Hydrogen fuel cells
Chemical engineer Dionisios Vlachos on the Apollo space missions, overpotentials, and proton exchange membranes
faq | March 14, 2016
Fuel cells represent a way of electrochemically converting hydrogen fuel into energy, producing only water as a by-product. The hydrogen fuel used in the fuel cell today is typically produced from steam methane reforming, although solar-powered water electrolysis will offer a green energy route. The base components of a fuel cell include:
In order to power a vehicle, multiple fuel cells are assembled into a fuel cell stack where the total power delivered is a function of the electrode area and number of cells in the stack.The fuel cell generates power due to electrons flowing from the anode, where hydrogen is oxidized, to the cathode, where oxygen is reduced. The electrons produced from the oxidation of hydrogen at the anode have a larger chemical potential than the electrons which are used to reduce oxygen at the cathode. This difference in the chemical potential of electrons enables energy to be extracted from the fuel cell by an external load.
The discovery of fuel cells
Fuel cells have a rich history dating back to the 1830s when the first hydrogen fuel cell was synthesized by William R. Grove utilizing a sulfuric acid electrolyte. The discovery was made due to Grove attempting to precipitate copper from an aqueous copper sulfate solution onto an iron surface. He noticed that water decomposed to form hydrogen and oxygen under an applied potential. Following this discovery, Grove, as well as Christian F.
These and other improvements in individual components has been the focus of research leading ultimately to the realization of fuel cell technologies, such as in the Toyota Mirai.
Hydroxide exchange membrane fuel cells
Hydroxide exchange membrane fuel cells (HEMFCs) are one more direction of research, in which, in particular, the University of Delaware is interested. HEMFCs offer an advantage over proton exchange membrane fuel cells (PEMFCs) since the stability of catalyst in acidic media is a major challenge and many more non-noble metal catalysts are stable in alkaline solutions. The increased stability of catalysts in alkaline solutions is owed to metal dissolution being, in general, more exothermic at lower pH than at high pH. Much of this work has been on developing new hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) anode and cathode catalysts, which are capable of speeding up the reaction. In addition, novel hydroxide exchange membranes have been developed, as the conductivity and long-term durability of HEMs need to be improved to compete with Nafion membranes in PEMFCs.
Search for new catalysts
Due to the excellent PEMs which have been invented, and also due to the relative ease of oxidizing hydrogen in an acidic environment, others have often focused on developing new cathode catalysts for ORR in acid. ORR is the largest source of overpotential losses in the fuel cell. Overpotential means that an additional thermodynamic driving force must be provided in order to achieve a given current density (increasing current density implies that a larger driving force is needed). In the fuel cell, this is equivalent to decreasing the chemical potential of electrons at the anode side, and increasing the chemical potential of electrons at the cathode side. As a result, the achievable electromotive force of the fuel cell decreases as the current density increases. The origin of overpotential losses for the ORR has been explained in terms of linear scaling relations (LSRs) which exist among the ORR intermediates. In a traditional four-electron ORR mechanism, O2 is sequentially reduced to form OOH*, O*, and OH* intermediates on the catalyst surface before ultimately evolving H2O from the catalyst surface. Because the adsorption energies of the ORR intermediates on a particular catalyst are highly correlated, a catalyst which is theoretically capable of requiring no additional overpotential has not been demonstrated as of yet. Even though the rate of ORR is low, when switching from the acidic to the basic environment of the HEMFC, the rate of ORR remains relatively unchanged. However, the rate of HOR is decreased by about two orders-of-magnitude, and thus, motivates a need to understand the origin of this decrease in activity and find better catalysts.
Advantages over other energy technologies
In contrast to fossil fuels, fuel cells are environmentally more or completely benign and do not produce greenhouse gases as a result of their operation. Furthermore, their fuel (hydrogen) is, in principle, renewable, namely through the hydrolysis of water to produce H2 and O2. As a result, the fuel cell offers the promise of being an integral part of a future renewable energy portfolio where solar or wind energy is used to produce the hydrogen fuel source, which is then used in the fuel cell to produce water, thus creating a closed loop with no carbon footprint.
The challenge of hydrogen storage
Outside of the issues of designing better catalysts and membranes, other technical challenges for fuel cells include the storage and transportation of the hydrogen source. Hydrogen has a very poor volumetric energy density (the amount of energy contained per unit volume at a given temperature and pressure), and therefore must be stored at extreme pressures in order for it to be incorporated into vehicle technologies. Otherwise, the size of the tank needed to store the hydrogen fuel would be prohibitively large for vehicle purposes.Due to this fundamental limitation of hydrogen storage, there has been work done on how to obtain hydrogen from sources other than gaseous H2, such as metal-hydride fuel cells. Nevertheless, current consumer fuel cell technologies, like the Toyota Mirai, have resorted to using supercritical H2(hydrogen which is at a pressure and temperature above its critical point of 13.3 atm and 33K as this is still the most viable option.
Future directions
As a result of the current challenges, as well as the unique challenges of solar production of H2 from H2O, future work will likely focus on alternative hydrogen sources that can be used to power a fuel cell. One popular idea we have been studying over the years is to use either ammonia directly in a fuel cell (in place of hydrogen), or to produce H2 from ammonia due to the capability of storing ammonia as a liquid at significantly lower pressures, and thus being much more facile to store and transport. In addition, ammonia is particularly attractive as a hydrogen source for fuel cells due it being free of carbon. As a result, the issue of catalyst poisoning due to trace amounts of CO in the methane-derived H2 can be eliminated.Ammonia oxidation, however, is itself not well-understood and requires large over potentials to achieve a sufficient current density due to the relative stability of NH3 in comparison to the reaction intermediates of the ammonia oxidation process. Interestingly, the same problem which limits ORR also is a limiting factor for ammonia oxidation: the difficulty of designing the optimal catalyst due to the existence of LSRs. As a result, by achieving a fundamental understanding of under what criteria these LSRs are broken, and through subsequent design of these catalysts in a laboratory, we might in a relatively short period of time solve many of the issues limiting the kinetics of these processes.
In the future, fuel cells may find an increased presence in vehicle applications and distributed power generation, e.g., in residential neighborhoods.While the use of fuel cells as a primary source of energy is currently cost-prohibitive, future advancements in the discovery of cheaper and more efficient catalysts, highly conductive and stable membranes, and alternative hydrogen sources offer the opportunity to make fuel cells significantly more attractive economically.
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