Renewable generation technologies, such as solar and wind, pose a fundamental challenge to centralized power generation due to variability and intermittency, ARPA-E noted. In addition, centralized generation frequently requires long transmission distances that result in power losses and leave lines susceptible to disruption during natural disasters. Many of these challenges can be mitigated through a distributed system, where power is generated in close proximity to the end-user.

REBELS addresses these challenges by developing innovative, low-cost distributed generation technologies using electrochemical power generation that can also act as a storage device, or, potentially as a means to generate liquid fuels—essentially, electrochemical gas-to-liquids (GTL).

Current fuel cell research generally explores technologies that either operate at high temperatures for grid-scale applications or low temperatures for vehicle technologies. Over the past ten years, this work has advanced high-temperature solid oxide fuel cell (SOFC) and low-temperature proton exchange membrane (PEM) technologies in both performance and cost.

Advantages and challenges of low temperature PEMFCs and high temperature SOFCs. Source: ARPA-E. Click to enlarge.

REBELS projects will focus on developing intermediate-temperature fuel cells through innovative designs, fuel activation approaches, and low-cost materials to facilitate widespread distributed power generation.

Potential benefits of fuel cell operation in an intermediate temperature range. Source: ARPA-E. Click to enlarge.

While the technologies that emerge from the REBELS program will be at earlier stages of their learning curves than current PEM and SOFC technologies, ARPA-E’s view is that fuel cell operation in an intermediate temperature regime could enable unique opportunities for cost reduction and performance improvement with multiple pathways to market adoption.

REBELS builds on materials advances over the past decade that have broadened the number of available electrolytes and electrodes beyond traditional PEM and SOFC temperature ranges. ARPA-E intends to bring together different scientific communities, such as fuel cell materials scientists, inorganic and polymer chemists, researchers working on novel approaches to activate carbon/hydrogen bonds for fuel processing, and experts in fuel cell fabrication methods to quickly advance intermediate temperature fuel cells to working prototypes and engage with stakeholders who can drive these devices towards market adoption.

ARPA-E also aims to alter fundamentally the paradigm of fuel cell systems by creating new functionality in fuel cell technology such as battery-like response to transient loads and electrochemical production of liquid fuels.

The use of such a device for liquid fuels production would depend on its chemistry and location. One example would be for the device to be coupled to a variable renewable generator such as a wind turbine or solar panel, similar in concept to the power-to-gas systems emerging in Germany. Excess electricity generated by the renewable resource could be used to electrochemically convert gaseous fuel such as methane to a liquid chemical to be stored in bulk.

In another embodiment, a fuel cell could be located at a natural gas wellpad or digester, with the fuel cell providing power (electrical output) and also converting excess natural gas to a more easily transported, and higher value liquid chemical. In these examples, the electrochemical cell can be operated with electricity as an input or an output, depending on the electrochemical reactions coupled by the device. If the reaction is thermodynamically favorable, the device could potentially produce chemicals and electricity simultaneously.

Categories Of Interest

REBELS is focused on supporting efficient, reliable, and fast-response ITFCs in one or more of the following three categories:

ITFCs for DG applications. This category focuses on the creation of a 100 W short stack prototype that demonstrates high efficiency and reliability, as well as a pathway to lower cost via a combination of inexpensive materials and reduction of overall system components. The final performance metrics must be met with the use of a non-hydrogen gas or liquid fuel.

Projects in this category will focus on two of the three subsystems in an overall fuel cell system: the fuel processor and the fuel cell stack. The third subsystem, power electronics, is the focus of other ARPA-E programs such as Agile Delivery of Electrical Power Technology (ADEPT), and will not be a focus of this program.

Technical targets for Category 1. Click to enlarge.

ITFCs with in-situ charge storage for dynamic response. This category focuses on fuel cells that also store charge in an electrode, enabling battery-like response to transient loads. For example, an electrochemical cell consisting of a metal hydride anode, proton-conducting electrolyte, and cathode could operate either as a fuel cell or a rechargeable metal hydride/air battery.

An intermediate operating temperature increases the number of potential anode materials, as there are many more materials available above 100 °C with hydrogen storage capacities > 7.5 wt%. Such a device could have a much faster response to transient loads that are currently addressed by integrating fuel cells with either batteries or ultracapacitors.

This new concept would integrate fuel cells and charge storage at the device-level rather than system-level, thus reducing the number of system components required for a given functionality. Similar functionality is envisioned for oxygen-based electrolytes with redox-active electrode species.

ARPA-E emphasized that these are only examples, and not meant to prescribe or limit the technical approaches that might receive an award though the REBELS program.

Technical targets for Category 2. Click to enlarge.

ITFCs with fuel production capability. This category focuses on ITFCs that can also convert methane or other gaseous hydrocarbons to liquid fuels using excess renewable energy. Whereas high temperature operation typically results in reversible conversation of H₂ and O₂ to water or complete oxidation of CH₄ to H₂ and CO₂, intermediate temperatures could enable partial oxidation of CH₄ to CH₃OH or the formation of carbon-carbon bonds to make other liquid fuels or higher value chemicals.

Examples could include conversion of methane or another hydrocarbon fuel to syngas, methanol, benzene, ethers, olefins, or other organics. The proposed choice of electrochemical half-reactions would determine whether electricity is an input or output in this device. Either would acceptable for this category.

This particular use of an electrochemical cell likens it to a small-scale gas-to-liquids reactor (GTL). The economics of GTL reactors were presented in the ARPA-E Reducing Emissions Using Methanotrophic Organisms for Transportation Energy (REMOTE) FOA. (Earlier post.)

Traditional GTL plants can only be built at large scale in order to achieve economic payback. These plants generally have a production capacity of >104 barrels of oil equivalent per day (bpd), and high capital cost of the reactor per unit capacity, usually >$100,000/bpd. Electrochemical GTL has the potential to outperform these systems in cost, throughput, and efficiency while keeping the footprint of the reactor small. A competitive system would have lower cost per capacity, high process intensity, high selectivity, and long lifetime.

Technical targets for Category 3. Click to enlarge.