RESEARCH HIGHLIGHTS. Si Microwire Solar Water Splitting Devices Matthew Shaner

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1 RESEARCH HIGHLIGHTS From the Resnick Sustainability Institute Graduate Research Fellows at the California Institute of Technology Si Microwire Solar Water Splitting Devices

2 Global Significance Inexpensive and abundant fossil fuels have been THE source of worldwide human prosperity during the last century. However, their finite quantity will demand a replacement that is also inexpensive and abundant for continued prosperity for future generations. Solar energy is one of few sources that has the capacity to serve as a replacement, but is intermittent when compared to the energy demands of society. Developing cost effective solar technologies that can supply energy to meet society's demands, even at night, has the opportunity of maintaining our prosperity and increasing the prosperity of developing nations. Project Summary To address solar energy intermittency, we are developing a technology that, reminiscent of plants, directly converts sunlight and water into hydrogen and oxygen, effectively storing solar energy in molecular hydrogen bonds for ondemand energy production. Inexpensive design formats, materials and processing techniques are being implemented to achieve low cost solutions in a manner that does not sacrifice efficiency. Fusing semiconductor physics, electrochemistry, chemical engineering, optoelectronics and materials science, this project developed a single integrated unit containing all of the active elements necessary for efficient solar water splitting. The design architecture was developed through engineering trade-offs and a balance of fabrication complexity with optical and electronic operating efficiencies. Scanning electron micrograph (SEM) images of the tandem junction wire array on the growth wafer. A water splitting device that consists of a tandem junction microwire array embedded in an ion-selective, gas impermeable membrane with integrated electrocatalysts that will produce separated gaseous hydrogen and oxygen with inputs of water and sunlight only.

3 Potential Impact If successful, this research will provide a robust technical solution for renewable hydrogen production. Such hydrogen could be used for ammonia production necessary to feed the world population, passenger vehicle transportation and electric grid storage and management. To achieve this potential and become truly disruptive many non-active technical and nontechnical system and business components will require significant attention in addition to this scientific research. Tandem junction Si microwire array device design with a second absorber coating each Si microwire to produce the second junction. The microwire array is embedded in a membrane that provides mechanical support, gas impermeability and ionic conductivity.

4 The Science Efficient solar water splitting entails conversion of sunlight into high currents at a voltage sufficient to overcome the reaction thermodynamics and system inefficiencies (~1.7 V total). Minimizing system inefficiencies requires short ionic transport lengths between each half reaction site, state-of-the-art catalysts and persistent product separation. Balancing and optimizing for these requirements resulted in a design consisting of a tandem junction Si microwire array based semiconductor device embedded in a gas impermeable, ionically conductive polymeric membrane. Photoelectrochemical measurements and electron microscopy at key fabrication steps were used to characterize progress and performance. Scanning electron micrograph (SEM) images of the tandem junction wire array on the growth wafer and cross section images showing the layered structure for both WO 3 and TiO 2 second absorbers and junctions. The ITO and FTO layers serve as contact layers for efficient charge transfer between Si and the outer absorber. Current density versus potential behavior of both devices with the TiO 2 device producing higher efficiency due to the increased photovoltage from TiO 2 versus WO 3.

5 Key Results Proof of principle tandem junction Si microwire devices have been demonstrated with WO 3 and TiO 2 as the second absorber and junction. Each system has been shown to have the ability to split water with sunlight as the only energy input. Separately, integration with gas impermeable, ionically conductive polymeric membranes and removal from the growth substrate has been demonstrated. Future Steps Future work will focus on integrating the two proof of principle demonstrations, semiconductor design and membrane integration, to realize the complete device design. Specific work will focus on improving fabrication processes that combine materials compatible under the same operating conditions: acidic or basic solutions. Nafion membrane embedded Si/TiO 2 microwire array that has been removed from the growth substrate. The membrane provides the mechanical support to maintain the arrays integrity.

6 RESEARCH HIGHLIGHTS From the Resnick Sustainability Institute Graduate Research Fellows at the California Institute of Technology Si Microwire Solar Water Splitting Devices Global Significance Inexpensive and abundant fossil fuels have been THE source of worldwide human prosperity during the last century. However, their finite quantity will demand a replacement that is also inexpensive and abundant for continued prosperity for future generations. Solar energy is one of few sources that has the capacity to serve as a replacement, but is intermittent when compared to the energy demands of society. Developing cost effective solar technologies that can supply energy to meet society s demands, even at night, has the opportunity of maintaining our prosperity and increasing the prosperity of developing nations. Fusing semiconductor physics, electrochemistry, chemical engineering, optoelectronics and materials science, this project developed a single integrated unit containing all of the active elements necessary for efficient solar water splitting. Project Summary To address solar energy intermittency, we are developing a technology that, reminiscent of plants, directly converts sunlight and water into hydrogen and oxygen, effectively storing solar energy in molecular hydrogen bonds for on-demand energy production. Inexpensive design formats, materials and processing techniques are being implemented to achieve low cost solutions in a manner that does not sacrifice efficiency. If successful, this research will provide a robust technical solution for renewable hydrogen production. Such hydrogen could be used for ammonia production necessary to feed the world population, passenger vehicle transportation and electric grid storage and management. To achieve this potential and become be truly disruptive many non-active technical and nontechnical system and business components will require significant attention in addition to this scientific research. The Science Efficient solar water splitting entails conversion of sunlight into high currents at a voltage sufficient to overcome the reaction thermodynamics and system inefficiencies (~1.7 V total). Minimizing system inefficiencies requires short ionic transport lengths between each half reaction site, state-of-the-art catalysts and persistent product separation. Balancing and optimizing for these requirements resulted in a design consisting of a tandem junction Si microwire array based semiconductor device embedded in a gas impermeable, ionically conductive polymeric membrane. Photoelectrochemical measurements and electron microscopy at key fabrication steps were used to characterize progress and performance. Key Results Proof of principle tandem junction Si microwire devices have been demonstrated with WO 3 and TiO 2 as the second absorber and junction. Each system has been shown to have the ability to split water with sunlight as the only energy input. Separately, integration with gas impermeable, ionically conductive polymeric membranes and removal from the growth substrate has been demonstrated. Future work will focus on integrating the two proof of principle demonstrations, semiconductor design and membrane integration, to realize the complete device design. Specific work will focus on improving fabrication processes that combine materials compatible under the same operating conditions: acidic or basic solutions. Publications Shaner, M. R. et al. (2014) Photoelectrochemistry of core shell tandem junction n p+-si/n-wo3 microwire array photoelectrodes. Energy Environ. Sci. 7, 779. Shaner, M. R., Hu, S., Sun, K. & Lewis, N. S. (2014) Energy & Environmental Science. Energy Environ. Sci. 8,