Session: 17-06: Symposium Steinfeld - Radiative and materials characterization and solar technology development

Paper Number: 133247

133247 - Efficient Solar Thermochemical Reactor for Fuel Production Enabled by Natural Convection Enhanced Oxygen Mass Transfer 

Abstract: 

High-performance thermochemical redox cycles for H₂O/CO₂ splitting require a high-temperature reduction step (typically ~1773K for benchmark material as ceria) with low oxygen partial pressure (po₂) environment. Enhancing the oxygen mass transfer in the reduction step is important in pushing the reaction thermodynamics for higher production rates as well as in alleviating temperature requirement. The integration of the reactor with high temperature electrochemical oxygen pump (EOP) for in-situ oxygen removal was found to be both flexible in tuning po₂ (from 1 to 10⁻²⁴ atm) and efficient in energy consumption (only ~ 2% of the total energy needed).

The challenges in practical engineering and operation of the EOP assisted reactor lie in (i) the limited oxygen transfer from ceria to the EOP governed by pure diffusion and hence limits the fuel production efficiency, (ii) the extremely high operation temperature of reduction step (~ 1773 K) which may lead to the failure of ceramic-based solid oxide EOP, e.g., function layers’ sintering and thermal failure of the clamp. To further enhance the oxygen mass transfer as well as to protect the EOP operation at high temperatures, we proposed an oxygen mass transfer enhancement method by inducing natural convection in the gap (local gas flow rate within 10^-5-10^-3 m·s^-1) to assist the otherwise pure oxygen diffusion (local gas flow rate within 10^-7-10^-5 m·s^-1) which can be achieved by active cooling the EOP to create the temperature difference in the gap. We define the two operation schemes: (i) Pure diffusion (PD) scheme without natural convection, and (ii) Convection enhanced (CE) scheme with natural convection induced by the temperature difference in the gap.

We demonstrated that the EOP electrode sintered at 1373K for 5 hours showed a highly porous structure achieving a current density of 315 A·m^-2 with an applied potential of 0.4 V at 1373 K (po_2,cathode = 10² ppm,  po_2,anode = 2.1×10⁶ ppm). Operating the EOP at higher temperature, i.e., 1673K which is relevant to ceria reduction temperature, led to a serious sintering problem in which the porous electrode degraded into a sintered structure after re-sintering at 1673K for 5 hours. The EOP current density reduced to 77.5 A·m^-2 at 0.4 V and 1473 K. Additionally, thermo-mechanical failure of the EOP clamp made of 304 stainless steel of which the melting point ranges from 1672K to 1727K can't be avoided.

This necessitates the implementation of the CE scheme for both mass transfer enhancement and the stable operation of the EOP of which the benefits are two-fold (i) natural convection in the gap introduces additional convective mass flux from ceria to EOP, (ii) the reduced EOP operation temperature can protect the device from thermal failure. During the reduction, the generated oxygen by ceria is transferred to the EOP across the gap either via pure diffusion in the PD scheme or combined convection and diffusion in the CE scheme. However, operating EOP at lower temperatures in CE scheme may lead to an increase in activation and ohmic overpotentials and a decrease in equilibrium potential. The quantitative analysis of the competing phenomena was discussed in detail in our study to guide the design and operation of the EOP. The ceria's thermochemical performance and the EOP’s electrochemical performance in PD and CE schemes were analyzed by numerically coupling the involved multiphysics. Results showed that the ceria δ is 2.24×10^-2 in the CE scheme and 2.26×10^-2 in the PD scheme after 1000s. Meanwhile, the current density exhibited a slight variance in the CE scheme compared with the PD scheme attributed to the mass transfer enhancement by natural convection, despite the EOP operation temperature in the CE scheme being 403 K lower than that in the PD scheme. Additionally, the EOP was fabricated and experimentally tested in a simulated environment  under various operation temperatures and po₂ to verify the performance.

Presenting Author: Runsen Wang Southern University of Science and Technology

Presenting Author Biography: Runsen Wang, a master's student from SUSTech, whose research interest is in solar thermochemical/electrochemical processes for hydrogen production.

Authors:

Runsen Wang Southern University of Science and Technology
Yuzhu Chen Southern University of Science and Technology
Meng Lin Southern University of Science and Technology