Nanotechnology for energy materials: Electrodes like leaf veins
SEM – model of a metallic nano-network with periodic arrangement ( left) and visual representation of a fractal pattern (right). © M. Giersig/HZB
Nano-sized metallic wires are attracting increasing attention as conductive elements for manufacturing transparent electrodes, which are employed in solar cells and touch screen panels. In addition to high electric conductivity, excellent optical transmittance is one of the important parameters for an electrode in photovoltaic applications. An international team headed by HZB scientist Prof. Michael Giersig has recently demonstrated for these applications that networks of metallic mesh possessing fractal-like nano-features surpass other metallic networks in utility. These findings have now been published in the most recent edition of the renowned journal Nature Communications.
Their new development is based on what is termed quasi-fractal nano-features. These structures have similarities to the hierarchical networks of veins in leaves. Giersig’s team was able to show that metallic networks with these features optimise performance of electrodes for several applications. They combine minimized surface coverage with ultra-low total resistance while maintaining uniform current density. In addition, it was demonstrated that these networks, inspired by nature, can surpass the performance of conventional indium tin oxide (ITO) layers. In experiments on artificially constructed electrode networks of different topologies, the scientists established that non-periodic hierarchical organisation exhibited lower resistance as well as excellent optical transmittance in comparison to periodic organisation. This led to elevated output power for photovoltaic components.
“On the basis of our studies, we were able to develop an economical transparent metal electrode", says Giersig, continuing “We obtain this by integrating two silver networks. One silver network is applied with a broad mesh spacing between the micron-diameter main conductors that serve as the “highway" for electrons transporting electrical current over macroscopic distances.” Next to it, additional randomly distributed nano-wire networks serve as local conductors to cover the surface between the large mesh elements. “These smaller networks act as regional roadways beside the highways to randomise the directions and strengths of the local currents, and also create refraction effects to improve transparency above that of classical shadow-limited performance”, according to Giersig. “Solar cells based upon these electrodes show exceptional a high efficiencies”.
Publication: Optimization of hierarchical structure and nanoscale-enabled plasmonic refraction for window electrodes in photovoltaics; Nature Communications, 7, 12825; doi:10.1038/ncomms12825
- Battery research with the HZB X-ray microscopeNew cathode materials are being developed to further increase the capacity of lithium batteries. Multilayer lithium-rich transition metal oxides (LRTMOs) offer particularly high energy density. However, their capacity decreases with each charging cycle due to structural and chemical changes. Using X-ray methods at BESSY II, teams from several Chinese research institutions have now investigated these changes for the first time with highest precision: at the unique X-ray microscope, they were able to observe morphological and structural developments on the nanometre scale and also clarify chemical changes.
- Hydrogen: Breakthrough in alkaline membrane electrolysersA team from the Technical University of Berlin, HZB, IMTEK (University of Freiburg) and Siemens Energy has developed a highly efficient alkaline membrane electrolyser that approaches the performance of established PEM electrolysers. What makes this achievement remarkable is the use of inexpensive nickel compounds for the anode catalyst, replacing costly and rare iridium. At BESSY II, the team was able to elucidate the catalytic processes in detail using operando measurements, and a theory team (USA, Singapore) provided a consistent molecular description. In Freiburg, prototype cells were built using a new coating process and tested in operation. The results have been published in the prestigious journal Nature Catalysis.
- Rutger Schlatmann re-elected as ETIP PV ChairThe European Technology and Innovation Platform for Photovoltaics (ETIP PV) was created by the European Commission in order to promote photovoltaic technologies and industries in Europe. Now, the ETIP PV Steering Committee elected a new Chair, as well as two Vice-Chairs for the term 2024 – 2026. Rutger Schlatmann, head of the division Solar Energy at the HZB, and professor at HTW Berlin, was re-elected as the ETIP PV Chair.