Solar hydrogen: Let’s consider the stability of photoelectrodes

Scalable large area BiVO<sub>4</sub> photoanode on FTO with Ni current collectors.

Scalable large area BiVO4 photoanode on FTO with Ni current collectors. © HZB

The results allow to assess differences in the stability of BiVO<sub>4</sub> in various pH-buffered borate, phosphate and citrate electrolytes.

The results allow to assess differences in the stability of BiVO4 in various pH-buffered borate, phosphate and citrate electrolytes. © https://pubs.acs.org/doi/10.1021/acsaem.0c01904

As part of an international collaboration, a team at the HZB has examined the corrosion processes of high-quality BiVO4 photoelectrodes using different state-of-the-art characterisation methods. The result is the first operando stability study of high-purity BiVO4 photoanodes during the photoelectrochemical oxygen evolution reaction (OER). This work shows how the stability of photoelectrodes and catalysts can be compared and enhanced in the future.

Hydrogen is a versatile fuel that can store and release chemical energy when needed. Hydrogen can be produced in a climate-neutral way by the electrolytic splitting of water into hydrogen and oxygen using solar energy. This can be achieved photo-electrochemically (PEC), and for this approach it is necessary to have low cost photoelectrodes that provide a certain photovoltage under illumination, and remain stable in aqueous electrolytes.

Stability of photoelectrodes

However, here lies the main obstacle; conventional semiconductors corrode very quickly in water. Metal-oxide thin films are much more stable, but still corrode over time. One of the most successful photoanode materials is bismuth vanadate (BiVO4), a complex metal oxide in which photocurrents are already close to the theoretical limit. But the biggest challenge for commercially viable PEC water splitting is now to assess and enhance the stability of photoelectrode materials during their PEC operation.

To this end, a team at the HZB Institute for Solar Fuels led by Prof. Roel van de Krol (HZB) together with groups from the Max Planck Institute for Iron Research, the Helmholtz Institute Erlangen-Nuremberg for Renewable Energy, the University of Freiburg and Imperial College London, have utilised a number of state-of-the-art characterisation methods to understand the corrosion processes of high-quality BiVO4 photo electrodes.

Observing the process from start to the end

"So far, we could only examine photoelectrodes before and after photoelectrochemical corrosion," says Dr. Ibbi Ahmet, who initiated the study together with Siyuan Zhang from the Max Planck Institute. "It was a bit like reading only the first and last chapters of a book, and not knowing how all the characters died". In a first step to solve this problem, the chemist provided a series of high-purity BiVO4 thin films that were studied in a newly designed flow cell with different electrolytes under standard illumination.

First operando stability study

The result is the first operando stability study of high-purity BiVO4 photoanodes during the photoelectrochemical oxygen evolution reaction (OER). Using in-situ plasma mass spectrometry (ICPMS), they were able to determine which elements were dissolved from the surface of the BiVO4 photoanodes during the photoelectrochemical reaction, in real time.

Stability number S

"From these measurements we were able to determine a useful parameter, the stability number (S)," says Ibbi. This stability number is calculated from the ratio between the O2 molecules produced and the number of dissolved metal atoms in the electrolyte and it is in fact a perfect comparable measure of photoelectrode stabilities. The stability of a photoelectrode is high if the splitting of water is proceeding rapidly (in this case the evolution of O2) and few metal atoms enter the electrolyte. This parameter can also be used to determine the change in photoelectrode stability during their lifetime or assess differences in the stability of BiVO4 in various pH-buffered borate, phosphate and citrate (hole scavenger) electrolytes.

This work shows how the stability of photoelectrodes and catalysts can be compared in the future. The authors have continued the collaboration and are now using these valuable techniques and insights to design viable solutions to enhance the stability of BiVO4 photoanodes and enable their use in long term practical applications.

arö

You might also be interested in

  • Stability of perovskite solar cells reaches next milestone
    Science Highlight
    27.01.2023
    Stability of perovskite solar cells reaches next milestone
    Perovskite semiconductors promise highly efficient and low-cost solar cells. However, the semi-organic material is very sensitive to temperature differences, which can quickly lead to fatigue damage in normal outdoor use. Adding a dipolar polymer compound to the precursor perovskite solution helps to counteract this. This has now been shown in a study published in the journal Science by an international team led by Antonio Abate, HZB. The solar cells produced in this way achieve efficiencies of well above 24 %, which hardly drop under rapid temperature fluctuations between -60 and +80 Celsius over one hundred cycles. That corresponds to about one year of outdoor use.
  • Scientists Develop New Technique to Image Fluctuations in Materials
    Science Highlight
    18.01.2023
    Scientists Develop New Technique to Image Fluctuations in Materials
    A team of scientists, led by researchers from the Max Born Institute in Berlin and Helmholtz-Zentrum Berlin in Germany and from Brookhaven National Laboratory and the Massachusetts Institute of Technology in the United States has developed a revolutionary new method for capturing high-resolution images of fluctuations in materials at the nanoscale using powerful X-ray sources. The technique, which they call Coherent Correlation Imaging (CCI), allows for the creation of sharp, detailed movies without damaging the sample by excessive radiation. By using an algorithm to detect patterns in underexposed images, CCI opens paths to previously inaccessible information. The team demonstrated CCI on samples made of thin magnetic layers, and their results have been published in Nature.
  • Recommended reading: Bunsen magazine with focus on molecular water research
    News
    13.01.2023
    Recommended reading: Bunsen magazine with focus on molecular water research
    Water not only has some well-known anomalies, but is still full of surprises. The first issue 2023 of the Bunsen Magazine is dedicated to molecular water research, from the ocean to processes in electrolysis. The issue presents contributions from researchers cooperating within the framework of a European research initiative in the "Centre for Molecular Water Science" (CMWS). A team at HZB presents results from the synchrotron spectroscopy of water. Modern X-ray sources can be used to study molecular and electronic processes in water in detail.