**4. Conclusions**

This article describes the detailed design and unique capabilities of the test facility for large-scale electrolysis stacks that produce substantial quantities of hydrogen with a high degree of efficiency and virtually interact with various other facilities in a future renewable energy system.

In contrast to the methods using ceramic or alkaline electrolytes, electrochemical water splitting in cells with PEMs is distinguished by the fact that large systems can be established on an MW scale and operated in a broad power range between 5%, 100% and beyond. To limit performance losses and degradation effects, iridium oxide (IrO2) and platinum (Pt) are used as catalysts for the electrodes in PEM electrolyzers. Intensive efforts are being made to reduce the current catalyst loading to 0.3 mg/cm2 at 6 W/cm<sup>2</sup> by 2035. Implementing measures for highly efficient, stable and durable H2 production in high-MW technical systems should help reduce costs fourfold, to around €585/kW by 2030.

For the reproducible production of 1056 cm2 MEAs, a catalyst-containing dispersion was applied in a controlled manner to a PTFE substrate using a slot die. The solvent of the dispersion layer was subsequently expelled from the roll-to-roll coating system as part of a continuous drying step, ensuring that no layer defects occurred. The coated and dried sheets were then rolled up and cut. The PTFE-supported anode and cathode layers were subsequently formed into an MEA in a discontinuous hot press using a Nafion membrane. Characterization measurements on samples from the produced electrode sheets revealed a cell voltage of 1.66 V and a cell efficiency rate of 76% at a current density of 2 A/cm2. The proportion of hydrogen measured in the oxygen was less than 0.05% across the entire current density range.

The 400 kW stack was designed for the MW electrolysis system based on an active cell area of 1056 cm2 and comprised a three-part bipolar unit. Two distribution structures were attached on both sides to a central plate with drill holes that enabled the supply of media. The two structures were comprised of a frame, which in one option consists of an expanded metal sandwich and in another, an expanded metal sandwich with an additional Ti fleece. In addition to a bipolar unit, the easy-to-assemble repeating unit consisted of a flat gasket, carbon fiber sheet, and an MEA.

A new test facility, which is unique in its ability to investigate the performance, dynamics and durability of PEM electrolysis stacks with a cell area of up to 1200–3000 cm2, enables comprehensive operational testing at an electrical power input of up to 500 kVA and a current strength of up to 4000 ADC, or even up to 10,000 ADC in a subsequent expansion step. Highly sensitive pressure and temperature control ensures safe operation during highly dynamic load changes that correspond to the operating characteristics of renewable power sources. The recording and processing of measurement data enables automatic operation with predefined load profiles and ensures the transfer of data to the control systems of the EnergyLab 2.0 and LLEC living labs on the basis of a message queuing telemetry transport (MQTT) network protocol.

The Energy Lab 2.0 is used by pertinent researchers and technicians as a real-life laboratory and simulation platform for the energy-related testing of sustainable conversion and storage technologies on a technological demonstration level. This includes all relevant aspects, such as renewable electricity production and energy storage, grid integration, and power-to-molecule technologies, which are needed for the sustainable operation of a renewable-based energy system in future. Experimental research and development task work is complemented by extensive systems analyses. Using a 1.5 MWh Li-ion battery and an 800 kWh redox flow storage system, positive effects on the efficiency and lifetime of systems are achieved and further developed with innovative cooling concepts, the optimization of operating strategy, and the use of self-learning models. With the synthesis of methane or Fischer–Tropsch-based (FT) fuels consisting of hydrogen and CO2, there is a focus on enhancing the process chain efficiency and dynamic operation to cope with intermittent power supplies. Operation of the microchannel-based FT plant was successfully simulated under stationary and dynamic operating conditions with real data from the PV field interconnected to electrolysis and the hydrogen buffer. On a relevant laboratory scale, the suitability of micro-structured reactors was demonstrated. Due to their efficient temperature control for FT synthesis, even with a fluctuating supply of hydrogen, neither the quality of the product nor the stability of the catalyst is affected. In the Energy Lab 2.0, the Smart Energy System Simulation and Control Center (SEnSSiCC) simulates the coupling of energy sectors with fluctuating energy producers of the future and supports developments and innovations under realistic conditions for a pioneering smart grid. The SEnSSiCC brings together numerous laboratories and institutions that will help depict the energy network of the future. Alongside the development and testing of new control algorithms, control strategies under critical operation conditions and variable network topologies can be safely and extensively investigated.

The Living Lab Energy Campus (LLEC) is an integrated research platform through which innovative production, distribution, and storage systems are investigated while integrated into the energy supply of the research campus, as well as being monitored and controlled by an adaptive, cloud-based, and model-predictive IT platform. Through connection to the LLEC electrolysis system in terms of data and gas, the aim is to investigate and further develop disruption-free processes and process efficiencies for the coupling of electricity and hydrogen generation, as well as for the production and storage of hydrogen in various forms, and H2 storage and reconversion.

**Author Contributions:** Conceptualization, B.E.; methodology, B.E.; software, R.K., V.H. and S.W.; formal analysis, B.E. and M.M.; investigation, M.H., H.J., R.K., M.S., A.S., S.W., M.R. and N.M.; resources, B.E., V.H., R.D. and S.K.; writing—original draft preparation, B.E.; writing—review and editing, M.M., M.H., H.J., R.K., M.S., A.S., P.P., S.W., M.R., N.M. and S.K.; visualization, B.E., M.H., H.J., R.K., M.S., A.S., P.P., S.W., M.R., N.M. and S.K.; supervision, B.E. and M.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** All of the authors would like to thank the unnamed colleagues from the participating institutes of Jülich, KIT, and DLR for their fruitful discussions and excellent cooperation as part of the Energy Lab 2.0 and Living Lab Energy Campus projects. The Energy Lab 2.0, the Living Lab Energy Campus (LLEC), and the MW electrolysis system are funded by the Federal Ministry of Education and Research (BMBF), the Federal Ministry for Economic Affairs and Energy (BMWi), the Baden-Württemberg Ministry of Science, Research and the Arts (MWK-BW), and the Helmholtz Association of German Research Centers (HGF). All work related to the Solar Power Storage Park of the Energy Lab 2.0 contributes to the re-search performed at (KIT-BATEC) KIT Battery Technology Center and CELEST (Center for Elec-trochemical Energy Storage Ulm-Karlsruhe).

**Conflicts of Interest:** The authors declare no conflict of interest.
