Late summer 2015 has seen the publication of 3 different devices for solar water splitting. These publications illustrate the global investment in moving to a carbon free (or at least carbon neutral) fuel. Release of the publications spanned a little over a month, August 11th to September 15th. For those not in the field, this means it was likely none of the groups knew the results of the others work when submitting and testing their research. This is close publication timeline provides us a rare opportunity to compare these devices side by side and highlight the work being done in the field of solar fuels.
August 11th, 2015 – Monash University:
Submitted in July and published on the 11th of August, in Energy & Environmental Science, Shannon A. Bonke, Mathias Wiechen, Douglas R. MacFarlane and Leone Spiccia from Monash University in Clayton, Australia, presented their work on a non-integrated “modular” water splitting device. This Monash team chose to work with a modular device architecture, where the light capture unit (PV) is wired to the catalysis unit (electrolyzer).
Spiccia and coworkers create a robust and efficient modular device through their efforts to engineer and optimize commercially available PV technology with a proven water splitting catalyst. The team refines their modular approach by “matching” the performance capabilities of the catalyst to the PV output parameters, arguing that when the systems are matched, solar-to-fuel efficiencies increase. In this regard, they feel making the effort to match the voltage and current output from the PV to the catalyst capabilities will greatly increase solar-to-fuel conversion efficiencies, likely matching that of the PV conversion efficiencies.
- Solar-to-fuel Efficiency: 22.4% at 100 suns
- Stability: >24 hr @ 100 suns – diurnal cycle trial of 72 hr at 100 suns
- pH: 13.5 (1.0M NaOH) to 7 (1.0M phosphate buffer)
- Catalyst: nickel foam (10-20 cm2)
- Light Capture: GaInP/GaAs/Ge multi-junction 10 mm CPV Receiver (Suncore Photovoltaic Tech.)
- Interesting additional information: Filtered river water (Yarra River, Melbourne) is also used to generate hydrogen at ~22% solar-to-fuel efficiency
Take home: In non-integrated modular systems, matching the catalyst and PV output is essential. Current commercially available material can be assembled into a solar hydrogen system that maintains 22% conversion efficiency over a broad range of pHs.
August 17th, 2015 – JCAP & CalTech:
In June, the Joint Center for Artificial Photosynthesis (JCAP) submitted their manuscript describing work on a fully integrated hydrogen evolution device. It was published August 17th, in the journal Energy & Environmental Science. The work culminates in the development of a “monolithically integrated, intrinsically safe solar-hydrogen prototype system”. Atwater and coworkers present a detailed analysis of the field of integrated devices as well as describe how JCAP’s efforts at developing robust electrodes lead to the successful engineering of the prototype.
Through effective protection, the JCAP team is able to incorporate an efficient light absorber, GaAs/InGaP, into a NiMo/Ni catalytic system, resulting in a stable solar hydrogen device. Further engineering of the system allows Atwater and coworkers to incorporate anion exchange membranes (AHA-type, NEOSEPTA membrane) with a NiMo/GaAs/InGaP/TiO2/Ni active layer in an “unassisted” solar hydrogen prototype. Areas where authors suggest improvements in efficiency and stability can be made are: novel architectures, current matching, and minimizing reflective losses.
- Solar-to-fuel Efficiency: 10% at 1 sun
- Stability: ~40 hr electrosynthetic cell – ~10 hr prototype (1cm2)
- pH: 13.7 (1.0M KOH)
- Catalyst: Ni and NiMo
- Light Capture: GaAs/InGaP – protected with TiO2
- Interesting additional information: Hydrogen flow rates for the 1cm2 prototype was 0.81 mL/s and stable over ~10 hrs
Take home: Protecting corrosion susceptible moieties of fully integrated devices will increase stability. Once stable systems have been identified, prototyping can provide insight into engineering. Fully integrated devices can allow for safe, unassisted, solar hydrogen production.
September 15th, 2015 – Fraunhofer Institute for Solar Energy Systems ISE & JCAP/CalTech:
Submitted in February and published in Nature Communications September 15th, the collaborative efforts of the Helmholtz-Zentrum Berlin Institute for Solar Fuels, the Joint Center for Artificial Photosynthesis at CalTech, the Technische Universität Ilmenau, and the Fraunhofer Institute for Solar Energy Systems ISE produced a report on a novel approach to synthesizing active moieties for fully integrated devices. By using a “low temperature, ambient pressure wet processing method” the authors believe that the process is likely industrially scalable. The two-step process simultaneously functionalizes the surface and deposits the catalyst.
This in-situ solution processing approach allows Hannappel and coworkers to deposit rhodium (Rh) catalysts on tandem GaInAs/GaInP photovoltaics, achieving devices with solar-to-hydrogen efficiencies of >14%. Reducing the amount of Rh used, moving to more earth abundant catalysts, and developing novel techniques for substrate fabrication are the author proposed pathways to a more cost effective device.
- Solar-to-fuel Efficiency: 14% at 1 sun
- Stability: >10 hr – observed 50% decrease in current over 35 hr
- pH: 0 (1.0M HClO4)
- Catalyst: Rh and RuO2
- Light Capture: InGaAs/InGaP – with AlInP layer
- Interesting additional information: Orientation of the device change stability reducing catalyst detachment in horizontal position
Take home: Solution processing techniques provide a new pathway to stabilizing materials as well as depositing catalysts in fully integrated unassisted solar hydrogen devices.
SOFI’s “two cents”:
These three publications highlight the global efforts directed at moving towards a carbon-free fuel. Each focuses on a different aspect of the same problem. From working with current technology to develop solar hydrogen sources for use today, to more fundamental approaches of subtract passivation and processing with various catalysts, the field is moving ever closer to developing solar hydrogen devices for commercial use.