Over the past few decades, the tremendous demand for energy caused by industrialization has resulted in a substantial increase in CO₂ emissions. Global CO₂ emissions are closely correlated with the burning of coal, which has been a primary fossil energy source for centuries and will remain one for decades more. The extensive consumption of coal leads to dramatic CO₂ emissions (Xu and Zhang 2012).

There are several options for reducing CO₂ emissions: (1) adopting renewable energy sources such as wind, solar, and geothermal energy; (2) improving the power generation efficiency of existing fossil energy sources, particularly coal; and (3) realizing the conversion and utilization of CO₂. Integrated gasification fuel cell (IGFC) systems combine coal gasification with high-temperature fuel cells to increase power generation efficiency and improve environmental sustainability compared to conventional coal-fired power generation systems (Peng and Han 2009). IGFC is regarded as the most promising process to achieve near-zero CO₂ emissions from coal power generation in the twenty-first century (Li et al. 2018).

In the last decade, both the U.S. and Japan have made tremendous investments in the development and application of IGFC systems (Damo et al. 2019; Discepoli et al. 2012; Li et al. 2010). In 2017, the China energy group launched the IGFC for near-zero CO₂ emissions program with support from the National Key R&D Program of China and 11 other organizations. This project focuses on coal gasification purification, high-temperature fuel cells, CO₂ capture and conversion, and IGFC system integration. The project will conclude with the demonstration of a megawatt-level (input heat value) IGFC system with near-zero CO₂ emissions.

Syngas produced by coal gasification, which has been used as a fuel for solid oxide fuel cell (SOFC) stacks, usually retains a small amount of unreacted CO and H₂ after the electrochemical reaction. Both power generation and CO₂ enrichment require the complete conversion of CO and H₂; thus, the catalytic combustion of the fuel gas is an essential component of the IGFC process (Sung et al. 2018; Kawabata et al 2012). Meanwhile, the thermal efficiency can be increased by using the exhaust heat in the reforming process, which also benefits from the efficient combustion of the fuel gas (Leea et al. 2013; Liese 2010).

Conventional catalysts including Pt and Pd have long been used in flue gas combustion. To mitigate the deactivation caused by water and sulfur compound at high temperature, Pd/Pt bimetallic catalysts that can achieve stable and complete combustion in an SOFC stack were recently developed (Hoque et al. 2012). In addition to the high cost of noble metal catalysts, catalytic deactivation at high vapor content and high temperature is a severe problem (Vepřek et al. 1986; Rudra and Kim 2010; Trembly et al. 2007). Considering these difficulties, we developed perovskite-based catalysts for kW-level catalytic combustion in this work.

To realize zero CO₂ emissions and energy storage, the electrochemical transfer of CO₂ into chemicals can be implemented using renewable energy resources, including wind and solar energy. Solid oxide electrolysis cells (SOECs), which are based on the inverse process of SOFCs, have been experimentally demonstrated to directly co-electrolyze H₂O/CO₂ into syngas (CO + H₂). Moreover, SOECs have the benefits of low energy consumption and high efficiency (Yang et al. 2019a, b).

One key challenge in the application of SOECs is the unreliability of SOEC stacks, which consist of, at a minimum, a metallic interconnect, sealing material, single SOEC, and electrode contact materials. Research on SOEC stacks is still limited. Ebbesen et al. (2011) reported SOEC stacks for steam electrolysis and CO₂/steam co-electrolysis. Zhang et al. (2013) demonstrated the long-term durability of SOEC cells and stacks. In China, SOEC stacks for hydrogen production have only been reported once: Zheng et al. (2014) manufactured and tested 30-cell nickel-yttria-stabilized zirconia hydrogen electrode-supported planar SOEC stacks at 800 °C in steam electrolysis mode.

In this work, we developed kW-level SOEC stacks and evaluated their electrochemical performance for H₂O electrolysis and H₂O/CO₂ co-electrolysis.

2.1 Objective

The aims of the IGFC for near-zero CO₂ emissions program include coal gasification, syngas purification, CO₂ capture & utilization, and IGFC system integration, as schematically shown in Fig. 1.

Schematic diagram of the IGFC for near-zero CO₂ emissions program

The objectives of the IGFC for near-zero CO₂ emissions program are to: (1) further develop coal gasification and syngas purification technology; (2) realize the efficient conversion of chemical energy from syngas in fuel cells; (3) establish a kW-level catalytic combustion system with a ≥ 99% conversion rate of combustible components; (4) explore a new method for CO₂ conversion; and (5) establish a kW-level SOEC verification platform. The ultimate goal is near-zero CO₂ emissions.

2.2 Scope of work

This research mainly focuses on catalytic combustion and SOFC/SOEC technology with the following objectives: (1) establish a simulation model for SOFC and SOEC to analyze the mechanisms of internal heat and mass transfer, optimize the fuel cell working mode, and improve fuel utilization; (2) investigate the mechanism of catalytic combustion involving exhaust gas, catalyst materials, and catalytic burners; (3) develop new exhaust gas catalysts; (4) study the catalytic combustion characteristics of exhaust gas to inform the construction of high-performance exhaust gas catalytic combustors; (5) establish a kW-level SOEC verification platform that optimizes the surface and interface microstructures of the electrode catalysts, reduces the polarization electromotive force and resistance of the CO₂ reduction reaction, improves the reaction efficiency, and reduces energy consumption; and (6) develop a SOEC module for CO₂/H₂O co-electrolysis.

3.1 SOFC/SOEC model

By establishing the simulation model for SOFC and SOEC, it is possible to deeply analyze the mechanism of internal heat and mass transfer and improve fuel utilization. The primary function of the simulation was to optimize the design of the electrodes, cells, and stacks. The microscopic model was used to optimize the electrode mirostructure and analyze the long-term stability of the cell.

The main progress is summarized in Fig. 2 and elaborated as follows:

Development of the SOFC/SOEC model

1. (1)

   A comprehensive steady-state model was developed to investigate the effects of electrode structure on SOFC performance, specific heat and mass transfer, and electronic and ionic charge transport. Percolation theory was used to evaluate the transport properties of the electrodes. The results show that for small particle sizes less than 0.4 μm, the optimal thickness of the functional layer is between 5 and 30 μm. For thicker functional layers, a relatively larger diameter results in better cell performance. The uniform and non-uniform distributions of electronic/ionic conducting materials in the anodic/cathodic functional layers were comprehensively compared. The findings also provide an alternative microstructural design in consideration of the non-uniform distribution of conducting materials, which is meaningful for fuel cell optimization.

2. (2)

   An enhanced quasi-two-dimensional, non-isothermal model for SOFC parametric simulation and optimization was proposed. The dependence of electric power generating efficiency on microstructural parameters is fully considered in this model. In addition, an elementary effect approach based on Monte Carlo experiments was adopted to comprehensively evaluate the sensitivity of all parameters. Subsequently, a feasible non-uniform distribution method in allusion to the functional layers was proposed to further improve cell performance along the channel direction.

3. (3)

   Three-dimensional (3D) SOFC/SOEC models were developed, and the effects of the new flow field structure of the oxygen electrode were analyzed. The effects of different cell operating parameters on the CO₂/H₂O co-electrolysis characteristics in SOECs were studied, and corresponding optimization strategies were proposed. The adoption of a new type of porous material could alter the flow field to reduce the electrolysis voltage (0.026 V at 25,000 A/m²) and increase the electrolysis efficiency (4.78% at 25,000 A/m²). In addition, from the theoretical point of view, the mechanism of CO₂ enrichment and conversion in SOFCs/SOECs has been clarified.

4. (4)

   A phase-field model was established to describe the morphological evolution of a porous electrode in a SOFC. The reductions in the three-phase boundary (TPB) density and performance caused by Ni coarsening at high temperature were evaluated using this model. At the operating current density of 4000 A/m², the total overpotential increased from the initial value of 0.176–0.191 V after 3750 h, and the activation overpotential increased by 0.012 V at 850 °C. In addition, the effects of the Ni content and other microstructural parameters were considered. Using this model, the real microstructure of the porous electrode and the related parameters can be characterized.

5. (5)

   A 3D kinetic Monte Carlo (KMC) model was developed to study the sintering kinetics and microstructural evolution of SOFC composite electrodes during the sintering process. The catalytic activity of the cathode materials, which based on the triple-point-boundary length, porosity, and tortuosity were calculated during KMC sintering. This model also provides a real microstructure for further research on the effects of microstructure on fuel cell electrochemistry and performance.

6. (6)

   A pore-scale Lattice–Boltzmann model was established to simulate the reactive transport processes in the cathode functional layer of the SOFC. The coupled effects of oxygen diffusion and charge transport in the nanometer-scale functional layer are fully considered. The effects of microstructure on cell performance were investigated in terms of species distribution, reactive area, and reaction rate, thereby providing a theoretical basis for optimizing the electrode structure.

3.2 Exhaust gas catalytic combustion technology

Water- and CO₂-resistant perovskite and hexaaluminate catalysts (La_0.8Sr_0.2Al_0.5Mn_0.5O_3−δ and LaMnAl₁₁O₁₉, respectively) for SOFC off-gas combustion were successfully synthesized and deposited on a honeycomb ceramic substrate using a dip-coating technique. Figure 3 shows a schematic diagram of the preparation process. To reduce the difficulty in sealing the SOFC stack, the pressure drop of the catalytic combustor was reduced to the greatest extent possible by using a monolithic catalyst with a hole through it.

Preparation process of the monolithic catalyst

A kW-level SOFC catalytic combustion burner and the testing system were established, as shown in Fig. 4.

Picture and schematic of the kW-level catalytic combustion burner

The conversion rates of H₂ and CO achieved using the self-developed monolithic catalyst and catalytic burner are shown in Fig. 5 for typical SOFC simulated exhaust gas with high water content. During the operation period, the conversion rates of H₂ and CO were higher than 99% and 95%, respectively, without any significant decay.

H₂ and CO conversion rates achieved by the kW-level catalytic combustion burner at at 800 °C

3.3 SOEC materials, cells, and stacks for CO₂ conversion

3.3.1 SOEC materials

New perovskite electrode materials such as La_0.4Sr_0.6Co_0.2Fe_0.7Nb_0.1O_3−δ (LSCFN) and Sr₂Fe_1.3Co_0.2Mo_0.5O_6−δ have been developed via the in situ precipitation of nanoparticles (Yang et al. 2019a, b) and demonstrated to facilitate CO₂ reduction in SOECs. The precipitation process is shown in Fig. 6. In our previous work, a high current density of 0.442 A/cm² and a low polarization resistance were obtained at 1.5 V with pure CO₂ at 800 °C due to the excellent CO₂ adsorbability of LSCFN.

The catalytic activity of perovskite electrode improved with the exsolved nanoparticles

Yang et al. (2020) reported a one-step synthetic method to fabricate Sr₂Fe_1.3Co_0.2Mo_0.5O_6−δ-Gd_0.1Ce_0.9O_2−δ composite electrode materials for symmetrical SOFCs. These materials also show great promise for application in SOECs.

3.3.2 Anode-supported single cells

Anode-supported SOFC/SOEC single cells with sizes of 156 mm × 70 mm were successfully fabricated with good quality and high yield by tape casting and sintering (Fig. 7). The active area of the oxygen electrode was approximately 85 cm². The cell production process can also be scaled up for stack development.

Anode-supported SOFC/SOEC single cell

3.3.3 Stack development and performance

We have developed a unique stack design with an open-air structure that can be integrated into a more extensive SOFC/SOEC system. As shown in Fig. 8, the kW-level stack for SOEC applications consists of 30 cells.

Image of the kW-level stack for SOFC/SOEC applications

Figure 9a shows the electrolysis results obtained with different steam contents. Higher steam contents resulted in lower electrolytic voltage. At 750 °C, the electrolytic voltage was only 35.6 V with 90%H₂O/10%H₂ under 29 A, a hydrogen electrode flux of 11.62 L/min, and an oxygen electrode flux of 83 L/min. The calculated rates of H₂O conversion and hydrogen production are shown in Fig. 9b. The conversion rate of H₂O was 58%, and the hydrogen production rate reached 6.06 L/min. The electrolytic efficiency \(\eta_{LHV}\) was determined using the following equation:

Electrochemical performance of the stack for H₂O electrolysis

where, V_H2 is the electrolyzing rate of H₂, LHV_H2 is the lower heating value of H₂ (LHV_H2 = 3.00 kWh/Nm³ for electrolysis), P_el is the electric energy consumption, U_c is the applied voltage per single cell, and \(\eta_{\rm F}\) is the Faraday efficiency, which was close to 100% in our experiments. The electrolytic efficiency during hydrogen production in this study reached 105.3%. The results indicate that that as the steam content increased, the electrolytic efficiency increased, while the conversion rate of H₂O slightly decreased.

Figure 10 shows the electrochemical performance of the stack for H₂O/CO₂ co-electrolysis. The results indicate that the electrolytic voltage of H₂O/CO₂ co-electrolysis was negatively correlated with the steam content. At 750 °C, the electrolytic voltage was 37, 38.1, or 38.8 V for 60%H₂O/30%CO₂/10%H₂, 45%H₂O/45%CO₂/10%H₂, or 30%H₂O/60%CO₂/10%H₂ under 32 A, respectively.

Electrochemical performance of the stack for H₂O/CO₂ co-electrolysis

In this work, a set of multi-scale SOFC and SOEC models was established to analyze the mechanisms of internal heat and mass transfer and optimize the structure and operating parameters. SOFC stack flue gas catalytic combustion and SOEC technology were evaluated for CO₂ capture and conversion in IGFCs. Perovskite and hexaaluminate catalysts were successfully fabricated. The kW-level catalytic combustion burner, the perovskite electrode materials, and a single SOEC were developed. Furthermore, the kW-level stack was assembled, and the electrochemical performance was evaluated. In future studies, the conversion rates of H₂ and CO in the catalytic combustion burner will be improved, and the long-term stability of the SOEC system will be verified.