Chemistry Reference
In-Depth Information
have a limited solubility in water; therefore they form oxygen and hydro-
gen gas bubbles at the surface of the anode and the yarn, resulting in an
increase of the specific resistance of the system. In addition, reduction of
water to hydrogen consumes hydrogen ions in the vicinity of the cathodes;
therefore the pH increases locally at that level causing precipitation of
Ni(II) as Ni(OH)
2
at the surface of the cable and cathodes. This effect also
increases the specific resistance of the cables because of deposition of a non-
conductive Ni(OH)
2
layer. The presence of this compound at the surface of
the fibres is confirmed by Raman spectroscopy, and this is only when a
maximum is observed in the current-potential curves. This also confirms the
fact that at lower applied potentials, the decomposition of water does not
occur.
Minor differences between the three electrolyte solutions are also
observed. First, electrolyte number 3 only shows a peak maximum in the
current-potential curves at potentials higher than 8 V. However, this is very
clear because its pH value is smaller, indicating that this electrolyte solu-
tion possesses a higher buffer capacity against consumption of hydrogen
ions in the vicinity of the fibre surface, avoiding hydrogen gas formation
and Ni(OH)
2
precipitation. Secondly, at a potential of 4 V, no deposition
occurred in electrolyte solution number 3, indicated by the absence of an
increase in the measured electrical current and confirmed by XPS data.
Additionally in this case, the lower pH plays an important role; because of
the lower pH value, the applied potential difference does not overlap with
the potential window in which the reduction of Ni(II) occurs. Therefore no
deposition is observed.
In an industrial galvanisation line, it is not the potential difference
applied between anode and cathode that counts but the current that is mea-
sured, because this current reveals information about how much of the com-
pound is deposited. Therefore in industry, a constant electrical current is
applied instead of a potential difference. However, one has to take into
account the fact that the applied electrical current does not cause high
potential differences, as otherwise the unfavourable effects of increase of
specific electrical resistance of the PAN fibres will appear. This was verified
in an additional experiment, and it was found that a current density of about
0.025 A dm
-2
could be applied at the first cathodic roller (1 in Fig. 11.5). This
does not seem particularly large, but can be explained by the relatively high
resistance of the fibres at the initial stage of the process. At the second
cathodic roller (2 in Fig. 11.5), a current density of 0.5 A dm
-2
could be
applied, which is much larger because at this roller the electrical resistance
of the cables should be taken into account, resulting in a large ohmic drop.
Finally, experiments were performed using current densities of
0.025 A dm
-2
and 0.2 A dm
-2
applied at the first (1 in Fig. 11.5) and second
(2 in Fig. 11.5) cathodic rollers respectively.
Galvanisation time was
