High Performance and Durable (Zn1-xCox)O:N Nanowires As Photoanode for Efficient Hydrogen Production Via Photoelectrochemical Water Splitting

The rapid depletion of fossil fuels and increasing strain on the environment due to excess consumption of the fossil fuels has necessitated the exploration of new renewable energy sources to solve the global energy crisis.¹ Hydrogen has been considered as the future promising energy source, capable of providing clean and reliable energy supply and thus, meeting the global energy demand. However, economic production of hydrogen still remains a challenge, limiting its commercialization as a fuel. Hence, the efficient and economic production of hydrogen using renewable energy sources, such as solar, wind, etc. has received special interest, in the aim of finding clean (low carbon footprint) fuel, that will have minimum impact on the environment. Among all promising renewable energy sources such as solar, geothermal, wind, solar derived energy remains a highly attractive approach since it is a de-concentrated and inexhaustible energy source. Hence, solar energy driven water splitting, also known as photo-electrochemical (PEC) water splitting is considered as a promising approach for economic and efficient hydrogen production, since water splitting does not involve any greenhouse gas emission or any toxic byproducts.    

           In PEC water splitting, a semiconductor is used as photoanode, which absorbs solar energy and thus, provides, photogenerated carriers (electrons and holes) for hydrogen evolution reaction (HER) and water oxidation  or oxygen evolution reaction (OER), respectively. The development of semiconductor material with low band gap (1.23 eV < E_g < 3 eV) to absorb visible light and thus, drive the HER and OER reactions with minimum over-potential, and minimum density of lattice defects is of considerable interest. This will also facilitate recombination of photo-generated carriers and thus, lower the PEC response, high electrical conductivity and high stability in aqueous electrolyte solution leading to efficient utilization of solar energy, resulting in efficient and economic hydrogen production. Thus far, well studied semiconductor materials such as TiO₂, ZnO, Fe₂O₃ have issues such as wide band gap and poor stability for long term PEC water splitting operation. However, among all these materials, ZnO is a promising material due to its higher electron mobility than TiO₂ (~155 cm² V^-1 s^-1 for ZnO vs. ~10^-5 cm² V^-1 s^-1 for TiO₂). The systematic band gap engineering of ZnO will help in lowering the band gap of ZnO (3.2 eV), which will improve light absorption in the visible region of interest and number of carriers available for the reaction.

           In this study, vertically aligned ZnO nanowires (NWs) co-doped with Co and N are explored as photoanode (semiconductor) for PEC water splitting for the first time. These NWs are synthesized using well-known hydrothermal route, followed by heat treatment in NH₃ at 400^oC, 500^oC 600^oC and 700^oC. These compositions are denoted as (Zn_0.95Co_0.05)O:N-y (y=400, 500, 600) NWs, where y is the temperature of heat treatment in NH₃ atmosphere. The cross-sectional SEM image of (Zn_0.95Co_0.05)O:N-600 NWs is shown in Fig. 1.

           The photoelectrochemical characterization of (Zn_0.95Co_0.05)O:N-y has been carried out in 0.5 M Na₂SO₄ as an electrolyte, Pt wire as counter electrode and Ag/AgCl as the reference electrode (+0.197 V vs. NHE), using a scan rate of 10 mV/sec and temperature of 26⁰C in H-type cell, where photoanode and cathode (Pt/C) are separated by Nafion membrane (Dupont).

           Thus, this study explores (Zn_1-xCo_x)O:N NWs as potential semiconductor material for economic and efficient production of hydrogen via PEC water splitting. The results of the synthesis, microstructural and photoelectrochemical activity of these semiconductor materials will be presented and discussed.

References:

1.             J. R. Miller, Science, 2012, 335, 1312-1313.

Acknowledgement:

The authors gratefully acknowledge the financial support of NSF grant (CBET-0933141). The authors also acknowledge the Edward R. Weidlein Chair Professorship funds and the Center for Complex Engineered Multifunctional Materials for partial support of this research.