As the global energy transition advances, energy storage becomes vital for energy security and addressing renewable intermittency. This thesis investigates the Power-to-X-to-Power (PtXtP) framework across technological, environmental, and economic dimensions to enhance long-term storage. The research spans manufacturing, system optimization, and techno-economic evaluation of both mature and emerging electrolyzer and fuel cell technologies.

The thesis begins by screening both short- and long-term storage solutions, highlighting their environmental impacts (Chapter 1). Despite non-negligible footprints, hybrid storage remains critical as a future energy security solution. Low-TRL technologies like reversible solid oxide cells (rSOC) show the lowest emissions (0.155 kg CO2/kWh). Zeolite as the hydrogen carrier or methane as the chemical carrier (0.168 kg CO2/kWh) also proves competitive. The study then further deeply assesses four electrolyzers, including AEL, PEMEL, AEMEL, and SOEC (Chapter 2). Material use in the stack production phase, particularly nickel, steel, and critical metals like platinum, drives manufacturing impacts. AEMEL powered by wind performs best for human health and ecosystems, while SOEC excels in climate impact but depends on the heat source. Integrating SOEC with biomass-produced heat yields over 70% efficiency, supporting its PtX role. Further analysis screens SOFC and PEMFC technologies within the XtP framework (Chapter 3). SOFCs outperform in efficiency and fuel flexibility, reaching 69.55% efficiency and 0.145 USD/kWh LCOE. Fuel type dominates the SOFC system's environmental impact. Ammonia-fed SOFCs achieve ~65% efficiency, slightly higher than hydrogen (62%), but require over 85% cracking. Recirculation offers limited gains at high single-pass fuel utilization.

Chapter 4 returns to PtXtP using the rSOC system. A novel rSOC with hybrid tank design (CH4/CO2 with ejectors) achieves 82.1% (SOEC), 72.6% (SOFC), and 60% round-trip efficiency. Coupling with batteries yields hybrid strategies tailored to national demand and renewable sources, cutting the energy storage costs up to 60%. Country-level metrics for storage size and cost are provided. An integrated SOFC-SOEC layout is further proposed, capable of capturing CO2 at 100% purity with just 1.3% SOEC stack area, enabling modular, multi-mode operation. Finally, the last part of this thesis (Chapter 5) explores modular scale-up strategies for SOFC. A hybrid design, decentralizing stacks and reformers, centralizing BoP, reduces LCOE to 0.10 USD/kWh, versus 0.16 (centralized) and 0.40 (decentralized). Beyond 300 kW, efficiency dominates cost, underscoring the need for further technological innovation and scale-driven economic improvements.

In conclusion, this thesis positions PtXtP systems as key to a sustainable energy future. It delivers actionable insights for improving efficiency, reducing emissions, and achieving cost-effective deployment through modular design. Future research should focus on rSOC degradation, rSOC grid integration, and SOEC cost reduction strategies to support commercial viability.