text: The interest in reducing as much as possible the costs of the catalyst for electrochemical processes led to the use of fine catalyst powders since the specific surface area increases as the average particle size decreases. For instance, most common PEM fuel cells and electrolyzers design is based on a polymeric membrane charged in platinum nanoparticles as an electrocatalyst (the so-called platinum black).
  Although the surface area to volume ratio is commonly considered to be the main parameter relating electrocatalyst size with its activity, to understand the particle-size effect, several more phenomena need to be taken into account:
  * Equilibrium shape: for any given size of a nanoparticle there is an equilibrium shape which exactly determines its crystal planes
  * Reaction sites relative number: a given size for a nanoparticle corresponds to a certain number of surface atoms and only some of them host a reaction site
  * Electronic structure: below a certain size, the work function of a nanoparticle changes and its band structure fades away
  * Defects: the crystal lattice of a small nanoparticle is perfect; thus, reactions enhanced by defects as reaction sites get slowed down as the particle size decreases
  * Stability: small nanoparticles have the tendency to lose mass due to the diffusion of their atoms towards bigger particles, according to the Ostwald ripening phenomenon
  * Capping agents: in order to stabilize nanoparticles it is necessary a capping layer, therefore part of their surface is unavailable for reactants
  * Support: nanoparticles are often fixed onto a support in order to stay in place, therefore part of their surface is unavailable for reactants