A team of researchers led by Northwestern Engineering professor and fuel cell pioneer Sossina Haile has created a new protonic ceramic fuel cell (PCFC) offering both exceptional power densities and long-term stability at optimal temperatures, a discovery that heightens the viability of incorporating fuel cells into a sustainable energy future. A paper describing their work is published in the journal Nature Energy.
The team overcame PCFC performance and stability challenges with a high-activity cathode (PrBa0.5Sr0.5Co1.5Fe0.5O5+δ, PBSCF), in combination with a chemically stable electrolyte, BaZr0.4Ce0.4Y0.1Yb0.1O3, BZCYYb4411). They deposited a thin dense interlayer film of the cathode material onto the electrolyte surface to mitigate contact resistance, an approach which is made possible by the proton permeability of PBSCF. The peak power densities of the resulting fuel cells exceed 500 mW cm−2 at 500 °C, while also offering exceptional, long-term stability under CO2.
|Scanning electron microscopy cross-section image of PBSCF/BZCYYb4411/ cermet anode fuel cell. Choi et al. Click to enlarge.|
Protonic ceramic fuel cells (PCFCs) incorporate a proton-conducting oxide as the electrolyte material. Like other fuel cells, they enable direct electrochemical conversion of chemical fuels to electricity at high efficiency and with zero emissions. They are particularly attractive in comparison to other fuel cells as a consequence of the high ionic conductivity of the electrolyte at at intermediate temperatures (400–600 °C). It is widely recognized that high-power operation in this temperature regime is key to lowering fuel cell costs. However, only a handful of studies report peak power densities of PCFCs exceeding 200mW cm−2 at 500 °C, whereas such performance is routine for traditional solid oxide fuel cells based on lower conductivity oxide-ion conducting electrolytes. A further challenge in PCFC development lies in the reactivity of many protonic ceramic electrolytes with CO2, precluding their use at intermediate temperatures with carbon containing fuels.
… In the present work we integrate three advances in PCFCs. First, we demonstrate exceptional proton solubility and transport through PBSCF, properties which render it ideal for oxygen electroreduction in PCFCs. Second, we establish that high ohmic losses have been a large part of the cause of poor cell performance to date and design a strategy to address this challenge. … Third, to address stability and processability challenges with known electrolytes, we introduce a compositional variant in the barium zirconate—barium cerate class, combining high stability with excellent processability and outstanding conductivity.—Choi et al.
Though recent research had demonstrated the potential of some protonic ceramic fuel cells to offer environmentally sustainable and cost-effective electric power generation, those cells’ high electrolyte conductivities failed to produce anticipated power outputs.
The Haile-led team overcame this persistent challenge by combining a high-activity cathode—the double-perovskite cathode PBSCF—with a new composition of matter, a chemically stable electrolyte labeled BZCYYb4411, to produce exceptional power density and stability in the highly prized intermediate temperature regime. This novel electrolyte allowed ions to move quickly and, unlike many previous electrolytes, remained stable even when operated for many hundreds of hours.
After years of scientists chasing high-power operation at 500-degrees Celsius—“a commercialization sweet spot,” Haile called it—the researchers’ discovery presents a significant step toward lower fuel cell costs and more sustainable energy.
The next challenge, Haile said, is to develop scalable manufacturing routes. Currently, getting the excellent contact between electrode and electrolyte requires a costly processing step. To bolster commercialization efforts, Haile and her team have ideas on how to approach this in a more cost-effective manner. Haile’s team will also investigate making the fuel cells reversible, which would transfer electricity back into hydrogen for placement on grid backup.
The research, funded in part by the US Department of Energy through the Advanced Research Projects Agency-Energy and by the National Science Foundation, included members of Haile’s lab team at Northwestern as well as scientists from the University of Maryland and the California Institute of Technology.
Sihyuk Choi, Chris J. Kucharczyk, Yangang Liang, Xiaohang Zhang, Ichiro Takeuchi, Ho-Il Ji & Sossina M. Haile (2018) “Exceptional power density and stability at intermediate temperatures in protonic ceramic fuel cells” Nature Energy doi: 10.1038/s41560-017-0085-9