A team led by Dr. Michael Grätzel at EPFL (Ecole Polytechnique Fédérale de Lausanne) in Switzerland has developed a highly efficient and low-cost water-splitting cell combining an advanced perovskite tandem solar cell and a bi-functional Earth-abundant catalyst.

The combination of the two delivers a water-splitting photocurrent density of around 10 milliamperes per square centimeter, corresponding to a solar-to-hydrogen efficiency of 12.3%. (Currently, perovskite instability limits the cell lifetime.) Their paper is published in the journal Science.

In a companion Perspective in the journal, Dr. Thomas Hamann of Michigan State University, who was not involved with the study, called Grätzel’s team’s work “an important step towards achieving [the] goal” of quickly developing alternative sources of energy that can replace fossil fuels.

Hydrogen, which is the simplest form of energy carrier, can be generated renewably with solar energy through photoelectrochemical water splitting or by photovoltaic (PV)–driven electrolysis. Intensive research has been conducted in the past several decades to develop efficient photoelectrodes, catalysts, and device architectures for solar hydrogen generation. However, it still remains a great challenge to develop solar water-splitting systems that are both low-cost and efficient enough to generate fuel at a price that is competitive with fossil fuels.

Splitting water requires an applied voltage of at least 1.23 V to provide the thermodynamic driving force. Because of the practical overpotentials associated with the reaction kinetics, a substantially larger voltage is generally required, and commercial electrolysers typically operate at a voltage of 1.8 to 2.0 V. This complicates PV-driven electrolysis using conventional solar cells—such as Si, thin-film copper indium gallium selenide (CIGS), and cadmium telluride (CdTe)—because of their incompatibly low open-circuit voltages. To drive electrolysis with these conventional devices, three to four cells must be connected in series or a DC–DC power converter must be used in order to achieve reasonable efficiency. … In contrast, perovskite solar cells have achieved open-circuit voltages of at least 0.9 V and up to 1.5 V according to recent reports, which is sufficient for efficient water splitting by connecting just two in series.

The EPFL team used a perovskite solar cell based on CH₃NH₃PbI₃. The cell has a short-circuit photocurrent density, open-circuit voltage, and fill factor of 21.3 mA cm^?2, 1.06 V, and 0.76, respectively, yielding a solar-to-electric power conversion efficiency (PCE) of 17.3%.

To overcome the large water-splitting overpotentials that are typically required to generate H₂ and O₂ at a practical rate, the EPFL researchers looked to implement efficient electrocatalysts.

They sought to avoid conventional expensive noble metals of low abundance, such as Pt, RuO₂, and IrO₂. For sustained overall water splitting, the catalysts for the H₂ evolution reaction (HER) and O₂ evolution reaction (OER) must be operated in the same electrolyte—which should be either strongly acidic or alkaline to minimize overpotentials, they noted. This requirement is a challenge for most of the Earth-abundant catalysts because a highly active catalyst in acidic electrolyte may not be active or even stable in basic electrolyte.

Thus, it is crucial to develop a bifunctional catalyst that has high activity toward both the HER and OER in the same electrolyte (either strongly acidic or strongly basic). Moreover, the use of a bifunctional catalyst simplifies the system, lowering the manufacturing cost and thus the cost of the resulting hydrogen.

To solve this, they incorporated iron (Fe) into Ni(OH)₂ to form NiFe layered double hydroxides (LDHs). The resulting catalyst electrode exhibited high activity toward both the oxygen and hydrogen evolution reactions in alkaline electrolyte.

Overall, the NiFe LDH/Ni foam electrode shows nearly the same performance as the Pt/Ni foam electrode, with 10 mA cm^?2 water-splitting current reached by applying just 1.7 V across the electrodes. To confirm the bifunctional activity of the NiFe LDH/Ni foam electrodes, the evolved gaseous products were quantified by means of gas chromatography. We confirmed quantitative Faradaic gas evolution at the predicted 2:1 ratio for hydrogen and oxygen, within experimental error. The exceptional bifunctionality, high activity, and low cost of the NiFe LDH/Ni foam electrode make it highly competitive for potential large-scale industrial applications.

Commenting on the EPFL team’s work, Dr. Hamann noted that:

While the 12% water-splitting efficiency reported is already exceptional, there are several paths to improvement. Use of a single band-gap material in a tandem configuration is not ideal, and combining a perovskite cell with a smaller band-gap semiconductor such as silicon could produce over 20% STH efficiencies. Some loss in available photovoltage by substituting a lower-voltage silicon cell for one of the high-voltage perovskite cells in order to increase the photocurrent may be compensated by the use of a better HER catalyst that requires a smaller overpotential. The NiFe LDH catalyst is also opaque and not amenable to an integrated photoelectrochemical system. It is not yet clear if alternative transparent catalysts are absolutely necessary or if the separated PV/electrolyzer configuration used here will ultimately be viable.