ABSTRACT
Experimental processes include electrolysis at very high temperatures (800°C), therefore, much of the energy required to release hydrogen is supplied as heat instead of electricity. Various catalytic agents are being studied to improve the efficiency of high-temperature electrolysis. Water spontaneously dissociates at around 2500°C, but this thermolysis occurs at temperatures too high for usual process piping and equipment. Catalysts are required to reduce the dissociation temperature [348]. If electrolysis occurs in pure water, H+ cations accumulate at the anode and OH− anions accumulate at the cathode. This can be proved by plunging a pH indicator to the water: the water near the anode is acidic whereas the water near the cathode is basic. The negative hydroxyl ions that approach the anode mostly combine with the positive hydronium ions (H3O+), forming water. The positive hydronium ions that approach the negative cathode mostly combine with negative hydroxyl ions to form water. Relatively few hydronium (or hydroxyl) ions reach the cathode (or anode). This can cause a concentration overpotential at both electrodes. Overpotential is an electrochemical term which refers to the potential difference (voltage) between a half-reaction’s thermodynam-ically determined reduction potential and the potential at which the redox event is experimentally observed. Pure water is a fairly good insulator since it has a low dissociation constant Kw = 1.0 × 10−14 at room temperature and thus pure water conducts current poorly, the value of its specific conductivity being ~0.055 µS·cm−1. Unless a very large potential is applied to provide an increase in the dissociation of water, the electrolysis of pure water proceeds very slowly limited by the overall conductivity [348]. If a water-soluble electrolyte is added, the conductivity of the water is enhanced considerably. The electrolyte dissociates into cations and anions; the anions move towards the anode and neutralize the buildup of positively charged H+ there; similarly, the cations move toward the cathode and neutralize the buildup of negatively charged OH− there. This provides the continuous flow of electricity. An electrolyte must be chosen carefully, since an anion from the electrolyte is in competition with the hydroxide ions to give up an electron. An electrolyte anion with less standard electrode potential than hydroxide will be oxidized instead of the hydroxide,
and no oxygen gas will be produced. A cation with a greater standard electrode potential than a hydrogen ion will be reduced instead, and no hydrogen gas will be produced. The following cations have lower electrode potential than H+ and are therefore suitable for use as electrolyte cations: Li+, Rb+, K+, Cs+, Ba2+, Sr2+, Ca2+, Na+, and Mg2+. Sodium and lithium are frequently used, as they form inexpensive, soluble salts. If an acid is used as the electrolyte, the cation is H+, and there is no competitor for the H+ created by disassociating water. The most commonly used anion is sulfate (SO2 4−), as it is very difficult to oxidize this ion to the peroxodisulfate ion, the standard potential for oxidation being −2.05 V [348]. Strong acids, such as sulfuric acid, and strong bases, such as potassium hydroxide (KOH) and sodium hydroxide (NaOH), are frequently used as electrolytes due to their strong conducting abilities. A solid polymer electrolyte, such as Nafion, can also be used. When a special catalyst is applied on each side of the solid electrolyte, it can efficiently split the water molecule with as little potential as 1.8 V [348]. Water electrolysis does not convert 100% of the electrical energy into the chemical energy of hydrogen. The process requires more extreme potentials than what would be expected based on the cell’s total reversible reduction potentials. This excess potential accounts for various forms of overpotential by which the extra energy is eventually lost as heat. For a well-designed cell the largest overpotential is the reaction overpotential for the four electron oxidation of water to oxygen at the anode. Reaction overpotential is an activation overpotential that specifically relates to chemical reactions that must formally precede electron transfer. An effective electrocatalyst to facilitate this reaction has not been developed. Platinum alloys are the default state of the art for this oxidation. Developing a cheap and effective electrocatalyst for this reaction would be a great advance. The simpler two-electron reaction to produce hydrogen at the cathode can be electrocatalyzed with almost no reaction overpotential by platinum or, in theory, a hydrogenase enzyme. If other less-effective materials are used for the cathode then another large overpotential must be paid for.
