Alkaline water electrolysis | |
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Typical Materials | |
Type of Electrolysis: | Alkaline Water Electrolysis |
Style of membrane/diaphragm | NiO [1] /Asbestos/polysulfone matrix and ZrO2 (Zirfon)/polyphenil sulfide [2] [3] |
Bipolar/separator plate material | Stainless steel |
Catalyst material on the anode | Ni/Co/Fe |
Catalyst material on the cathode | Ni/C-Pt |
Anode PTL material | Ti/Ni/zirconium |
Cathode PTL material | Stainless steel mesh |
State-of-the-art Operating Ranges | |
Cell temperature | 60-80 °C [4] |
Stack pressure | <30 bar [4] |
Current density | 0.2-0.4 A/cm2 [4] [5] |
Cell voltage | 1.8-2.40 V [4] [5] |
Power density | to 1.0 W/cm2 [4] |
Part-load range | 20-40% [4] |
Specific energy consumption stack | 4.2-5.9 kWh/Nm3 [4] |
Specific energy consumption system | 4.5-7.0 kWh/Nm3 [4] |
Cell voltage efficiency | 52-69% [4] |
System hydrogen production rate | <760 Nm3/h [4] |
Lifetime stack | <90,000 h [4] |
Acceptable degradation rate | <3 μV/h [4] |
System lifetime | 20-30 years [4] |
Alkaline water electrolysis is a type of electrolyzer that is characterized by having two electrodes operating in a liquid alkaline electrolyte. Commonly, a solution of potassium hydroxide (KOH) or sodium hydroxide (NaOH) at 25-40 wt% is used. [6] These electrodes are separated by a diaphragm, separating the product gases and transporting the hydroxide ions (OH−) from one electrode to the other. [4] [7] A recent comparison showed that state-of-the-art nickel based water electrolyzers with alkaline electrolytes lead to competitive or even better efficiencies than acidic polymer electrolyte membrane water electrolysis with platinum group metal based electrocatalysts. [8]
The technology has a long history in the chemical industry. The first large-scale demand for hydrogen emerged in late 19th century for lighter-than-air aircraft, and before the advent of steam reforming in the 1930s, the technique was competitive.[ citation needed ]
The electrodes are typically separated by a thin porous foil (with a thickness between 0.050 to 0.5 mm), commonly referred to as diaphragm or separator.[ citation needed ] The diaphragm is non-conductive to electrons, thus avoiding electrical shorts between the electrodes while allowing small distances between the electrodes. The ionic conductivity is supplied by the aqueous alkaline solution, which penetrates in the pores of the diaphragm. The state-of-the-art diaphragm is Zirfon, a composite material of zirconia and Polysulfone. [9] The diaphragm further avoids the mixing of the produced hydrogen and oxygen at the cathode and anode, [10] [11] respectively.
Typically, Nickel based metals are used as the electrodes for alkaline water electrolysis. [12] Considering pure metals, Ni is the least active non-noble metal. [13] The high price of good noble metal electrocatalysts such as platinum group metals and their dissolution during the oxygen evolution [14] is a drawback. Ni is considered as more stable during the oxygen evolution, [15] but stainless steel has shown good stability and better catalytic activity than Ni at high temperatures during the Oxygen Evolution Reaction (OER). [5]
High surface area Ni catalysts can be achieved by dealloying of Nickel-Zinc [5] or Nickel-Aluminium alloys in alkaline solution, commonly referred to as Raney nickel. In cell tests the best performing electrodes thus far reported consisted of plasma vacuum sprayed Ni alloys on Ni meshes [16] [17] and hot dip galvanized Ni meshes. [18] The latter approach might be interesting for large scale industrial manufacturing as it is cheap and easily scalable, but unfortunately, all the strategies show some degradation. [19]
In alkaline media oxygen evolution reactions, multiple adsorbent species (O, OH, OOH, and OO–) and multiple steps are involved. Steps 4 and 5 often occur in a single step, but there is evidence that suggests steps 4 and 5 occur separately at pH 11 and higher. [20] [21]
Overall anode reaction: |
Where the * indicate species adsorbed to the surface of the catalyst.
The hydrogen evolution reaction in alkaline conditions starts with water adsorption and dissociation in the Volmer step and either hydrogen desorption in the Tafel step or Heyrovsky step.
Volmer step: |
Tafel step: |
Heyrovsky step: |
Overall cathode reaction: |
In comparison to Proton exchange membrane electrolysis, the advantages of alkaline water electrolysis are mainly:
One disadvantage of alkaline water electrolyzers is the low-performance profiles caused by the commonly-used thick diaphragms that increase ohmic resistance, the lower intrinsic conductivity of OH− compared to H+, and the higher gas crossover observed for highly porous diaphragms.
In chemistry and manufacturing, electrolysis is a technique that uses direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from naturally occurring sources such as ores using an electrolytic cell. The voltage that is needed for electrolysis to occur is called the decomposition potential. The word "lysis" means to separate or break, so in terms, electrolysis would mean "breakdown via electricity."
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Proton-exchange membrane fuel cells (PEMFC), also known as polymer electrolyte membrane (PEM) fuel cells, are a type of fuel cell being developed mainly for transport applications, as well as for stationary fuel-cell applications and portable fuel-cell applications. Their distinguishing features include lower temperature/pressure ranges and a special proton-conducting polymer electrolyte membrane. PEMFCs generate electricity and operate on the opposite principle to PEM electrolysis, which consumes electricity. They are a leading candidate to replace the aging alkaline fuel-cell technology, which was used in the Space Shuttle.
High-temperature electrolysis is a technology for producing hydrogen from water at high temperatures or other products, such as iron or carbon nanomaterials, as higher energy lowers needed electricity to split molecules and opens up new, potentially better electrolytes like molten salts or hydroxides. Unlike electrolysis at room temperature, HTE operates at elevated temperature ranges depending on the thermal capacity of the material. Because of the detrimental effects of burning fossil fuels on humans and the environment, HTE has become a necessary alternative and efficient method by which hydrogen can be prepared on a large scale and used as fuel. The vision of HTE is to move towards decarbonization in all economic sectors. The material requirements for this process are: the heat source, the electrodes, the electrolyte, the electrolyzer membrane, and the source of electricity.
A proton-exchange membrane, or polymer-electrolyte membrane (PEM), is a semipermeable membrane generally made from ionomers and designed to conduct protons while acting as an electronic insulator and reactant barrier, e.g. to oxygen and hydrogen gas. This is their essential function when incorporated into a membrane electrode assembly (MEA) of a proton-exchange membrane fuel cell or of a proton-exchange membrane electrolyser: separation of reactants and transport of protons while blocking a direct electronic pathway through the membrane.
Electrolysis of water is using electricity to split water into oxygen and hydrogen gas by electrolysis. Hydrogen gas released in this way can be used as hydrogen fuel, but must be kept apart from the oxygen as the mixture would be extremely explosive. Separately pressurised into convenient 'tanks' or 'gas bottles', hydrogen can be used for oxyhydrogen welding and other applications, as the hydrogen / oxygen flame can reach approximately 2,800°C.
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A membrane electrode assembly (MEA) is an assembled stack of proton-exchange membranes (PEM) or alkali anion exchange membrane (AAEM), catalyst and flat plate electrode used in fuel cells and electrolyzers.
The hybrid sulfur cycle (HyS) is a two-step water-splitting process intended to be used for hydrogen production. Based on sulfur oxidation and reduction, it is classified as a hybrid thermochemical cycle because it uses an electrochemical reaction for one of the two steps. The remaining thermochemical step is shared with the sulfur-iodine cycle.
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Water oxidation is one of the half reactions of water splitting:
Proton exchange membrane(PEM) electrolysis is the electrolysis of water in a cell equipped with a solid polymer electrolyte (SPE) that is responsible for the conduction of protons, separation of product gases, and electrical insulation of the electrodes. The PEM electrolyzer was introduced to overcome the issues of partial load, low current density, and low pressure operation currently plaguing the alkaline electrolyzer. It involves a proton-exchange membrane.
Dioxide Materials was founded in 2009 in Champaign, Illinois, and is now headquartered in Boca Raton, Florida. Its main business is to develop technology to lower the world's carbon footprint. Dioxide Materials is developing technology to convert carbon dioxide, water and renewable energy into carbon-neutral gasoline (petrol) or jet fuel. Applications include CO2 recycling, sustainable fuels production and reducing curtailment of renewable energy(i.e. renewable energy that could not be used by the grid).
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Anion exchange membrane(AEM) electrolysis is the electrolysis of water that utilises a semipermeable membrane that conducts hydroxide ions (OH−) called an anion exchange membrane. Like a proton-exchange membrane (PEM), the membrane separates the products, provides electrical insulation between electrodes, and conducts ions. Unlike PEM, AEM conducts hydroxide ions. The major advantage of AEM water electrolysis is that a high-cost noble metal catalyst is not required, low-cost transition metal catalyst can be used instead. AEM electrolysis is similar to alkaline water electrolysis, which uses a non-ion-selective separator instead of an anion-exchange membrane.