Electrocatalysis for energy: from nanostructured to molecular approach

Hydrogen has the largest energy density of all fuels and is considered the more suitable energy source (properly H2 is an energy vector) for matching a clean and carbon neutral future energetic scenario. This rather old technology was theorized almost 50 years ago but still doesn't have a widespread application due to severe limitations. The high cost and the poor sustainability for large scale application of electrochemical devices for hydrogen production and conversion to electricity are the main limitations. In fact, proton exchange membrane electrolyzers (PEMs) and fuel cells (PEMFCs) employ catalysts based on high amounts of rare noble metals, such as Pt, Ir, Ru and Pd. In addition, proton exchange membranes, such as the DuPont Nafion®, are very expensive materials. The reduction of precious metal loadings to negligible amounts keeping constant catalyst activity is a possible route for making fuel cells and electrolyzers sustainable devices. Traditional electrocatalysts are based on metal nanoparticles dispersed on conductive supports where only the particles surface atoms are involved in electrocatalysis. Replacing nanoparticles with metal complexes is a way for making accessible each metal center of the catalyst. A molecular catalyst offers other advantages with respect to nanosized materials, such as control of the selectivity of the oxidation reaction occurring in direct fuel cells fed with liquid and renewable fuels such as alcohols and formic acid. So direct fuel cells can convert a biomass-derived fuel not only into electricity but also into high purity chemicals. A second route to make fuel cells and electrolyzers sustainable devices is the replacement of proton exchange membranes with anion exchange membranes (AEMs) because in alkaline environment several nanostructured catalysts based on cheap metals can be used (in acidic environment most of the transition metals would be subject to corrosion phenomena). Thanks to the development over the last few years of high efficiency and stable alkaline membranes, we have developed anodic and cathodic nanostructured catalysts based on cheap metals like iron and nickel which are assembled together in alkaline fuel cells and eletrolyzers able to reach an activity close to the state of the art PEM based devices. As example an iron phthalocyanine cathode based H2/O2 fed fuel cell set up in our laboratory delivered a remarkable power density of 1 W cm-2.