(491d) Self-Contained Electrochemical Process to Produce Pure Compressed Hydrogen from Hydrocarbons and Steam without an External Energy Supply

This paper develops and applies a physics-based model to predict the behavior of a concept that tightly couples a hydrocarbon-fueled solid-oxide fuel cell (SOFC) and an electrochemical protonic-ceramic hydrogen-separation cell (PSM). The system, called Compound Hydrogen Generation (CH2Gen), is designed to produce separated and compressed hydrogen without any external electrical energy supply.

The SOFC is configured as a typical anode-supported membrane-electrode assembly (MEA), with a relatively thick (order millimeter) porous composite anode, a thin (order 20 microns) oxygen-ion membrane, and a thin (order 20 microns) composite cathode. The role of the PSM is to electrochemically separate and compress hydrogen from reformed methane, such as reported by Malerød-Fjeld, et al. [1] The protonic-ceramic membrane is a doped-perovskite material such as yttrium-doped barium zirconate (BaZr_0.8Y_0.2O_3-d, BZY20). Because both sides of the PSM operate in a reducing gas-phase environment, the cell can be symmetric with both electrodes using materials such as Ni-BZY.

The SOFC and the PSM could each be operated alone, but the SOFC would need to be connected to an external load and the PSM connected to an external power supply. The innovation with the CH2Gen is associated with the electrical and gas-flow connections between the SOFC and the PSM.

The present paper considers two alternative gas-handling coupling strategies. In both cases, the anode (negatrode) of the SOFC is connected electrically to the cathode (positrode) of the PSM. The cathode (positrode) of the SOFC is connected directly to the anode (negatrode) of the PSM. An important consequence of the electrical coupling is that the SOFC and PSM must operate at the same voltage and current.

Parallel configuration

Figure 1 illustrates a parallel CH2Gen implementation. In this case, the SOFC and the PSM share the same fuel chamber, with catalytic fuel reforming within both the SOFC anode and the PSM positrode structures. As a result of oxygen-ion transport through the SOFC membrane, oxygen enters the fuel chamber. As a result of proton transport through the PSM membrane, hydrogen leaves the fuel chamber. As illustrated in Fig. 1, the air chamber, the fuel chamber, and the hydrogen-collection chambers are all represented as perfectly stirred reactors [2].

Although not strictly necessary, both the SOFC and the PSM should operate at the same temperature. Considering catalytic steam reforming, protonic membrane performance, and oxygen-ion membrane performance, operating temperatures should be around 600--700 ËšC. Thus, at relatively low operating temperatures, the SOFC should likely be based on a doped-ceria (e.g., Ce_0.9Gd_0.1O_2-d, GDC10) membrane rather than a yttria-stabilized (YSZ) membrane.

Serial configuration

In a serial CH2Gen implementation, the SOFC and PSM are physically separated, with exhaust gas from the SOFC anode chamber feeding directly to the PSM anode chamber. Electrically, however, the SOFC cathode is wired directly to the PSM anode. As illustrated for the parallel implementation, all the gas chambers are idealized as perfectly stirred reactors (PSR).

The SOFC is fed by a hydrocarbon-steam mixture and air. Steam reforming proceeds within the catalytic surfaces of the porous composite SOFC anode (e.g., Ni-BZY). Hydrogen is electrolyzed by oxygen ions O^2- emerging from the ion-conducting membrane (e.g., GDC). The gas-phase composition of the SOFC chamber depends on the fuel feed composition and rates, reforming rates, oxygen-ion flux, and gas-phase residence time.

The exhaust of the SOFC anode chamber is fed directly to the negatrode-side of the PSM. In addition to further reforming on the PSM composite electrode to produce H₂, electrochemical processes deliver protons to the proton-conducting membrane. Because the PSM membrane is polarized (i.e., same voltage as the SOFC), protons migrate across the membrane to deliver H₂ into the hydrogen-collection chamber. Assuming a back-pressure regulator on the exhaust of the H₂-collection chamber, the H₂ may be electrochemically compressed.

Operational constraints

The CH2Gen electrical configuration requires that the SOFC and PSM operate at the same voltage and current (electrical and ionic). A further practical constraint is that the exhaust from system be dominantly CO₂ and H₂O, with only trace levels of H₂, CO, or other hydrocarbon side products.

Assume that the hydrogen-collection chamber is held at an elevated pressure (e.g., 5 bar) and that the PSM fuel chamber has only small H₂ partial pressure. Thus, there is a strong electrochemical driving potential to transport H₂ from the hydrogen-collection chamber back to the PSM fuel chamber. The operating voltage for the PSM (and thus the SOFC) must be sufficiently high as to force low-concentration H₂ into the elevated-pressure H₂-collection chamber. In other words, the SOFC voltage must be greater than the open-circuit voltage associated with the PSM MEA.

Mathematical models

The CH2Gen system tightly couples the SOFC with PSM through the fuel flow and the electrical connections. The present model treats the SOFC and PSM as button cells with the gas compartments modeled as perfectly stirred reactors (PSR). The electrochemical button-cell models build upon previous models for fuel cells, electrolysis cells, membrane separations, etc. [3-12]. The SOFC and PSM models are coupled with an iterative algorithm that forces the SOFC and PSM to operate at the same voltage and current.

Summary and conclusions

A model is developed and applied to evaluate the feasibility of a novel electrochemical cell architecture that is designed to reform hydrocarbons, separate and electrochemically compress H₂, and emit only CO₂ and H₂O---all without any external electrical power. The cell combines a solid-oxide fuel cell (SOFC) with a protonic-ceramic separation membrane (PSM) in a unique way. The negatrode of the SOFC is wired to the positrode of the PSM and the negatrode of the PSM is wired to the positrode of the SOFC. In this way, the SOFC provides the power needed to drive the PSM.

Because of the unique electrical connection, both the SOFC and the PSM must necessarily operate at the same voltage and current. Two alternative gas-connection architectures are considered. In the parallel case, the SOFC and PSM share the same fuel chamber. In the serial case, the exhaust of the SOFC fuel chamber feeds the downstream fuel chamber of the PSM. In both cases, all the gas flows are modeled as perfectly stirred reactors.

The results establish the feasibility of the concept, called CH2Gen. Assuming that the concept is viable, process scale-up will likely require alternative gas-handling approaches. As is typical in fuel-cell technology, channel architectures will likely be appropriate. However, the next steps in technology development are beyond the scope of the present feasibility study.

References

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