Compendium

Electrochemistry — Cells, Batteries, Fuel Cells, Electrolysis, Corrosion

29 min read

Electrochemistry — Cells, Batteries, Fuel Cells, Electrolysis, Corrosion

Electrochemistry is the chemistry of charge transfer between phases — typically a solid electrode and a liquid electrolyte — and the technology built atop it. Half of the periodic table’s energy storage and energy conversion stems from electrochemical principles: every battery in every phone, electric vehicle, and grid-scale storage facility; every fuel cell in Toyota Mirai and Bloom Energy Server; every electrolyzer producing green hydrogen; every electroplating bath, chlor-alkali plant, and aluminum smelter; every glucose biosensor and pH electrode; and the slow, ubiquitous decay of metal infrastructure called corrosion.

This note opens with the thermodynamic framework (cell potentials, Nernst), continues into kinetics (Butler-Volmer, Marcus), then surveys the major application classes: batteries (Li-ion, beyond Li-ion, flow), fuel cells (PEMFC, SOFC, AFC), electrolysis (water, chlor-alkali, aluminum, organic), and corrosion. The chemistry connects to physical-chemistry (thermodynamics, kinetics, transport) and to engineering practice in energy-storage-systems.

1. Electrochemical cells — galvanic and electrolytic

A cell consists of two electrodes (anode and cathode) immersed in an electrolyte (sometimes two electrolytes separated by a membrane or salt bridge). Half-reactions occur at each electrode:

  • Anode: oxidation (loss of electrons) — supplies electrons to the external circuit
  • Cathode: reduction (gain of electrons) — receives electrons from the external circuit

The mnemonic “AnOx, RedCat” (anode oxidation, reduction cathode) is universal. Polarity convention can confuse: in a galvanic cell (spontaneous, like a battery discharging) the anode is the negative terminal; in an electrolytic cell (driven by external power, like an electrolyzer charging) the anode is positive.

Cell notation (Daniell cell)

Zn(s) | Zn²⁺(aq, 1 M) || Cu²⁺(aq, 1 M) | Cu(s)

Left of the double bar = anode side, right = cathode side, single bar = phase boundary, double bar = salt bridge or membrane. Overall reaction: Zn(s) + Cu²⁺ → Zn²⁺ + Cu(s); E°_cell = +1.10 V.

Standard electrode potentials

Tabulated against the standard hydrogen electrode (SHE): 2 H⁺(1 M) + 2 e⁻ → H₂(g, 1 bar) on platinized Pt; defined as exactly 0 V at all temperatures.

Selected E° (volts vs SHE, 25 °C):

  • F₂/F⁻ +2.87
  • O₃/O₂ +2.07
  • MnO₄⁻/Mn²⁺ +1.51 (acid)
  • Cl₂/Cl⁻ +1.358
  • O₂/H₂O +1.229 (acid)
  • Ag⁺/Ag +0.7996
  • Fe³⁺/Fe²⁺ +0.771
  • Cu²⁺/Cu +0.342
  • 2 H⁺/H₂ 0 (definition)
  • Pb²⁺/Pb -0.126
  • Fe²⁺/Fe -0.44
  • Zn²⁺/Zn -0.7626
  • Al³⁺/Al -1.66
  • Mg²⁺/Mg -2.37
  • Na⁺/Na -2.71
  • Li⁺/Li -3.04 (most negative — basis of Li battery cell voltage)

E°_cell = E°_cathode - E°_anode (both as reduction potentials). For Daniell: 0.342 - (-0.763) = 1.10 V. Positive E°_cell ⇒ spontaneous galvanic reaction; ΔG° = -nFE°_cell, where F = 96485.33 C/mol (Faraday constant, named for Michael Faraday’s electrolysis laws, 1834).

Nernst equation

For a half-reaction Ox + ne⁻ → Red:

E = E° - (RT/nF) ln([Red]/[Ox]) = E° - (RT/nF) ln Q

At 25 °C (298.15 K), RT/F = 0.02569 V, and (RT/F) ln 10 = 0.0592 V, so:

E = E° - (0.0592/n) · log₁₀ Q (volts at 25 °C)

A factor-of-10 change in concentration ratio shifts a one-electron potential by 59.2 mV. Concentration cells, pH electrodes, and ion-selective electrodes (ISEs) all exploit this.

Equilibrium constant from E°

At equilibrium ΔG° = -nFE° = -RT ln K, hence:

ln K = nFE° / RT

For Daniell (n=2, E° = 1.10 V): ln K = 2·96485·1.10/(8.314·298) = 85.6, so K ≈ 1.5 × 10³⁷. Effectively complete reaction — but only after surmounting kinetic barriers.

Reference electrodes

Practical alternatives to the cumbersome SHE:

  • Saturated calomel (SCE): Hg(l) | Hg₂Cl₂(s) | KCl(sat) — +0.241 V vs SHE; long history; toxic Hg now restricted
  • Silver/silver chloride (Ag/AgCl): Ag | AgCl | KCl(aq) — +0.210 V vs SHE in 3 M KCl, +0.197 V in sat KCl; non-toxic, used in pH electrodes (BASi, Pine Research, Gamry electrodes), biopotentials (ECG, EEG)
  • Mercury/mercurous sulfate (MSE): Hg | Hg₂SO₄ | K₂SO₄(sat) — +0.641 V vs SHE; chloride-free
  • Cu/CuSO₄: +0.318 V vs SHE; cathodic protection field reference

Always report potentials with the reference electrode! “E = -0.5 V vs Ag/AgCl” is meaningful; “E = -0.5 V” is not.

2. Conductivity and ion transport

Conductivity κ (S/m, siemens per meter). For an aqueous solution, κ = Σ_i (|z_i| F u_i c_i), summed over ions, where u_i is ionic mobility (m²/(V·s)).

Molar conductivity Λ_m = κ/c (S·m²/mol). At infinite dilution Λ_m → Λ_m⁰. Kohlrausch’s law of independent ionic migration: Λ_m⁰ = ν_+ λ_+⁰ + ν_- λ_-⁰.

Selected limiting molar ionic conductivities at 25 °C (S·cm²/mol):

  • H⁺ 349.8 (anomalously high; Grotthuss proton-hopping via H-bond network)
  • OH⁻ 198 (also Grotthuss)
  • K⁺ 73.5
  • Na⁺ 50.1
  • Li⁺ 38.7
  • Cl⁻ 76.4
  • SO₄²⁻ 160

Conductivity sensors widely used for water quality, ionic strength of process streams; instruments from Mettler Toledo (InLab series), Hach, Hanna Instruments. Conductivity-temperature compensation (typically 2%/°C for dilute electrolytes) built into modern probes.

3. Electrochemical kinetics

Thermodynamics says whether a reaction can occur; kinetics says how fast. Most electrochemical reactions are sluggish without a driving overpotential.

Butler-Volmer equation

i = i₀ · [exp(α n F η / RT) - exp(-(1-α) n F η / RT)]

  • i — current density (A/m²)
  • i₀ — exchange current density (intrinsic activity at equilibrium; A/m²)
  • α — symmetry/charge-transfer coefficient (0 ≤ α ≤ 1; often ~0.5)
  • η = E - E_eq — overpotential (V)
  • n — electrons per reaction

At large η (|η| > 50-100 mV), one exponential dominates and the Tafel equation results:

|η| = b · log₁₀(|i| / i₀)

where Tafel slope b = 2.303 RT / (α n F). At 25 °C with α = 0.5 and n = 1, b ≈ 118 mV/decade. Tafel slope is the workhorse diagnostic of electrochemical kinetics: ~30 mV/dec for fast multi-electron, ~120 mV/dec for one-electron rate-limited, ~60 mV/dec common in alkaline OER. Plot log|i| vs η and extract i₀ from intercept.

Exchange current densities (cm⁻² A; H₂/H⁺ in 1 M H₂SO₄):

  • Pt ~10⁻³ A/cm² — best HER catalyst
  • Pd ~10⁻³
  • Rh, Ir ~10⁻³
  • Ni ~10⁻⁵
  • Fe ~10⁻⁶
  • Hg ~10⁻¹³ — extreme overpotential, classical polarography

Marcus theory of electron transfer

Rudolph Marcus, Nobel chemistry 1992, gave a quantitative theory of outer-sphere electron transfer:

k_ET = κ ν_n exp(-ΔG‡/k_B T), with ΔG‡ = (ΔG° + λ)² / (4λ)

λ is the reorganization energy — the cost of distorting the inner-sphere coordination + outer-sphere solvent shell to the transition state geometry. Three regimes as a function of -ΔG°:

  • Normal region: rate increases with driving force (-ΔG° < λ)
  • Optimum: -ΔG° = λ (activationless, fastest)
  • Marcus inverted region: rate decreases with further increase in driving force (-ΔG° > λ); experimentally verified by Closs & Miller (1984) for intramolecular ET in donor-acceptor molecules

Marcus theory underlies photosynthetic and respiratory electron-transport chains, mixed-valence chemistry, molecular electronics.

4. Mass transport

Three transport mechanisms move species to/from the electrode:

  • Diffusion (Fick’s laws): J = -D ∇c; ∂c/∂t = D ∇²c
  • Migration (under electric field): J = -(z F D c / RT) ∇φ; suppressed by adding excess supporting electrolyte
  • Convection (stirring, RDE rotation, natural)

Aqueous diffusion coefficients D are typically 10⁻⁵ cm²/s; in non-aqueous solvents 10⁻⁶-10⁻⁵. Excess “supporting” electrolyte (0.1-1 M LiClO₄, TBAPF₆, KCl) is added in analytical electrochemistry to eliminate migration so transport is diffusion-only.

Cottrell equation — diffusion-limited transient

For a planar electrode after a potential step into the diffusion-limited regime:

i(t) = nFA c√(D/(π t))

i ∝ t⁻¹/². Chronoamperometry uses this to extract D.

Rotating disk electrode (RDE) — Levich

A rotating disk creates a controlled hydrodynamic boundary layer δ ∝ ω⁻¹/². The mass-transport-limited current:

i_lim = 0.620 n F A D^(2/3) ω^(1/2) ν^(-1/6) c

where ω is the rotation rate (rad/s) and ν is the kinematic viscosity. Plot i_lim vs ω^(1/2) (Levich plot) — linear for purely mass-transport-limited; deviation gives kinetic info via Koutecky-Levich (1/i = 1/i_k + 1/i_lev). RDE is the standard tool for ORR catalyst benchmarking (Pt, Pt-alloys, non-PGM Fe-N-C — Pine WaveDriver, Gamry RRDE-3A).

5. Voltammetry techniques

Cyclic voltammetry (CV)

Sweep potential E linearly from E₁ to E₂ and back, plot i vs E. The most-used electrochemical technique.

For a reversible (Nernstian) couple:

  • Peak separation ΔE_p = 59/n mV at 25 °C
  • Peak current ratio i_pa/i_pc = 1
  • Peak potentials independent of scan rate v
  • Randles-Sevcik: i_peak = (2.69 × 10⁵) · n^(3/2) · A · D^(1/2) · v^(1/2) · c (i in A, A in cm², D in cm²/s, v in V/s, c in mol/cm³)

For quasi-reversible or irreversible systems, ΔE_p widens with scan rate. Diagnostic plots: i_p vs v^(1/2) (linear ⇒ diffusion-controlled), log i_p vs log v (slope 0.5 diffusion, 1 surface-bound).

Instruments: BioLogic VSP / SP-300, Gamry Reference 600+, Pine WaveDriver, Princeton Applied Research VersaSTAT, Metrohm Autolab, CH Instruments 760E.

Electrochemical impedance spectroscopy (EIS)

Apply a small AC perturbation (5-10 mV) at varying frequency (typically 100 kHz to 10 mHz) and measure complex impedance Z(ω) = Z’ + jZ”.

Nyquist plot: -Z” vs Z’. Classic features:

  • High-frequency intercept ≈ ohmic resistance R_s (electrolyte + contacts)
  • Semicircle of diameter R_ct (charge-transfer resistance); related to i₀ by R_ct = RT/(nFi₀)
  • Low-frequency Warburg tail (45° line) — semi-infinite diffusion
  • 90° vertical line — blocking double-layer capacitance

Bode plot: |Z| and phase vs log frequency.

Equivalent circuit fitting (Randles cell: R_s + (R_ct ∥ C_dl) + W) extracts kinetic parameters. Constant phase element (CPE) accounts for non-ideal capacitance from surface roughness/heterogeneity. Software: ZView (Scribner), EC-Lab (BioLogic), Echem Analyst (Gamry), RelaxIS.

EIS is critical for battery state-of-health diagnostics, fuel-cell membrane characterization, corrosion monitoring (3LP, Tafel-Stern-Geary), and coating quality.

6. Batteries — primary, secondary, the modern Li-ion ecosystem

Lead-acid (1859, Gaston Planté)

PbO₂ + Pb + 2 H₂SO₄ → 2 PbSO₄ + 2 H₂O (discharge). E_cell ≈ 2.1 V/cell. 12 V automotive battery = 6 cells in series. Specific energy 30-50 Wh/kg, energy density 60-110 Wh/L, low cost (~$150/kWh installed), >99% recycled in the developed world (Exide Technologies, Clarios — formerly Johnson Controls Power Solutions, EnerSys). Workhorse for SLI (starting-lighting-ignition), UPS, forklifts.

NiCd and NiMH

NiCd (Edison/Jungner ~1899): Cd + 2 NiOOH + 2 H₂O → Cd(OH)₂ + 2 Ni(OH)₂; 1.2 V/cell; toxic Cd, memory effect; phased out under EU RoHS for most consumer uses.

NiMH (1990s; Stanford Ovshinsky, ECD Ovonics): MH + NiOOH → M + Ni(OH)₂; same 1.2 V; 60-120 Wh/kg. Toyota Prius (first generation 1997) used Panasonic NiMH for ~25 years across hybrid models; replaced by Li-ion only for plug-in/EV applications. Eneloop (Panasonic) AA rechargeables for consumer.

Li-ion — the modern dominant chemistry

Stanley Whittingham (TiS₂ cathode 1976), John Goodenough (LiCoO₂ 1980), Akira Yoshino (commercial cell 1985), shared Nobel chemistry 2019. Sony commercialized in 1991 (8 mm camcorder). Now powers essentially all consumer electronics, EVs, drones, e-bikes, grid storage.

Standard cell: graphite anode | electrolyte (LiPF₆ in EC/DMC/EMC) | layered oxide cathode. Discharge: Li deintercalates from anode, intercalates into cathode; charge reverses.

Cathode chemistries (3.7 V nominal except LFP):

  • LCO (LiCoO₂): 145 Wh/kg cell, narrow stability window, expensive Co; legacy laptops, phones; safety risk if abused; Sumitomo, Umicore
  • NCA (LiNi₀.₈Co₀.₁₅Al₀.₀₅O₂): ~250 Wh/kg cell; Panasonic 18650 & 21700 (Tesla Model S/X/3/Y NA-built); fine balance of power and energy
  • NMC (LiNi_xMn_yCo_zO₂; x+y+z=1): NMC111, NMC532, NMC622, NMC811 (higher Ni reduces Co), NMC9-series (low/no Co); ~200-260 Wh/kg cell; dominant EV cathode globally; suppliers Umicore, BASF, LG Chem, POSCO, Sumitomo, Tanaka; cell-makers CATL, LG Energy Solution, Samsung SDI, SK On, BYD, Northvolt
  • LFP (LiFePO₄): 3.2 V nominal; 130-180 Wh/kg cell; longer cycle life (>3000 cycles); thermally stable to 270 °C; no cobalt or nickel; lower cost; BYD Blade, CATL LFP, EVE — adopted by Tesla Standard Range, Ford Mustang Mach-E SR, all Chinese-market BYD vehicles; grid storage (Tesla Megapack 3.9 MWh, Fluence Gridstack Pro 8 MWh)
  • LMFP (Mn substitution into LFP): slightly higher voltage (3.7 V plateau), modest energy density gain; Gotion, CATL M3P shipping 2024

Anodes:

  • Graphite: 372 mAh/g theoretical; ubiquitous; synthetic (BTR, Shanshan) or natural (Northern Graphite, Syrah)
  • Silicon (or Si-C composites): 3579 mAh/g theoretical (Li_3.75Si); volume expansion ~300% is the challenge; Sila Nanotechnologies (Mercedes EQG with Sila anode 2025), Group14 Technologies, Nexeon, Amprius (high-Si cells with 450 Wh/kg in pouches, used by AALTO HAPS)
  • Lithium titanate (LTO): 1.55 V vs Li/Li⁺ — gives lower cell voltage but exceptional cycle life and power; Toshiba SCiB used in Honda Fit EV and Mitsubishi i-MiEV power packs
  • Lithium metal: target for high-energy cells but plagued by dendrites

Electrolytes: LiPF₆ (1 M) in EC (ethylene carbonate) + DMC/EMC/DEC/PC; FEC (fluoroethylene carbonate) additive stabilizes Si anode SEI; LiFSI replacing LiPF₆ in premium cells for stability. Solid-state and gel electrolytes promise safer cells.

Cell formats: 18650 (18 mm × 65 mm — classic, Panasonic for early Tesla), 21700 (Tesla Model 3, larger), 4680 (46 mm × 80 mm; Tesla Cybertruck, Berlin Gigafactory; tabless design ~5× capacity per cell), pouch (LG, Samsung — Hyundai Ioniq 5, Kia EV6), prismatic (BYD Blade — long, thin, structural).

Pack-level pricing: 2025 average ~80-100/kWh, NMC cells 80/kWh enables EV-ICE cost parity at retail without subsidy.

Beyond Li-ion

Solid-state batteries (SSB): replace flammable liquid electrolyte with solid Li-ion conductor. Three families:

  • Sulfide electrolytes (Li₁₀GeP₂S₁₂ “LGPS”, argyrodite Li₆PS₅Cl): highest conductivity (~10⁻² S/cm) approaching liquids; air/moisture sensitive; Toyota (commercial target 2027-2028), Samsung SDI ASSB pilot 2027, Solid Power (BMW partnership), SES AI
  • Oxide (garnet Li₇La₃Zr₂O₁₂ “LLZO”): air-stable but high interfacial resistance; QuantumScape (Volkswagen, samples shipping 2024)
  • Polymer (PEO-based): lower temperature operation; Blue Solutions (Bolloré, Mercedes-Benz EQS-class as add-on)

Key challenges: Li dendrite penetration of solid electrolyte at high current densities, manufacturing scale-up, cathode-electrolyte interfacial stability.

Lithium-sulfur (Li-S): theoretical 2600 Wh/kg, practical 350-500 Wh/kg cells (Sion Power Licerion, OXIS Energy — went bankrupt 2021, NEXTECH Batteries, Lyten 3D Graphene Li-S, Theion 4D quantum graphite cathode in Germany). Polysulfide shuttle, lithium anode dendrites, low cycle life are the foes. Niche aerospace use (Stratospheric Platforms HAPS, Airbus Zephyr).

Lithium-air (Li-O₂): theoretical 11 kWh/kg (rivals gasoline). Practical cycling has remained elusive for >25 years due to side reactions, electrolyte oxidation, clogged cathodes. Active research at IBM, Argonne, MIT but no commercial product.

Sodium-ion (Na-ion): Na abundant (~1000× cheaper raw material than Li), broadly similar architecture to Li-ion with hard carbon anode + layered oxide or Prussian blue analogue cathode. CATL launched mass production 2023 (160 Wh/kg cells, debuted in JMEV E and Sehol E10X), Northvolt Nyköping pilot 2023, HiNa Battery (Sehol, JAC), Faradion (acquired by Reliance India), Natron Energy (Prussian blue analogue, US grid). Lower energy density (110-160 Wh/kg cells) limits passenger EV range but suits stationary storage, low-cost urban EVs, two-wheelers. Cobalt and nickel optional.

Multivalent (Mg²⁺, Ca²⁺, Al³⁺): theoretical advantage from 2-3 electrons per ion; Mg metal anodes feasible without dendrites, but Mg²⁺ insertion into oxides slow. Toyota Research, Pellion (shut down 2020), Bar-Ilan group active.

Zinc-air (primary and rechargeable): theoretical 1086 Wh/kg; Duracell hearing-aid cells are zinc-air (e.g., Duracell ActivAir 312 ~570 mAh, two-week life). Rechargeable Zn-air for grid: Zinc8 Energy Solutions, Eos Energy Enterprises (Znyth aqueous Zn-MnO₂ Z3 Cube, 600 kWh modules; AEC Solar PPAs in Texas 2024-2025), e-Zinc (Toronto; long-duration 50-100 h).

Flow batteries — for stationary grid

Decoupled energy (tank volume) from power (stack area). Tanks of redox-active liquid pumped past electrodes.

  • Vanadium redox (VRFB): V²⁺/V³⁺ | V⁴⁺/V⁵⁺ in H₂SO₄; 1.26 V; no cross-contamination because single element on both sides. Sumitomo Electric (Hokkaido Demo 60 MWh installed 2022), Invinity Energy Systems (UK, scaling), Largo Clean Energy. Capex ~$500-1000/kWh; 20+ year life. Limited by vanadium price volatility.
  • Iron-chromium: Westinghouse 1980s, EnerVault (defunct), Storen Tech (China).
  • All-iron: ESS Inc. (Wilsonville, OR; ESS Energy Warehouse 50 kW/250 kWh, ESS Energy Center grid-scale; iron + saline electrolyte; no rare elements; public 2021 SPAC).
  • Zinc-bromine: Redflow (Australia, ceased 2023), EnSync, Primus Power (zinc-bromine flow battery); plating-type Zn anode.
  • Organic flow: avoid metals entirely; quinone/anthraquinone (Harvard Aziz lab spin-out Quino Energy), viologen (Lockheed Martin’s GridStar Flow), XL Batteries (oligomeric organic flow); active research; commercial scale modest.

Battery management systems (BMS)

A BMS protects, monitors, and optimizes a battery pack:

  • State-of-charge (SOC) estimation: coulomb counting (∫i dt) + voltage-OCV lookup + Kalman filtering / unscented Kalman filter (UKF). Modern packs estimate within 1-3%.
  • State-of-health (SOH): capacity fade tracking, internal resistance growth (via EIS-lite or pulse tests).
  • Thermal management: liquid cooled (glycol/water mixture in cold plates — Tesla, Porsche, Ford; immersion cooling XING Mobility “IMMERSIO XM25” Taiwan; M&I Materials Engineered Fluids Coolanol; Shell Diala dielectric oils; 3M Novec retired but Solvay Solef PVDF lines active), forced-air (less common in EVs, used in Nissan Leaf early), phase-change (academic).
  • Cell balancing: passive (dissipate excess charge on high cells through resistors during charge — simple, lossy), active (transfer charge from high to low cells with DC-DC converters — efficient, costly; common in commercial-vehicle and stationary).
  • Safety functions: overcharge / over-discharge / overcurrent / overtemperature cutoff; pyrotechnic fuse (Tesla, Mersen Versa-Trip); contactor control; isolation monitoring (HV bus to chassis); thermal runaway mitigation — cell-to-cell propagation barriers (CATL “no propagation” cell-to-pack), pressure-relief vent, gas exhaust paths.

BMS chips: Analog Devices LTC68xx / ADBMS68xx family, Texas Instruments BQ76952, Maxim MAX17852, NXP MC33772. ASIL-D safety integrity for automotive.

7. Fuel cells

Fuel cells convert fuel chemical energy directly to electricity (no Carnot limit) through controlled electrochemical oxidation.

PEMFC — proton-exchange membrane / polymer-electrolyte membrane

H₂ + ½ O₂ → H₂O; E° = 1.229 V (real cells deliver 0.6-0.8 V at useful current). 80-100 °C operation.

  • Membrane: Nafion (Dupont, now Chemours; sulfonated PTFE; PFSA family); alternatives Gore-Select reinforced (W. L. Gore), Asahi Kasei Aciplex, Solvay Aquivion
  • Catalyst: Pt or Pt-alloy nanoparticles on carbon (Pt loading target 0.3 g/kW total for transportation by 2030, currently ~0.5 g/kW)
  • Bipolar plates: graphite (Pocograph, SGL Carbon), coated stainless (Cellimpact, Borit), titanium (aerospace)

Toyota Mirai (gen 2, 2021): 114 kW stack, 330 cell pairs, 5.6 kg H₂ at 700 bar (10000 psi), 650 km / 400 mi NEDC range. Hyundai Nexo, Honda CR-V e:FCEV (2024 — replaces Clarity), Ballard Power Systems (transit buses, trucks), Plug Power (forklifts ProGen, GenDrive). MAN Hydrogen Engine and Cummins HyPM HD systems for trucks; Hyundai XCIENT fuel-cell truck deployed in Switzerland by H2 Mobility coalition.

SOFC — solid-oxide fuel cell

O²⁻ ion conducting YSZ (yttria-stabilized zirconia) electrolyte; 600-1000 °C operation; can run on H₂, CH₄, CO, syngas via internal reforming.

  • Bloom Energy Server: 250 kW Bloom Box (USA — eBay, Apple, Walmart, Equinix data center contracts); newer versions 80% electrical efficiency in CHP, ~50% pure electrical
  • Mitsubishi Hitachi Power Systems MEGAMIE: hybrid SOFC + gas turbine
  • Ceres Power: UK; 30-50 kW Steel Cell modules; partners with Bosch (Bosch SOFC pilot Salzgitter), Doosan, Weichai for distributed CHP and shipboard auxiliary power
  • FuelCell Energy SureSource: molten carbonate (MCFC) variant at 650 °C — for example POSCO subsidiaries in South Korea, Pfizer Connecticut campus

SOFCs and reversible SOECs (solid-oxide electrolyzer cells) are receiving major investment for grid-flexible H₂/electricity and synthetic-fuel pathways.

AFC, PAFC, DMFC, AEM, MCFC

  • AFC (alkaline): OH⁻ conducting electrolyte (30-45% KOH); Apollo program 1960s; Bacon cell; CO₂ contamination of alkali a problem
  • PAFC (phosphoric acid): 200 °C; H₃PO₄ in SiC matrix; UTC PureCell 400 (former United Technologies; now Doosan PureCell Hartford) deployed in commercial buildings
  • DMFC (direct methanol): methanol + water → CO₂ + 6 H⁺ + 6 e⁻ at anode; portable power (SFC Energy EFOY Pro for off-grid telecom and military)
  • AEM (anion-exchange membrane): OH⁻ conducting polymer; lower-cost non-PGM catalyst possible; Hyzon Motors trucks, Enapter AEM electrolyzer modules (Italy/Germany)
  • MCFC (molten carbonate, 650 °C): CO₃²⁻ transport; tolerates CO₂ as fuel feed (carbon-capture utility); FuelCell Energy DFC ERG 3.7 MW units (Connecticut), Doosan FuelCell Korea

8. Electrolysis — water splitting, chlor-alkali, aluminum, organic

Water electrolysis (H₂ production)

2 H₂O → 2 H₂ + O₂; E°_cell = -1.23 V (so a minimum 1.23 V of overpotential needed plus kinetic/ohmic overpotentials → real cells 1.8-2.2 V at typical current densities).

  • Alkaline electrolysis (AEL): cheapest mature commodity; 30% KOH; Ni-based electrodes; sub-MW to 20+ MW per stack; Nel Hydrogen (Norway; A-Series, M-Series), Cummins (acquired Hydrogenics 2019; HyLYZER), John Cockerill (DQ Series, Belgium), Topsoe, Tianjin Mainland, Sungrow Hydrogen (China). Capex ~$700-1200/kW, response time minutes-to-hours.
  • PEM electrolysis: pure water + acidic PEM; iridium oxide anode (Ir scarce, ~1000-1500/kW; ramp seconds.
  • Solid-oxide electrolysis (SOEC): 700-850 °C; high efficiency (90% LHV theoretical) by using waste heat; Sunfire (HyLink SOEC 220 kW arrays; Salzgitter Flachstahl SALCOS); Topsoe SOEC (500 MW factory Herning, Denmark, commissioning 2025); Bloom Electrolyzer (reversible Bloom Box; commercial deployments).
  • AEM (anion-exchange membrane): between AEL & PEM; Enapter AEM Multicore modular 1-MW shipping container; Hyzon Hyzer-x AEM.

LCOH (levelized cost of hydrogen) target 1/kg by 2031). Electricity cost (~50-60% of LCOH) dominates; capex utilization (capacity factor) matters as much as capex itself. EU REPowerEU: 10 Mt domestic + 10 Mt imported green H₂ by 2030.

Chlor-alkali

NaCl(aq) → NaOH(aq) + ½ Cl₂(g) + ½ H₂(g). Three historical cell types: mercury (phased out 2017 under Minamata Convention except for limited derogations), diaphragm (asbestos, declining), and the modern membrane cell with a perfluorinated cation-exchange membrane:

  • Membrane suppliers: Chemours Nafion N2030/2050, Asahi Kasei Aciplex F, AGC Flemion
  • Cell technology: Asahi Kasei IM Series, AGC ACILYZER, thyssenkrupp Nucera BiTAC (formerly Uhde; recent IPO 2022); chlor-alkali capacity approximately 100 Mt/y Cl₂ globally, ~3% of electricity in chemical industry.

Aluminum smelting — Hall-Héroult process

Al₂O₃ dissolved in cryolite Na₃AlF₆ at ~950 °C; electrolyzed with carbon anode and molten aluminum cathode pool:

2 Al₂O₃ + 3 C → 4 Al + 3 CO₂

Discovered independently by Charles Martin Hall (US) and Paul Héroult (France) in 1886 — same year, age 23 both. Process basically unchanged in 138 years.

  • Energy: ~13 kWh/kg Al at the cell (~26 kWh/kg primary including auxiliaries) — about 1% of global electricity goes to aluminum
  • Producers: Rio Tinto (Pacific NW, Canada, Iceland), Alcoa, Norsk Hydro, Chalco (China — controls ~55% world production), EGA (UAE), Rusal
  • Inert anode (oxygen-evolving, no CO₂): ELYSIS (Alcoa-Rio Tinto JV with Quebec, Apple investment; Apple iPhone enclosures since 2018 used some ELYSIS metal); achievable 2026-2028 commercial scale — would eliminate ~9% of global industrial CO₂ from primary Al

Other industrial electrolysis

  • Chlorate (NaClO₃) for pulp & paper bleaching: NaCl + 3 H₂O → NaClO₃ + 3 H₂
  • Copper electrorefining (electrowinning ER) from anode mud: 99.99% Cu cathode; Codelco Chuquicamata Chile, Glencore, Aurubis
  • Zinc electrowinning: from H₂SO₄ leach solution; Korea Zinc, Boliden, Nyrstar
  • Nickel/cobalt: from sulfate or chloride leach; key for battery raw materials
  • Electro-organic synthesis: Sigma Technical PFAS destruction, Aspect Energy Tech electro-DEC adipic acid, Pajarito Powder non-PGM ORR catalysts; rapidly growing field with kg-scale Monsanto adiponitrile (since 1965 → nylon-6,6), modern academic work by Phil Baran, Shannon Stahl, Kevin Moeller.

9. Corrosion electrochemistry

Corrosion is the unwanted oxidation of metals. Global cost ~3-4% of GDP per year (NACE/AMPP IMPACT 2016 study: $2.5 trillion globally).

Galvanic series in seawater (most active → most noble)

Mg, Zn, Al alloys, mild steel, cast iron, Pb, Sn, Cu alloys, Ni alloys, stainless steel (active), Ag, Ti, stainless (passive), graphite, Pt, Au.

Coupling dissimilar metals in a conducting electrolyte → galvanic corrosion; the more active metal sacrifices itself. Magnitude depends on area ratio (small anode + large cathode = catastrophic; opposite is benign — basis of sacrificial-anode design).

Pourbaix diagrams

Plot of electrode potential E vs pH showing thermodynamically stable species (metal, oxide, ion, hydroxide). Three regions: immunity (metal stable), passivation (insoluble oxide forms), corrosion (soluble cation or oxyanion).

  • Iron: passivates above pH ~9 (Fe₃O₄, Fe₂O₃), corrodes in acid as Fe²⁺/Fe³⁺
  • Aluminum: amphoteric — passivates pH 4-9 (Al₂O₃·xH₂O), corrodes both acid (Al³⁺) and base (AlO₂⁻)
  • Stainless steel: Cr₂O₃ passive film stable across very wide pH range; basis of “stainless”

Localized corrosion

  • Pitting: breakdown of passive film at chloride ions; small anode (pit) / large cathode (passive surface); critical pitting temperature CPT — measured via ASTM G48 for stainless and Ni-alloys; 316L SS ~25 °C, super-duplex 2507 ~75 °C, alloy C-276 ~150 °C
  • Crevice corrosion: differential aeration cells in tight gaps (under gaskets, threads); same alloys ranked by CCT (critical crevice temperature)
  • Stress-corrosion cracking (SCC): combined tensile stress + corrosive environment; austenitic stainless in chloride (boiling MgCl₂ test; many real-world swimming-pool ceiling collapses), brasses in NH₃, carbon steel in caustic, Inconel in PWR primary water
  • Microbially induced corrosion (MIC): sulfate-reducing bacteria Desulfovibrio in oil pipelines, marine fouling; refer to NACE TM0212

Cathodic protection

Apply electrons to the structure to force it into the immunity region:

  • Sacrificial anodes: Zn, Mg, Al alloys connected to steel (ship hulls — Mg “candy bars” bolted to underwater plating, Cathwell, MME Group, Galvotec); replaced periodically
  • Impressed current cathodic protection (ICCP): external DC power source forces electrons into the structure via inert anodes (Pt-coated Ti, mixed-metal oxide MMO — De Nora, Permascand, Cathwell). Used for buried pipelines (regulated under PHMSA 49 CFR Part 192), reinforced concrete (galvanic or impressed), offshore platforms.

Pipeline cathodic-protection survey: close-interval potential survey (CIPS) measures pipe-to-soil potential at ~1 m intervals; -0.85 V vs Cu/CuSO₄ criterion (NACE SP0169).

Other protection

  • Barrier coatings: paint, epoxy (3M Scotchkote pipeline FBE — fusion-bonded epoxy), polyurethane, polyethylene tape, hot-dip galvanizing (Zn on steel — 30-100 years atmospheric life depending on environment, ASTM A123)
  • Inhibitors: amines, phosphates, molybdates, organic-film formers (cooling-water chemistry — ChampionX, Solenis, Ecolab Nalco Water, Veolia); volatile corrosion inhibitors (VCI) for shipping
  • Alloying: weathering steel Cor-Ten (US Steel) self-protects via Cr-Cu-P-rich rust; stainless steel families 304, 316, duplex 2205, super-duplex 2507, super-austenitic 254 SMO; Ni-alloys Inconel 625, Hastelloy C-276 for severe environments

10. Electrochemical sensors

Glucose biosensor — the canonical case

Leland Clark, 1962, proposed an oxygen-electrode-based enzyme sensor for glucose using glucose oxidase. Modern iterations:

  • Strip-based finger-stick: 2nd-generation mediator chemistry (ferrocene, Os mediators); LifeScan OneTouch Verio, Roche Accu-Chek, Abbott FreeStyle; chronoamperometric detection
  • Continuous glucose monitors (CGM) — subcutaneous, 7-14 day wear:
    • Dexcom G7: 30-minute warm-up, factory calibrated, 24-day Real-Time Audio Alerts; integrated with Tandem t:slim X2 and Omnipod 5 pumps in hybrid closed-loop
    • Abbott FreeStyle Libre 2/3: 14-day, 1-minute reading interval (Libre 3); >50 million users worldwide as of 2024
    • Medtronic Guardian 4 / Simplera: 7-day, integrated MiniMed 780G AID pump
    • Senseonics Eversense E3: 6-month implanted (US/EU); fluorescence-based with electrochemical reference

pH and ion-selective electrodes

  • Glass pH electrode: Corning Hammett, Beckman, Mettler Toledo InLab, Hanna FC; H⁺-selective glass membrane; 59.16 mV/pH unit at 25 °C
  • ISEs for Na⁺, K⁺, Ca²⁺, Cl⁻, F⁻, NH₄⁺, NO₃⁻ — clinical blood-gas/electrolyte analyzers (Radiometer ABL90, Siemens RAPIDPoint 500e, Werfen GEM Premier 5000) measure simultaneously
  • LAQUAtwin handheld (Horiba) for soil, irrigation water, hydroponics

Other gas/liquid sensors

  • Clark oxygen electrode (1956): Pt cathode, Ag/AgCl anode behind O₂-permeable membrane; dissolved O₂ measurement; used in cell culture bioreactors (Hamilton VisiFerm), blood-gas analyzers, environmental monitoring
  • Breathalyzer: small fuel-cell Pt electrode oxidizes ethanol → current; police evidential units (Intoximeters EC/IR II, Alcotest 9510, Drager Alcotest 7510)
  • CO sensor: amperometric, low cross-sensitivity; Figaro TGS5042, City Technology
  • NOₓ sensor for diesel SCR control: ZrO₂ solid electrolyte; NGK, Bosch
  • Toxic gas badges: H₂S, HCN, NH₃, Cl₂ — Industrial Scientific Ventis Pro5, Honeywell BW MaxXT II; amperometric electrochem cells with 12-24 month life

11. Electroplating and electrowinning

Electroplating (decorative & functional)

Pass current through electrolyte containing metal cation; deposit at cathode (workpiece). Workpieces: from costume jewelry to integrated-circuit interconnects.

  • Cu: PCB through-hole, IC interconnect (Cu damascene since IBM 1997); CuSO₄ + H₂SO₄ baths
  • Ni: corrosion barrier, wear resistance; Watts bath (NiSO₄ + NiCl₂ + H₃BO₃); decorative electroless Ni-P
  • Cr: hard chrome (industrial — hexavalent Cr⁶⁺ chromic acid baths, being phased out under REACH 2024; trivalent Cr³⁺ alternatives by Atotech/MKS, Coventya, MacDermid)
  • Au, Ag: jewelry, contacts; cyanide-based (Au(CN)₂⁻) industrial; non-cyanide Au sulfite for IC bonding pads
  • Zn: corrosion protection (electrogalvanizing of automotive body sheet — ArcelorMittal Extragal, Tata Steel Zincoat, Nippon Steel SuperDyma); acid Zn or alkaline Zn baths

“Throwing power” — uniformity of deposit thickness across complex geometry; high throwing power baths (cyanide Cu, alkaline Zn) needed for PCBs and intricate parts.

Electrowinning

Extract metals from leach solutions. Cu from solvent-extraction electrolytes (SX-EW; Codelco, Freeport-McMoRan El Abra); Ni and Co for batteries (key to the EV supply chain); Zn from leached ore. Highly energy-intensive: Al ~13 kWh/kg, Cu ~2 kWh/kg, Zn ~3 kWh/kg.

12. Supercapacitors and hybrid devices

Electrochemical capacitors (EDLCs, “supercaps”) store charge in the electrochemical double layer rather than via Faradaic reactions. Energy density is lower than batteries (5-10 Wh/kg vs 150-250 Wh/kg for Li-ion) but power density is much higher (10 kW/kg vs 1-3 kW/kg) and cycle life is far longer (>500,000 cycles vs ~1000-3000 for Li-ion).

  • EDLC electrodes: activated carbon (BET 1500-2500 m²/g), templated carbons, carbide-derived carbons (CDC; Y. Gogotsi, Drexel), CNTs, graphene; electrolyte typically acetonitrile + TEABF₄ or propylene carbonate-based (2.7-3.0 V max), or ionic liquid (3.5-4 V max)
  • Pseudocapacitors: surface Faradaic charge storage; RuO₂ (highest C but expensive), MnO₂, conducting polymers (PEDOT, polyaniline); MXenes (Ti₃C₂T_x — also Gogotsi/Barsoum 2011)
  • Hybrid capacitor (Li-ion capacitor, LIC): graphite/pre-lithiated anode + activated carbon cathode; ~30 Wh/kg, fast charge; JM Energy (Toyota), Vinatech, Eaton XLR/XLM modules

Applications: KERS in motorsport (Formula 1 2009-13 hybrid systems), regenerative braking buses (Maxwell Technologies, acquired by Tesla 2019; Skeleton Technologies in Estonia/Germany with graphene-derived “curved graphene”; Eaton; Nesscap; CAP-XX), grid frequency-regulation (short bursts), uninterruptible power for IT racks, automatic-stop-start in passenger cars (no battery wear).

13. Electrochemical CO₂ reduction and N₂ fixation

Two of the largest open frontiers — turning power and water into chemicals without combustion.

Electrochemical CO₂ reduction (CO2RR)

CO₂ + n e⁻ + n H⁺ → products. Product spectrum depends critically on catalyst, potential, and electrolyte:

  • Cu is the only metal producing C₂+ products at significant rates (ethylene, ethanol, n-propanol); 70+ years since Hori’s seminal screening (1985-90)
  • Au, Ag: selective for CO at low overpotential (Au better at smaller particles, Buonsanti EPFL, Strasser Berlin)
  • Sn, Pb, Bi, In: formate (HCOO⁻) via HER pathway
  • Single-atom Ni, Fe, Co on N-doped carbon (M-N-C): CO selectivity, Sargent Northwestern/Toronto

Commercial efforts: Twelve (Berkeley spinout — CO and methanol from CO₂, polycarbonate-grade syngas demo with Mercedes-Benz, partnerships with Procter & Gamble for surfactants, Tide laundry detergent CO₂-based ingredients, LanzaTech for ethanol), Dioxycle (Paris), Carbon Re (UK), Mitsubishi Chemical, Siemens-Evonik Rheticus project for butanol/hexanol. Bipolar membrane electrolyzers and gas-diffusion-electrode flow cells achieve > 200 mA/cm² in lab — 1 A/cm² target for economic operation. Energy efficiency 35-50% (HHV) typical; capital-cost battle vs methanol/Fischer-Tropsch via syngas from electrolysis.

Electrochemical nitrogen fixation

N₂ + 6 H⁺ + 6 e⁻ → 2 NH₃ (vs the energy-intensive Haber-Bosch process — 1-2% of global energy, ~1.5% of CO₂ emissions). Many high-profile claims have failed reproducibility tests when traceable ¹⁵N-labeled feedstock is used (Choi/Norskov 2020 standards). Lithium-mediated pathway (Li plates, reacts with N₂, then protonated) is the leading credible chemistry: Jens Nørskov, Karthish Manthiram (MIT/Caltech), Aleks Nikiforov DTU — efficiencies improving from ~0% to >60% Faradaic in 2023-24. Aspirational commercial scale remains a decade away; near-term green ammonia uses green-H₂ Haber-Bosch (Yara Pilbara, CF Industries Donaldsonville blue/green ramp, Topsoe-Casale dynamic Haber-Bosch).

14. Electroanalytical workflow — practical considerations

Several “gotchas” trip new electrochemists; documenting them here as field-tested knowledge.

  • iR compensation: real measurement includes ohmic drop = i × R_uncomp. Modern potentiostats apply positive feedback or current interruption; ZIR (instantaneous EIS) the standard. Always verify CV peak separations after compensation, especially for low-conductivity media.
  • Electrolyte purity: trace water, oxygen, transition metals confound kinetic measurements. Standard practice: Ar/N₂ purging 15-20 min, then headspace blanket; pre-dried solvents (acetonitrile from Sigma-Aldrich anhydrous bottle, then activated alumina column); recrystallized supporting electrolyte (TBAPF₆, NaClO₄).
  • Electrode pretreatment: glassy carbon polished on 0.05 μm alumina then ultrasonic in EtOH and water; Pt/Au electrochemically cleaned with potential cycling in 0.5 M H₂SO₄ until characteristic H_UPD/O-region voltammetry sharp.
  • Reference-electrode drift: leakage of KCl from saturated SCE/Ag-AgCl changes junction potential over hours; monitor with ferrocene internal standard (Fc/Fc⁺ recommended IUPAC reference for non-aqueous, +0.40 V vs SHE in MeCN).
  • Two-electrode vs three-electrode: lab CV/EIS always three-electrode (counter electrode size > working area); two-electrode appropriate for batteries, fuel cells, full-cell impedance.
  • Glove box vs Schlenk line: lithium chemistry, alkali metals, air-sensitive intermediates require Ar glove box (<0.1 ppm H₂O, O₂); MBraun and Innovative Technology dominant suppliers.

15. Quick-reference numerical anchors

  • F = 96485.33 C/mol; 1 mol electrons = 26.80 A·h.
  • RT/F at 25 °C = 25.69 mV; (RT/F) ln 10 = 59.16 mV.
  • Tafel slope = 2.303 RT/(αnF) = 59 mV/dec (α=1, n=1, 25 °C), ≈ 118 mV/dec (α=0.5, n=1, 25 °C).
  • Specific energy benchmarks (cell level): gasoline 12,200 Wh/kg (thermal), diesel ~12,700; Li-ion NMC811 250 Wh/kg, LFP 170 Wh/kg, lead-acid 35 Wh/kg, Ni-MH 90 Wh/kg, Na-ion 140 Wh/kg.
  • Faraday units in industrial scale: 1 kA·h ≈ 37.3 mol electrons; aluminum cell at ~350 kA produces ~117 kg Al/h per cell (3 e⁻ per Al).
  • Hydrogen energy density: 33.3 kWh/kg (LHV) or 39.4 kWh/kg (HHV); at 700 bar 5.6 MJ/L (~1.4 kWh/L) — gravimetric champion, volumetric weak vs gasoline 9.4 kWh/L.
  • Coulombic efficiency for a quality Li-ion cell: >99.9% per cycle; even 99.5% kills cycle life since after 1000 cycles 0.995^1000 ≈ 0.007 capacity left.
  • Practical Nernst sanity: increase [Ox]/[Red] by 10× → E shifts +59 mV/n at 25 °C. Common pH electrode response 59.16 mV/pH unit (Nernstian) is the benchmark for slope check during calibration.

16. Connections and outlook

The electrochemistry stack reaches into nearly every engineering vertical:

Looking ahead, three structural trends are pulling on the field: (1) the relentless cost-decline curve of lithium-ion (~85% in the last decade, projected another 30-50% by 2030) pulling EVs and stationary storage past parity with incumbents; (2) green hydrogen electrolysis scaling from megawatt-class to multi-gigawatt projects (Saudi NEOM Helios 2.2 GW, UK Aberdeen Hydrogen Hub, Spain Iberdrola Puertollano); and (3) the electrification of industrial processes that historically used fossil-fired heat — green steel via direct reduction of iron with H₂ (HYBRIT Sweden, Boston Metal molten-oxide electrolysis), low-temperature electrochemical CO₂ reduction (Twelve, Carbon Re), green ammonia (Casale, Topsoe, Yara, CF Industries — Donaldsonville).

Adjacent

  • physical-chemistry — thermodynamic, kinetic, and statistical-mechanics foundations underlying every cell potential, rate law, and transport equation
  • inorganic-chemistry — coordination chemistry of redox-active transition-metal complexes; cathode oxide structures; catalyst design
  • analytical-chemistry-methods — voltammetry and EIS as quantitative analytical techniques; chromatographic and spectroscopic characterization of electrolytes and electrode films
  • materials-overview — alloys, ceramics, and polymers of electrochemical interfaces; coatings; passive films
  • energy-storage-systems — battery pack engineering, BMS, thermal management, grid integration
  • power-electronics — DC/DC and AC/DC stages for chargers, inverters, electrolyzer rectifiers
  • water-treatment — electrocoagulation, electrochlorination, electrodialysis for desalination and reuse

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