From waste to value: The right electrolytes can enhance glycerol oxidation
The glycerol’s hydroxyl groups are attracted to the Bi3+ ions on the surface of the BiVO4 photoanode. The electrolyte plays a decisive role in mediating these interactions. © HZB
When biomass is converted into biodiesel, huge amounts of glycerol are produced as a by-product. So far, however, this by-product has been little utilised, even though it could be processed into more valuable chemicals through oxidation in photoelectrochemical reactors. The reason for this: low efficiency and selectivity. A team led by Dr Marco Favaro from the Institute for Solar Fuels at HZB has now investigated the influence of electrolytes on the efficiency of the glycerol oxidation reaction. The results can help to develop more efficient and environmentally friendly production processes.
In 2023, around 16 billion litres of biodiesel and HVO diesel were produced in the European Union*, based on maize, rapeseed, or partially on waste materials from agricultural production. A by-product of biodiesel production is glycerol, which can be used as a building block for the production of valuable chemicals such as dihydroxyacetone, formic acid, glyceraldehyde and glycolaldehyde via a glycerol oxidation reaction (GOR). Glycerol can be oxidised electrochemically in (photo)electrochemical (PEC) reactors, which are currently being developed in particular for the production of green hydrogen. However, this path in PEC-plants is still hardly exploited at present, even though it could significantly increase the economic efficiency of the PEC Power-to-X process, since the oxidation of glycerol requires a much lesser energy input than hydrogen production through water splitting, but at the same time produces more valuable chemicals.
Examining the influence of different electrolytes
Many studies have already investigated the role of photocatalysts in PEC electrolyzers, while the role of the electrolyte had not yet been systematically analysed. A team led by Dr Marco Favaro at the Institute for Solar Fuels has now unveiled the influence of electrolyte composition on the efficiency and stability of the glycerol oxidation.
They used a PEC cell with photoanodes made of nanoporous bismuth vanadate (BiVO4). They tested acidic electrolytes (pH = 2) with various cations and anions, including sodium nitrate (NaNO3), sodium perchlorate (NaClO4), sodium sulphate (Na2SO4), potassium sulphate (K2SO4) and potassium phosphate (KPi). "Our results showed that BiVO4 photoanodes perform best in NaNO3 and outperform the commonly used Na2SO4 in terms of photocurrent, stability, and production rates of high-quality glycerol oxidation reaction products," summarises Favaro.
Sodium nitrate performs best
The team also investigated the reasons for this difference in performance: their hypothesis is that the size of the ions, their different salting in/out capabilities (Hofmeister series), and their different pH buffering capacity play a role. "The composition of the electrolyte has a surprising clear effect on the glycerol oxidation efficiency, and we were able to observe this trend in both bismuth vanadate and polycrystalline platinum anodes," says PhD student Heejung Kong. This supports the assumption that these findings could generally apply to different materials and processes.
The choice of electrolyte is therefore of great importance for the efficiency and stability of glycerol oxidation. "Our research could help to convert biomass by-products into valuable chemicals more efficiently and to produce valuable chemicals from waste materials while minimising the impact on the environment," says Favaro.
Note: This work was supported by the European Innovation Council (EIC) via OHPERA project (grant agreement 101071010).
- Battery research with the HZB X-ray microscopeNew cathode materials are being developed to further increase the capacity of lithium batteries. Multilayer lithium-rich transition metal oxides (LRTMOs) offer particularly high energy density. However, their capacity decreases with each charging cycle due to structural and chemical changes. Using X-ray methods at BESSY II, teams from several Chinese research institutions have now investigated these changes for the first time with highest precision: at the unique X-ray microscope, they were able to observe morphological and structural developments on the nanometre scale and also clarify chemical changes.
- BESSY II: New procedure for better thermoplasticsBio-based thermoplastics are produced from renewable organic materials and can be recycled after use. Their resilience can be improved by blending bio-based thermoplastics with other thermoplastics. However, the interface between the materials in these blends sometimes requires enhancement to achieve optimal properties. A team from the Eindhoven University of Technology in the Netherlands has now investigated at BESSY II how a new process enables thermoplastic blends with a high interfacial strength to be made from two base materials: Images taken at the new nano station of the IRIS beamline showed that nanocrystalline layers form during the process, which increase material performance.
- Hydrogen: Breakthrough in alkaline membrane electrolysersA team from the Technical University of Berlin, HZB, IMTEK (University of Freiburg) and Siemens Energy has developed a highly efficient alkaline membrane electrolyser that approaches the performance of established PEM electrolysers. What makes this achievement remarkable is the use of inexpensive nickel compounds for the anode catalyst, replacing costly and rare iridium. At BESSY II, the team was able to elucidate the catalytic processes in detail using operando measurements, and a theory team (USA, Singapore) provided a consistent molecular description. In Freiburg, prototype cells were built using a new coating process and tested in operation. The results have been published in the prestigious journal Nature Catalysis.