Hi, I have a question about electrodeposition. After determining the reduction potential of ion X using cyclic voltammetry (let's say -0.4 V), I plan to apply a potential around that value to deposit metal X from an ion X solution. I notice that the sign of the current can be negative or positive depending on the value of the potential I apply (for example, -0.6 V and -0.8 V). What does the sign of the current indicate, and does it need to be positive or negative for successful electrodeposition? Additionally, how does the current profile look after a period of deposition-does it decrease, increase, or remain constant over time?
The sign of the current will depend on whether you are using the US or IUPAC convention for plotting a cyclic voltammogram. If you are using the IUPAC convention then positive current refers to an oxidation reaction and negative current refers to a reduction reaction. It is very strange that you would observe a different sign in the current if you applied a -0.6 vs -0.8 V potential for electrodeposition. If you are using the IUPAC convention you should observe a negative current associated with the reduction of your ion X. Unless there is another process occurring at those potentials that induces an oxidation reaction, although that seems very unlikely. If you hold the potential and measure the current for the electrodeposition, you should see (Again IUPAC convention) and sharp negative spike followed by a gradual increase in the current. The current should still be negative, but less negative over time, meaning that the current increases slowly over the course of the deposition. I hope this helps.
Thank you so much for your answer. Could you explain me more why, in the oxidation process, the sign of the current is positive, while in the reduction process, the sign of the current is negative? Is it always true that if we apply any potential more negative than the reduction potential of a metal ion, the ion will be reduced to form a metal?
@@Alex-t98 To reiterate the previous point, there is no real "reason" why oxidation current is positive. This is simply convention in the IUPAC system. In fact, in a very fundamental sense, there is no such thing as + or - current. Think about what current physically is: simply put, the flow of electrons (or more broadly, the flow or movement of charge over time). The unit of current is Ampere, which is equivalent to Coulombs per second, or the flow of charge over time. The sign simply indicates which direction it is flowing, nothing more. If you have 2 electrodes, the flow of current can either be from electrode A to electrode B, or vice versa. So we arbitrarily assign + to one of those directions, and - to the other. That is quite literally all it is. The other thing to recognize in relation to a real electrochemistry experiment using a potentiostat is that the current you observe is the *NET* current. As noted in the previous answer, depending on your applied potential and its relation to any number of redox processes that can occur in your experiment, you will get current indicating some oxidation (+) or reduction (-) taking place. But let's say you have some reduction occurring (like the deposition of X) as well as simultaneously some other oxidation happening too. The current the potentiostat reports to you will be the aggregate, or net, current from both those processes. Now, in many cases, you try to design your experiment such that there is only 1 main reaction that can take place, like say the deposition of X. So you can presume all or most of your current is arising from that process and that process alone. But this is not always the case, since if you had -100 mA from X deposition, but also +30 mA from some auxiliary oxidation reaction for instance, your potentiostat would report to you -70 mA.
@@Pineresearch I also observed the trend of current as you said but still dont really know the reason why the current spikes, then gradually increases and becomes less negative overtime. Please help me clarify it
@@raven-tn I am not completely certain what you are referring to - are you running a chronoamperometry experiment at a set potential and observing a negative current? And the current you say starts at some large negative number, then goes towards zero over time? It seems like you are describing where it looks like a current decay but it's "upside-down" so to speak. If this is accurate, it is just the phenomenon described by the Cottrell equation, which defines the current behavior over time due to diffusional effects. This is most commonly applied to a chronoamperometry experiment under normal conditions, and the fact that the current is negative simply goes back to my previous comment explaining that it is just convention describing a reduction process.
