The following description is provided to assist the understanding of the reader. None of the information provided or references cited is admitted to be prior art to the present technology.
The development of new materials that can solve the challenging problems of clean energy production, conversion and storage is of paramount importance in the quest for alternatives to fossil fuel use. One promising candidate is a fuel cell, a device that converts chemical energy directly into electrical energy. In a polymer electrolyte membrane fuel cell (PEMFC), an alkaline fuel cell (AFC), or a phosphoric acid fuel cell (PAFC), the main fuel is hydrogen, which, when reacted with oxygen, produces water as the only reaction product. However, to make hydrogen-based energy systems viable on a large scale, many problems still need to be resolved. These are mainly connected with new catalyst materials focusing primarily on three characteristics: activity, stability and selectivity. Improvement of these features presents the major roadblock to a wide commercialization of fuel cells.
Presently the state of the art approach for changing these properties undoubtedly entails changing the electronic properties of the catalyst in some way, shape or form. This approach rests on the premises that changing the catalyst's electronic structure will (i) change the adsorption free energy of reactants and products thus increasing the activity for a desired reaction, (ii) change the stability of catalyst by making the metal (or other active material) less soluble in relatively aggressive electrolytes and (iii) only effect activity for one reaction at the catalyst's surface. The possible beneficial effect of this approach has been extensively supported and advertised by theoretical work.
In the recent past, it has been shown many times that for platinum group and platinum based catalyst, the activity is determined by the solution side rather than the metal side of the catalyst. The term spectator species has been introduced for molecules, which come from the supporting electrolyte and essentially block the surface sites so that they are unavailable for the electrochemical reaction. These species do not alter the electronic properties of the surface nor do they participate in the reaction, hence they are spectators. In general these species greatly influence all three characteristics of the catalyst. By introducing the concept of chemically modified electrodes (CME) it is possible to enhance catalyst's activity, stability and selectivity without changing its electronic properties.