Hydrogen Notes

Abundance and Isotopes

Hydrogen is found in various environments:
- Atmosphere: 5 \cdot 10^{-5} vol.%, mostly at 100 km altitude.
- Earth crust: 0.74 wt.%
- Sun: 50 wt.%
- Gas planets and the universe.
Hydrogen has three isotopes:
- Protium (^1H): 99.9855%
- Deuterium (^2H): 0.0145%
- Tritium (^3H): 10^{-15} Vol.%

Stable Isotope Method

The ratio between Deuterium (D) and Hydrogen (H) can reveal information about:
- Type of plant
- Origin of the plant
- Temperature during harvest
- Rainfall before harvest
- Climate
Similar analyses can be done with other elements like O, N, S, and P.

Tritium Method

Tritium (^3H) is used for dating water.
It is generated in the atmosphere and decays with a half-life of t_{1/2} = 12.32 years.
Atmospheric nuclear bomb tests between 1953 and 1963 increased Tritium levels by a factor of 1000.
Applications:
- Dating old wine (before 1954)
- Determining mineral water age
- Hydrogeology

Physical Properties of Hydrogen

Relative atomic mass: 1.00794
Atomic number: 1
Melting point: -259.14°C
Boiling point: -252.76°C
Oxidation states: +1, 0, -1
Density: 0.08988 g/l
Electronegativity: 2.20 (Pauling)
Atomic radius: 37.5 pm

Peculiar Physical Properties

Diffusibility:
- Physical: v1/v2 = \sqrt{M2/M1}
- Chemical: Through Pd, Fe
Thermal Conductivity: High
Solubility:
- Physical: 21 ml per liter of water (0 °C, 0.1 MPa)
- Chemical: Pd, TiFe, LaNi5 (up to LaNi5H_{6.7} at RT, 0.85 MPa), CNT, BB.

Hydrogen as a Permanent Gas

Critical temperature: -239.96°C = 33.19 K
Critical density: 0.0310 g/cm³
Critical pressure: 1.31 MPa

Hydrogen as a Filling Gas

1 liter of hydrogen at 0 °C, 0.1 MPa: 0.09 g
1 liter of air: 1.29 g
Buoyancy: 1.20 g/l = 1.20 kg/m³
Ideal Gas Law: pV = nRT
Disadvantages: Combustible (forms explosive mixtures), high diffusion rate (losses).

Thermal Decomposition of Hydrogen

Requires very high temperatures due to high enthalpy of formation and binding dissociation energy.
Examples:
- 300 K: 10^{-34}% decomposition
- 1500 K: 10^{-3}% decomposition
- 2000 K: 0.081% decomposition
- 3000 K: 7.85% decomposition
- 4000 K: 62.2% decomposition
- 5000 K: 95.4% decomposition
- 6000 K: 99.3% decomposition (approximate surface temperature of the sun)

Occurrence and Production of Hydrogen

Occurrence:
- Water (H_2O)
- Methane (CH_4) in natural gas
- Carbohydrates (CmHn) in crude oil
Energy Sources:
- Thermal
- Electrical (electrolysis)
- Chemical (metal/acid, decomposition of hydrides)

Thermal Decomposition of Water

Successful only at very high temperatures due to high enthalpy of formation and binding dissociation energy.
No technical relevance.

Electrolytic Decomposition

Some 5 kWh yield 1 m³ H2 and 1/2 m³ O2. Very pure products, directly usable for chemical applications like catalytic hydrogenation.

Hydrogen Evolution Reactions

Volmer reaction
Tafel reaction
Heyrovsky reaction
Volmer-Tafel mechanism
Volmer-Heyrovsky mechanism

Electrode Kinetics

Butler-Volmer equation: j = j0 \exp{\frac{\alpha nF}{RT} \eta} - j0 \exp{\frac{-\alpha nF}{RT} \eta}
- j: current density
- \eta: overpotential (\eta = E - E_0)
- j_0: exchange current density
- \alpha: charge transfer coefficient

Tafel Equation Derivation

Consider high overpotentials where oxidation reaction can be neglected, simplifying the Butler-Volmer equation:
- j = -j_0 \exp{\frac{\alpha nF}{RT} \eta}
- \ln{j} = \ln{j_0} - \frac{\alpha nF}{RT} \eta
This equation is called the Tafel equation

Polarization Resistance

Consider low overpotentials where oxidation and reduction rates are similar. The Butler-Volmer equation can be simplified to:
- j = j_0 \frac{nF}{RT} \eta
This is the so-called polarization resistance.

Hydrogen Overpotential

Hydrogen overpotential at 1 mA/cm² for various materials:
- Pt: 0.015 V
- Pd: 0.120 V
- Fe: 0.40 V
- Pb: 0.52 V
- Graphite: 0.60 V
- Hg: 0.80 V

Volcano Plot

Metals with weak M-H bonds show low reaction rates towards Had formation, resulting in low Had coverage.
Strong M-H bonds hinder the reaction rate of the Tafel or Heyrowsky reaction.
The optimum is found at intermediate M-H bond energies (e.g., noble metals like Pt, Ru, Rh, Re, Ir).

Hydrogen Embrittlement

Particularly dangerous for high-strength steels (> 1400 MPa).
Low hydrogen concentrations (0.5 - 1 ppm) can cause damage.
Hydrogen entrance can occur during:
- Production and processing (pickling, coating, welding).
- Corrosion, exposure to H_2 gaseous or liquid (e.g., spaceships), high-pressure hydrogen (e.g., pipelines with sulphur-containing natural gas).
Result: Damage through hydrogen that recombines within the metal causing intergranular brittle fracture.

Hydrogen Quantification

Hot extraction / melt extraction to determine hydrogen content in metallic samples.
- Sample is heated in a vacuum or under carrier gas.
- Increase in temperature increases diffusion of hydrogen in metals to its surface.
- Trapped hydrogen starts diffusing further.
- Hot extraction determines diffusible hydrogen.
- Melt extraction determines overall hydrogen.
Hot extraction: T < T_{melting}
Melt extraction: T > T_{melting}
Detection: IR detector, heat conductivity detector, or mass spectrometer.

Hydrogen Detection by Mass Spectrometry

H_2 is pumped into the MS (may be by carrier gas).
Ionization (electron impact).
Mass filter (quadrupole).
Ion detection (e.g., SEA).
Ionization energies: 15.6 eV, 21.9 eV, 15.5 eV, 25.2 eV.

Devanathan-Stachurski Cell

Electrochemical determination of hydrogen diffusion coefficients in metals.
Sample (thickness d) is charged electrochemically with hydrogen on one side, and detected on the other.
Diffusion coefficient can be calculated from measured hydrogen permeation transients.
Equations:
- MH{ads} \leftrightarrow MH{abs}
- c_{H,0} = const.
- c_{H,L} = 0
Hydrogen entrance side (cathode):
- Acidic electrolyte: H3O^+ + M + e^- \rightarrow MH{ads} + H_2O
- Neutral/alkaline electrolyte: H2O + M + e^- \rightarrow MH{ads} + OH^-
H_{ads} \rightarrow H^+ + e^-
Hydrogen exit side (anode): Hydrogen detection through measurement of the oxidation current (potentiostatically).

Water Electrolysis

Alkaline Water Electrolysis:
- Electrolyte: 20-30 wt.% potassium hydroxide solution.
- Temperature: 80°C
- Separation: Ion-permeable separator (diaphragm).
- Pressure: Pressure-less to 1-3 MPa.
- Load gradient: Seconds, suitable for wind and PV units.
- Power: Few Nm³/h up to few hundreds Nm³/h.
Membrane Electrolysis:
- Electrolyte: Solid polymer electrolyte (SPE) = thin proton-conducting polymer membranes.
- Separation: SPE.
- Pressure: Goal up to 14 MPa.
- Conversion factor: Only some 50%.
- Power: Small units with few Nm³/h, low investment costs.
High-Temperature Electrolysis:
- Temperature: 700 – 1000 °C.
- Advantage: Parts of the dissociation energy are taken from thermal energy (solar thermal or PV-solar coupling).
- Load Gradient: Sluggish due to high temperature; suitable only for non-intermittent use.

High-Pressure Electrolysis

Target pressure: 35-70 MPa.
Traditional mechanical pressurizing is energy-intensive and technically complicated.
Idea: Use electrochemistry.
Nernst Equation:
- E = E0 + \frac{RT}{zF} \ln{\frac{c{ox}}{c_{red}}}
Where:
- E_0 = Electrochemical Potential
- E_{00} = Standard Potential
- R = Gas Constant
- T = Temperature
- F = Faraday Constant
- z = Number of electrons transferred per formula unit
- c_{ox} = concentration of the oxidised form
- c_{red} = concentration of the reduced form.

Water Splitting Photo Electrodes

Advantage: Direct conversion of light into hydrogen.
Materials: InP, III/V semiconductors, nanoporous Si, ZnO, II/VI semiconductors, TiO2 nanotubes, WO3 + MeOx (Me=Fe, Co, Ni).
Reactions:
- h \nu \rightarrow e^- + h^+
- 2 H2O + 2 e^- \rightarrow H2 + 2 OH^-
- 2 OH^- + 2 h^+ \rightarrow O2 + H2
Problems: Photocorrosion, overvoltage of hydrogen formation.

Chemical Formation of Hydrogen

From metals and nonmetals in alkalines:
- Al + OH^- + 3 H2O \rightarrow Al(OH)4^- + 1.5 H_2 \uparrow
- Si + 2 OH^- + H2O \rightarrow SiO3^{2-} + 2 H_2 \uparrow
From metals in acids:
- Zn + 2 H3O^+ \rightarrow Zn^{2+} + 2 H2O + H_2 \uparrow
Through hydrolysis of hydrides:
- CaH2 + 2 H2O \rightarrow Ca(OH)2 + 2 H2 \uparrow

Hydrogen Formation Through Steam Reforming

Chemical carbohydride decomposition:
- 206.2 \text{ kJ} + CH4 + H2O(g) \leftrightarrow CO + 3 H_2
- Conditions: 700 – 830 °C, 4 MPa, Ni-catalysis, 8 % methane.
- High temperatures: 1200 – 1500 °C, without catalyst, 0.2 % methane.
Shift reaction:
- CO + H2O \rightarrow CO2 + H_2

Hydrogen Production Through Steam Reforming

Sources: Coal, coke, crude oil, natural gas.
Processes: Gasification of coal, coking, chemical carbohydride decomposition, carbon oxide conversion.
Pollutants: Hydrogen sulfide, carbon dioxide, carbon monoxide.

Hydrogen Purification

Hydrogen sulfide absorption in methanol, binding on bases (ZnO, Na2O, K2CO_3), oxidation to sulphur, oxidative adsorption on active coal or iron(III)-hydroxide.
Carbon monoxide conversion to carbon dioxide, carbon dioxide elutriation with liquid nitrogen, conversion to methane at 250-300 °C, 3 MPa, Ni-catalyst (CO + 3 H2 \leftrightarrow CH4 + H2 O and CO2 + 4 H2 \leftrightarrow CH4 + 2 H_2 O), finally methane condensation (-162 °C).
High purity hydrogen achieved through electrolysis, direct production.
Pd-diffusion (300 °C).
Uranium purification route: U + 1.5 H2 \leftrightarrow UH3 (forward 250 °C, return 500 °C).
Lanthanum nickel purification route: LaNi5 + x H \leftrightarrow LaNi5H_x (x max. 6.7).

Hydrogen Production From Biomass

Gasification of biomass (woodchips, straw):
- Advantages: Sustainable, high efficiency.
- Units: Güssing, Austria; Herten, Ruhr industrial area, Germany; Burlington, USA.
- Economical power sizes: 2.5 – 10 MW.

Biological Hydrogen Production

Advantages: Renewable source.
Source: Bacteria, micro algae, microbes.
Origin: Photosynthesis.
Research area: Identification of enzyme systems, modification.
Problem: Scalability not yet realized; indirect process.

Rainbow of Hydrogen Colors

White hydrogen: Naturally occurring, fracking of underground deposits.
Black hydrogen: From black coal.
Brown hydrogen: From lignite (brown coal).
Grey hydrogen: Steam reforming of natural gas.
Blue hydrogen: Grey hydrogen with CCS(U), carbon capture storage.
Turquoise hydrogen: Methane pyrolysis yields hydrogen and solid carbon.
Red hydrogen: Nuclear high-temperature catalytic water splitting.
Pink hydrogen: Nuclear power plant electricity for water electrolysis.
Purple hydrogen: Combined nuclear chemo thermal electrolysis of water.
Green hydrogen: Water electrolysis from renewable energy (< 0.1 %).
Yellow hydrogen: Solely from electrolysis using solar power.

Hydrogen Storage

Hydrogen storage under pressure:
- Pressure: 20 - 70 MPa.
- Tank materials: Steel cylinder, aluminum with carbon fiber coating, HDPE with carbon fiber coating.
- Problem: Pressure stability required; cylinders have non-conformal geometry.
- Application: Cars.
Cryogenic hydrogen storage:
- Temperature difference: 250 °C isolation:
- Mobile units need less than 1 % withdrawal
- Large storage has significantly lower specific losses due to increased volume surface ratio.

Ortho and Parahydrogen

H2 molecule: protons unpaired (↑↑) or paired (↑↓).
Equilibrium relation: o-H2 ⇄ p-H2 + 0.08 kJ.
Temperature dependent equilibrium: T↓ equilibrium shifts toward p-H2. Catalyst: active carbon.
Different properties: p-H2 higher specific heat.
Property (p-H2 vs o-H2):
- Boiling point / K: 13.813 vs 14.05
- parts H2 at RT /%: 25 vs 75

Hydrogen Storage as Metal Hydride

Storage material: Metal hydride tank.
High weight (meaningful for e.g. ships).
Container: Steel cylinder or aluminum with carbon fiber coating.
Problem: Charging and discharging kinetics, maybe cartridge durability (ca. 1000 cycles).
Heat of reaction: Metal + hydrogen \leftrightarrow metal hydride + heat
Requires chemical stability.
Two-tank approach is necessary for hydride storage.

Hydrogen Transport

Methods: Pipeline, trailer, train, ship.
Rhein-Ruhr-pipeline: 225 km length, diameter 200 mm, intermittent storage, minimum pressure 4 MPa, maximum pressure 8 MPa (326 MWh per 100 km length).

Hydrogen Fuel Cell

Consists of anode, electrolyte membrane, and cathode, producing DC current.