Transitioning from fossil fuels to sustainable and green energy sources in mobile applications is a difficult challenge and demands sustained and highly multidisciplinary efforts in R&D. Liquid organic hydrogen carriers (LOHC) offer several advantages over more conventional energy storage solutions, but have not been yet demonstrated at scale. Herein we describe the development of an integrated and compact 25 kW formic acid-to-power system. Formic acid is introduced as a viable liquid energy carrier for future mobile and static applications. Formic acid is effectively one CO2 molecule carrying one H2 molecule. In an energy storage system this CO2 is hydrogenated and subsequently dehydrogenated. This second part, a formic acid to power system, was developed and is schematically presented.
In short: the formic acid is dehydrogenated inside a generator, the hydrogen/CO2 gas flow is used in a tailored fuel cell system to produce power. Several dehydrogenation-catalysts were screened. A Ru-TPPTS water-based catalyst was eventually chosen because of its stability, commercial availability, ease of activation and low CO production. Tests show remarkably fast activity upon formic acid input and give stable gas outputs at moderate temperatures. Challenges remaining are: Purification of the hydrogen gas flow and energy integration to increase system efficiency. Economic viability requires sustainable produced formic acid at a target price below $450 per ton. Few companies are working on formic acid production using renewable sources of energy. More scale up projects are needed to accelerate this development. CO2 capture and storage at location would ensure a closed carbon cycle.
Audience Take Away:
We highlighted a number of key engineering challenges encountered during scale-up of the technology and discuss several aspects commonly overlooked by academic researchers
- High intrinsic catalytic activity was found to have little impact on the viability of the developed technology, whereas availability and stability of the utilized catalytic solution were found to be of critical importance. A high TOF catalyst is not the first objective for a scale up system. Many catalysts have been reported with TOFs exceeding a million. Often, they are not stable or very sensitive to e.g. water, oxygen or other common impurities in for example the formic acid feed. Research can focus more on the application of the catalyst. System design should be considered when developing catalyst and can compensate for low turnover frequencies.
- Lab scale experiments do not automatically show the same behaviour in scaled up systems. For example, we encountered CO production (poisoning the fuel cell) at larger scale but at lab scale the catalyst was tested to be 100% selective towards CO2 . Also, the scale up system at elevated pressure was less sensitive to foaming and more responsive upon formic acid input or depletion.
- The goal is not to reach a high reaction rate but a high productivity. Catalyst costs are relatively small compared to e.g. fuel cell costs. Catalyst concentrations can be varied and often deviate from research. In scientific publications, concentrations are deliberately kept low to reach high TOF, while higher concentrations increase productivity and give more insight in the applicability of the catalyst.
- The solvent choice has great influence on the required system design. Water is the preferred solvent but requires above-ambient-operating pressures to prevent unacceptable boil-off rates and high cooling duties. Other solvents such as ionic liquids degrade over time and poison the fuel cell with organic residues. Effluent gas flow purification is required to protect the fuel cell.