Researchers at the University of Exeter (UK) have developed a novel p-type LaFeO₃ photoelectrode using an inexpensive and scalable spray pyrolysis method. The nanostructured photoelectrode results in spontaneous hydrogen evolution from water without any external bias applied with a faradaic efficiency of 30% and excellent stability.

The researchers believe this new type of photoelectrode is not only cheap to produce, but can also be recreated on a larger scale for mass and worldwide use. An open-access paper on the work is published in Scientific Reports.

A promising way of storing solar energy is via chemical fuels, in particular hydrogen as it is considered as a future energy carrier. The greatest challenge is to develop a suitable technology for large scale and cost effective solar fuel production to compete with fossil fuel. One way this could be achieved is by using photoelectrochemical (PEC) water splitting which directly converts water and sunlight to solar fuel (hydrogen).

… Cost effective solar fuel generation is hindered by the semiconductor material not meeting certain essential criteria to achieve highly efficient solar to hydrogen conversion. These criteria are as follows: (i) the visible part of the solar spectrum must be absorbed for higher efficiency of hydrogen production and the band edges should ideally straddle the redox potential of water splitting, (ii) the photoexcited carriers must separate and migrate to the surface without recombination, (iii) adsorbed species must be reduced and oxidized by the photogenerated electrons and holes to produce H₂ and O₂. Also for cost effective, environmental and scalability issues, earth abundant non-toxic materials should be the focus of research into new semiconductor materials.

… To the best of our knowledge, we report for the first time the nanostructured LaFeO₃ photoelectrode for spontaneous hydrogen evolution from water without any external applied bias.

—Pawar and Tahir

To prepare the photoelectrode, the researchers sprayed precursor solution at 150 °C and then annealed at different temperature from 475 °C to 625 °C with an increment of 25 °C to get single phase crystalline LaFeO₃ material.

The team analyzed the photoelectrochemical (PEC) performance of LaFeO₃ in 0.1 M aqueous NaOH (pH 13) solution by illuminating the photoelectrode from the electrolyte side.

They found the annealing temperature was found to be an important factor which directly affected the photocurrent density of the LaFeO₃ photoelectrodes.

The water splitting was conducted in a glass reactor vessel. The LaFeO₃ working electrode and Pt counter electrode were connected by a single looped wire, without any external bias being applied. Hydrogen was produced spontaneously during the water splitting test during the first 6 hour cycle where the photoelectrode generated 0.18 μmol/cm₂ of hydrogen after 6 hours, with a faradaic efficiency of 30%.

It then underwent a second cycle of water splitting test to determine if the electrode was re-usable and how much the performance varied. After a further 6 hours illumination, the LaFeO₃ thin film generated 0.08 μmol/cm² of hydrogen (Figure S8). This provided additional evidence that the film is re-useable, although the amount of hydrogen produced is almost halved, the researchers said.

These findings demonstrate that LaFeO₃ is a potential candidate to act as a photoelectrode for unassisted PEC water splitting to generate solar fuel (hydrogen) cost effectively. However further work is required to investigate and improve slow charge carrier dynamics and low light absorption challenges of LaFeO₃ photoelectrodes.

—Pawar and Tahir

Resources

• Govinder S. Pawar & Asif A. Tahir (2018) “Unbiased Spontaneous Solar Fuel Production using Stable LaFeO₃ Photoelectrode” Scientific Reports volume 8, Article number: 3501 doi: 10.1038/s41598-018-21821-z