Another popular concept is "electric current-potential relationship", a standard concept used when referring to the voltammetry measurements of the cell or half-cell performance utilized in performance evaluation, as can be seen to its link with the "Impedance" cluster that includes impedance, current density, conductivity, etc. Out of the individual fuel cell components, the data shows that although "Fuel Cell electrodes" is a relatively popular topic, higher interest in "Fuel Cell Cathodes" exists. We suspect this is due to research efforts directed to the Oxygen Reduction Reaction (ORR), which occurs in the cathode of most fuel cells, being one of the major challenges when it comes to reducing cost. One can also see the interest in tackling this challenge with other concepts, particularly "electrochemical reaction catalysts" and "electrochemical reduction" cluster. Similar to water splitting, the inclusion of concepts such as "nanoparticles" (also reflected on Table ) and "Surface area" in this blue cluster confirms morphology and surface research as important parameters when optimizing catalysts. This is also supported by the key role of carbon and graphene in hydrogen fuel cell research (Table .)