Over-voltages of Nickel

V. O. Krenig and V. N. Uspenskaya observed that when a base metal, immersed in acid, is coupled with a nobler metal - e.g. aluminium and nickel - there is a difference in the amount of hydrogen evolved against the results when the metals are not coupled. Observations were also made in cells with three electrodes - e.g. Al-Ni-Pt.

The shape of the polarization curves for hydrogen discharge involves the subject of over-voltage. In order to discharge gaseous hydrogen upon any metal surface, it is necessary to make the potential of the latter at least as negative as that of a reversible hydrogen electrode, i.e., the equilibrium potential of gaseous hydrogen in that soln. It is equally true, however, that if the metal of the cathode, e.g., zinc, has a higher soln. press, than hydrogen in that soln., it is also necessary to counteract the tendency of the metal to dissolve, and to replace hydrogen. This requires the application of a potential at least as negative as that of the metal (in this case of zinc). Ordinarily the hydrogen over-voltage is defined as the excess potential beyond that of a reversible hydrogen electrode required to discharge gaseous hydrogen from that soln. either at some minimum current density or at some specified current density. If the latter definition is used, the hydrogen over- voltage is the same as hydrogen polarization as above defined. For metals more noble than hydrogen, such as copper, silver, and gold, this over-voltage is probably due principally to the concentration and state of the hydrogen in or on the metal. For metals such as tin, lead, nickel, and zinc, however, the so-called hydrogen over- voltage is largely a function of the potential of the metal itself. The potential required to deposit hydrogen with a given current density upon any cathode is a function not only of the metal (as usually emphasized), but also of the hydrogen-ion conc. of the soln., or of the cathode film. Hence the cathode efficiency as well as the character of the deposited metal may depend directly upon the H^•-ion conc. of the soln. In nickel deposition the pH of the soln. probably has a greater effect than that of any other factor. Similarly the metal anode efficiency is determined by the respective polarization curves for metal soln. and for oxygen evolution, which latter is influenced by oxygen over-voltage. With metals such as nickel, anode polarization is affected by passivity, i.e., the tendency of the metal to assume an abnormally positive potential. In Fig., curve F represents oxygen evolution upon nickel from a nickel sulphate soln. If the nickel becomes passive, as represented by the curve E, the anode efficiency will be lowered by the simultaneous evolution of oxygen. W. A. Caspari gave for the over-voltage of hydrogen on nickel 0.21; J. Tafel, 0.07; E. Muller, 0.03; A. Coehn and K. Dannenberg, 0.14; W. D. Harkins, 0.15; E. Newbery, 0.19; and A. Thiel and W. Hammerschmidt, 0.14. H. M. Cassel and E. Krumbein, N. Thon, H. Ficke, O. Essin, and F. Meunier studied this subject.

E. Newbery found the cathodic over-voltages of nickel, referred to the hydrogen electrode, in normal soln. of the salts, to be with current densities D milliamperes per sq. cm.:

D	2	6	10	20	50	100	200	400
NiSO4	0.58	0.63	0.65	0.76	0.79	0.80	0.80	0.80
(NH4)2Ni(SO4)2	0.45	0.68	0.72	0.74	0.77	0.78	0.77	0.75
Ni(NO3)2	0.40	0.44	0.47	0.52	0.70	0.91	0.94	0.88
NiCl2	0.71	0.74	0.75	0.77	0.81	0.87	0.95	0.96
H2SO4	0.29	0.31	0.30	0.29	0.26	0.24	0.21	0.18
NaOH	0.18	0.20	0.20	0.20	0.21	0.21	0.21	0.22

Reduction occurs in soln. of the nitrate. Hydrogen is liberated at quite moderate current densities. The anodic over-voltages were:

D	2	6	10	20	50	100	400	1200
NaOH	0.45	0.51	0.53	0.54	0.55	0.55	0.57
H2SO4	-	-	-	-	-	0.62	0.67	0.68
NiSO4	0.04	0.09	0.13	0.16	0.22	1.65*	1.68*	1.69*
Ni(NO3)2	0.12	0.15	1.60*	1.64*	1.67*	1.68*	1.69*	1.71*
NiCl2	0.07	0.09	0.09	0.08	0.08	0.08	0.09	0.10

The asterisk indicates that the metal became passive. The anodic over-voltage of nickel is low and fairly constant, and it has similar values in acidic and in alkaline soln. It shows a slight tendency to increase with time. Nickel is a useful metal to replace platinum in cases where a low cathodic over-voltage is required, but it is very difficult to keep the over-voltage down. Roughening the surface mechanically or depositing a thin coating of rough silver on the metal depresses the over-voltage for a time. With a current density of 10 milliamperes per sq. cm. the over-voltage in acid rose from 0.18 to 0.38 volt in ten minutes, and then fell to 0.33 volt after a further twenty minutes. Interrupting the current for a few seconds causes a temporary fall in the over-voltage, but it rapidly rises to a still higher value on renewing the current. In alkali the over-voltage is much more constant, the time effect being almost nil at all current densities, although a small rise is produced by subjecting the metal to the action of a very high current density. The subject was studied by W. D. Harkins and H. S. Adams, M. Knobel, E. Denina and G. Ferrero, T. Onoda, A. Gunther-Schulze, F. Meunier, N. Kobozeff and N. I. Nekrasoff, E. Liebrich and W. Wiederholt, A. Thiel and W. Hammerschmidt, G. Jones and S. M. Christian, and T. Onoda. According to S. Glasstone, the electrodeposition potential and the over-voltage of nickel in N-soln. were respectively -0.57 and 0.33 volt at 15°; -0.43 and 0.19 volt at 55°; and -0.29 and 0.05 volt at 95°. I. Slendyk and P. Herasymenko studied the separation of hydrogen at the nickel cathode; C. Marie and G. Lejeune, the influence of colloids on the over-voltage; G. R. Hood and F. C. Krauskopf, the over-voltage of nickel cathode in connection with the electrolytic reduction of potassium chlorate; and T. Erdey-Gruz and M. Volmer, the effect of occluded hydrogen.

E. Newbery measured the cathodic over-voltage of nickel in N-NSO₄ in the presence of the indicated percentages of gelatin, gum arabic, and dextrin, and found:

			Gelatin		Gum arabic		Dextrin
Current density	Nil	0.1	1.0	0.1	1.0	0.1	1.0
2	0.77	0.82	0.88	0.82	0.83	0.84	0.80
6	0.81	0.87	0.91	0.87	0.87	0.87	0.83
10	0.83	0.87	0.92	0.88	0.88	0.88	0.84
50	0.82	0.86	0.90	0.87	0.87	0.86	0.84
100	0.82	0.86	0.89	0.86	0.87	0.85	0.83
200	0.81	0.85	0.88	0.86	0.86	0.84	0.83
400	0.81	0.84	-	0.85	0.86	0.84	0.83
Increase	-	0.04	0.09	0.05	0.05	0.04	0.02

The over-voltage of nickel in soln. of ammonium nickel sulphate is raised 0.11 volt by the presence of 1.0 per cent, of gum arabic, whilst 0.02 per cent, of gelatin raises the over-voltage by 0.5 volt at low current densities, but depresses it by 0.01 volt at high current densities.

J. N. Pring and J. R. Curzon observed that the over-voltage increases with rise of temp, and is higher with cast and rolled sheet metal than with the electrolytic metal. The over-voltage of nickel was studied by J. W. Richards, A. Thick and E. Bruening, S. Koidzumi, F. P. Bowden and E. K. Rideal, and N. Kobozeff and N. I. Nekrasoff. According to W. J. Muller, active nickel anodes require an over-voltage of 0.4 volt to pass into soln. G. Patoux observed the relation between the current density and polarization of nickel in soln. of nickel sulphate to be:

Current density	0.9	1.3	2.3	3.0	4.4	6.7	7.1	8.6
Polarization	2.27	2.33	2.36	2.38	2.39	2.39	2.39	2.39 volts

Experiments with very conc. and very dil. soln. of the sulphate show that the conc. of the soln. has little influence on the result. A soln. containing a gram of ammonium chloride in 50 c.c. of a soln. of 62 grms. of nickel sulphate per litre, and with a gram of citric or boric acid each in place of ammonium chloride, gave:

NH₄Cl

				Citric acid			Boric acid
Current density	0.6	1.7	4.6	0.8	3.1	6.3	1.2	2.0	3.37
Polarization	0.58	0.61	0.66	1.66	1.66	1.69	1.80	1.83	1.83 volts

D. Reichenstein attributed polarization to a slow reaction in or at passive as well as reversible electrodes due in part to the formation of compounds with the gases set free at the electrodes. U. C. Tainton found that a high current density increases the hydrogen over-voltage at the cathode. R. L. Dorrance and W. C. Gardiner found that the anodic polarization is considerably reduced in the presence of chlorides or bromides. According to F. Forster and F. Kriiger, in the electrodeposition of nickel, the addition of chlorides hinders the passage of the nickel anodes into the passive state - vide swpra, the electrodeposition of nickel. The transitory passivity of nickel anodes is recognized by the irregularity of the potential difference, the current strength, the acidity, the current-yield, the formation of anode slime, and, in some cases, by the formation of a thick layer of nickel peroxide. C. Russo observed that chromous sulphate hinders but does not prevent the passivation of nickel anodes. The subject was discussed by F. Forster and F. Kriiger, G. Grube and H. Metzger, A. Coehn and M. Glaser, E. Vogel, W. G. Ellis, H. E. Haring, A. D. Garrison and J. F. Lilley, and K. Murata. M. Schade observed that if the conc. of the acid is too small, some hydroxide as well as nickel separates out during the electrolysis of soln. of nickel salts. The polarization depends on the kind of nickel salt employed; it is greater in sulphate soln. than it is in chloride soln. The polarization is increased by the presence of H'-ions, and decreased by raising the temp, and conc. of the soln. A. Oberbeck discussed the polarization in soln. containing potassium sulphate, chloride, bromide, and iodide; and M. Krieg, various other salts. According to N. A. Isgarischeff and C. M. Ravikovitsch, the cathode polarization voltage obtained in the electrolytic deposition of nickel from nickel chloride soln. is affected by the addition of various chlorides in the order: CdCl₂ < AlCl₃ < LiCl < CoCl₂ < MgCl₂ < NCl₂ < NaCl < KCl < CaCl₂ < SrCl₂ < NH₄Cl < BaCl₂ < ZnCl₂. The yields of nickel deposited per unit of current are as follow: in the presence of NH₄Cl, 68 per cent.; NaCl, 66 per cent.; LiCl, 64 per cent.; KCl, 60.1 per cent.; MgCl₂, 55.9 per cent.; BaCl₂, 52 per cent.; SrCl₂, 32.4 per cent.; NCl₂, 35.4 per cent.; and CaCl₂, 30.8 per cent, of the theoretical. Of the above cations, only zinc and cadmium are deposited together with nickel on the cathode. The most lustrous deposits are obtained in the presence of lithium, sodium, ammonium, and magnesium, whilst the toughest deposits are given by the addition of the first three of these metals and of cadmium. Evolution of gas at the cathode occurs to any considerable extent only in the presence of alkaline-earth metals. N. A. Isgarischeff and N. Kudrjawzeff studied the effect of an alternating current in the electrolysis of soln. of zinc sulphate. J. W. Shipley and C. F. Goodeve found that in alternating current electrolysis there is a critical density, 4.6 amp. per sq. cm., which must be exceeded before gas is evolved from a soln. of sodium hydroxide. A. P. Rollet discussed the anodic reactions of nickel in acidic and alkaline soln.; and S. Veil, in the presence of gelatin.

According to B. Bruzs, when a current is passed between nickel electrodes in a soln. of a nickel salt the anode is at a higher temp, than the cathode, and the difference is directly proportional to the current density up to a certain value of the latter, at which there is a sudden increase in the temperature difference, attributed to the evolution of oxygen at the anode. If two nickel electrodes through which a small current has been passed for a few minutes are short-circuited, a chemically induced current, in the same direction as the impressed current, is observed when the polarization has been dissipated. This induced current is ascribed to disintegration of the lattice giving rise to enhanced activity of the loosened lattice atoms.

U. Sborgi examined the anodic behaviour of nickel in soln. of sodium chloride in methyl alcohol; U. Sborgi and G. Cappon, in soln. of ammonium and calcium nitrates in ethyl alcohol; U. Sborgi and P. Marchetti, in soln. of lithium chloride and silver nitrate in acetone; and E. Siegler and R. Cernatesco, in quinoline, glycerol, o-toluidine, aniline, nitroethane, propyl alcohol, and water.

J. Nickles observed that the passivity of nickel towards fuming nitric acid is produced by superficially oxidizing the metal by heating it in an alcohol flame. Purified nickel, said H. St. C. Deville, is passive in ordinary conc. nitric acid; and E. St. Edme found that whereas sheet nickel is passive in ordinary nitric acid, sp. gr. 1.4, iron is passive only in the fuming acid. Iron becomes passive slowly, nickel rapidly, but if the two metals are introduced in the acid together, both become passive instantly. Electrolytic nickel deposited from an ammoniacal soln. of the chloride or sulphate becomes passive immediately. Iron loses its passivity when heated in hydrogen, but passive nickel remains passive although it yields a small quantity of ammonia, and acquires a silvery lustre. W. W. Hollis found that above 80°, nickel is no longer passive. H. N. Huntzicker and L. Kahlenberg said that nickel is normally passive, but immediately after cathodic treatment in a soln. of phosphoric acid, it will displace copper and silver from soln. of their salts, and reduce potassium permanganate, ferric chloride, and nitric acid. V. Rothmund observed that nickel becomes passive in sulphuric, perchloric, nitric, acetic, boric, phosphoric, citric, tartaric, oxalic, hydrofluosilicic, thiocyanic, and the halogen acids; F. Eisenkolb, in oxy-acids; and E. S. Hedges observed the periodic passivity of nickel.

According to M. le Blanc and M. G. Levi, at room temp, and with a current density of 0.5 amp. per sq. dm., nickel dissolves quantitatively in halides and cyanide soln., and sulphuric acid; but in soln. of other salts it is passive; above 80°, however, it generally loses its passivity. Nickel is passive in potash-lye even at 80°; and it is partially passive in soln. of ammonium oxalate or sodium acetate. Increasing the current increases the passivity, but changes in the conc. of the soln. have no effect. According to M. G. Levi, if the passivity of a metal depends on the formation of an insoluble coating, it should be removed by the addition of the soln. of another salt, the anion of which forms with the metal a readily soluble salt. The behaviour of nickel in such mixtures of electrolytes indicates that the passivity of the metal in sodium carbonate or potassium hydroxide soln. may be due to the formation of a protective layer. This could, however, not be observed in the other soln. examined, so that here the passivity appears to be due to the smallness of the reaction velocity. A small addition of sodium chloride to a soln. causing passivity brings the velocity of ion-formation up to the value required for the quantitative dissolution of the metal. Sulphuric acid also acts in this way, but to a less extent than sodium chloride. The addition of sugar or acetone is without influence, but carbamide destroys the passivity, although it is uncertain whether its action is a direct one or whether it is due to its decomposition products. According to E. Newbery, nickel becomes passive in dil. sulphuric acid when a current density of 100 milliamperes per sq. cm. is applied. If the current density is lowered to 50 milliamperes, the well-known periodic passivation and activation are observed, being more easily produced with this than with any other metal. The potential falls from about 0.6 volt (referred to a standard oxygen electrode) slowly at first and then very rapidly to a value of about -0.5 volt, immediately rising again to near the highest value. The lower limit is very uncertain, as it is almost impossible to follow the rapid changes of potential with the usual over-voltage apparatus. The period of each " vibration " is usually from 6 to 10 seconds, and the nature is such as to suggest strongly an alternate formation and breakage of a continuous protective film over the surface of the electrode. In alkaline soln. pure nickel passivates at once, even at the lowest current densities. If the nickel contains a small percentage of copper, however, the passivity is only partial, and the nickel is dissolved and deposited in a black, sooty form on the cathode, leaving the copper adhering to what is left of the anode. For this reason, samples of nickel from different manufacturers may show widely differing resistance to disintegration. The anodic over-voltage of nickel is low and fairly constant, and has similar values in acidic and alkaline soln. W. Rathert observed that the abrupt change of the anodic potential which occurs when active nickel becomes passive has not the same value as that which occurs when the passive metal becomes active. F. Kriiger and E. Nahring, W. Liebereich, C. T. Thomas and W. Blum, G. Russo, and K. Georgi discussed the subject. H. Dietrich examined the periodic phenomena in the electrolysis of nickel salts. A. Brochet and J. Petit found that with nickel as soluble anode with an alternating current, in a soln. of potassium cyanide, the metal dissolves quantitatively so long as tlie current density does not exceed 2 amp. per sq. dm.; beyond this, the dissolution decreases, reaching a minimum of 80 per cent, of the theoretical when the current density is 8 amp. per sq. dm. The subject was studied by 0. Sackur, and M. le Blanc, G. C. Schmidt, W. J. Muller and co-workers, E. Muller and F. Spitzer, C. W. Bennett and W. S. Burnham, F. Eisenkolb, A. D. Garrison and J. F. Lilly, D. Reichenstein, K. Murata, N. Isgari- scheff, A. Adler, H. Eggert, E. Becker and H. Hilberg, 0. Grube, W. Hittorf, A. S. Russell, E. Liebreich, T. Onoda, V. Rothmund, E. Grave, E. P. Schoch, E. Baumann, etc. - vide the passivity of iron. W. J. Muller and W. Machu studied the time of passivation of nickel - vide iron.