Half reactions are bookkeeping devices to keep track of oxidation and reduction components of a system. The oxidation counterpart, however, does not happen independently of the reduction component. In a real electrochemical cell, there is usually a salt bridge between the anode side and the cathode side that balances the production of cations (or anions) in the respective cell solutions. As for whether electrons built up in an electrode - no, not generally. In a complete cell, the electrons that are retained in one electrode due to electrode oxidation flow to the other electrode via a wire, which is what produces current. At the other electrode, these electrons perform more chemistry (reduction of cations in solution), which completes the circuit. When the cell is depleted, equilibrium is reached and the cell no longer has enough potential energy to generate current flow (here's where you Gibb energy equations from the other thread come into play). At that point the cell is dead. However it can be recharged by applying an electric potential in the other direction, which causes electrons to flow in reverse. This is only spontaneous with the application of the external potential. Once a new non-equilibrium situation is produced, the external potential is removed and the flow of current in the battery is spontaneous again. In principle this process can be repeated indefinitely, but certain irreversible reactions at the electrode surfaces reduce the efficiency of the battery after each recharge, which means that batteries do have a limit to the number of times they can be recharged.