Applied electrochemistry

AUTOCATALYTIC DEPOSITION

It is used to deposit a metallic coating or powder in an aqueous or non-aqueous solution; the reducing agent

is a species intentionally added into the bath. A catalyst is required for the deposition process, and it is

typically a metal: even in this case the reaction is of a RedOx type between metal salts in the solution and

reducing agents. Bath instability may occur when reduction takes place in the bath itself instead of on the

surface of the component: this can be avoided increasing the reducing agents’ concentration or increasing

the temperature. A filter is required to allow the purification of the bath from the impurities. In particular, in

case of Ni/P, byproducts called ortophosphites help the deposition process, but only up to a critical

concentration: in this phase, called turnover, the bath doesn’t work properly anymore. So an initial transient

is required for the production of these byproducts.

Comparing electroless plating and electrodeposition, the following table of pros and cons can be obtained.

The main advantage of the first one is the possibility of coating non-conductive substrates, but it is also a

more complex technique which requires lots of additives, and so a higher cost.

The surface of the component must be activated (if it is not autocatalytic) and cleaned to remove the oxides

and increase the roughness. This allows to increase the most the adhesion through mechanical interlocking.

If the substrate is metallic, it can be activated by applying a palladium film (clusters), by using a momentary

cathodic current, by immersing it in a reducer solution or by putting it in contact with steel, nickel or

aluminium.

We have been talking about this process even if there is not a circulation of current because it can still be

considered an electrochemical process which can be described with the same considerations done before

(like the characteristic curve I vs V for the reducing agent and the ionic species).

So autocatalytic deposition requires: 30

- An activated surface, which must be cleaned, degreased, etched, rinsed and neutralised to increase

the adhesion with the deposited layer, increasing the roughness and inducing mechanical

interlocking.

- A source of metal.

- A reducing agent, which must be chosen according to the materials involved in the process. It may

be codeposited on the substrate, which could be exploited for the deposition of alloys (as for Ni/P).

- Additives like accelerants to increase the kinetics, or stabilizers like complexing agents, buffering

agents (constant pH) or inhibitors (to deactivate dusts and powders which may act as nucleation sites

for the deposition) which guarantee the stability of the bath in time.

- A temperature higher than the one of electrodeposition again to fasten the kinetics

BARREL PLATING

Nickel balls are mixed with the components in a barrel immersed in the solution, acting as catalytic material:

a micro-galvanic cell forms at the point of contact between the component and the metallic balls inducing a

uniform deposition. A continuous line is used to allow a mass production and it is exploited for small parts.

Also, in PCB production an electroless deposition step called ENIG takes place. It allows to produce a NiP

layer, followed by an Au one, on copper. So there is an autocatalytic deposition of NiP (medium phosphorus

content to increase the solderability of the components) followed by a galvanic immersion gold deposition.

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Ni, NiP AND NiP-COMPOSITE

Electroless Ni plating allows to plate an alloy of nickel and phosphorus due to the presence of sodium

hypophosphite acting as a reducing agent together with precursor nickel ions. By tuning the amount of

phosphorus co-deposited, it is possible to tailor the properties of the coating. An alloy is desired because it

provides better properties with respect to the pure metal, like a higher corrosion protection.

Depending on the content of phosphorus in the alloy, three categories can be distinguished:

- Low phosphorus alloys contain 2-4% in weight of P and they are characterized by high hardness, high

oxidation resistance (better solderability), low cost (with respect to high phosphorous) but lower

corrosion resistance. It is a nano-crystalline material.

- Medium phosphorus coatings (6-9% in weight) have worse properties with respect to low P ones, but

they are preferred because they are characterized by an amorphous structure, so a high corrosion

resistance.

- High phosphorus alloys (>10%) are completely amorphous, non-magnetic and display a high

resistance to temperature and corrosion.

Below 20% of phosphorus, we deposit a metastable structure (especially above 7%, that is for amorphous

structures) below the eutectic. It is therefore possible to apply a thermal treatment above the eutectic (one

hour at 400°C) generating two phases: nickel (so a magnetic one) and intermetallic compounds (high thermal

stability and hardness). The thermal treatment is exploited to improve the mechanical properties (hardness)

of the coating. 32

The amount of phosphorus can be tuned with the composition of the formulation, pH and temperature: the

acidic formulations are more common, while the alkaline ones are used only for substrate non-compatible

with the low pH. Electroless plating allows to reach a uniform thickness of the Ni-P alloy on the component,

independently on its shape, which is not possible with electroplating due to the distribution of the current

lines.

The substrate may be self-activated or may need an initiator, which may be a catalyst, galvanic displacement

or a current pulse (for conductive substrates). Clearly, both conductive and non-conductive substrates can

be exploited. Filters are very important to reduce the presence of contaminants.

Even composites can be co-deposited with the metallic layer through electroless plating: solid lubricants can

be embedded in the coating like particles (PTFE), or hardness may be increased by using ceramics. The

dimension of the particles can be from 100 nm to the order of micrometres and the amount of co-deposited

particles can be tuned by controlling their concentration in the bath. Too small particles are avoided for the

safety of the operators during handling. Other functionalities may be provided, like photoactivity (by

depositing titanium oxide, which is a photocatalyst).

ENERGY STORAGE SYSTEMS

Energy storage systems can be divided into two classes, batteries and capacitors. These devices can be

connected to an external component using two poles (electrodes) and they are able to store energy. Some

batteries are not rechargeable as they exploit internal reactions to generate energy.

The distinction between capacitors and batteries can be seen graphically on the Ragone plot, which compares

the specific power and the specific energy: the first ones are characterized by high values of power and

relatively low specific energy, so they can provide instantaneously high power, but for a short time (limited

total energy). Batteries on the other hand can provide a lower specific power, but for a much longer time.

They are usually combined. 33

The trend nowadays is to increase the specific energy of batteries in order to store a higher energy in smaller

devices (miniaturization).

Fuel cells are devices based on electrochemical processes having very small values of specific power, but high

specific energy. They are not considered storage devices because they exploit precursor gases to generate

energy, so they are considered power generators.

BATTERIES

They are electrochemical devices able to power an external electrical device, so it is able to store energy and

induce the circulation of current through a spontaneous electrochemical reaction. The Daniel cell and the

Volta pile (water with sodium chloride as electrolyte) are two examples of first battery. Later lead-acid cells

were created, which were the first example of rechargeable device, followed by many other devices. The

most recent devices are based on lithium and lithium ions.

A distinction in batteries can be made between primary and secondary devices:

- Primary batteries cannot be recharged, so they can be used only once. In fact, the potential to

introduce for the recharge may be too high, so not sustainable, or maybe the reactions are not

reversible (as for lithium batteries). This is true for example for lithium batteries, in which Li is

oxidised and the reverse reaction is not possible.

- Secondary batteries are rechargeable many times, and this is the case of lead-acid batteries in cars

and of lithium ions batteries, in which Li is not present in the metallic form, but as ion (no need to

reduce Li to the zero-valent state). Therefore, no oxidation of the metal occurs. They are also safer

than metallic lithium batteries, but safety problems may be present due to the liquid electrolyte

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(therefore we are moving towards solid electrolytes). Typically, they are discharged once produced,

so they must be charged prior using them. They can also work as accumulators because they are able

to store energy through reversible reactions.

Depending on the type of battery produced, it is necessary to accurately choose the adequate type of

material, as we can see from the example of lithium given above.

The batteries can have different shapes: the most common is the cylindrical one, but also coin cells (typically

non rechargeable) are available as well as prismatic ones (containing many cells in series or in parallel). A

different shape implies a different amount of active material and a completely different production process.

To implement the performances, it is possible to put the batteries in series (to increase the voltage) or in

parallel (to collect more current). In this case, the losses related to junctions can become extremely

important. A plot showing the cell voltage is reported on the left

(charging below, discharge above): the over voltages

must be included, and even the junction term (above all

for multi-cell configurations). The trend is to increase the

erogated potential, so to minimise the losses

(overpotential and so on).

To improve the performances, it is important to

minimise the losses, that is the resistances, in order to

maximise the cell voltage.

Below there is the discharging curve (the charging one is from low to high voltages): at the beginning, there

is a huge drop due to the losses. The central area is a sort of plateau which should be as prolonged in time as

much possible, as it represents the nominal voltage of the battery, that is the voltage at which it will operate.

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Another important value is the cut-off voltage, which is slightly lower than the nominal one and it represents

the lower working limit of the battery (outside the lab, it can only work between the nominal and the cut-off

values). Below this voltage, the system will indicate that the device is completely discharged to avoid any

degradation (irreversible reactions not recovered during charge) of the battery even if a residual energy is

present.

The second plot shows how the curve changes depending on how the battery is used, that is, at different C-

r