Solar fuels at M2N

Solar-to-chemical energy conversion requires three simple steps: absorption of light, creation of charges (electrons and holes), and catalytic chemical reactions in which the charges are used to oxidize and reduce compounds in endothermic reactions such as the splitting of water and the reduction of carbon dioxide. The reaction products, e.g. hydrogen, carbon monoxide, methanol, or methane can be used directly as fuels, or used as starting products for further chemical reactions. To enable solar energy production in yields exceeding the energy conversion of natural photosynthesis (typically <1%) with cheap and abundant materials is a tremendous challenge. This has not yet been achieved in artificial systems. The crucial issue is that absorption of light, creation of charges, and at least two chemical reactions have to work in concert at the same high rate, outperforming the various recombination processes and side reactions that can occur. It has been argued that the successful construction of a direct artificial system for efficient solar fuel generation is the most important research challenge in coming years.

Principle of photoelectrochemical energy conversion

Within M2N we are working on photoelectrochemical water splitting and carbon dioxide reduction:

Water splitting
We investigate photoelectrochemical solar to hydrogen conversion using an “artificial leaf” based on organic multi-junction solar cells and catalysts for the hydrogen and oxygen evolution reactions. Because a wide range of organic semiconductors is available, we can tune the maximum power point voltage of the cells to coincide with the operating point determined by the thermodynamic potential for water splitting and the overpotentials defined by the catalysts. By optimizing these parameters, a significant progress in the performance of organic artificial leaves can be achieved. With organic semiconductors we have achieved solar to hydrogen energy conversion efficiencies of about 6% in wireless and autonomous working devices. For more efficient photoelectrochemical water splitting both balancing the nature and surface area of the catalysts with the materials used in the solar cell is crucial.

Schematic device layout of an organic photo-electrochemical cell for wireless water splitting.

Oxygen evolution reaction
Electrolysis of water is a potential source for hydrogen production on a large scale. A measure of the inefficiency in electrolysis is the so called overpotential, the voltage needed to drive the process in excess of the thermodynamic equilibrium voltage. The contribution to the overpotential of the oxygen evolution dominates overwhelmingly. A mechanism or a proper understanding for the high catalytic activity of e.g. ruthenium oxide and oxide perovskites is still unknown, but it might be that magnetic moments are at play. We try to correlate the specific atomic sites and their magnetic identity with water dissociation on transition metal oxide surfaces with spin-polarized scanning tunneling spectroscopy and scanning tunneling spectroscopy

Carbon dioxide reduction
Towards this aim of direct conversion of carbon dioxide (CO2) into fuels by light we have selected, carbon monoxide (CO) and methane (CH4) as the our targets. Also here we are working on developing new catalysts to reduce overpotentials and increase selectivity, as well as developing new photovoltaic conversion systems tailored to the electrochemical potential. We have achieved solar-to-CO energy conversion of 6.8% during continuous operation using in which the Faradaic efficiency for CO formation was about 85% and where the power was delivered by three series connected metal halide perovskite solar cells.

Set up used for electrochemical carbon dioxide reduction.

Current-density voltage characteristics for three series connected metal halide perovskite solar cells before, during, and after a 30 min. CO2 to CO conversion experiment.