Solar to fuel conversion could provide an alternative to mankind’s currently unsustainable use of fossil fuels. Solar fuel generation by photoelectrochemical (PEC) methods is a potentially promising approach to address this fundamental and important challenge.

Experimental demonstrations of PEC systems which convert solar energy to hydrogen via water splitting and to carbon-based fuels via CO₂ reduction date back to the 1970s. Since that time, there have been improvements in both the functional understanding and performance of laboratory test devices. However, an approach which could be practical and scalable has not yet been developed. This level of technical progress could be contrasted with photovoltaic (PV) power conversion, which has progressed from laboratory demonstrations and niche applications to large scale deployment in a number of countries.

The key research bottlenecks that need to be addressed before practical and scalable solar fuel devices can become a reality will be outlined. The analysis will focus on a fundamental requirement of any sustainable solar conversion technology, which is to generate more energy over its useful lifetime than was required to manufacture and maintain it (positive return on energy investment).

Reported laboratory solar to hydrogen (STH) conversion efficiencies range from <1% to over 20%, with a number of approaches that yield efficiencies comparable to solar PV [1]. However, there are few reports of extended operational stability, which is a clear prerequisite for a positive energy return on investment [2]. Efficient separation of the product hydrogen is another critical bottleneck.

Electrochemical reduction of CO₂ to fuel molecules such as methanol and ethanol could form the basis for the production of renewable and sustainable transportation fuels, replacing the fossil fuels which are used today. There has been substantial recent progress in solar driven PEC CO₂ reduction, with a number of reports of energy conversion efficiencies of over 1% [3,4]. While these values are larger than those estimated for biomass generation by plants and algae (up to 1%) [5], the natural system utilizes dilute CO₂ in the atmosphere as a feed stock and generates a separated fuel product. Thus, carbon capture and the selective production and separation of fuels remain as outstanding challenges for this PEC technology.  

This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, under Award Number DE-SC0004993. 

(1)   Ager et al., Energy Environ. Sci. 2015, 8, 2811–2824.

(2)   Sathre et al., Energy Environ. Sci. 2014, 7, 3264–3278.

(3)   Rongé et al., Chem. Soc. Rev. 2014, 43, 7963–7981.

(4)   Schreierr et al., Nat. Commun. 2015, 6, 7326.

(5)   Blankenship et al., Science. 2011, 332, 805–809.