Active and Stable Mxene Modified Cu₂o Composite Films for Photoelectrochemical Water Splitting

Although limited in supplies and non-renewable, fossil fuels currently provide 80% of the global annual energy. 1 Furthermore, the combustion of fossil fuels emits lingering gas emissions that exacerbates global warming. Unlike fossil fuels, the solar energy is abundant, globally accessible and renewable, therefore developing technologies which can efficiently and sustainably harness and utilise this energy source is imperative. Solar-driven water splitting has been one of the most studied reactions because its product (H 2 ) is a clean energy source with 4-fold higher mass energy density than fossil based fuels like gasoline. However, achieving good solar to hydrogen conversion efficiency (STH) in a single device remains a big challenge. Cu 2 O is a very attractive photocathode material for the solar-driven water splitting as it has a narrow direct band gap of ca. 2 eV, which is capable to absorb most of the visible portion of the sunlight. It has favourable band positions for the hydrogen evolution reaction, with a theoretical 18% STH efficiency. 2 However, one major drawback which limits the wider use of this material is its lack of stability in aqueous solutions when exposed to light. In this study, we aim to introduce MXene’s as a cocatalyst on to Cu 2 O films. MXene’s are a family of 2-dimensional transition metal carbides, nitrides and carbonitrides. They have good electronic conductivity, good chemical stability and abundant active sites, 3 making them a suitable co-catalyst for solar-driven water splitting reactions. We observed a 3.7-fold increase in photocurrent density during photoelectrochemical (PEC) measurements at 0 V (vs. reversible hydrogen electrode) on MXene modified Cu 2 O compared to the plain Cu 2 O film. We also found that MXene addition, along with deposition of ZnO charge transfer layer, significantly improves the photo-stability of Cu 2 O photocathodes during simulated solar-driven water splitting reactions. We attribute this outcome mainly to the enhancement of the interfacial charge transfer and electrode photoconductivity upon illumination imparted by MXenes. References 1 World Energy Counc. , 2013, 1–28. 2 A. Paracchino, V. Laporte, K. Sivula, M. Grätzel and E. Thimsen, Nat. Mater. , 2011, 10 , 456–461. 3 M. Naguib, M. Kurtoglu, V. Presser, J. Lu, J. Niu, M. Heon, L. Hultman, Y. Gogotsi and M. W. Barsoum, Adv. Mater. , 2011, 23 , 4248–4253.