Electrochemistry, Batteries, and Corrosion

Comprehensive vocabulary flashcards covering electrochemical cells, electrode potentials, Nernst equation derivation, primary and secondary batteries, fuel cells, corrosion mechanisms, and corrosion control methods.

Oxidation/Reduction Reaction

A chemical reaction in which electrons are transferred from one reactant to another; it can be split into two half-reactions that show which species gains electrons and which loses them.

Oxidizing Agent (Oxidant)

A substance that has a strong affinity for electrons and tends to remove them from other species, such as $Ce^{4+}$.

Reducing Agent (Reductant)

A species that readily donates electrons to another species, such as $Fe^{2+}$.

Electrochemical Cell (Galvanic or Voltaic Cell)

A device used to convert the chemical energy produced in a redox reaction into electrical energy, consisting of two half-cells connected externally by a metal wire and solutions connected indirectly by a salt bridge.

Electrolytic Cell

A device that utilizes an external power source to drive chemical reactions, converting electrical energy into chemical energy.

Electrode Potential

The tendency of an electrode to lose or gain electrons when it is in contact with its own ions in solution.

Standard Hydrogen Electrode (SHE)

A primary reference electrode represented as $Pt, H_2(1\,\text{atm}), H^+(c=1\,\text{M})$ whose reduction potential is arbitrarily assigned as zero.

Standard Electrode Potential ($E^\circ$)

The potential of an electrode at a given temperature ($25^\circ\text{C}$) when the concentration of its surrounding ions is unity ($1\,\text{M}$), determined on the hydrogen scale.

Electrochemical Series

A series formed by arranging standard electrode potentials in increasing order; higher positive values reflect a greater tendency to get reduced, while greater negative values indicate a greater tendency to get oxidized.

Nernst Equation

An equation giving the relationship between electrode potential and electrolyte concentration: $E = E^\circ - \frac{RT}{nF} \ln\left(\frac{[\text{Products}]}{[\text{Reactants}]}\right)$, which at $25^\circ\text{C}$ becomes $E = E^\circ - \frac{0.0592}{n} \log\left(\frac{[\text{Products}]}{[\text{Reactants}]}\right)$.

Cell Electromotive Force ($\text{EMF}$)

The overall potential of a cell calculated from the reduction potentials of the two electrodes: $E_{\text{cell}} = E_{\text{cathode}} - E_{\text{anode}} = E_R - E_L$.

Battery

A device that transforms chemical energy into electrical energy, consisting of two or more electrical cells connected in series to act as a source of direct electric current at a constant voltage.

Primary Battery (Primary Cell)

A battery in which the cell reaction is not reversible; once reactants are mostly converted to products, electricity production ceases and the battery is dead.

Secondary Battery (Secondary Cell)

A battery in which the cell reaction can be reversed by passing direct electric current in the opposite direction, allowing it to be recharged through numerous charging and discharging cycles.

Zinc-Air Battery

An alkaline metal-air battery producing a voltage of $1.25\text{--}1.35\,\text{V}$ that utilizes oxygen directly from the air as the cathodic active material rather than storing it inside the cell.

Fuel Cell

A primary galvanic cell that directly converts the chemical energy of a fuel into electrical energy by continuously and separately supplying fuel and oxidant to the electrodes from an external source.

Hydrogen-Oxygen Fuel Cell

A fuel cell with porous graphite electrodes impregnated with a catalyst ($Pt$, $Ag$, or a metal oxide) and an aqueous $KOH$ or $NaOH$ electrolyte, operating under a gas pressure of about $50\,\text{atm}$ to generate an $\text{EMF}$ of $1\,\text{V}$.

Polymer Electrolyte Membrane Fuel Cell (PEMFC)

A hydrogen-oxygen fuel cell using a solid proton exchange membrane electrolyte, operating at relatively low temperatures ($60\text{--}80^\circ\text{C}$) with instant start-up and high power density.

Nafion

A sulfonated tetrafluoroethylene-based fluoropolymer commonly used as the solid proton exchange membrane electrolyte in PEM fuel cells.

Lithium-Ion Battery

A rechargeable battery with a nominal cell voltage of $3.7\,\text{V}$, featuring a lithium metal oxide cathode ($LiCoO_2$, $LiFePO_4$, or $LiNiO_2$), a graphite anode, and a non-aqueous electrolyte with a lithium salt ($LiPF_6$ or $LiBF_4$).

Lead-Acid Storage Cell

A secondary storage cell producing approximately $2.0\,\text{V}$ ($12\,\text{V}$ for six cells in series) using a spongy lead ($Pb$) anode, a lead dioxide ($PbO_2$) cathode, and dilute $H_2SO_4$ electrolyte ($20\text{--}21\%$ by weight, density $1.2\text{--}1.3\,\text{g/cm}^3$).

Corrosion

The gradual disintegration or deterioration of a metal caused by chemical or electrochemical reactions with its surrounding environment.

Dry Corrosion (Chemical Corrosion)

Corrosion brought about by the direct chemical interaction of a metal surface with atmospheric gases or anhydrous liquid in the absence of moisture.

Liquid Metal Corrosion

Corrosion caused by the chemical action of flowing liquid metal at high temperatures on a solid metal or alloy, resulting in dissolution or liquid metal penetration.

Wet Corrosion (Electrochemical Corrosion)

Corrosion that takes place when a conducting liquid contacts a metal or two dissimilar metals/alloys, causing oxidation at separate anodic areas and reduction at cathodic areas connected by an electron current.

Galvanic Corrosion (Bimetallic Corrosion)

Electrochemical corrosion occurring when two dissimilar metals are connected by an electrical conductor in a conductive solution, causing the metal higher in the electrochemical series to act as the anode and corrode.

Galvanic Series

A practical series reflecting the actual corrosion behavior of metals and alloys in a given environment, taking into account passivity where oxide films make effective electrode potentials more noble/cathodic.

Differential Aeration Corrosion (Concentration Cell Corrosion)

Corrosion that occurs when a metal surface is partially immersed in an electrolyte and exposed to air; the poorly oxygenated area acts as an anode and corrodes, while the well-oxygenated area acts as a cathode.

Overvoltage (in Corrosion)

The excess electrical potential required to evolve gas at an electrode surface (such as $0.70\,\text{V}$ for hydrogen on zinc); reducing overvoltage (e.g., to $0.33\,\text{V}$ by adding copper sulfate) accelerates the corrosion rate.

Pilling-Bedworth's Rule

The rule stating that an oxide film is protective and impervious if the specific volume of the oxide is greater than that of the metal atom, but porous and non-protective if the specific volume of the oxide is less than that of the metal atom.

Cathodic Protection (Electrochemical Protection)

A corrosion mitigation technique that prevents the dissolution of a metal by forcing the entire metal structure to behave as a cathode, eliminating anodic areas.

Sacrificial Anodic Protection (Galvanic Protection)

A cathodic protection method where a more active metal ($Mg$, $Zn$, $Al$, or their alloys) is electrically connected to a structure so that all corrosion concentrates on the sacrificial metal, sparing the structure.

Impressed Current Cathodic Protection

A cathodic protection method where direct current from an external source is applied in the opposite direction to nullify the natural corrosion current, turning the protected structure into a cathode via an inert or consumable anode.