Understanding the oxygen evolution reaction (OER) activity and stability of the NiFe-based materials is important for achieving low-cost and highly efficient electrocatalysts for practical water splitting. Here, we report the roles of Ni and Fe on the OER activity and stability of metallic NiFe and pure Ni thin films in alkaline media. Our results support that Ni(OH) 2 /NiOOH does not contribute to the OER directly, but it serves as an ideal host for Fe incorporation, which is essential for obtaining high OER activity. Furthermore, the availability of Fe in the electrolyte is found to be important and necessary for both NiFe and pure Ni thin films to maintain an enhanced OER performance, while the presence of Ni is detrimental to the OER kinetics. The impacts of Fe and Ni species present in KOH on the OER activity are consistent with the dissolution/re-deposition mechanism we proposed. Stability studies show that the OER activity will degrade under prolonged continuous operation. Satisfactory stability can, however, be achieved with intermittent OER operation, in which the electrocatalyst is cycled between degraded and recovered states. Accordingly, two important ranges, that is, the recovery range and the degradation range, are proposed. Compared to the intermittent OER operation, prolonged continuous OER operation (i.e., in the degradation range) generates a higher NiOOH content in the electrocatalyst, which is likely related to the OER deactivation. If the electrode works in the recovery range for a certain period, that is, at a sufficiently low reduction potential, where Ni 3+ is reduced to Ni 2+ , the OER activity can be maintained and even improved if Fe is also present in the electrolyte.

Temperature affects the efficiency of an integrated photovoltaic-electrolyser significantly more than the photovoltaic cells alone.

Direct solar hydrogen generation via a combination of photovoltaics (PV) and water electrolysis can potentially ensure a sustainable energy supply while minimizing greenhouse emissions. The PECSYS project aims at demonstrating a solar‐driven electrochemical hydrogen generation system with an area >10 m2 with high efficiency and at reasonable cost. Thermally integrated PV electrolyzers (ECs) using thin‐film silicon, undoped, and silver‐doped Cu(In,Ga)Se2 and silicon heterojunction PV combined with alkaline electrolysis to form one unit are developed on a prototype level with solar collection areas in the range from 64 to 2600 cm2 with the solar‐to‐hydrogen (StH) efficiency ranging from ≈4 to 13%. Electrical direct coupling of PV modules to a proton exchange membrane EC to test the effects of bifaciality (730 cm2 solar collection area) and to study the long‐term operation under outdoor conditions (10 m2 collection area) is also investigated. In both cases, StH efficiencies exceeding 10% can be maintained over the test periods used. All the StH efficiencies reported are based on measured gas outflow using mass flow meters.

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The outdoor operation of an up‐scaled thermally photovoltaic electrolyzer (PV EC), constructed using a heat exchanger (HE) made of low‐cost materials, compared to its nonintegrated counterpart to quantify heat transfer and its effects, is studied. Thermal coupling of the PV and EC can reduce the difference between their temperatures, benefitting device performance. Such devices can produce hydrogen at rooftop installations of small‐to‐medium‐sized nonindustrial buildings. The devices are tested outdoors using automated real‐time monitoring. Under ≈880 W m−2 peak irradiance, they produced hydrogen at ≈120 and ≈110 mL min−1 rate with and without HE, respectively, corresponding to about 8.5% and 7.8% solar‐to‐hydrogen efficiencies. During about 700 h of testing, the HE is beneficial at over ≈500 W m−2 due to cyclic device operation. Under lower irradiance levels, pumping previously heated electrolyte through the HE increases the PV and reduces the electrolyte temperature, reducing the device performance. The HE increases the cumulative hydrogen production (≈800 L from both devices), so even relatively modest heat transfer rates can improve the PV EC operation. Improving the HE should further increase the benefits, but additional measures may be needed to maximize the hydrogen production.

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Direct coupling of photovoltaic (PV) modules to electrolyser(s) (ECs) can benefit from reduced component costs by omitting power electronics. Thermal integration of the PV to the EC could potentially enhance performance by cooling the PV and heating up the EC. However, conventional PV and EC constructions need an additional heat exchanger to make this possible. Without concentrating optics, the temperature of the PV remains comparatively low, which could be an added challenge for effective use of the waste heat. Considering this, the question is, how much thermal integration can benefit the PV-EC operation, and how complex a sufficiently efficient heat exchanger would be. To study the effect of heat exchange on the device operation, we compared the simultaneous operation of two identical sets of PV modules directly electrically coupled to EC stacks. One of the devices was also thermally coupled using a heat exchanger at the back of the PV module to heat up the electrolyte (1.0 M KOH) before it enters the EC (Figure 1.a, the thermally coupled device is on the left in Figure 1.b). The heat exchanger prevented contact between the corrosive KOH and the PV module but enabled heat transfer from the PV module to the electrolyte. The PV modules consisted of nine series-connected 6-inch wafer silicon heterojunction solar cells, and the total collection area, of each, was ca. 2600 cm2 (51 cm × 51 cm), of which ca. 2480 cm2 was active. The devices were operated outdoors in Berlin, Germany (52° 25ʹ 53.3ʺ N, 13° 31ʹ 25.9ʺ E) for a total of about 700 hours, of which the last about 500 hours were continuous, except for few short maintenance breaks. During testing, the solar to hydrogen efficiency of both devices was typically in the 8 – 12 % range, reducing with increasing irradiance. Typical peak hydrogen production rate on a sunny day (800 – 850 W/m2) was about 120 ml/min with the heat exchanger and about 110 ml/min without, the highest measured values being about 10 ml/min higher. The heat exchanger improved the performance under irradiance over about 500 W/m2, and at over 800 W/m2 irradiance, the enhancement corresponded to about 10 % increase in the hydrogen production rate. On the other hand, interestingly, the heat exchanger also reduced the hydrogen production rate at low irradiance conditions. This, together with the fact that most of our testing days were comparatively cloudy, probably explains why the total hydrogen yields over the 700-hour period were very similar for both devices, with a slight 10 litre advantage for thermal integration (ca. 770 litres vs ca. 760 litres). Nevertheless, based on our results, even a moderately efficient heat exchanger enhances the PV-EC operation in sunny conditions. Since much of the annual hydrogen yield would be produced in such conditions, the concept of transferring heat from PV to EC shows definite promise, but further development and optimization is needed to extract the full benefits of our approach. The authors acknowledge support from the German Federal Ministry of Education and Research in the framework of the project CatLab (03EW0015A). The present study benefits from work started under the PECSYS project (ended December 2020) funded by the FUEL CELLS AND HYDROGEN 2 JOINT UNDERTAKING under grant agreement No. 735218. This Joint Undertaking receives support from the EUROPEAN UNION’S HORIZON 2020 RESEARCH AND INNOVATION programme and Hydrogen Europe and N.ERGHY. Figure 1

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