Electrolytic hydrogenation, cathodes

Cathodic hydrogen evolution is one of the most common electrochemical reactions. It is the principal reaction in electrolytic hydrogen production, the auxiliary reaction in the production of many substances forming at the anode, such as chlorine, and a side reaction in many cathodic processes, particularly in electrohydrometallurgy. It is of considerable importance in the corrosion of metals. Its special characteristic is the fact that it can proceed in any aqueous solution particular reactants need not be added. The reverse reaction, which is the anodic ionization of molecular hydrogen, is utilized in batteries and fuel cells. 

In alkaline electrolyzers, hydrogen is obtained at the cathode with a purity of approximately 98 vol%, with oxygen and water vapor as the only impurities. Hydrogen may be further purified to almost 100% by the removal of oxygen in a catalytic deoxidizer and the subsequent removal of water vapor in a dryer. In the purification step, 5-10% of the produced hydrogen may be lost therefore, the use of electrolytic hydrogen without purification should always be considered in priority for each application. 

Dyer C.K., Improved cathodes for industrial electrolytic hydrogen production, Int.. Hydrogen Energ., 9(12), 993-995,1984. 

S. Trasatti provides a detailed review of the advances in the production of cathodes with favorable characteristics for the manufacture of electrolytic hydrogen. A careful analysis is given of performance characteristics on the basis of electrode reaction kinetics and catalysis. 

If an alkali chromate and turkey red oil be added to the electrolyte, the cathode reduction amounts only to that due to 3 per cent, of the hydrogen discharged. A patent was granted in 1913 for producing alkali perborate by electrolysis, and others have followed. 

Attempts have been made to cheapen the process by producing electrolytic oxygen without the simultaneous liberation of hydrogen by the adoption of depolarising electrolytes, or cathodes also of cathodes winch absorb hydrogen and may subsequently Ik- employed as elements in gas cells.8... 

The stereochemical feature of electrolytic hydrogenation of acetylenes to the corresponding olefins changes drastically with electrolytic conditions, such as supporting electrolyte, solvent, and cathode material [83-88]. 

Detection of CO2-" anion radical was conducted with a Pb electrode in CO2 saturated aqueous, acetonitrile and propylene carbonate electrolytes during cathodic polarization by ultraviolet (UV) spectroscopic measurements by Aylmer-Kelly et al. CO2- anion radical is mostly present freely in both aqueous and nonaqueous electrolyte solutions. Stabilization of CO2- due to hydrogen bond formation in aqueous electrolyte solution was suggested on the basis of the red shift of the observed absorption band. 

The basic structure and principle of all fuel cells is similar the cell consists of two electrodes which are separated by an electrolyte. The electrodes are connected through an external circuit. The electrodes are exposed to gas or liquid flows to supply fuel and oxidant (for instance hydrogen and oxygen). The electrodes have to be gas or liquid permeable and therefore possess a porous structure. The electrolyte should have a gas permeability as low as possible. For fuel cells with an acid electrolyte, hydrogen is oxidized at the negative electrode (the anode) according to the following equation. The protons formed enter the electrolyte and are transported to the cathode ... 

Hydrogen ions move through electrolyte to cathode. Electrons stream Into cathode from load. Cathode Is bathed with oxygen. 

Figure 19. Relation between m and cathodic overpotential during electrolytic hydrogenation of...

In cases where more than one product may be formed, particuarly when the relative yield is potential dependent, potentiostatic electrolysis is particularly desirable. If a controlled constant current electrolysis is carried out in such a case, the potential will generally increase with time as the reactant is consumed, so that eventually the diffusion-limited current is reached, which itself eventually becomes smaller and smaller and the remaining fraction of the current is then passed by decomposition of the solvent or supporting electrolyte, usually with evolution of hydrogen (cathodic processes) or oxygen (anodically) in the case of an aqueous medium. 

Failure modes. C.P. titanium metal and its alloys are susceptible to hydrogen pickup and hence extremely sensitive to embrittlement by nascent hydrogen gas moreover in corrosive electrolytes the cathode must be polarized cathodically during shutdowns. 

Calcium is produced by electrolysis in molten hydroxide electrolytes or by a thermal process. In the case of electrolysis in molten hydroxide electrolyte, hydrogen evolution is likely to be an important cathodic side reaction. 

However, hydrogen production from acetate oxidation, as aimed for in an MEC, is thermodynamically not feasible. Indicated by the equilibrium potentials of the individual half reactions at microbial conditions (pH 7, acetate 1 M, 1 bar), the electromotive force (emf) (= cathode potential - anode potential) of this reaction is -0.14 V, which means that additional electrical energy is required to support electrolytic hydrogen formation (Fig. 2). This is provided by applying a circuit voltage that is... 

By supplying the ions electrolytically, the cathodic reaction (3) generates hydrogen gas micro bubbles, which capture the flocculated pollutants and float them to the surface. Under the right conditions, this combination of processes captures the pollutants as a stable surface floe layer that is easily separated from the treated water. Reaction (3) also means that the water s pH will increase with greater treatment dosing, and its pH must be pre-adjusted such that it is always near neutral at discharge. 

---