An electrocatalyst is a catalyst that participates in electrochemical reactions. Catalyst materials modify and increase the rate of chemical reactions without being consumed in the process. Electrocatalysts are a specific form of catalysts that function at electrode surfaces or may be the electrode surface itself. An electrocatalyst can be heterogeneous such as a platinum surface or nanoparticles, or homogeneous like a coordination complex or enzyme. The electrocatalyst assists in transferring electrons between the electrode and reactants, and/or facilitates an intermediate chemical transformation described by an overall half-reaction.
There are multiple ways for many transformations to occur. For example, hydrogen and oxygen can be combined to form water through a free-radical mechanism commonly referred to as combustion. Useful energy can be obtained from the thermal heat of this reaction through an internal combustion engine with an upper efficiency of 60% (for compression ratio of 10 and specific heat ratio of 1.4) based on the Otto thermodynamic cycle. It is also possible to combine the hydrogen and oxygen through redox mechanism as in the case of a fuel cell. In this process, the reaction is broken into two half-reactions which occur at separate electrodes. In this situation the reactant's energy is directly converted to electricity.
|H H2(g) 2H+ + 2e−||0 ≡ 0|
|O2(g) + 4 H+ + 4 e− 2 H2O||+1.23|
This process is not governed by the same thermodynamic cycles as combustion engines, it is governed by the total energy available to do work as described by the Gibbs free energy. In the case of this reaction, that limit is 83% efficient at 298K. This half-reaction pair and many others don't achieve their theoretical limit in practical application due to lack of an effective electrocatalyst.
One of the greatest drawbacks to galvanic cells, like fuel cells and various forms of electrolytic cells, is that they can suffer from high activation barriers. The energy diverted to overcome these activation barriers is transformed into heat. In most exothermic combustion reactions this heat would simply propagate the reaction catalytically. In a redox reaction, this heat is a useless byproduct lost to the system. The extra energy required to overcome kinetic barriers is usually described in terms of low faradaic efficiency and high overpotentials. In the example above, each of the two electrodes and its associated half-cell would require its own specialized electrocatalyst.
Half-reactions involving multiple steps, multiple electron transfers, and the evolution or consumption of gases in their overall chemical transformations, will often have considerable kinetic barriers. Furthermore, there is often more than one possible reaction at the surface of an electrode. For example, during the electrolysis of water, the anode can oxidize water through a two electron process to hydrogen peroxide or a four electron process to oxygen. The presence of an electrocatalyst could facilitate either of the reaction pathways.
Like other catalysts, an electrocatalyst lowers the activation energy for a reaction without altering the reaction equilibrium. Electrocatalysts go a step further than other catalysts by lowering the excess energy consumed by a redox reaction's activation barriers.
Ethanol-powered fuel cells
An electrocatalyst of platinum and rhodium on carbon backed tin-dioxide nanoparticles can break carbon bonds at room temperature with only carbon dioxide as a by-product, so that ethanol can be oxidized into the necessary hydrogen ions and electrons required to create electricity.
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