Hydrogen Storage and Fuel Cell Electrochemistry

Hydrogen Storage

• Hydrogen can be stored in gaseous or liquid form
• Gaseous storage options include containers and composite tanks
• Composite tanks at 350/700 bar are primarily for transportation
• Rock salt caverns can be used for bulk storage.
• Exploited oil and gas fields have sluggish dynamics and contamination issues
• Pipelines are unsuitable for hydrogen storage due to low volumetric energy density and allowable pressure drop
• Liquid hydrogen storage involves compounds like methanol or ammonia (as chemical precursors)
• Hydrates are a niche market option, but are heavy and require temperatures unsuitable for dehydration
• Liquid organic hydrogen carriers (LOHC) require about 30% energy for dehydration and us aromatic carriers
• Liquefaction of H2 will have about 21% losses in modern plants.

Renewable energy needed

• Renewable power needed equals the capacity factor times the average power demand
• Germany needing about ~60GW

Fuel Cell Electrochemistry

• In a hydrogen fuel cell, hydrogen is oxidized at the anode, releasing electrons
• At the cathode, oxygen is reduced using the electrons
• The anode has a lower electrical potential, while the cathode has a higher electrical potential

Half-Cell Reactions

• In a proton conduction membrane fuel cell (cation conduction), hydrogen ions (H+) are produced at the anode
• In an oxygen ion conduction membrane fuel cell (anion conduction), oxide ions (O2-) are consumed at the anode

Electrolysis

• Oxidation occurs at the anode
• Requires energy input to force the reaction to happen
• Ionic transportation in an electrolysis is driven by the electrical potential difference, as Anode potential is greater than Cathode potential.
• Electrons have to be transported to the cathode for reduction to happen

Fuel Cell Types

• Solid Oxide Fuel Cells (SOFC) simple in process, have proven longevity up to 100,000 hours and uses CO as fuel
• Molten Carbonate Fuel Cells (MCFC) has simple process design, large cell areas, and uses CO as fuel.

Fuel Cell Stack Component Requirements

• Electrolytes require high ionic conductivity and gas tightness
• Cathodes requires high electro-catalytic activity and electronic conductivity with long-term stability in an oxidizing atmosphere
• Anodes requires high electro-catalytic activity, electronic conductivity, and long-term stability in a reducing atmosphere

Kinetics and Overpotentials

• Overpotential is the additional voltage required to drive a reaction at a certain rate in a fuel cell
• Activation overpotential is caused by the kinetics of the electrode reactions at the electrodes
• Ohmic overpotential is caused by the electrical resistance of the electrolyte and can be affected by altering its dimensions.
• Concentration overpotential is caused by mass transport limitations and can be addressed through optimizing the fuel cell design

Activation Energy

• Activation energy cannot be used to generate electricity in the fuel cell.

Deviations from Nernst Potential

• Short circuits
• Gas crossover from reactants
• Gas leakage

Mitigation of concentration polarization

• Avoid low reacting gas concentration

Overpotentials Types in Fuel Cells

• Activation overpotential: limited velocity of transport at phase boundary, reactors, electrolytes, electrodes, and temperature
• Ohmic Overpotential: ohmic resistances, electrolyte, electrodes, interconnectors
• Concentration Overpotential: low reactant concentration or low porosities of electrodes

Exchange Current Density

• Exchange current is a measurement of the rate at which electrons are exchanged between the electrode and the electrolyte at equilibrium
• Increasing temperature helps reactions to occur more effectively

Meaning of Transfer Coefficient

• a < 0.5: Cathodic branch steeper than anodic branch
• a = 0.5: Anodic and cathodic branches are symmetrical
• a > 0.5: Anodic branch steeper than cathodic branch

Fuel Cell Power Setting

• Clamping voltage described as UK = UN - IR
• UN: is determined by the gas composition
• R: const. (isothermal case)

Local Current Density in Fuel Cells

• Higher the power of the cell, the higher the gas consumption

Drivers of Cross-current in a Cell

• Electrodes and bipolar plates are quite good, but not ideal equipotential surfaces.

Stack Cross Current Occurs when

• There's inadequate flow
• Clogging

Fuel cell test diagnostics:

• EZ (Single cell measurement): Evaluates total performance
• HZ (Half-cell measurement): Isolates electrode performance and uncovers fundamental kinetics

Fuel Processing

• Goal is pure H2, via a reformer
• The Water Gas Shift (WGS) reaction reduces CO
• A PROX or SeLOx stage can further reduce CO

Desulfurization Methods

• Adsorption: active carbon or alumina
• Gases: hydrogenation
• Liquids: Selective Oxidations

Direct Fuel Cells

• No reformer:
  • Higher theoretical energy density.
  • Convenient
  • Low BoP
  • Not efficient

DMFC Catalysts:

• Platinum-Ruthenium
• Precious metals as higher stability required

Hydrogen as a fuel

• H2S above 50 ppm severely and irreversibly poisons the catalyst

SOFC materials

• Electrolyte is gas tight and an ion conductor made of Y2O3-doped ZrO2
• Interconnect is a gas tight and electronic conductor with high conductivity.
• Cathode/Anode with a porous media with good electrical conductivity

Overview of hydrogen storage methods including gaseous, liquid, and chemical carriers. Covers renewable energy requirements and fuel cell electrochemistry. Includes information on composite tanks, rock salt caverns, and liquid organic hydrogen carriers (LOHC).