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Chapters / Chapter 8, Photoelectrochemistry / Lesson 8.2

8.2 Measuring photoelectrodes

Light calibration, photocurrent and chopped-light measurements, hole scavengers, IPCE and APCE, ABPE and STH, and stability, with calculators for each efficiency.

About 30 minutesOpen to everyoneQuiz at the end

Getting the light right

  • Simulated sunlight. Use a solar simulator with an AM 1.5G filter, set to 100 mW/cm² at the sample position with a calibrated reference cell. A thermopile alone does not account for the spectrum.
  • Area. Define the illuminated area with a mask, and give the direction of illumination, from the front or through the substrate.
  • The electrolyte and cell window absorb some light, so calibrate through the same window and path length.

Photocurrent measurements

  • LSV in the dark and under light, at a slow scan rate, often 10 to 20 mV/s, with potentials converted to RHE (Lesson 1.1). The photocurrent at 1.23 V vs RHE is a common comparison for photoanodes.
  • Chopped light shows how fast the photocurrent rises and falls. Spikes when the light switches on, followed by a decay, point to surface recombination.
  • Hole scavengers, such as sulfite or H2O2, react much faster than water. Comparing photocurrents with and without them separates charge separation from charge injection efficiency.

IPCE and APCE

The incident photon-to-current efficiency (IPCE, also called external quantum efficiency) is the fraction of incoming photons at one wavelength that give an electron in the circuit. Dividing by the fraction of light absorbed gives the absorbed photon-to-current efficiency (APCE), which shows how well the absorbed light is used.

A good check is to integrate the IPCE over the AM 1.5G spectrum. The result should match the photocurrent measured under the solar simulator within a few percent.

Efficiency numbers and what they mean

Chen and co-workers (2010) set out definitions that avoid the most common mix-ups.

  • ABPE, the applied bias photon-to-current efficiency, is computed from a three-electrode measurement. It describes the electrode under a given bias, not a device.
  • STH, the solar-to-hydrogen efficiency, is the benchmark for a device. It is only valid for a two-electrode cell with no applied bias and no sacrificial reagent, under AM 1.5G light, with the hydrogen measured.

Stability

Many photoelectrodes corrode under light, for example by photo-oxidation of the semiconductor itself. Report long chronoamperometry under light at a fixed potential, the photocurrent before and after, the Faradaic efficiency for O2 or H2 measured during the test, and the electrolyte composition after it, for example dissolved metal from the film. Protective layers and co-catalysts often decide how long a photoelectrode lasts.

Check your understanding

Quiz for lesson 8.2

Pick one answer per question. You see the explanation straight away.

References and further reading

  1. Z. Chen et al., Accelerating materials development for photoelectrochemical hydrogen production, standards for methods, definitions, and reporting protocols, J. Mater. Res. 25 (2010) 3–16.
  2. M. G. Walter, E. L. Warren, J. R. McKone, S. W. Boettcher, Q. Mi, E. A. Santori, N. S. Lewis, Solar water splitting cells, Chem. Rev. 110 (2010) 6446–6473.
  3. T. Hisatomi, J. Kubota, K. Domen, Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting, Chem. Soc. Rev. 43 (2014) 7520–7535. doi:10.1039/c3cs60378d