Novel systems for combinatorially screening an array of electrochemical cells are disclosed. The screening aparatus may comprise a system of electrodes and electrolytes common to each of the members of an electrochemical cell array. An electrochemical probe is used to supply a reactive fluid (or reactive species) to only one of the cells in the array at a time. The probe is configured to address each of the cells of the array individually, and to be chemically isolated from all of the other cells of the array. Thus, the cells of the array that are not being addressed by the probe are in a non-active condition. Such a system is capable of combinatorially screening a library of materials, including catalyst, electrode, and electrolyte materials, wherein each of the members of the library defines one of the cells of the array.