔𝑀𝑎𝑡𝑒𝑟𝑖𝑎𝑙𝑀𝑛𝑂2 ,𝑝𝑜𝑤𝑑𝑒𝑟𝑒𝑑

𝑍𝑛+2𝑂𝐻−→𝑍𝑛𝑂+𝐻2𝑂+2𝑒−

2𝑀𝑛𝑂2+𝑍𝑛→𝑀𝑛2𝑂3+𝑍𝑛𝑂

2𝑀𝑛𝑂2+𝐻2𝑂+2𝑒−→𝑀𝑛2𝑂3+2𝑂𝐻−

• Overall Reaction very similar to Thermite, with a net transfer of oxygen from one metal to another.

• Zinc is really good at bonding with reactive oxygen species in solution, leading to its use as a Sacrificial Anode on marine applications. It is linked to a steel structure through a half-reaction, and prevents corrosion without coating the steel.

By utilizing two reactions that share a common electrolyte, alkaline batteries eliminate the need for a

Salt-Bridge.electrons

Net Negative Charge Accumulates

Net Positive Charge Accumulates

𝑁𝑎𝐶𝑙→𝑁𝑎+¿+𝐶𝑙−¿

• By contacting each chamber with an aqueous salt solution (in a gel that does not allow the electrolyte to pass) the ions from the salt can disperse into the electrolyte solutions and prevent a net charge from developing in either chamber.

• The salt ions do not participate in the electro-chemical reaction driving the battery.

A- B+

Electrolyte Choice

• Must be common between metal choices to eliminate salt bridge.• Must be able to overcome activation energy of oxidation/reduction at

operating conditions.• Thermodynamics, rate of reaction and electrode area determines

ideal power output of cell.• Power output further limited by diffusion of ions between

electrodes. Charge accumulation opposes the voltage across the electrodes.

Source: NASA John Glenn Research Center

Molten Metal Batteries

• By Stepping up from aqueous acid/base electrolyte to molten metals, metal reactions with higher activation energies can be used.

• Largest NaS battery in the world was built in 2010 in Presidio, TX. The battery supplies the city during power outages, as Presidio has a single line connection to the US power grid.

Fuel Cells: If a battery oxidizes metals, why can’t we oxidize fuel?

Theoretically, any reaction involving the transfer of electrons and a metal catalyst could be used.. The chemists decided to start with a simple one:

𝐻2+12𝑂2→𝐻 2𝑂

Splitting into half reactions:

𝐻2→2𝐻+¿+ 2𝑒−¿ 1

2𝑂2+2𝑒

−→𝑂2−

Any incompletely oxidized organic can theoretically be used. In fact, CO Detectors employ fuel cells. The rate of electrical energy production from the conversion of CO into CO2 is proportional to the CO concentration in the air. It is enough power to produce a detectable signal, but the unit must be plugged in to operate the electronics of the alarm and a simple logic circuit.

Fuel Cell Construction

𝐻+¿ ¿

𝐻2

𝑒−

𝑂2

𝐻2𝑂

Membrane to Pass H+

Catalyst for H+ formation from adsorbed H2

Catalyst for H20 formation from adsorbed O2

Compare Battery and Fuel Cell Membrane..

𝑂𝐻−

𝐻2𝑂

𝑀𝑒𝑡𝑎𝑙 𝑃𝑜𝑤𝑑𝑒𝑟

𝑒−

𝐻+¿ ¿

𝐻2

𝑂2

𝑒−

Alkaline Storage Battery Hydrogen Fuel Cell

Metal Powder Particles are millions of times larger than dissolved

aqueous species.Diameter(H2)≈2*Diameter(H+)

~Filter Paper Polymer Electrolyte Membrane (PET)

Why don’t we see Fuel Cells all over the place?

• Membrane technology is the limiting factor for..– Cost. Labor and material intensive production.– Durability. Current technologies degrade and reduce unit

performance and lifetime.– Sensitivity.• Require narrow temperature range.• Clean O2 flow required to prevent fouling; some

cannot intake room air without processing.

Fuel Cells have the advantage of flowing reactants. Applying this to Batteries results in..

Flow Battery

Cathode Anode

A

A- B+

B

electrons

Porous Medium

Electrodes as Catalysts or Reactants?

• In alkaline batteries, the metal electrodes are used as reactants and consumed as the battery produces electrical power.

• In fuel cells, the metal electrodes act as catalysts, and are not consumed, though they still degrade with time.

• Flow batteries tend to use metal as catalysts, but there are some hybrids which consume the metal.

Comparison

Storage Battery Flow Battery• Batch (Fill it, seal it, run it)• Voltage determined by

reaction.• Decreasing reactant

concentration and max power output with use.

• No pumping.

• Steady State (Control feed)• Voltage determined by

reaction.• Control of flow rate directly

controls power output as long as tanks hold out.

• Pumping cost effects efficiency.

+ -

Vs.+ -

Power Grid Use of Flow Batteries

• Load Balancing- Charge system during non-peak hours.– Storing Energy from renewables.– Peak Sharing: Output during highest demand to

diminish load on power plants.

Voltage Adding in Series: Flow Batteries as Powerful DC Converters

At steady state, the power transfer between subsystems must be equal. • For electricity, Power = Current*Voltage

Charge at High Voltage, Low Current from Transmission Line

Produce Power at Low Voltage, High Current (Step down of voltage to usable form by DC to AC converters)

Electrolyte transferred to smaller cell bank

22

Storage technologies

Flow batteries are now acknowledged to cover the “sweet spot” – highest storage capacity for 20kW – 10MW applications

Source: Plurion Systems Media- Public Domain