An electrocatalyst is a catalyst that participates in electrochemical reactions. Electrocatalysts are a specific form of catalysts that function at electrode surfaces or, most commonly, may be the electrode surface itself. An electrocatalyst can be heterogeneous such as a platinized electrode. Homogeneous electrocatalysts, which are soluble, assists in transferring electrons between the electrode and reactants, and/or facilitates an intermediate chemical transformation described by an overall half reaction. Major challenges in electrocatalysts focus on fuel cells.