AVS 60th International Symposium and Exhibition
    Energy Frontiers Focus Topic Tuesday Sessions
       Session EN+AS+PS-TuA

Invited Paper EN+AS+PS-TuA7
Power Curves of the Artificial Leaf

Tuesday, October 29, 2013, 4:00 pm, Room 101 A

Session: Water Splitting and Carbon Dioxide Conversion
Presenter: D. Nocera, Massachusetts Institute of Technology
Correspondent: Click to Email
An artificial leaf can perform direct solar-to-fuels conversion via water splitting. The artificial leaf is a buried junction, in which the rectifying junctions are protected from solution or “buried”. Whereas water splitting catalysis is combined with charge separation, current rectification, and photovoltage generation in a solution junction PEC device, in a buried junction device, catalysis is separated from the current rectification, charge separation, and photovoltage generation, which occur at the internal junction. The buried junction photoelectrochemical (BJ–PEC) cell is free from many of the design limitations of a traditional solution junction photoelectrochemical (SJ–PEC) cell. First and foremost, in a SJ–PEC, water splitting catalysis is combined with charge separation, current rectification, and photovoltage generation. Accordingly, most candidate materials are based on metal–oxides. Decades of research have shown that it is extremely difficult to produce a competent photovoltaic (PV) material that at the same time is capable of facilitating the demanding four–electron, four–proton chemistry of water splitting. Second, in a SJ–PEC, the band edges of the flatband potentials of the semiconductor must straddle the thermodynamic potentials of OER and HER under the conditions of operation. These foregoing limitations are circumvented in a buried junction device. In a BJ–PEC, water–splitting catalysis is separated from the internal junction where current rectification, charge separation, and photovoltage generation occur. Accordingly, the OER and HER catalysts may be optimized independently from the PV device such that the maximum power characteristics of the PV and catalyst may be matched independently. Of equal significance, in a BJ–PEC, the potential drop across the outer Helmholtz layer will adjust automatically to move the Fermi levels to energetic positions that allow the water splitting reaction to proceed. For this reason, the photovoltages produced at buried junctions need not be fixed relative to a specific material flatband potential and consequently there is no requirement for the flatband potentials of the semiconductors to straddle the thermodynamic potentials of the OER and HER. There simply has to be sufficient potential generated by the PV device to enable water splitting. This talk will focus on the analysis of Tafel and photovoltaic power curves. The presented analysis highlights the importance of matching the electrochemical load of the water splitting catalyst to the onset of maximum current of the PV component, drawing a clear link between the kinetic profile of the water splitting catalyst and the SFE of devices such as the artificial leaf.