Metal sulfates such as copper sulfate (CuSO4) and iron sulfate (FeSO4) play a critical role in sulfur-driven thermochemical cycles for hydrogen production. These cycles leverage the redox properties of metal sulfates to split water molecules, releasing hydrogen while recovering sulfur for reuse. The process offers a promising alternative to oxide-based thermochemical cycles, particularly when integrated with concentrated solar power (CSP) systems. This article examines the decomposition pathways of metal sulfates, their sulfur recovery efficiency, and their performance in solar-driven applications, while comparing them to oxide-based systems in terms of energy inputs and byproduct management.

Thermochemical cycles using metal sulfates typically involve two key steps: the thermal decomposition of the sulfate and the subsequent hydrolysis of the resulting metal oxide to produce hydrogen. For CuSO4, the decomposition occurs at temperatures around 700–900°C, yielding copper oxide (CuO) and sulfur trioxide (SO3), which further dissociates into sulfur dioxide (SO2) and oxygen. The CuO is then hydrolyzed at lower temperatures (200–400°C) to regenerate CuSO4 and release hydrogen. Similarly, FeSO4 decomposes at 500–700°C, producing iron oxide (Fe2O3) and SO3, followed by hydrolysis to complete the cycle. The efficiency of sulfur recovery is crucial, as any loss of sulfur species reduces the cycle’s sustainability. Studies indicate that CuSO4-based cycles achieve sulfur recovery rates exceeding 90%, while FeSO4 systems reach slightly lower efficiencies due to side reactions forming stable sulfides.

Integration with solar concentrators enhances the viability of sulfate-based cycles by providing the high temperatures required for decomposition. Parabolic troughs and solar towers can deliver heat at the necessary ranges, with peak efficiencies observed when the solar flux is optimized to match the decomposition kinetics. Pilot projects, such as those conducted at the Solar Tower Jülich in Germany, have demonstrated the feasibility of coupling sulfate cycles with CSP, achieving hydrogen production rates of 5–10 kg per day. The use of solar energy reduces reliance on fossil fuels, lowering the carbon footprint compared to conventional steam methane reforming.

Compared to oxide-based thermochemical cycles, sulfate systems exhibit distinct advantages and challenges. Oxide cycles, such as those using cerium oxide (CeO2) or zinc oxide (ZnO), often require higher temperatures (above 1,200°C) for reduction, increasing energy demands and material degradation risks. Sulfate cycles operate at more moderate temperatures, reducing thermal stress on reactor materials. However, sulfate cycles must manage corrosive sulfur species, necessitating specialized containment materials like silicon carbide or nickel-based alloys. Oxide systems, in contrast, produce fewer corrosive byproducts but face challenges in oxygen separation during reduction.

Byproduct handling is another critical differentiator. Sulfate cycles generate SO2, which must be captured and recycled to prevent emissions. Advanced scrubbing systems and catalytic converters can recover over 95% of sulfur species, ensuring environmental compliance. Oxide cycles produce oxygen as a byproduct, which is less hazardous but offers limited economic value unless utilized in secondary processes. The energy penalty for sulfur recovery in sulfate cycles is offset by the lower overall thermal input compared to oxide systems, making them competitive in life-cycle assessments.

Material optimization efforts focus on improving the reactivity and stability of metal sulfates. Doping CuSO4 with small amounts of cobalt or manganese enhances decomposition kinetics, reducing the required temperature by 50–100°C. Similarly, nanostructured FeSO4 particles exhibit faster hydrolysis rates, boosting hydrogen yield. Research at the European Solar Thermal Energy Association has shown that tailored sulfate composites can achieve cycle efficiencies of 25–30%, approaching the theoretical limits for thermochemical water splitting.

Pilot-scale projects highlight the progress in scaling sulfate-based hydrogen production. The HYTHEC initiative in Europe tested a CuSO4 cycle coupled with a 100 kW solar concentrator, achieving continuous operation for 200 hours with minimal sulfur loss. In Japan, the Sunshine Project demonstrated FeSO4 cycles in a 50 kW reactor, emphasizing the potential for industrial waste sulfates as feedstocks. These efforts underscore the adaptability of sulfate cycles to diverse solar and waste-recovery applications.

In summary, metal sulfates offer a viable pathway for thermochemical hydrogen production, particularly when integrated with solar energy. Their moderate temperature requirements and high sulfur recovery rates make them competitive with oxide-based systems, though challenges in corrosion control and byproduct management persist. Ongoing material innovations and pilot projects are critical to advancing this technology toward commercial viability. As solar concentrator efficiency improves and reactor designs mature, sulfate-driven cycles may emerge as a key component of the sustainable hydrogen economy.