Coupling Chemical and Electrical Energy with Half-Reactions Conceptual Exploration of Battery and Fuel Cell Technology Collin Schmidt Chemical Engineering Undergraduate

What is a half-reaction? 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: 𝐹𝑒 2 𝑂 3 +2𝐴𝑙→2𝐹𝑒+ 𝐴𝑙 2 𝑂 3 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+ 3 2 𝑂 2 → 𝐹𝑒 2 𝑂 3 2𝐴𝑙+ 3 2 𝑂 2 → 𝐴𝑙 2 𝑂 3 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+ 3 2 𝑂 2 → 𝐹𝑒 2 𝑂 3 ) 2𝐴𝑙+ 3 2 𝑂 2 → 𝐴𝑙 2 𝑂 3 𝐹𝑒 2 𝑂 3 +2𝐴𝑙→2𝐹𝑒+ 𝐴𝑙 2 𝑂 3

What is a half-reaction? (cont.) 2𝐹𝑒+ 3 2 𝑂 2 → 𝐹𝑒 2 𝑂 3 2𝐴𝑙+ 3 2 𝑂 2 → 𝐴𝑙 2 𝑂 3 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: 𝐹𝑒 3+ +3 𝑒 − →𝐹𝑒(𝑠) 𝐴𝑙 3+ +3 𝑒 − →𝐴𝑙(𝑠) 3 2 𝑂 2 +6 𝑒 − → 𝑂 2− 3 3 2 𝑂 2 +6 𝑒 − → 𝑂 2− 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 Electricity Batteries and Fuel Cells operate by separating charged molecules from electrons at the mircoscopic level, and accumulate macroscopic flows of electrons and ions. Electron Ion Conducts through Metal Diffuses through Solution 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.

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? 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. 𝐴𝑙 2 𝑂 3 𝐴𝑙 (𝑠)

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 Electrons Ions Electrons 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.

Modern Alkaline Battery 𝑍𝑛 (𝑠) ,𝑝𝑜𝑤𝑑𝑒𝑟𝑒𝑑 𝑃𝑜𝑟𝑜𝑢𝑠 𝐼𝑜𝑛 𝑇𝑟𝑎𝑛𝑠𝑓𝑒𝑟𝑖𝑛𝑔 𝑀𝑎𝑡𝑒𝑟𝑖𝑎𝑙 𝑀𝑛 𝑂 2 , 𝑝𝑜𝑤𝑑𝑒𝑟𝑒𝑑 𝑍𝑛+2 𝑂𝐻 − →𝑍𝑛𝑂+ 𝐻 2 𝑂+2 𝑒 − 2𝑀𝑛 𝑂 2 +𝑍𝑛→ 𝑀𝑛 2 𝑂 3 +𝑍𝑛𝑂 2𝑀𝑛 𝑂 2 + 𝐻 2 𝑂+2 𝑒 − → 𝑀𝑛 2 𝑂 3 +2 𝑂𝐻 − 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 A- B+ 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.

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.

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. Source: NASA John Glenn Research Center

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: 𝐻 2 + 1 2 𝑂 2 → 𝐻 2 𝑂 Splitting into half reactions: 1 2 𝑂 2 +2 𝑒 − → 𝑂 2− 𝐻 2 →2 𝐻 + +2 𝑒 − 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 Membrane to Pass H+ Catalyst for H+ formation from adsorbed H2 Catalyst for H20 formation from adsorbed O2 𝐻 2 𝑂 2 𝐻 + 𝐻 2 𝑂 𝑒 −

Compare Battery and Fuel Cell Membrane.. Alkaline Storage Battery Hydrogen Fuel Cell ~Filter Paper Polymer Electrolyte Membrane (PET) 𝑂𝐻 − 𝐻 2 𝑂 𝑀𝑒𝑡𝑎𝑙 𝑃𝑜𝑤𝑑𝑒𝑟 𝑒 − 𝐻 + 𝐻 2 𝑂 2 𝑒 − Metal Powder Particles are millions of times larger than dissolved aqueous species. Diameter(H2)≈2*Diameter(H+)

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 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 Produce Power at Low Voltage, High Current (Step down of voltage to usable form by DC to AC converters) Charge at High Voltage, Low Current from Transmission Line Electrolyte transferred to smaller cell bank

Source: Plurion Systems Media- Public Domain 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