Colorado Fuel Cell Center wins follow-on award for Electrofuels

Electrofuels: Storage of renewable-but-intermittent electricity in the form of chemical bonds and fuels

Colorado School of Mines has received a follow-on award to develop proton-conducting ceramics for electrochemical synthesis of ammonia for storage of renewable solar and wind energy. The project is entitled “Protonic Ceramics for Energy Storage and Electricity Generation with Ammonia” and is funded by the U.S. Department of Energy, Advanced Research Projects Agency–Energy (DOE ARPA-E) as part of the REFUELS program: “Renewable Energy to Fuels Through Utilization of Energy-Dense Liquids.” The program is led by FuelCell Energy (FCE, Danbury, CT), with Colorado School of Mines serving as subcontractor to FCE. Mechanical Engineering Associate Professor Neal Sullivan serves as Primary Investigator on the program. Co-PIs include Professors Ryan O’Hayre of (Metallurgical and Materials Engineering), and Robert J. Kee (Mechanical Engineering). Effort is distributed across two of Mines’ leading research centers: the Colorado Fuel Cell Center (CFCC) and the Colorado Center for Advanced Ceramics (CCAC).

Reversible electricity-to-ammonia technology based on a proton-conducting electrochemical cell integrated with an NH3 catalyst.

Figure 1: Reversible electricity-to-ammonia technology based on a proton-conducting electrochemical cell integrated with an NH3 catalyst. The steam electrolyzer at left harnesses a novel protonic-ceramic materials set to produce H2 from water vapor and electricity feedstocks. The electron micrograph reveals a 5-μm-thick BaCe0.4Zr0.4Y0.1Yb0.1O3-d protonic-ceramic electrolyte sandwiched between the nano-scale steam electrode (top) and the micron-scale fuel electrode (bottom). Nitrogen sweeps the product H2 from the fuel electrode into the ammonia-synthesis reactor, where the two react over a ruthenium catalyst on a (BaO)2(CaO)(Al2O3) support to form NH3. The fine particle size of the catalyst over refractory insulation fibers is captured in the SEM at right. Image courtesy of Prof. R. J. Kee.

This research focuses on the electrochemical conversion of intermittent renewable electricity (solar & wind) into chemical bonds as means of energy storage.  Such “electrofuels,” or fuels synthesized using renewable electricity, can address the challenges of intermittent renewable energies in matching generation with demand. In this context, electrochemical energy storage together with reversible electrochemical cells are focusing the attention as the solution towards the final consolidation of intermittent renewable energies.

We harness the electricity generated by solar and wind to drive electrochemical cells that split water vapor into hydrogen and oxygen, and then react the product hydrogen with nitrogen to for ammonia (NH3). Ammonia is the world’s second-most-produced chemical, containing significant hydrogen in a high-density liquid form that is easily transported. Further, ammonia lacks any elemental carbon, making for a carbon-neutral storage media for renewable electricity.

An illustration of the process is shown in Figure 1. Separate reactors are used for electrochemical water splitting and thermochemical ammonia synthesis. Water vapor and nitrogen are fed to the positive and negative electrodes, respectively, of a proton-conducting ceramic electrolyzer (16, 23, 24). Renewable electricity drives the electrochemical water-splitting reaction at the steam electrode. Protons (H+) transport across the dense ceramic membrane to the fuel electrode, where they recombine to form H2 and mix with the N2 feedstock. The H2 / N2 mixture exits the electrochemical cell and enters a downstream reactor containing a novel ammonia-synthesis catalyst, forming NH3. The process is reversible. In “energy-storage” mode, feedstocks of H2O, N2 and electricity form NH3 fuel. In “energy-generation” mode, NH3 fuel is electrochemically oxidized to generate electricity, presenting a potentially fungible energy-storage device.

Figure 1 also provides a cross-sectional scanning-electron micrograph (SEM) of the reversible protonic-ceramic electrochemical cell (RePCEC). The “button cell” is centered on a 5-μm-thick, dense electrolyte of BaCe0.7Zr0.1Y0.1Yb0.1Ni0.04O3-δ (BCZYYbN). This electrolyte is mechanically supported on a ~ 500-μm thick porous composite Ni- BaCe0.7Zr0.1Y0.1Yb0.1O3-δ (BCZYYb) cermet fuel electrode, with a ~ 20-μm-thick porous BaCo0.4Fe0.4Zr0.1Y0.1O3-δ (BCFZY) triple-conducting oxide air-steam electrode.

Ruthenium dispersed on a novel (BaO)2(CaO)(Al2O3) (B2CA) support serves as the ammonia catalyst. The Ru-B2CA catalyst is mixed with refractory insulation fibers (Fig. 1) and packaged within a quartz reactor placed in the electrolyzer fuel exhaust.

While Figure 1 depicts the NH3-synthesis / energy-storage mode of operation, the process is reversible. The spatial decoupling of the electrochemical and catalytic processes presents benefits over the conventional integrated catalyst-cell design. The separation enables independent optimization of the size and operating conditions for each assembly. For example, the electrolyzer operates at 600 ºC for efficient H2 production, while the downstream NH3 synthesis is executed at 400 ºC for highest NH3-synthesis rates. Further, decoupling can be exploited to balance the NH3-Faradaic efficiency, limited by thermodynamics, and the H2-synthesis rate, driven by electric current.

The process shown in Figure 1 features ammonia as the working fluid.  Alternate fuels can be considered.  For example, the Colorado Fuel Cell Center is now funded by the National Energy Technology Laboratory to utilize the same electrochemical process for upgrading carbon dioxide into fuels. The products (CH4, CO + H2) can then be catalyzed into a CO2-neutral synthetic fuel. Such research has the potential to capture and utilize CO2 greenhouse gases as a renewable energy-storage medium.

Electrofuels synthesis is facilitated through higher-pressure electrochemical operation.  This presents an engineering and operational challenge, as few ceramic electrochemical devices have been characterized at elevated pressures and temperatures (20 barg, 500 ºC).  A team of students, engineers and scientists at the Colorado Fuel Cell Center has developed this technical capability, and is using it to advance reversible proton-conducting electrochemical cells.

Pressurized test stand assembly

Figure 2. Pressurized test stand experimental assembly.

Figure 2 illustrates our approach to high-pressure, high-temperature, electrochemical-performance characterization and fuels synthesis.  The protonic-ceramic electrolyzer is bonded to a composite ceramic frame. The MEA-ceramic frame assembly is packaged between two 0.5 mm-thick chemically exfoliated vermiculated gaskets (Thermiculite 870, Flexitallic) that provide the sealing between the frame and the interconnects. Metallic-mesh current collectors coated in silver paste are used to connect the cell electrodes with the metallic interconnects. The metallic interconnects are silver-paste bonded to the two end plates that host all the parts described.

The assembly is placed in a preloaded spring-based compression. Compression forces are transferred through the endplates, interconnects, gaskets and ceramic frame; the protonic-ceramic electrolyzer does not receive any direct compression force.

The assembly is packaged within a stainless-steel pressure vessel. The vessel is sealed with a crown that connects with gas supply and exhaust to the gas manifolds at the bottom end-plate in order to feed the gas reactants in and take the products out of the cell. Mass flow controllers regulate the mass flow that goes through the fuel and air inlet lines. The pressure vessel includes four heating elements that enable 600 ºC operating temperature.

Support for the research comes from the U.S. Department of Energy, The information, data, or work presented herein was funded in part by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy, under Award Number DE-AR0000808. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.  The authors wish to thank Professor Robert J. Kee for providing art illustration in Figure 1.