Electrolysis feels the heat Electricity infrastructure powered by sunlight and wind requires flexible storage capacity to compensate for the intermittency of these sources. In this context, Hauch et al. review progress in solid oxide electrolyzer technology to split water and/or carbon dioxide into chemical fuels. These devices, which rely on oxide conduction between cathode and anode, use nonprecious metals as catalysts and operate above 600°C, thereby benefiting from thermodynamic and kinetic efficiencies. The authors highlight recent optimizations of cell components as well as systems-level architecture. Science , this issue p. eaba6118 BACKGROUND Alleviating the worst effects of climate change requires drastic modification of our energy system: moving from fossil fuels to low-carbon energy sources. The challenge is not the amount of renewable energy available—energy potential from solar and wind exceeds global energy consumption many times over. Rather, the key to a 100% renewable energy supply lies in the integration of the growing share of intermittent sources into a power infrastructure that can meet continuous demand. The higher the share of renewables, the more flexible and interconnected the energy system (the electric grid, the gas and heat networks, etc.) needs to be. Critically, a future energy system where the supply of electricity, heat, and fuels is based solely on renewables relies heavily on technologies capable of converting electricity into chemicals and fuels suitable for heavy transport at high efficiencies. In addition, higher electrolysis efficiency and integrated fuel production can decrease the reliance on bioenergy further than conventional electrolysis can. ADVANCES Electrolysis is the core technology of power-to-X (PtX) solutions, where X can be hydrogen, syngas, or synthetic fuels. When electrolysis is combined with renewable electricity, the production of fuels and chemicals can be decoupled from fossil resources, paving the way for an energy system based on 100% renewable energy. Solid oxide electrolysis cell (SOEC) technology is attractive because of unrivaled conversion efficiencies—a result of favorable thermodynamics and kinetics at higher operating temperatures. SOECs can be used for direct electrochemical conversion of steam (H 2 O), carbon dioxide (CO 2 ), or both into hydrogen (H 2 ), carbon monoxide (CO), or syngas (H 2 +CO), respectively. SOECs can be thermally integrated with a range of chemical syntheses, enabling recycling of captured CO 2 and H 2 O into synthetic natural gas or gasoline, methanol, or ammonia, resulting in further efficiency improvements compared with low-temperature electrolysis technologies. SOEC technology has undergone tremendous development and improvements over the past 10 to 15 years. The initial electrochemical performance of state-of-the-art SOEC single cells has more than doubled, while long-term durability has been improved by a factor of ∼100. Similar improvements in performance and durability have been achieved on the stack level. Furthermore, SOEC technology is based on scalable production methods and abundant raw materials such as nickel, zirconia, and steel, not precious metals. Performance and durability improvements as well as increased scale-up efforts have led to a hundredfold gas production capacity increase within the past decade and to commissioning of the first industrially relevant SOEC plants. Over the next 2 to 3 years, plant size is expected to further increase by a factor of almost 20. In recent years, SOEC systems have been integrated with downstream synthesis processes: examples include a demonstration plant for upgrading of biogas to pipeline quality methane and the use of syngas from an SOEC plant to produce fuels for transport via the Fischer-Tropsch process. OUTLOOK Improved understanding of the nanoscale processes occurring in SOECs will continue to result in performance and lifetime gains on the cell, stack, and system levels, which in turn will enable even larger and more efficient SOEC plants. In Germany, the share of intermittent renewables in the electricity supply has passed 30%, while in Denmark, intermittent sources account for almost 50% of the electricity supply. As this happens for a growing number of countries, demand for efficient energy conversion technologies such as SOECs is poised to increase. The increasing scale will help bring down production costs, thereby making SOECs cost-competitive with other electrolysis technologies and, given sufficiently high CO 2 emissions taxation, cost-competitive with fossil-based methods for producing H 2 and CO. SOECs offer an opportunity to decrease the costs of future renewable energy systems through more efficient conversion and enable further integration of renewables into the energy mix. Solid oxide electrolyzers: From nanoscale to macroscale. The splitting of H 2 O or CO 2 occurs at solid oxide electrolysis cell (SOEC) electrodes. Multiple cells are combined into SOEC stacks, which are in turn combined into SOEC plants. When renewable electricity is used, the production of transport fuels and chemicals can be decoupled from fossil resources. SOECs operate at elevated temperatures, resulting in electrolysis efficiencies unattainable by other electrolysis technologies. In a world powered by intermittent renewable energy, electrolyzers will play a central role in converting electrical energy into chemical energy, thereby decoupling the production of transport fuels and chemicals from today’s fossil resources and decreasing the reliance on bioenergy. Solid oxide electrolysis cells (SOECs) offer two major advantages over alternative electrolysis technologies. First, their high operating temperatures result in favorable thermodynamics and reaction kinetics, enabling unrivaled conversion efficiencies. Second, SOECs can be thermally integrated with downstream chemical syntheses, such as the production of methanol, dimethyl ether, synthetic fuels, or ammonia. SOEC technology has witnessed tremendous improvements during the past 10 to 15 years and is approaching maturity, driven by advances at the cell, stack, and system levels.