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
A Basic Electrolyzer Two electrodes: –Cathode (negatively charged) –Anode (positively charged) An Electrolyte External circuit Diaphragm
Polymer Electrolyte Membrane (PEM) Electrolyzers: 1.Uses a solid plastic material as an electrolyte. 2.Water reacts at the anode to form oxygen, electrons, and positively charged hydrogen ions (protons).
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
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
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
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
Necessary Water Inputs For Electrolysis Amount of water needed to meet average US person’s 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
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.
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.
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 %.
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.
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.
Photoelectrolysis Photoelectrolysis: Clean and renewable means of deriving hydrogen Also known as ‘Water Splitting’ (2 processes): 1)Conversion of solar radiation to electricity in photovoltaic cells 2)Electrolysis of water in a separate cell Conversion efficiency = 3% - 32%
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,
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 < 1.7 eV - for efficient absorption.
Photoelectrolysis Possible solution: Oxide based conductors - Titanium oxide Advantage – robust in aqueous environments but have Disadvantage - wide bandgaps ~ 3.0 eV
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.
Other Applications of Electrolysis: pH meters -
Other Applications of Electrolysis: Electroplating
Other Applications of Electrolysis: Anionic polymerization