Periodic Reporting for period 1 - CHALCON (Chalcogenide-Silicon tandem PEC for CO2 reduction)
Reporting period: 2022-09-01 to 2024-08-31
Fossil fuels dependent growth has led to an unprecedented rise in the atmospheric CO2 levels. This has triggered climate change which emerged as one of the biggest global challenges of our time. Artificial photosynthetic systems, such as photoelectrochemical cells (PEC) offer affordable solution to reduce CO2 by converting it to valuable products directly using only sunlight as energy input. This strategy can substantially reduce CO2 and generate greener fuels and chemicals to propel sustainable growth. For practical realization of such PEC systems, it is necessary to integrate inexpensive semiconducting materials that not only fully utilize the solar energy spectrum but also drive CO2 reduction (CO2R) efficiently for many hours. Various semiconducting materials such as oxides, nitrides, phosphides have been studied so far. However, most of them do not satisfy the essential criteria and often require expensive materials. Cumulative factors including insufficient light absorption, unsuitable energy band alignment, poor charge separation and transport, and slow catalytic conversion process at the surface are predominant limitations to the current PEC systems. Moreover, to the best of our knowledge, there are very few semiconductors that are even stable in aqueous media used for CO2R. The project started with clear questions to address –
Is it possible to design a stable semiconductor addressing the above challenges and drive photoelectrochemical CO2R?
Is it possible to fabricate a single integrated tandem device comprising of inexpensive materials and drive PEC reaction in an standalone configuration?
Is it possible to design a stable semiconductor addressing the above challenges and drive photoelectrochemical CO2R?
Is it possible to fabricate a single integrated tandem device comprising of inexpensive materials and drive PEC reaction in an standalone configuration?
The objective of this project (CHALCON) was to develop a multilayer tandem architecture comprising of all inexpensive materials in an interdisciplinary pursuit towards efficient PEC CO2R operation. The proposed architecture was CIGS/Si tandem capable of performing CO2R chemistry and alternative oxidation reaction such as Glycerol and halide oxidation to produce value-added reactions. The work performed within this project includes (i) development of a wide bandgap (1.8 eV) chalcogenide absorbers; p-type Cu(In,Ga)S2 and intrinsic Sb2S3 absorber suitable for top cell in tandem PEC devices, (ii) development of a scalable and robust CIGS photocathode for efficient and stable PEC CO2 reduction, (iii) demonstration of a semiconductor system that is stable in aqueous media and active for PEC CO2 reduction, producing CO and Formate, (iv) established the origin of the catalytic behavior and identified the active sites on CIGS surface (through both experimental and theoretical approaches) responsible for PEC CO2R, (v) development of a scalable process for all inorganic monolithic Si/Sb2S3 tandem photoelectrode fabrication, (vi) demonstration of a standalone PEC system for hydrogen evolution reaction combined with alternative oxidation reaction (iodide oxidation).
The main target of developing multilayer protected photocathode with selective catalyst for CO2 reduction to CO has been achieved beyond expectation, In this project, a compositionally engineered wide bandgap (1.8 eV) CIGS thin films were successfully synthesized with suitable conduction band alignment with CO2 redox potential, showing high surface affinity for CO2R intermediates, and high stability in the aqueous electrolyte. The project demonstrated CO and Formate production with faradaic efficiency of 32% and 14 % respectively, from a bare CIGS photocathode without the need of any transport or protection or cocatalyst layer. The device showed promising stability of over 80 mins in an aqueous media with no signs of degradation. The stable performance of CIGS photocathodes for CO2 reduction in aqueous media provides a rare opportunity to understand the complex catalytic process at the semiconductor-electrolyte interface, covering broader interest of the community. The project has led to the development of a multilayer engineered tandem PEC device based on Sb2S3/Si which covers the entire solar spectrum owing their suitable band gaps and demonstrated a tandem PEC device. The tandem PEC device can drive water splitting reaction combined with iodide oxidation reaction with standalone solar-to-hydrogen (STH) efficiency of 2.1 %. Very few tandem systems exist in monolithic architecture that combine cost-effective materials with suitable properties. Material design and integration strategy, fundamental knowledge, and scientific outcomes from the project can expedite the development of highly efficient, selective and stable PEC devices.