Metal (1): Electrolysis

Well, since chemistry is nothing than electron transferring, electrolysis can act as both the strongest oxidant and reductant, but if controlled it can help us obtain high-quality crystals or something nearly impossible from normal chemical reactions. Since almost all of these were performed long long ago, we can only try our best to recall.

Of course, having positive oxidation state, metals can only be deposited on the cathode.

Na

Being one of the most reactive metals, it reacts very violently with water and many weaker acids like NH3, so obtaining from aqueous solution is surely impossible.

It's very soft so the shape does not matter.

NaOH(l) - stainless steel anode

The industrial way, but our equipment was very very terrible.

However, when we do so the produced Na violently exploded. Too dangerous!

NaCl - FeCl3 - Pt anode - propylene carbonate(PC) solvent

FeCl3 reacts with NaCl to form soluble Na[FeCl4], while PC is a good polar aprotic solvent.

Solution was nearly opaque and much FeCl3 remained there. We had no way to exclude air or water.

Something less dense than solution was formed on the cathode. Current was very small. The produced stuff reacted violently with water(even exploded sometimes) and even trace water in paraffin oil, so it cannot be molten in paraffin oil. Also it seems that something like Fe is also in it. Chlorine produced on the cathode strongly corroded both the metal cathode and the wire connecting power source and electrodes.

Clearly solubility is still too low.

Cu

Cu comes after H in the activity series, so obtaining it should be of no problem. In fact, many problems can ruin your process completely.

Cu obtained can be protected with some kits that are sold in online shops, though I sometimes fail to do so at that time. If a crystal is oxidized, it would never become bright again even if it's washed with acid.

Cathode should be a Cu rod or something, though anything would be covered with Cu.

Device should not be moved during electrolysis, of course. Although most people consider current variation to be fatal, we did not observe any visible difference.

CuSO4 - Cu anode

This is the first method you can think of, as the amount of Cu2+ reduced is exactly the same as Cu oxidized, so composition of solution remain intact.

Species of Cu: [Cu(H2O)6]2+, Cu2(OAc)4(H2O)2(if AcOH is added)

Current must be kept at a very low value, 20mA or lower, for example by adding resistors, otherwise an extremely fragile tree or even hydrogen would be formed.

It's said that a "wrong" concentration can result in microcrystals, but this is not the biggest problem.

Oxygen in the solution can oxidize Cu into CuO even if it's the cathode and thus kept reduced. As a result its crystal structure is destroyed and no visible crystal grain can be observed on the huge "crystal". Paraffin oil cannot prevent this as even trace of oxygen causes serious problems. Acidification can solve this by turning any CuO into soluble Cu2+, but since we have no H2SO4 we used AcOH, which solved this problem a little and may turn Cu2+ into neutral complex, but something red, probably Cu2O, is sometimes observed. Although at such condition Cu2O should be unstable, even a trace can destroy the crystals, so the result is still very terrible. The effect of H2SO4 might be good, see below.

CuSO4 - Pb anode

This is just a process for H2SO4 preparation. Pb surface is quickly oxidized into PbO2 which is the real anode.

Species of Cu: [Cu(H2O)6]2+

Since no Cu is dissolved on the anode(instead O2 is evolved), Cu is gradually depleted and pH falls very quickly. This is now a CuSO4-H2SO4 solution. Even if the current is extremely high(no control measures were made and solution became very hot), at some point we still observed shiny crystal grains of at least 1mm on the cathode. However, of course, as further Cu depletion occurred, hydrogen started to form, the cathode surface became a sponge and we did not collect the crystals. All in all, from an extremely acidic, or probably hot and unsaturated solution, very large, chunky Cu crystals might be grown and this could be tested in the near future.

CuSO4 - NaCl - Cu anode

Changing ligand would change many things...

Species of Cu: [CuCl2]-

During my experiment, I added NaCl to about 4x volume(since no balance was available) of CuSO4.5H2O, and added some AcOH, then added water to dissolve all. NaCl was nearly saturated. Solution was green due to presence of [CuCl4]2-. This was covered with paraffin oil(since Cu(I) is highly air-sensitive) and electrolyzed. Solution quickly became dark brown, then gradually colorless, now it's ready.

Current can be as high as 200mA as long as the surface of cathode is already ready(by electrolyzing at low current). Sword-shaped, extremely large crystals can be obtained. After a few days I turned off the power and harvested the large crystals with chopsticks, then continued. This is really amazing!

At low current like 15mA, after a few months very well-formed, dense, chunky crystal up to ~5cm can be made. Few intervention is needed, mainly to adjust anode position, and power cut is of course needed during this.

Zn

Zn comes far before H and even Fe, but the high overpotential for H2 evolution makes electrodeposition possible in water.

ZnSO4 - Zn anode

Species of Zn: [Zn(H2O)6]2+, [ZnCl4]2-(if chloride is added) and so on

Following the same principles as the Cu one, we tried this. However even at very low current the Zn is still spongy, and bubbles are constantly formed on it. Addition of acid or base is obviously forbidden, and addition of NaCl does almost nothing.

Due to the production of Zn(OH)2 precipitation, we concluded that Zn reacted with H+ from Zn2+ hydrolysis. In fact, according to the Pourbaix diagram, Zn reacts with water at any pH, but especially so at low or high pH.

Interestingly, when some I2 is added, the evolution of H2 almost completely ceased, and the deposited Zn became a dark node. Maybe I2 competed with H+, or quenched any formed H. radical.

It's said that using NH3 as ligand Zn crystals can be grown at pH=8.0, and changing solvent might be another way.