Efficient conversion of solar energy into molecules for energy storage and other purposes is one of the most important challenges for sustainable energy systems of the future. Particularly directly coupled photovoltaic (PV) assisted electrochemical (EC) processes, like e.g., PV-assisted water splitting, are of interest. In most practical cases, PV and EC devices are developed separately and then merged at a later stage to verify their performance as a PV-EC system. Enormous efforts of many groups in catalyst development produce increasing number of new catalysts every year. In turn photovoltaic community continues optimization of variety of PV technologies. In this situation, it would be useful to be able evaluate potential of one or other catalyst pair in combination with different PV technologies without building and optimizing an experimental device. To address this issue, we have recently developed a simple method to assess limit of solar-to-hydrogen efficiency (STH) of a specific electrolyzer based on the “reverse analysis” of its polarization curve [1]. We show that despite the complexity of the parameter space, there is a surprisingly simple way to estimate the limit of efficiency in any PV-assisted electrolyzer system.

The maximum STH for a specific electrolyzer is achieved when the current-voltage (IV) characteristics of the PV device crosses polarization curve of the EC device at maximum power point (MPP). Conversely, each point on the EC polarization curve can be considered the MPP of a PV device optimally coupled to the EC device. Therefore, at each point on the polarization curve, the minimum PV efficiency and maximum EC efficiency can be calculated for a specific irradiance. The product of both efficiencies generates the STH limit that can be attained at that specific point on the polarization curve. The result of the transformation is the dependence of the STH limit on the PV efficiency at the assumed irradiance level and optionally ratio of the active areas of PV and EC devices. This "reverse analysis," carried out with elementary math, does not involve any modeling or analysis of PV IV characteristics and provides a quick simple way to quantify the potential of any electrolyzer.

In our paper we present the principle of the reverse analysis using an example NiMo/NiFeO_X catalyst pair. Next, we show how using this analysis losses in an experimental PV-EC combination can be identified and quantified. We present extended set of results of reverse analysis applied to a variety of PV-EC combinations described in the literature. Finally, we discuss how this analysis can be applied to different electrochemical devices coupled to PV like e.g., PV-assisted CO₂ reduction.

TOC Figure. Principle of the reverse analysis: (a) at each point on the polarization curve, minimum PV efficiency and maximum EC efficiency are calculated; (b) the product of both efficiencies generates the STH limit at the specific point; (c) the result of the transformation presented as STH limit vs. PV efficiency.