The Center for Alkaline-Based Energy Solutions (CABES), a Cornell University-led Energy Frontier Research Center (EFRC) established in 2018, is advancing the scientific understanding of electrochemical energy conversion in alkaline media at the fundamental level. Using this new knowledge, CABES is aiming to solve the technological challenges that have hindered widespread integration of fuel cells into our energy infrastructure.
One major global challenge is building an energy infrastructure that can sustainably meet our ever-growing energy demands. A vital component of an energy infrastructure is a chemical fuel (e.g., gasoline) because chemical fuels are easily transportable and energy dense. Currently, fossil fuels fill the role of chemical fuels, however, these fuels come with a high environmental cost: burning fossil fuels releases carbon dioxide into the atmosphere, which acts as a powerful greenhouse gas.
As opposed to fossil fuels, hydrogen gas is a chemical fuel that combusts without releasing carbon dioxide. Furthermore, it has three times the energy density of gasoline, making it an ideal energy carrier. Hydrogen can be produced in an environmentally benign process by splitting water. Devices that do this are known as water electrolyzers, and devices which utilize hydrogen as a fuel to produce electrical energy are known as hydrogen fuel cells, see Figure 1. There are several flavors of fuel cells and electrolyzers, but two of the most readily utilized architectures rely on acidic and alkaline electrolytes. Though alkaline fuel cells have been used as far back as the 1960’s to power the Apollo spacecrafts, they have yet to gain the large-scale adoption required to offset fossil fuel usage.
Why alkaline electrolytes
As its name implies, the scientific focus of CABES is understanding and advancing catalysis and transport in alkaline electrolytes. High-performance catalysts are required for low-temperature polymer electrolyte fuel cells because the rate of a chemical reaction decreases exponentially with decreasing temperature. Further, catalysts increase the energy efficiency of the hydrogen cycle depicted in Figure 1. Acidic electrolytes tend to eat through most non-noble metals, leaving scarce and expensive metals like platinum as the catalyst choice. Alkaline electrolytes can alleviate this cost burden by enabling the use of a diverse selection of inexpensive and abundant non-noble metal catalysts.
Catalyst design principles
CABES has employed several strategies to design high performance and inexpensive catalysts for fuel cells and electrolyzers. One strategy is to take advantage of the groupings in the periodic table. Palladium is significantly more cost effective than platinum and located directly above platinum on the periodic table, suggesting they share chemical properties. Experiments have shown that palladium does indeed have good catalytic activity for oxygen reduction, but lags behind platinum, offsetting much of its cost benefit. To improve the catalytic activity of this material, researchers have explored nickel-palladium alloys.1 Specifically, nickel-palladium alloy nano-particles were synthesized, and their surface was enriched with palladium by removing surface nickel atoms through electrochemical dealloying. This improves the activity of the catalyst through two separate mechanisms: lattice strain and atom efficiency.
Lattice strain refers to the fact that the distance between atoms in the alloy is smaller than in pure palladium because the atomic radius of nickel is smaller than palladium. Since the arrangement of atoms influences the electronic structure, and electronic structure dictates activity, this is a method for tuning the activity of the metal. Specifically, theory has shown that the effect of lattice strain is to change the energetic position of the metal d bands, which in turn regulates the adsorption energy of reaction intermediates.2 There is a 'Goldilocks' adsorption energy—an energy strong enough to cause reactant adsorption and reaction initiation, but weak enough to allow products to desorb from the surface. Product desorption is particularly important, otherwise the catalyst will be poisoned and its effectiveness ruined.
Atom efficiency refers to the fact that only atoms on the surface of the catalyst participate in the catalytic cycle. The atoms in the bulk of the material are ‘dead weight’ as they are insulated from the interface and do not participate in the catalysis. Synthesizing small (8 nm) particles maximizes the surface area to volume ratio, and the core-shell structure ensures the active and expensive palladium atoms are on the surface while nickel fills the center of the particle to provide structural support. The electrochemically dealloyed Pd-Ni catalysts showed enhanced activity compared to commercial palladium catalysts, and a negligible decrease in activity after 4000 potential cycles in alkaline media.
Transition metal spinel oxides are another class of alkaline catalysts explored by CABES researchers. These dispense with platinum group metals entirely and are essentially free in comparison to Pt. A systematic study investigated 15 types of spinel oxide electrocatalysts for the oxygen reduction reaction in alkaline fuel cells.3 It was found that the mixed metal oxides MnCo2O4 and CoMn2O4 significantly outperformed their single metal counterparts Co3O4 and Mn3O4, suggesting a synergistic effect between Mn and Co. The fundamental chemistry behind this effect was explored with high-resolution analytical electron microscopy and in situ X-ray absorption spectroscopy.
Electron microscopy was used to track the atomic structure (position of atoms) and revealed a core-shell structure where the Co preferentially segregates to the center and the shell is enriched in Mn. This distribution of Mn and Co could be continuously tuned by varying the composition and synthesis temperature. This highlights the advanced degree of control over catalyst structure that can be achieved.
In situ X-ray absorption spectroscopy was used to track the electronic structure (oxidation state) and revealed both Mn and Co valences changed cyclically with applied potential, suggesting the two atoms may serve as co-active sites as part of a synergistic catalyst mechanism.4 An in depth follow-up study based on controlled catalyst structure, isotope effects, and theoretical calculations provided strong evidence for a synergistic mechanism where Mn sites bind O2 and the Co sites activate H2O, so as to facilitate the proton-coupled electron transfer processes.5
Importantly, Mn-Co spinel catalysts exhibit phenomenal catalytic activity, outperforming commercial platinum catalysts in low humidity (less than 100% relative humidity) environments. Such testing conditions are more representative of real-world fuel cell applications. With Mn-Co spinel serving as the cathode catalyst, a power density of over 1 W/cm2 was achieved in an alkaline fuel cell. This is a value that begins to compete with platinum-based, state-of-the-art acidic fuel cells.
Conclusion
Though fuel cells have been a research focus for the past few decades, these devices have yet to see large-scale commercialization. Recent developments in state-of-the-art alkaline anion exchange membrane fuel cells have the potential to remedy the major drawbacks of liquid electrolyte (low power density) and proton exchange membrane (high platinum requirement) fuel cells. Once these last few challenges are solved, we are likely to see a significant shift towards a renewable hydrogen energy economy. CABES is working to understand the fundamental science governing alkaline electrocatalysis and to produce functional materials that will realize the full potential of alkaline based energy technologies. For more information on alkaline electrocatalysis and polymer electrolytes, CABES researchers recently published a comprehensive review in Chemical Reviews.6
