During my talk I will discuss four topics, over which I have felt concern over the past few years:

- The concept of applied bias photon-to-current efficiency (ABPE).

The ABPE is described as ‘a diagnostic measurement in materials development’. However, the widely accepted ABPE equation is not formulated as a true energetic efficiency. Furthermore, negative ABPEs are obtained for applied potentials > 1.23 V, resulting in increasingly negative efficiencies for increasingly high photocurrents, which is non-sensical. Both of these issues invalidate the ABPE equation for the very condition it is designed to represent. It is also noticeable that a lot of recent reviews and systematic studies of photoelectrode materials in biased systems do not use the ABPE as an effective way to compare their performances. I will discuss the issues of the currently accepted ABPE formulation and suggest a more rigorous formulation.

- The perpetual reporting of incident radiation on photoelectrochemical cells as AM 1.5 G and 100 mW cm^-2

The accepted basic requirement for the characterisation of solar water splitting systems is the same as for benchmarking the performances of solar cells: the incident spectral irradiance has to match that of the Air Mass 1.5 Global (AM 1.5 G) spectrum of natural sunlight, amounting to 100 mW cm^-2 when integrated. The spectra of light sources is rarely an exact match to the AM 1.5 solar spectrum, even if an AM 1.5 filter is used.  I will discuss the importance rigorous calibration and the need to report the actual spectra of light sources used during the characterization of photoactive materials and assessment of photoelectrochemical devices.

- The construction of Pourbaix diagrams to predict photoelectrode and catalyst stability – example of BiVO₄ degradation

There are some puzzling aspects in the Pourbaix diagrams being used for demonstrating the reactions responsible for light-induced BiVO₄ degradation. I will present the published Pourbaix diagrams for the combined aqueous bismuth and vanadium systems and discuss why some postulated reactions that are presented as electrochemical (i.e. involving a change in oxidation state) cannot actually be so. I will also consider why chemical speciation in different electrolytes can contribute to electrode degradation.  

- Effect of temperature on (photo)electrolyser performance

Solar cells must be maintained at 25 °C during characterisation and here the characterisation of water splitting systems ceases to follow the same standards. The question is whether this matters. The deviation of photoabsorber temperature from 25°C in solar water splitting systems is not necessarily unreasonable or preventable, since the presence of electrolyte makes it harder to fix the photoabsorber temperature to one specific constant value; photoelectrochemical cells are not usually jacketed or designed to be immersed in water baths and so the electrolyte (and hence photoabsorber) temperature could readily vary by > 5 °C, depending on the ‘room temperature’ of a laboratory at the start of the experiment, and can increase by > 10 °C relative to the starting temperature, depending on the photocurrent density, size of the absorber relative to the volume of electrolyte and the duration of the experiment. It therefore seems more important for the electrolyte temperature to be measured, rather than controlled, since temperature affects Gibbs free energy of formation of the water splitting reaction and the volumetric molar densities of evolving gases. I will show, from the thermodynamic viewpoint, the importance of electrolyte temperature in (photo)electrolysis.