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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Electrolysis | 4/5 | https://en.wikipedia.org/wiki/Electrolysis | reference | science, encyclopedia | 2026-05-05T10:47:31.576904+00:00 | kb-cron |
the electrode potential for the reduction producing hydrogen is −0.41 V, the electrode potential for the oxidation producing oxygen is +0.82 V. Comparable figures calculated in a similar way, for 1 M zinc bromide, ZnBr2, are −0.76 V for the reduction to Zn metal and +1.10 V for the oxidation producing bromine. The conclusion from these figures is that hydrogen should be produced at the cathode and oxygen at the anode from the electrolysis of water—which is at variance with the experimental observation that zinc metal is deposited and bromine is produced. The explanation is that these calculated potentials only indicate the thermodynamically preferred reaction. In practice, many other factors have to be taken into account such as the kinetics of some of the reaction steps involved. These factors together mean that a higher potential is required for the reduction and oxidation of water than predicted, and these are termed overpotentials. Experimentally it is known that overpotentials depend on the design of the cell and the nature of the electrodes. For the electrolysis of a neutral (pH 7) sodium chloride solution, the reduction of sodium ion is thermodynamically very difficult and water is reduced evolving hydrogen leaving hydroxide ions in solution. At the anode the oxidation of chlorine is observed rather than the oxidation of water since the overpotential for the oxidation of chloride to chlorine is lower than the overpotential for the oxidation of water to oxygen. The hydroxide ions and dissolved chlorine gas react further to form hypochlorous acid. The aqueous solutions resulting from this process is called electrolyzed water and is used as a disinfectant and cleaning agent.
== Research trends ==
=== Electrolysis of carbon dioxide ===
The electrolysis of carbon dioxide usually gives formate or carbon monoxide, but sometimes it produces more elaborate organic compounds such as ethylene. This finding has motivated intensive study.
=== Electrolysis of acidified water ===
Electrolysis of water produces hydrogen and oxygen in a ratio of 2 to 1 respectively.
2 H2O(l) → 2 H2(g) + O2(g) E° = +1.229 V The energy efficiency of water electrolysis varies widely. The efficiency of an electrolyser is a measure of the enthalpy contained in the hydrogen (to undergo combustion with oxygen or some other later reaction), compared with the input electrical energy. Heat/enthalpy values for hydrogen are well published in science and engineering texts, as 144 MJ/kg (40 kWh/kg). Note that fuel cells (not electrolysers) cannot use this full amount of heat/enthalpy, which has led to some confusion when calculating efficiency values for both types of technology. In the reaction, some energy is lost as heat. Some reports quote efficiencies between 50% and 70% for alkaline electrolysers (50 kWh/kg); however, higher practical efficiencies are available with the use of polymer electrolyte membrane electrolysis and catalytic technology, such as 95% efficiency. The National Renewable Energy Laboratory estimated in 2006 that 1 kg of hydrogen (roughly equivalent to 3 kg, or 4 liters, of petroleum in energy terms) could be produced by wind powered electrolysis for between US$5.55 in the near term and US$2.27 in the longer term. About 4% of hydrogen gas produced worldwide is generated by electrolysis, and normally used onsite. Hydrogen is used for the creation of ammonia for fertilizer via the Haber process, and converting heavy petroleum sources to lighter fractions via hydrocracking. Onsite electrolysis has been utilized to capture hydrogen for hydrogen fuel-cells in hydrogen vehicles.
=== Electrocrystallization === A specialized application of electrolysis involves the growth of conductive crystals on one of the electrodes from oxidized or reduced species that are generated in situ. The technique has been used to obtain single crystals of low-dimensional electrical conductors, such as charge-transfer salts and linear chain compounds.
=== Electrolytic production of iron === Iron for steel production, is produced by reduction of iron oxides using carbon. The overall process is an example of carbothermal reduction. One study of steel making in Germany found that producing 1 ton of steel emitted 2.1 tons of CO2e with 22% of that being direct emissions from the blast furnace. As of 2022, steel production contributes 7–9% of global emissions. Electrolysis of iron would eliminate direct CO2 emissions. The electrolysis of iron has been demonstrated in molten oxide salts using a platinum anode. An idealized equation is:
2 Fe2O3 → 4 Fe + 3 O2 This method was performed a temperature of 1550 °C which presents a significant challenge to maintaining the reaction. Particularly, anode corrosion is a concern at these temperatures. Additionally, the low temperature reduction of iron oxide in alkaline solutions has been reported. The temperature is much lower than traditional iron production at 114 °C. The low temperatures also tend to correlate with higher current efficiencies, with an efficiency of 95% being reported. While these methods are promising, they struggle to be cost competitive because of the large economies of scale keeping the price of blast furnace iron low.