Solar simulator calibration and Solar Cell characterization

For the 1 sun measurement, we used a solar simulator (ABET Technologies, Model Sun2000) equipped with a 550 W xenon lamp as a light source. The GaInP/GaAs/GaInNAsSb multi-junction solar cell (0.316 cm2 area) was manufactured by Solar Junction.

As the cells were operated under concentrated sunlight, the calibration of the solar simulator for 1 sun conditions (100 mW cm−2) was carried out using the AM 1.5 Direct spectrum (ASTM G173). Although reference solar cells can be used to adjust a simulator for appropriate total power output, spectral control is crucial for accurate multi-junction cell measurements. For this reason, the external quantum efficiency (EQE) of each sub-junction of the multi-junction stack was measured using a grating monochromator (Newport CS260) calibrated with silicon and germanium photodetectors (Newport 918D-UV, 918D-IR). All light sources and photodetectors were calibrated by the manufacturers before the experiment. These EQE measurements were integrated with the AM 1.5D spectrum to determine short-circuit current densities and to understand the current-limiting junction of the cells. The EQE of each sub-junction and the AM 1.5D spectrum are shown overlaid in Supplementary Fig. 1. The cells were all top-junction limited, which allowed the simulator to be tuned without luminescent coupling impacting the current27. The 1 sun I–V characteristics of the cell were measured using this condition. These data are shown in Fig. 2a.

For the PV-electrolysis measurement, we used a multi-sun solar simulator (Newport, Model 66921) with a 1,000 W xenon lamp as the white light source. A water filter was applied to the light beam, to remove the excessive infrared component from the lamp spectrum and to better match the AM 1.5D spectrum. The cell package was mounted onto a water-cooling stage such that a surface temperature of 25 °C was maintained. The distance between the cell and the lamp was adjusted to achieve the desired short circuit current (J SC ). The intensity of the concentrated white light was determined from the ratio of the J SC under concentration to the J SC under 1 sun illumination, consistent with standard practices for characterizing concentrated PV cells32,33. No concentrator optics were placed between the simulator and the cell and therefore the possible effects of concentrator optics were not considered in efficiency calculation. As the cell was cut and installed in a standard CPV cell package, the whole cell area is active under illumination; therefore, no aperture or mask was used. Numerical modelling based on the junction ideality factor was later conducted to determine whether open-circuit voltage (V OC ) inflation due to the mismatch between the solar simulator spectrum and the AM 1.5D spectrum resulted in STH inflation36,37,38. The results of these calculations show that the lamp spectrum mismatch did not cause significant STH inflation in our experiments (details provided in Supplementary Note 4 and Supplementary Table 1).

During the duration of the experiment the solar cell performance was very stable, as expected for III–V solar cells. No hysteresis or time-transient behaviour was observed during the I–V measurement. The I–V characteristics results in Fig. 2 were measured with a forward voltage sweep rate of 50 mV s−1 and a sampling period of 0.02 s. Changing the sweep rate, direction or sampling rate did not generate a noticeable difference in the results.

Electrolyser fabrication and characterization

Membrane electrode assemblies were fabricated using a conventional catalyst-coated membrane technique. Nafion 115 membranes purchased from FuelCellsEtc were cut into 3.5 cm × 3.5 cm2 pieces. These membranes were pretreated by soaking in 3% H 2 O 2 at 80 °C for 1 h, then soaking in 0.5 M H 2 SO 4 at 80 °C for 1 h and finally soaking in Millipore (18.2 MΩ cm) water at 80 °C for 1 h. The membranes were removed from the water and blotted dry before the catalyst was deposited. Next, a Pt black catalyst (ETEK) and Nafion 117 ionomer solution (Aldrich) were mixed in a 3:1 weight ratio. Separately, an Ir black catalyst (Premetek) and Nafion 117 ionomer solution (Aldrich) were also mixed in a 3:1 weight ratio. The Pt and Ir catalyst/ionomer mixtures were both dispersed in 4:1 volume ratio mixtures of isopropanol and water. The catalyst/ionomer solutions were sonicated for several minutes and then deposited onto opposite sides of the Nafion membranes by spray casting. The Pt catalyst was loaded on the cathode side at 0.5 mg cm−2 and the Ir catalyst was loaded on the anode side at 2 mg cm−2 over a 2.5 cm × 2.5 cm area for a total device active area of 6.25 cm2. This catalyst-coated membrane was pressed between carbon paper (Sigracet GDL 35BC, Ion Power) on the cathode side and Ti mesh (Dexmet) on the anode side. Two identical assemblies prepared in this manner were loaded into cell assemblies (5 cm2, Fuel Cell Technologies, Inc.), which were maintained at a temperature of 80 °C for all measurements. The two electrolysers were connected in series. Millipore water (18.2 MΩ cm) preheated to 80 °C was fed into the anode side of the first electrolyser; there was no input to the cathode side. The cathode and anode outputs of the first electrolyser were connected to the cathode and anode inputs of the second electrolyser and both outputs of the second electrolyser were collected; thus, the H 2 and O 2 products could be quantified using a volume displacement Faradaic efficiency measurement apparatus.

PV-electrolysis system operation

To construct the PV-electrolysis system, the triple junction PV cell, the two electrolysers and a potentiostat (BioLogic, VMP3) were connected in series as follows: the working electrode port of the potentiostat was connected to the bottom contact of the solar cell, the top contact of the solar cell was connected to the anode of the first electrolyser, the cathode of the first electrolyser was connected to the anode of the second electrolyser and the cathode of the second electrolyser was connected to the counter electrode port of the potentiostat. The potentiostat reference lead was connected to the counter electrode lead so that it could measure the current passing through the closed system; no additional potential was applied. All electrical connections were made with standard copper cables, which introduced negligible resistance compared with other components of the system.

Before the start of the operation, preheated Millipore water was purged with H 2 and O 2 , and pumped into the two electrolysers with a Chem-tech Series 100 pump at a flow rate of 42 ml min−1. The solar cell was kept at 25 °C on a water cooler stage and positioned under the multi-sun solar simulator. The distance between the cell and the solar simulator was adjusted so that ∼42 suns of solar concentration was achieved. At this concentration, the solar cell output a short circuit photocurrent of 184 mA (∼583 mA cm−2) and aligned the solar cell I–V curve for an optimal operation point to match the electrode size and electrolyser capacity.

To begin operation, the shutter of the solar simulator was opened. The system current was recorded continuously by the potentiostat and these data were used to calculate the STH efficiency as a function of time as shown in Fig. 4. The system was run continuously for 48 h without interruption or modification. Periodically throughout the experiment, the gas products from the cathodes and anodes of the electrolysers were collected using volume displacement devices to calculate the Faradaic efficiency. At the end of the 48 h operation, the shutter of the solar simulator was closed.

Data availability

The data that support the findings of this study are available from the authors on request.