Metal Oxide-Based Tandem Cells for Self-Biased Photoelectrochemical Water Splitting

Abstract

Photoelectrochemical (PEC) water splitting represents a promising route to convert solar energy into clean hydrogen. Constructing tandem cells has emerged as a feasible approach and attracted tremendous attention for self-biased water splitting, especially using low-cost and stable metal oxides. Herein, a state-ofthe-art review of metal oxide-based PEC/photovoltaic (PV) tandem cells and PEC tandem cells is comprehensively presented, with a focus on crucial issues of designing efficient tandem devices from the aspects of the photoanodes, photocathodes, and photovoltaics. Different device configurations and efficiency limitations of tandem cells are introduced, and advances of metal oxide-based PEC/ PV and PEC tandem cells in terms of material design and device optimization are discussed. Metal oxide-based tandem cells have shown solar-to-hydrogen efficiencies of ∼8%, which is approaching practicality. Perspectives on remaining challenges and potential strategies are outlined.

Synergies of co-doping in ultra-thin hematite photoanodes for solar water oxidation: In and Ti as representative case

“…the band gap, there are other essential requirements that also need to be considered for efficient water splitting for hydrogen generation: the band edge positions with respect to the redox level of water, resistivity, minority carrier lifetime, atband potential, over-potential losses and stability of photoelectrodes against corrosion. [4][5][6][7] Metal oxide semiconductors are the most promising materials for the PEC reactions thanks to their excellent stability in many electrolytes over a wide pH range, low cost, non-toxic nature and their versatility in terms of fabrication techniques. [6][7][8][9] Among the various metal oxides, iron oxide (hematite; a-Fe 2 O 3 ) 10 is considered as one of the potential photoanode candidates for solar water oxidation, thanks to its unique properties such as a suitable bandgap (2.0-2.2 eV), low cost, non-toxicity and high stability.…”

“…[4][5][6][7] Metal oxide semiconductors are the most promising materials for the PEC reactions thanks to their excellent stability in many electrolytes over a wide pH range, low cost, non-toxic nature and their versatility in terms of fabrication techniques. [6][7][8][9] Among the various metal oxides, iron oxide (hematite; a-Fe 2 O 3 ) 10 is considered as one of the potential photoanode candidates for solar water oxidation, thanks to its unique properties such as a suitable bandgap (2.0-2.2 eV), low cost, non-toxicity and high stability. 4,[11][12][13] However, pristine a-Fe 2 O 3 exhibits poor water splitting efficiency, far below the maximum theoretical efficiency of 12.9%, because of the mismatch between the valence band energy level and the water reduction potential, the short hole diffusion length of 2-4 nm and the low electron mobility.…”

Enhancement of Titania Photoanode Performance by Sandwiching Copper between Two Titania Layers

“…Photoelectrochemical (PEC) water splitting is a promising strategy to convert renewable solar energy into chemical energy for tackling the growing energy shortage and environmental crisis. In comparison with other energy conversion approaches, PEC water splitting stands out due to its low cost, environmental friendliness and easy separation of hydrogen and oxygen from water [ 1 , 2 ]. Unsatisfactorily, the energy conversion efficiency is hindered by the sluggish reaction kinetics which involves four electrons oxidation process [ 3 , 4 ].…”

Semi-transparent quaternary oxynitride photoanodes on GaN underlayers

“…4 Additionally, the other advantages of the wireless PEC tandem cell are the (i) flexibility in materials selection, (ii) low resistance losses and (iii) high proton conductivity, 5,6 which target for high solar-to-hydrogen (STH) conversion efficiency. 7 Furthermore, the wireless configuration is more suitable for industrial manufacturing in comparison to the wired tandem configuration in which the photoabsorbers are connected via an external circuit. 8 With respect to the TCS materials required for a PEC tandem cell, the most commonly used substrates are transparent conductive oxides (TCOs) such as fluorine/indium doped tin oxide (FTO/ITO) and aluminum doped zinc oxide (AZO).…”

“…12 Among the often studied metal oxide-based photoanodes such as TiO 2 , WO 3 , ZnFe 2 O 4 , BiVO 4 , the most representative one is the latter, 13 exhibiting the highest STH conversion efficiency of 3.7% in a bias-free tandem cell for PEC water splitting. 7 To achieve higher STH efficiency, the front photoanode in wireless tandem PEC cells has to be semi-transparent. Thus, it must absorb photons with energy larger than its band gap and transmit the remaining photons to the photocathode behind it, avoiding scattering or parasitic absorption of the transmitted light.…”