Hydrogen evolution

Hydrogen evolution (HER) is a process with a sequence of several elementary reaction steps (Equations 1.140a through d). 

If the Volmer reaction is the slowest and thus the rate-determining step, the Butler-Volmer equation applies for the HER current. 

ItH i a and /Ch 1 c are the rate constants for the Volmer reaction in anodic or cathodic direction, respectively 

0H is the surface coverage by adsorbed H atoms 1 - 0H is the fraction of free surface accessible to hydrogen adsorption 

At the equilibrium potential the anodic partial current density is the opposite of fhe cathodic partial current density and its value equals the exchange current density /q h,i- 

FIGURE 5.22. The i-V curve and efficiency for hydrogen evolution T h during positive sweep of a p-S i (100) disk in 0.3 mol dm NH4F at pH 3,5. Rotation rate 40 Hz. Scan rate lOmV/s. (Reprinted from Blackwood et 1992, with permission from Elsevier Science.) 

The efficiency of hydrogen evolution and effective dissolution valence are directly correlated, and their relation varies with potential, illumination, and doping of the silicon. The overall relation among these two parameters and the factors are summarized in Fig. 5.23. 

The intrinsic limiting current for n-Si depends on the diffusion of the minority carrier as follows  

Due to the role of surface states, the dark limiting current of silicon electrode is extremely sensitive to surface defects and thus surface preparation. Any scratch even barely visible on the mirror like surface can result in a significant increase of the anodic limiting current. According to Chazalviel, defects associated with surface treatment are primarily responsible for the large limiting current values reported in the literature. The effect caused by surface states may, however, be reduced by the formation of a 

FIGURE 5.23. Summary of effective dissolution valence and hydrogen evolution efficiency as a function of potential for different materials and illumination conditions. L is illumination intensity. 

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A convenient form of apparatus, particularly for large classes, is shown in Fig. 84 it is identical with that used for the determination of the equivalent weight of metals by hydrogen evolution. A and H are glass tubes connected together by the rubber tubing J and securely fastened to the board B. The tube A is... 

A solution of 1.05 M diborane in THF (25 ml, 26 mraol) was added slowly to a stirred suspension of 3-acetyl-5-hydroxy-2-methylindole (1.0 g, 5.3 mmol) in THF (10 ml). After hydrogen evolution ceased, the mixture was heated at reflux for I h, cooled and poured into acetone (75 ml). The mixture was heated briefly to boiling and then evaporated in vacuo. The residue was heated with methanol (50ml) for 20min. The solution was concentrated and 3NHC1 (40ml) was added. The mixture was extracted with ether and the extracts dried (MgSO ) and evaporated to yield a yellow oil. Vacuum sublimation or recrystallization yielded pure product (0.76 g, 82%). 

Unlike many other borohydrides, lithium borohydride is highly soluble ia ethers including aUphatic ethers, THF, an d polyglycol ethers. It is also very soluble ia amines and ammonia. Dissolution ia water and lower aUphatic alcohols leads to extensive decomposition and hydrogen evolution. 

The standard electrode potential for zinc reduction (—0.763 V) is much more cathodic than the potential for hydrogen evolution, and the two reactions proceed simultaneously, thereby reducing the electrochemical yield of zinc. Current efficiencies slightly above 90% are achieved in modem plants by careful purification of the electrolyte to bring the concentration of the most harmful impurities, eg, germanium, arsenic, and antimony, down to ca 0.01 mg/L. Addition of organic surfactants (qv) like glue, improves the quaUty of the deposit and the current efficiency. 

Similar reactions can be written for other metallic additives. At the negative electrode two more reactions can occur. Hydrogen evolution... 

Some battery designs have a one-way valve for pressure rehef and operate on an oxygen cycle. In these systems the oxygen gas formed at the positive electrode is transported to the negative electrode where it reacts to reform water. Hydrogen evolution at the negative electrode is normally suppressed by this reaction. The extent to which this process occurs in these valve regulated lead —acid batteries is called the recombination-efficiency. These processes are reviewed in the Hterature (50—52). 

The low current efficiency of this process results from the evolution of hydrogen at the cathode. This occurs because the hydrogen deposition overvoltage on chromium is significantly more positive than that at which chromous ion deposition would be expected to commence. Hydrogen evolution at the cathode surface also increases the pH of the catholyte beyond 4, which may result in the precipitation of Cr(OH)2 and Cr(OH)2, causing a partial passivation of the cathode and a reduction in current efficiency. The latter is also inherently low, as six electrons are required to reduce hexavalent ions to chromium metal. 

The ions, M , formed by this reaction at a rate, may be carried into a bulk solution in contact with the metal, or may form insoluble salts or oxides. In order for this anodic reaction to proceed, a second reaction which uses the electrons produced, ie, a reduction reaction, must take place. This second reaction, the cathodic reaction, occurs at the same rate, ie, = 7, where and are the cathodic and anodic currents, respectively. The cathodic reaction, in most cases, is hydrogen evolution or oxygen reduction. 

If the potential of a metal surface is moved below line a, the hydrogen reaction line, cathodic hydrogen evolution is favored on the surface. Similarly a potential below line b, the oxygen reaction line, favors the cathodic oxygen reduction reaction. A potential above the oxygen reaction line favors oxygen evolution by the anodic oxidation of water. In between these two lines is the region where water is thermodynamically stable. 

Mercury Cells. The cathode material ia mercury cells, mercury [7439-97-6] Hg, has a high hydrogen overvoltage. Hydrogen evolution is suppressed and sodium ion reduction produces sodium amalgam [11110-32-4J, HgNa. 

Adequate ventilation is necessary for aH process lines to ensure worker safety. Electroless copper baths must have good ventilation to remove toxic formaldehyde vapors and caustic mist generated by the hydrogen evolution reactions and air sparging. Electroless nickels need adequate ventilation to remove nickel and ammonia vapors. Some states and municipalities requite the removal of ammonia from wastewaters. A discussion of printed circuit board environmental issues and some sludge reduction techniques is avaHable (25). 

When water pH is between about 4 and 10 near room temperature, iron corrosion rates are nearly constant (Fig. 5.5). Below a pH of 4, protective corrosion products are dissolved. A bare iron surface contacts water, and acid can react directly with steel. Hydrogen evolution (Reaction 5.3) becomes pronounced below a pH of 4. In conjunction with oxygen depolarization, the corrosion rate increases sharply (Fig. 5.5). 

Isobutyronitrile (2-methylpropionitrile, isopropyl cyanide) [78-82-0] M 69.1, b 103.6 , d 0.7650, n 1.378. Shaken with cone HCl (to remove isonitriles), then with water and aq NaHC03. After a preliminary drying with silica gel or Linde type 4A molecular sieves, it is shaken or stirred with CaH2 until hydrogen evolution ceases, then decanted and distd from P2O5 (not more than 5g/L, to minimize gel formation). Finally it is refluxed with, and slowly distd from CaH2 (5g/L), taking precautions to exclude moisture. 

In oxygen-free seawater, the J(U) curves, together with the Tafel straight lines for hydrogen evolution, correspond to Eq. (2-19) (see Fig. 2-2lb). A limiting current density occurs with COj flushing for which the reaction ... 

The sum of all the cathodic partial reactions is included in e.g., oxygen reduction according to Eq. (2-17) and hydrogen evolution according to Eq. (2-19). The intermediate formation of anode metal ions of anomalous valence is also possible ... 

Even in good alloys and under favorable conditions, the a value does not lie above about 0.6. In enamelled storage tanks where the current requirement is low, the a value can fall to as low as about 0.1. The cause of the high proportion of selfcorrosion is hydrogen evolution, which occurs as a parallel cathodic reaction according to Eq. (6-5b) or by free corrosion of material separated from the anode on the severely craggy surface [2-4, 19-21]. 

Fig. 6-9 (above) Apparatus for determining the weight loss of galvanic anodes by measuring hydrogen evolution. 

In oxygen-free water, the self-corrosion is practically solely due to hydrogen evolution... 

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