Electrolysis half-time

It is interesting to note that only acetic add, acetaldehyde, and CO2 have been detected by HPLC from the outlet of the anode compartment of a DEFC with Pt/C catalyst [56], while depending on electrode potentials, acetaldehyde, acetic acid, CO2, and trace amounts of CH4 can be found in electrolysis half-cell. It is also found that acetaldehyde can be exclusively produced at a potential <0.35 V versus RHE on a Pt catalyst in a long-time electrolysis experiment no acetic acid was detected in the potential range [6], This implies that the alcohol product distribution depends on electric energy input In an acid electrolyte, Pt-based catalysts have shown better EOR activity than other platinum group metal (PGM)-based ones. However, Pt itself is readily poisoned by various Ci, C2 intermediate species. Binary and ternary Pt-based... 

The amount of product in an electrolysis reaction is calculated from the stoichiometry of the half-reaction and the current and time for which the current flows. 

In the controlled (constant) potential method the procedure starts and continues to work with the limiting current iu but as the ion concentration and hence its i, decreases exponentially with time, the course of the electrolysis slows down quickly and its completion lags behind therefore, one often prefers the application of a constant current. Suppose that we want to oxidize Fe(II) we consider Fig. 3.78 and apply across a Pt electrode (WE) and an auxiliary electrode (AE) an anodic current, -1, of nearly the half-wave current this means that the anodic potential (vs. an RE) starts at nearly the half-wave potential, Ei, of Fe(II) - Fe(III) (= 0.770 V), but increases with time, while the anodic wave height diminishes linearly and halfway to completion the electrolysis falls below - / after that moment the potential will suddenly increase until it attains the decomposition potential (nearly 2.4 V) of H20 -> 02. The way to prevent this from happening is to add previously a small amount of a so-called redox buffer, i.e., a reversible oxidant such as Ce(IV) with a standard... 

In a run one-half the size here described, the checkers obtained similar results by using the same geometry of electrodes with the same concentrations of reactants in a 700-ml. tail-form beaker, with one-half the electrolysis time. 

Generally, sensitivity in the analytical sense is greater if the technique employed is faster, i.e. the electrolysis time is shorter, or the frequency of a periodic electrolysis is higher. Resolution of half-wave potentials, and thus accuracy of standard potentials and stability constants, is better if a derivative technique such as differential pulse polarography, a.c. polar-ography, and, preferably, the second derivative technique second-harmonic a.c. polarography, is employed. 

Polarography is valuable not only for studies of reactions which take place in the bulk of the solution, but also for the determination of both equilibrium and rate constants of fast reactions that occur in the vicinity of the electrode. Nevertheless, the study of kinetics is practically restricted to the study of reversible reactions, whereas in bulk reactions irreversible processes can also be followed. The study of fast reactions is in principle a perturbation method the system is displaced from equilibrium by electrolysis and the re-establishment of equilibrium is followed. Methodologically, the approach is also different for rapidly established equilibria the shift of the half-wave potential is followed to obtain approximate information on the value of the equilibrium constant. The rate constants of reactions in the vicinity of the electrode surface can be determined for such reactions in which the re-establishment of the equilibria is fast and comparable with the drop-time (3 s) but not for extremely fast reactions. For the calculation, it is important to measure the value of the limiting current ( ) under conditions when the reestablishment of the equilibrium is not extremely fast, and to measure the diffusion current (id) under conditions when the chemical reaction is extremely fast finally, it is important to have access to a value of the equilibrium constant measured by an independent method. 

AuC14. At the same time there exists in the solution a small number of simple ions, so that on electrolysis gold is deposited at the kathode, but the primary effect of the current is to send the aurichloric ions to the anode. The solution of mercuric iodide in potassium iodide, of which mention was madebefore,is a half-way example of the same kind. Its solu-... 

Fuel boxes can be distributed like soft drinks to multiple distribution channels, even dispensing machines. Consumers can get their fuel anywhere and any time. By 2025, one-quarter of the industrialized vehicle fleet uses fuel cells, which also account for half of new sales. Renewables start out slowly but pick up speed after 2025. Some one billion metric tons of co2 are sequestered in 2025, and, after 2025, hydrogen is widely produced from coal, oil and gas fields, with carbon dioxide extracted and sequestered cheaply at the source. Also, large-scale renewable and nuclear energy schemes to produce hydrogen by electrolysis become attractive by 2030. ... 

On substituting the term of the right half of the equation (XI-22) into the equation (XI-23) we obtain an equation which enables us to calculate the current efficiency at any time during electrolysis if the value of concentration ratio Cj/c2 at the cathode side of the diaphragm is known ... 

In the section on voltaic cells, we saw that the anode lost mass over time (as the metals were oxidized and went into solution), while the cathode gained mass over time (as the cations were reduced and plated on the surface). The voltaic cell, however, requires spontaneous reactions in each half-cell, which limits the types of electrodes that can be used. In an electrolytic cell, because we are adding electric current to the cathode and the anode, we can force nonspontaneous reactions to occur. In some cases, this allows us to use electrolysis for purposes other than separating a molten compound or aqueous solution. One of the more common alternate uses is the purification of different metals. 

Now suppose that we are to carry out the same separation at constant current. With the same apparatus, the current cannot exceed 2 mA, the limit imposed by the mass-transfer rate of lO MAg" ". This separation would require 5 x 10 s, or 5.8 days By starting with a higher initial current, say 1 A, which would make it possible to plate out half the silver in 500 s, the time could be shortened, but at the risk of plating out copper accidentally as the rate of mass transfer of silver gradually decreased. Only by interposing a reducible material to consume the excess current without forming solid products could the rate be increased. Such a material should be reducible at the proper current density in just the potential range calculated above. Indeed, such a procedure constitutes an internal form of controlled-potential electrolysis that would permit the same performance as that calculated above for controlled-potential electrolysis. 

To carry out the determination of a metal ion by means of anodic stripping, a fresh hanging drop is formed, gentle stirring is begun, and a potential is applied that is a few tenths of a volt more negative than the half-wave potential for the ion of interest. Deposition is allowed to occur for a carefully measured period, which can vary from a minute or less for 10 M solutions to 30 min or longer for 10 M solutions. These times seldom result in complete removal of the analyte. The electrolysis period is determined by the sensitivity of the method ultimately employed for completion of the analysis. 

Mechanistic studies can employ CPE if the coupled chemical reactions are slow. Conventional bulk electrolyses require typically 10-30 min for completion, longer than the typical longest time for voltammetric techniques (ca. 20 s maximum for cyclic voltammetry, CV, ca. 8 s for polarography, etc.). This is important to recall when comparing CPE with voltammetry data. An electrode reaction that is chemically reversible in a slow CV experiment may be irreversible in bulk electrolysis if the electrode product has a half-life of, e.g., a minute or two. Conversely, an electron transfer that is quasi- or irreversible in a relatively fast voltammetric experiment may be electrochemically reversible in the long timescale of bulk electrolysis. 

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