Challenging efficiency records of solar hydrogen production

US researchers have raised questions over how the photoelectrochemical community, including themselves, tests the efficiency of solar-to-hydrogen devices.¹

Photoelectrochemical hydrogen production is a promising source of sustainable energy but for this to be commercially viable, it needs to be more efficient. However, true like-to-like comparison between numbers reported around the world is nearly impossible. Without consistent techniques, we may be basing our future on inaccurate numbers.

Now, Henning Döscher, John Turner and colleagues from the National Renewable Energy Laboratory (NREL), US, have highlighted just how vital a standardised way to verify efficiency is.

The team analysed sources of error in measurements and used these to explain discrepancies amongst reported values. They considered light sources and a device’s active area, and how inconsistencies can cause wide variations in results. For example, in a laboratory, it is difficult to replicate sunlight’s natural fluctuations, and this is the ultimate light source when using these devices commercially.

Jan Rongé, an expert in solar fuels from KU Leuven, Belgium, says the definition of illuminated area is not yet sufficiently standardised in the photoelectrochemical cell field. ‘The analysis has made it clear that multijunction PEC cells require reliable light sources without spectral mismatch.’

The NREL team was even critical of their own previous work,² and revealed a relative over-illumination in laboratory experiments not reflective of real sunlight conditions. ‘We achieved excellent conversion efficiency results which were clearly too good to be true,’ comments Döscher.

Shane Ardo, an expert in photoelectrochemistry from the University of California, US, commends this key merit of the work. ‘By selflessly demonstrating that their own reported efficiencies from nearly two decades ago were erroneous, they have increased the urgency for standardised protocols by leaps and bounds.’

Döscher emphasises that obtaining accurate measurements is essential not only for commercialising photoelectrochemical cells, but also for showing a scientific advance. ‘The only way to measure and quantify consistent progress is with standardised reproducible measurement techniques.’ Turner agrees. ‘For different groups to be able to compare their results, definitive standardised measurement techniques needed to be developed.’

Ardo sees a bright future for more accurate and useable data in the photoelectrochemical field as a result. ‘Hopefully this work will drive initiatives for standardisation so that future demonstrations between groups can be compared.’