coupling chemical and electrical energy with half-reactions conceptual exploration of battery and...
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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?
๐น๐2๐3+2๐ด๐โ2๐น๐+๐ด๐2๐3
Chemistsโ 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:
2๐น๐+32๐2โ๐น๐2๐3
2 ๐ด๐+32๐2โ๐ด๐2๐3
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.
โ1(2๐น๐+32๐2โ๐น๐2๐3)
2 ๐ด๐+32๐2โ๐ด๐2๐3
๐น๐2๐3+2๐ด๐โ2๐น๐+๐ด๐2๐3
What is a half-reaction? (cont.)2๐น๐+
32๐2โ๐น๐2๐3 2 ๐ด๐+
32๐2โ๐ด๐2๐3
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:
๐น๐3+ยฟ+3๐โโ๐น๐(๐ )ยฟ ๐ด๐3+ยฟ+3๐โโ ๐ด๐(๐ )ยฟ
32๐2+6 ๐
โโ๐2โ3โ 3
2๐2+6 ๐
โโ๐2โ3โ
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.
โข The electrons are ions are separated by physical characteristics which determine their diffusion rate through other matter.โข Electrons move rapidly through conducting materials.โข Ions diffuse through solutions, but not through solid metals.
โข 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 isnโt there a Thermite Battery? The energy change for the reaction is extremely large, wouldnโt it make a strong battery?
๐ด๐2๐3
๐ด๐(๐ )
โข Aluminum Oxide nearly the same grain size as elemental aluminum.
โข Aluminum Oxide is extremely stable. Burning magnesium is one of the few convenient ways to penetrate it.
โข Thermite battery would have to work at extremely high temperatures, or require an extremely powerful catalyst.
Acid/Base Solutions as Electrolytes
โข Aqueous solutions of Acids/Bases can be used to transfer ions from one metal to another.
โข Diffusion and electron extraction/donation occurs at room temperature.
โข Materials and relatively cheap and well understood.
Storage Battery
โข Metal Electrodes as site of oxidation/reduction.
โข Electrolyte capable of oxidation/reduction.
โข Circuit path for electrons.
โข Diffusion path for ions.
โข In the past, these could be recharged by replacing electrolyte.
Ions
Electrons
Modern Alkaline Battery
๐๐(๐ ) ,๐๐๐ค๐๐๐๐๐๐ ๐๐๐๐ข๐ ๐ผ๐๐๐๐๐๐๐ ๐๐๐๐๐๐๐๐๐ก๐๐๐๐๐๐๐๐2 ,๐๐๐ค๐๐๐๐๐
๐๐+2๐๐ปโโ๐๐๐+๐ป2๐+2๐โ
2๐๐๐2+๐๐โ๐๐2๐3+๐๐๐
2๐๐๐2+๐ป2๐+2๐โโ๐๐2๐3+2๐๐ปโ
โข Overall Reaction very similar to Thermite, with a net transfer of oxygen from one metal to another.
โข 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
๐๐๐ถ๐โ๐๐+ยฟ+๐ถ๐โยฟ
โข 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.
โข The salt ions do not participate in the electro-chemical reaction driving the battery.
A- B+
Electrolyte Choice
โข Must be common between metal choices to eliminate salt bridge.โข Must be able to overcome activation energy of oxidation/reduction at
operating conditions.โข Thermodynamics, rate of reaction and electrode area determines
ideal power output of cell.โข 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
โข By Stepping up from aqueous acid/base electrolyte to molten metals, metal reactions with higher activation energies can be used.
โข 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 canโt 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:
๐ป2+12๐2โ๐ป 2๐
Splitting into half reactions:
๐ป2โ2๐ป+ยฟ+ 2๐โยฟ 1
2๐2+2๐
โโ๐2โ
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
๐ป+ยฟ ยฟ
๐ป2
๐โ
๐2
๐ป2๐
Membrane to Pass H+
Catalyst for H+ formation from adsorbed H2
Catalyst for H20 formation from adsorbed O2
Compare Battery and Fuel Cell Membrane..
๐๐ปโ
๐ป2๐
๐๐๐ก๐๐ ๐๐๐ค๐๐๐
๐โ
๐ป+ยฟ ยฟ
๐ป2
๐2
๐โ
Alkaline Storage Battery Hydrogen Fuel Cell
Metal Powder Particles are millions of times larger than dissolved
aqueous species.Diameter(H2)โ2*Diameter(H+)
~Filter Paper Polymer Electrolyte Membrane (PET)
Why donโt we see Fuel Cells all over the place?
โข Membrane technology is the limiting factor for..โ Cost. Labor and material intensive production.โ Durability. Current technologies degrade and reduce unit
performance and lifetime.โ Sensitivity.โข Require narrow temperature range.โข 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?
โข In alkaline batteries, the metal electrodes are used as reactants and consumed as the battery produces electrical power.
โข In fuel cells, the metal electrodes act as catalysts, and are not consumed, though they still degrade with time.
โข Flow batteries tend to use metal as catalysts, but there are some hybrids which consume the metal.
Comparison
Storage Battery Flow Batteryโข Batch (Fill it, seal it, run it)โข Voltage determined by
reaction.โข Decreasing reactant
concentration and max power output with use.
โข No pumping.
โข Steady State (Control feed)โข Voltage determined by
reaction.โข Control of flow rate directly
controls power output as long as tanks hold out.
โข Pumping cost effects efficiency.
+ -
Vs.+ -
Power Grid Use of Flow Batteries
โข Load Balancing- Charge system during non-peak hours.โ Storing Energy from renewables.โ 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. โข 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