Hydrogen Notes

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: $v<em>1/v</em>2 = \sqrt{M<em>2/M</em>1}$
  • Chemical: Through Pd, Fe
• Thermal Conductivity: High
• Solubility:
  • Physical: 21 ml per liter of water (0 °C, 0.1 MPa)
  • Chemical: Pd, TiFe, $LaNi<em>5$ (up to $LaNi</em>5H_{6.7}$ 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 ($C<em>mH</em>n$) 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 $H<em>2$ and $1/2$ m³ $O</em>2$. 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: $j = j<em>0 \exp{\frac{\alpha nF}{RT} \eta} - j</em>0 \exp{\frac{-\alpha nF}{RT} \eta}$
  • $j$: 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:
  • $MH<em>{ads} \leftrightarrow MH</em>{abs}$
  • $c_{H,0} = const.$
  • $c_{H,L} = 0$
• Hydrogen entrance side (cathode):
  • Acidic electrolyte: $H<em>3O^+ + M + e^- \rightarrow MH</em>{ads} + H_2O$
  • Neutral/alkaline electrolyte: $H<em>2O + M + e^- \rightarrow MH</em>{ads} + OH^-$
• $H_{ads} \rightarrow H^+ + e^-$
• 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:
  • $E = E<em>0 + \frac{RT}{zF} \ln{\frac{c</em>{ox}}{c_{red}}}$
• 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, $WO<em>3 + MeO</em>x$ (Me=Fe, Co, Ni).
• Reactions:
  • $h \nu \rightarrow e^- + h^+$
  • $2 H<em>2O + 2 e^- \rightarrow H</em>2 + 2 OH^-$
  • $2 OH^- + 2 h^+ \rightarrow O<em>2 + H</em>2$
• Problems: Photocorrosion, overvoltage of hydrogen formation.

Chemical Formation of Hydrogen

• From metals and nonmetals in alkalines:
  • $Al + OH^- + 3 H<em>2O \rightarrow Al(OH)</em>4^- + 1.5 H_2 \uparrow$
  • $Si + 2 OH^- + H<em>2O \rightarrow SiO</em>3^{2-} + 2 H_2 \uparrow$
• From metals in acids:
  • $Zn + 2 H<em>3O^+ \rightarrow Zn^{2+} + 2 H</em>2O + H_2 \uparrow$
• Through hydrolysis of hydrides:
  • $CaH<em>2 + 2 H</em>2O \rightarrow Ca(OH)<em>2 + 2 H</em>2 \uparrow$

Hydrogen Formation Through Steam Reforming

• Chemical carbohydride decomposition:
  • $206.2 \text{ kJ} + CH<em>4 + H</em>2O(g) \leftrightarrow CO + 3 H_2$
  • Conditions: 700 – 830 °C, 4 MPa, Ni-catalysis, 8 % methane.
  • High temperatures: 1200 – 1500 °C, without catalyst, 0.2 % methane.
• Shift reaction:
  • $CO + H<em>2O \rightarrow CO</em>2 + H_2$

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, $Na<em>2O$, $K</em>2CO_3$), 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 ($CO + 3 H<em>2 \leftrightarrow CH</em>4 + H<em>2 O$ and $CO</em>2 + 4 H<em>2 \leftrightarrow CH</em>4 + 2 H_2 O$), finally methane condensation (-162 °C).
• High purity hydrogen achieved through electrolysis, direct production.
• Pd-diffusion (300 °C).
• Uranium purification route: $U + 1.5 H<em>2 \leftrightarrow UH</em>3$ (forward 250 °C, return 500 °C).
• Lanthanum nickel purification route: $LaNi<em>5 + x H \leftrightarrow LaNi</em>5H_x$ (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.