Electrolysis

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Electrolysis

• Amy Jewel, Rob Larkin and Todd Haurin

Water will be the coal of the future. -
Jules Verne, 1874
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The process
of hair removal
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Is NOT our topic today!
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What is electrolysis?

• Definition of Electrolysis
• ? A chemical process in which bonded
  elements and compounds are dissociated by
  the passage of an electric current.
• The electrolysis of water
• ? 2H2O energy 2H2 2O2

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A Basic Electrolyzer

• Two electrodes
• Cathode (negatively charged)
• Anode (positively charged)
• An Electrolyte
• External circuit
• Diaphragm

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Polymer Electrolyte Membrane (PEM) Electrolyzers

• Uses a solid plastic material as an electrolyte.
• 2. Water reacts at the anode to form oxygen,
  electrons, and positively charged hydrogen ions
  (protons).

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Polymer Electrolyte Membrane (PEM) Electrolyzers
3. The electrons flow through an external circuit
to the cathode. 4. The hydrogen ions move across
the PEM to the cathode, where they combine with
the electrons to form hydrogen gas
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Alkaline Electrolyzers

• Similar to PEM electrolyzers, except that they
  use an alkaline solution as an electrolyte.
• Usually this solution is sodium hydroxide or
  potassium hydroxide.

• This type of electrolyzer has been in use
  commercially for several decades

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Solid Oxide Electrolyzers

• A solid ceramic material is used as the
  electrolyte.
• At the cathode, water combines with electrons
  from the external circuit to produce hydrogen gas
  and negatively charged oxygen ions.
• The oxygen ions move through the solid oxide
  membrane and release electrons to the external
  circuit.
• In order for this type of electolyzer to
  function properly, the solid oxide membrane must
  be between 500 800 degrees Celsius, which is
  much higher than the temperatures required by the
  other electrolyzers

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Energy Balance and Efficiency of Electrolysis

• The electricity needed for hydrogen
• production by electrolysis can currently be
• generated by a variety of sources,
• including
• fossil fuels
• wind power
• photovoltaic cells
• hydropower

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Necessary Water Inputs For Electrolysis

• Amount of water needed to meet average US
  persons energy demand though electrolysis 3,000
  liters of water per year
• Amount of water currently used by an average US
  person for indoor residential purposes 138,770
  liters a year

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Electrolysis Efficiency Basics

• Although hydrogen is a promising alternative
  fuel, hydrogen production by electrolysis is not
  extremely efficient.
• The primary energy inputs to be considered are
  the energy requirements for building and running
  an electrical generating facility.

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Energy Balance - Part I

• An input of 1.4 billion kW per hour of
  electricity is required to produce 1 billion kW
  per hour of hydrogen by electrolysis.
• Energy balance (Useful Energy Output)/(Energy
  Input)
• (1 kW/hr electricity)/(1.4 kW/hr hydrogen
  energy)
• 0.71, or 71 efficiency for the initial
  electrolysis process.

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Energy Balance - Part II

• The other main process to consider in production
  of hydrogen gas is the necessary cooling of
  hydrogen to about minus 253 degrees Celsius.
• This process demands considerable energy,
  resulting in a loss of approximately 30 percent
  of the hydrogen energy.
• As a result of each stage of the hydrogen
  production process, the total production
  efficiency is approximately 30 .

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High Temperature Electrolysis

• Process which could increase hydrogen efficiency
  to the range of 45 to 50
• The DOE is currently examining the use of high
  temperature electrolysis powered by fossil fuel,
  renewable, and even nuclear technologies.
• High temperature electrolysis utilizes the solid
  oxide electrolyzer described earlier.

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High Temperature Electrolysis

• The efficiency increase is achieved because high
  temperature electrolysis utilizes a significant
  amount of heat, for example from a nuclear
  reactor.
• The added heat decreases the amount of
  electricity required to separate the water into
  hydrogen and oxygen. 

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Photoelectrolysis

• Photoelectrolysis Clean and renewable means of
• deriving hydrogen Also known as Water Splitting
  
• (2 processes)
• Conversion of solar radiation to electricity in
  photovoltaic cells
• Electrolysis of water in a separate cell
• Conversion efficiency 3 - 32

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Photoelectrolysis

• However, the 2 processes can be combined in
• a single nanoscale process Photon
• absorption creates a local electron-hole pair
• that electrochemically splits a neighboring
• water molecule. In theory, rather than 2
• sequential process, the combination can allow
• for greater overall efficiency,

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Photoelectrolysis

• Challenges Finding a robust semiconductor to
  satisfy the competing requirements of nature.
  Solar photons are primarily visible light, a
  wavelength that requires semiconductors that
  require small bandgaps lt 1.7 eV - for efficient
  absorption.

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Photoelectrolysis

• Possible solution Oxide based conductors -
  Titanium oxide
• Advantage robust in aqueous environments but
  have
• Disadvantage - wide bandgaps 3.0 eV

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Photoelectrolysis

• Dye-sensitized photocells
• accumulate energy from multiple low-energy
  photons to inject higher-energy electrons into
  the semiconductor a promising direction for
  matching the solar spectrum.

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Other Applications of Electrolysis

• pH meters -

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Other Applications of Electrolysis

• Electroplating

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Other Applications of Electrolysis

• Anionic polymerization