What is the relationship between electrodes and ions in a voltaic cell?

Question

From what I understand, a voltaic cell involves two metals in two solutions. The metal and solution picked requires for the metal and the solution to interact. Atoms from the anode lose electrons and become cations, and cations from the cathode gain electrons and become solid metal atoms (I know I'm generalizing).

This explanation seems as though it requires for the electrodes and their respective solutions to be of the same initial atom, e.g. a zinc anode in a Zn2+ solution, and a copper cathode in a Cu2+ solution. This seems to be the only thing that makes sense.

However, in my homework there are various problems asking for a drawing of a "cobalt-hydrogen" standard cell. This seems ridiculous because there can't be a hydrogen electrode; hydrogen isn't a metal. I looked at the solution, and apparently the correct cathode to use is platinum. This seems completely arbitrary to me, what does platinum have to do with hydrogen? Virtually nothing, it's just some random metal that doesn't even appear on any electrode potential tables.

Also, how would this work? How is it possible for hydrogen ions to gain electrons, become hydrogen atoms, and then join platinum solid? That doesn't make any sense at all. Hydrogen is a gas, and even if it wasn't, there's no way it can become a platinum atom, of all things.

There are more examples than this: there are problems involving a tin(IV)-zinc standard cell (tin(IV) isn't a metal as it is written), a chromium (III)-hydrogen standard cell (both electrodes don't make sense in this case), and a slightly different problem which already writes the cell notation: Pt(s) | Fe2+(aq) || MnO2(aq), HCl (aq) | C(s), and I have no idea how MnO2 and H+ would react with carbon to form a solid.

Maybe an "X-Y" cell doesn't mean what I think it means, and X and Y don't have to be metals. In any case, I need these clarifications to be cleared up, with emphasis on the question: how is it possible for an electrode to be placed in a solution of ions that appear to have no relation to the electrode?

Answer 1

Hydrogen is not a metal, but it can make an electrode like a metal, which produces or absorbs electrons. A hydrogen electrode is obtained with a platinum plate which is covered by platinum sponge, and dipped in an acidic solution. The platinum metal has no chemical effect. It is used as an electron carrier. It has been chosen because platinum does not interfere with aqueous solutions. Any other metal may transmit electrons to the cathode. But it may react and produce its own ions that interfere with the desired reaction : $\ce{H2 <--> 2 H^+ + 2 e^-}$. Platinum has not this drawback.

Introducing now a glass tube under the platinum sponge, hydrogen can bubble in the solution. Of course, most of $\ce{H2}$ will go up the solution and burst at its surface. But a small proportion will be absorbed by the sponge. This will make a hydrogen electrode : $\ce{H2}$ can lose electrons to be sent into the platinum electrode, and the $\ce{H+}$ ions will go into solution. Here $\ce{H2}$ behaves exactly like a metal : it produces positive ions and electrons. And it can also act reversibly.

Answer 2

However, in my homework there are various problems asking for a drawing of a "cobalt-hydrogen" standard cell. This seems ridiculous because there can't be a hydrogen electrode; hydrogen isn't a metal. I looked at the solution, and apparently the correct cathode to use is platinum. This seems completely arbitrary to me, what does platinum have to do with hydrogen? Virtually nothing, it's just some random metal that doesn't even appear on any electrode potential tables.

In electrochemistry, electrode can be anything as long it can participate in electron transfer reactions. Later in your classes you will find glass electrodes! Yes, glass is a non-conductor but a very very thin glass can still serve as an electrode. The working of the glass electrode confused scientists for 100 years. These are very special cases.

The gas electrode is another strange case but not a ridiculous one. It is an interesting case. As you pointed out gases are not conductors of electricity under ordinary conditions, but it turns out that many gases when bubbled over platinum electrode coated with some platinum black (oxide), they start to participate in electrochemical reaction as if the electrode were made of the gas itself.

If I remember correctly, gases adsorb on the surface and participate in electrochemical reduction or oxidation. In electrochemistry, sometimes there are observations and but no explanation exists, so don't be surprised by the mysteries and wonders of this field.

In future you might encounter semi-conductor electrodes and some fancy experiments have been done with Teflon "electrodes".