Alkali metal carbonates, such as sodium carbonate (Na2CO3) and potassium carbonate (K2CO3), have emerged as promising molten salts for hybrid thermochemical-electrolytic hydrogen production systems. These materials serve as high-temperature reaction media, facilitating both thermal decomposition and electrochemical steps in hydrogen generation. Their unique properties enable efficient ion transport, thermal stability, and catalytic activity, making them suitable for advanced water-splitting processes.

In molten carbonate systems, alkali carbonates function as ionic conductors at elevated temperatures, typically between 450°C and 800°C. The molten state allows for high ionic conductivity, which is critical for electrolytic hydrogen production. The primary ion transport mechanism involves the movement of carbonate (CO3²⁻) and alkali metal cations (Na⁺ or K⁺). Under an applied electric field, CO3²⁻ ions migrate toward the anode, where they decompose into CO2 and O2, while water reduction occurs at the cathode, producing H2. The overall process can be represented as a combination of thermochemical and electrochemical reactions, where heat and electricity drive water splitting with higher efficiency than standalone methods.

One key advantage of molten carbonates is their ability to lower the operational voltage required for water electrolysis compared to conventional low-temperature systems. Research indicates that molten carbonate electrolysis can achieve voltages as low as 0.9–1.2 V at 750°C, significantly below the 1.8–2.0 V required in alkaline or proton-exchange membrane electrolyzers. This reduction in energy demand improves the overall efficiency of hydrogen production. Additionally, the high operating temperatures enable faster reaction kinetics, further enhancing process performance.

Electrode compatibility is a critical factor in molten carbonate systems. Nickel-based anodes and cathodes are commonly used due to their resistance to oxidation and corrosion in carbonate melts. However, long-term exposure to molten salts can still lead to material degradation, particularly at the anode where oxygen evolution occurs. Recent studies have explored the use of protective coatings, such as lanthanum-strontium cobalt ferrite (LSCF) or gadolinium-doped ceria (GDC), to mitigate electrode corrosion. These materials form stable interfaces that reduce oxidative damage while maintaining high electrochemical activity.

Another challenge in molten carbonate systems is salt evaporation at high temperatures. Alkali carbonates exhibit significant vapor pressure above 700°C, leading to gradual salt loss and system instability. To address this, researchers have investigated eutectic mixtures, such as Li2CO3-Na2CO3-K2CO3 blends, which lower the melting point and reduce evaporation rates. For example, a ternary eutectic composition (32.1% Li2CO3, 33.4% Na2CO3, 34.5% K2CO3) melts at approximately 400°C and shows improved thermal stability compared to single-component carbonates. Containment strategies, such as ceramic or metallic coatings on reactor walls, have also been tested to minimize salt loss.

The integration of thermochemical cycles with molten carbonate electrolysis offers further efficiency gains. For instance, the hybrid sulfur (HyS) cycle combines thermal decomposition of sulfuric acid with electrochemical steps in a molten carbonate electrolyte. In this process, sulfuric acid decomposes at high temperatures to produce SO2 and O2, while SO2 is electrochemically oxidized in the molten carbonate medium to regenerate sulfuric acid and release hydrogen. This approach leverages both heat and electricity to maximize hydrogen yield while minimizing energy waste.

Material compatibility extends beyond electrodes to the containment materials used in reactors. Molten carbonates are highly corrosive to many metals and ceramics, necessitating the use of specialized alloys like Inconel or Hastelloy for structural components. Advanced ceramics, such as yttria-stabilized zirconia (YSZ), have also been employed as liners to protect against salt-induced degradation. Recent developments in refractory materials aim to enhance durability while maintaining cost-effectiveness for large-scale deployment.

System scalability remains a hurdle for molten carbonate-based hydrogen production. While laboratory-scale experiments demonstrate feasibility, industrial implementation requires solutions for heat management, salt replenishment, and gas separation. Innovations in reactor design, such as rotating or fluidized-bed configurations, aim to improve heat transfer and reduce thermal gradients. Additionally, continuous salt recycling systems are being developed to maintain electrolyte composition over extended operational periods.

Recent research has explored the use of additives to enhance molten carbonate performance. For example, doping carbonates with rare-earth oxides (e.g., CeO2 or La2O3) has been shown to improve ionic conductivity and catalytic activity. These modifications can further reduce energy losses and increase hydrogen production rates. Similarly, the introduction of nano-sized particles into the melt has demonstrated potential for enhancing electrode kinetics and stability.

Environmental considerations also play a role in the adoption of molten carbonate systems. While these processes produce high-purity hydrogen, the release of CO2 during carbonate decomposition requires careful management. Coupling molten carbonate electrolysis with carbon capture technologies can mitigate emissions, creating a near-zero-carbon hydrogen production pathway. Alternatively, using renewable electricity to power the electrolysis step ensures that the overall process remains sustainable.

The future of molten carbonate-based hydrogen production lies in optimizing system integration and reducing costs. Advances in materials science, coupled with innovative reactor designs, are expected to address current limitations in corrosion, salt stability, and scalability. As renewable energy sources become more prevalent, the synergy between high-temperature electrolysis and intermittent power supply could position molten carbonate systems as a key technology for green hydrogen generation.

In summary, alkali metal carbonates offer a versatile and efficient medium for hybrid thermochemical-electrolytic hydrogen production. Their ability to facilitate ion transport, lower energy requirements, and integrate with advanced cycles makes them a compelling choice for next-generation hydrogen technologies. While challenges like salt evaporation and material degradation persist, ongoing research continues to develop solutions that enhance performance and durability. With further refinement, molten carbonate systems could play a pivotal role in the transition to a sustainable hydrogen economy.