Periodic current reversal
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Periodic current reversal (PCR) is a current modulation technique that can be used in various electrochemical processes. In the most basic sense PCR simply means that the electrical current applied to the cell is reversed periodically - but unlike pure AC the average (nett) current is still biased in one direction (otherwise there is no net electrochemical action).
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[edit] Equipment
The earliest (and simplest) PCR arrangements simply use an offset AC (sine wave) voltage where the potential will reverse for a time each cycle (e.g. a half wave rectifier with leaky diode). Modern PCR equipment uses electronic switching to do the reversal allowing a square wave form with precise control of the timing and currents for forward and reverse currents. Often PCR cycles will incorporate zero current phases as well (this could be seen as a variation on pulse charging adding the short reversals during rest periods).
[edit] Effects
PCR has been found to improve the physical properties of electrolytically deposited metals (the reverse current preferentially removes dendrites) and may also reduce polarization effects (can improve purity of product and cell efficiency). The reverse conduction may be quite small compared to the forward - for instance a typical modern PCR battery changer might only reverse the current one or a few few times a second for a few milliseconds at a current between 10% to 300% of the forward current but because of the short time the reverse charge conduction is from no more than 2% of the forward.
[edit] Uses
PCR techniques have been researched and/or used in electrowinning, electroplating and battery changers
