Concentration cells can be electrode concentration cells or electrolyte concentration cells.

Electrolyte Concentration cell- In this type of cell electrodes in both the half cells are made up of same substances and electrolyte is the solution of same substances but with different concentration.

Electrode Concentration cell- In this type of cell two electrodes of same substance but with different concentration are dipped in same solution.

working:

A concentration cell generates electricity from the reduction in thethermodynamic free energyof the electrochemical system as the difference in the chemical concentrations in the two half-cells is reduced. The same reaction occurs in the half-cells but in opposite directions, increasing the lower and decreasing the higher concentration. The energy is generated from thermal energy that the cell absorbs as heat, as the electricity flows. This generation of electricity from ambientthermal energy, without atemperature gradient, is possible because the convergence of the chemical concentrations in the two half-cells increases entropy, and this increase more than compensates for the entropy decrease when heat is converted into electrical energy.