Self-assembled Ni-Rh bimetallic catalyst for the exceptional performance and stability of direct methane protonic ceramic fuel cells

doi:10.21203/rs.3.rs-2522738/v1

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Self-assembled Ni-Rh bimetallic catalyst for the exceptional performance and stability of direct methane protonic ceramic fuel cells

https://doi.org/10.21203/rs.3.rs-2522738/v1

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published 18 Nov, 2023

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Direct methane PCFCs are promising electrochemical devices that address the technical and economic challenges associated with using pure hydrogen, such as the high cost of green production, transportation, and long-term storage. However, Ni, a conventional monometallic catalyst has sluggish reaction kinetics and a low tolerance for carbon cocking under CH₄ operation, limiting its wider applications. Herein, we develop a self-assembled Ni-Rh bimetallic catalyst through Ni exsolution and Rh surface decoration in a fuel electrode. The Ni-Rh bimetallic catalyst shows remarkably high catalytic activity with an exceptional performance of ~ 0.50 W/cm² at 500 ℃. Moreover, the catalyst significantly improves the stability with a degradation rate of 0.02%/h at 500 ℃; this value is ~ 20-fold lower than that of conventional PCFC (0.4%/h). Synchrotron-based in situ X-ray photoelectron spectroscopy reveals that the Ni-Rh bimetallic catalyst initiates a self-carbon cleaning process due to its high-water dissociation reaction, allowing sustainable operation.

Physical sciences/Energy science and technology/Fuel cells

Physical sciences/Engineering/Mechanical engineering

Physical sciences/Materials science/Materials for energy and catalysis/Fuel cells

Ni-Rh bimetallic catalyst

protonic ceramic fuel cells

methane steam reforming

self-carbon cleaning

high performance

high stability

Protonic ceramic fuel cells (PCFCs) are promising electrochemical devices with low operating temperatures (< 600 ℃), high electrochemical performance, high efficiency, and no fuel dilution^1–3. Moreover, the high fuel flexibilities with hydrocarbons and ammonia instead of pure hydrogen is a breakthrough for broad utilization since there is still a lack of required technologies for green hydrogen production, storage, and transportation^4–8. Among various approaches, a direct methane PCFC, which directly uses methane as a fuel through a methane steam reforming reaction (CH₄ + H₂O $\to$ CO + 3H₂, ΔH_293K = 206 kJ/mol) at its fuel electrode, is a rational strategy to overcome the technical and economic challenges of transitioning to a fully hydrogen-based economy⁹. Methane is a good candidate for alternative fuel due to its ~ 3-fold higher volumetric energy density (~ 10 MJ/L at 250 bar) than hydrogen (~ 3 MJ/L at 350 bar)¹⁰. Additionally, it is storable and transportable using existing infrastructures, such as tanks and gas pipelines.

Nevertheless, there are several limitations to utilizing direct methane PCFCs. At low temperatures (< 600 ℃), the catalytic activity of Ni, which is the monometallic catalyst at the fuel electrode of PCFC, for CH₄ activation decreases rapidly due to sluggish reaction kinetics, creating an insufficient hydrogen supply¹¹. Carbon-cocking, which causes significant performance degradation over long-term operation, is also a critical problem of direct hydrocarbon fuel utilization¹². Therefore, to achieve the high performance and stability of direct methane PCFCs at low temperatures, structural and/or material modification of the fuel electrode is necessary to meet the following requirements: (1) highly active catalyst for CH₄ activation, (2) self-carbon cleaning, and (3) large and evenly distributed active sites for both CH₄ activation and self-carbon cleaning. There are several approaches to modify the structures of the fuel electrode catalysts, including multistep infiltration, pulsed laser deposition, and atomic layer deposition^13–15. However, these engineering processes are complicated, time-consuming, and cost-ineffective for large-scale applications due to various manufacturing process parameters.

The development of Ni-based bimetallic catalysts containing small amounts of novel metals, such as Rh, Ru, and Pd, provide an effective strategy to address the limitations of direct methane PCFCs. These bimetallic catalysts significantly improve the catalytic activity for hydrocarbon reforming through the synergistic effect of two materials, facilitating H₂ spillover and increasing carbon resistance^{16,17,18−20}. Exsolved metal catalysts improve gas conversion with evenly dispersed nanosized particles and enlarge the metal catalyst/catalyst support interface, allowing for self-carbon cleaning through high water–catalyst interactions⁴. In addition, exsolved nanoparticles anchored at the catalyst support exhibit high structural stabilities without agglomeration, even at high temperatures²¹. Therefore, taking advantage of both approaches—bimetallic catalyst and exsolved nanoparticles—is a rational approach for designing a reactive and robust fuel electrode for a direct methane electrochemical cell.

In this study, we report a direct methane PCFC with exceptional performance and stability with a self-assembled Ni-Rh bimetallic catalyst. We decorated the fuel electrode with an extremely low amount of Rh (0.14 mg/cm²) through a one-step infiltration process; the price difference between Ni (6.6 $\times$ 10⁻³ $/cm²) and Ni + Rh (6.9 $\times$ 10⁻³ $/cm²) used in cell manufacturing is not large (see Methods section). This phenomenon enables self-assembly between infiltrated Rh and subsequently exsolved Ni particles during reduction. PCFC with a self-assembled Ni-Rh bimetallic catalyst exhibits an exceptional performance of 1.13 W/cm² at 650 ℃ and 0.50 W/cm² at 500 ℃ under direct methane fuel conditions, surpassing other previously reported direct methane PCFCs and solid oxide fuel cells (SOFCs). This phenomenon is attributed to significantly enhanced methane conversion by the Ni-Rh bimetallic catalyst, achieving nearly thermodynamic equilibrium. Moreover, this catalyst showed outstanding electrochemical stability with a degradation rate of 0.02%/h at 500 ℃, which is ~ 20-fold lower than that of the conventional PCFC (0.4%/h). Synchrotron-based in situ X-ray photoelectron spectroscopy (XPS) measurements reveal that the Ni-Rh bimetallic catalyst induces self-carbon cleaning adsorbed on Ni due to its high-water dissociation properties. We believe that the bimetallic catalyst self-assembled by a readily simple and cost-effective process, and it is extensively applicable in electrochemical cells with various gases, such as ammonia, carbon dioxide, and hydrocarbon fuels.

Main 1. Fuel cell structure and self-assembled Ni-Rh bimetallic catalyst

To architecture the fuel electrode with a self-assembled Ni-Rh bimetallic catalyst, we combine the exsolution and one-step infiltration processes on the Ni/BaZr_0.4Ce_0.4Y_0.1Yb_0.1O_3−δ (BZCYYb) anode-support single cell configuration (see Supplementary Fig. 1), as summarized in Fig. 1(a). We design a fuel electrode with a Ni-diffused BZCYYb catalyst support using the interdiffusion mechanism of Ni. During sintering, Ba is evaporated from the BZCYYb lattice, creating an A-site vacancy (ABO_3−δ → A_1−αBO_3−δ + α(${\text{V}}_{A-site}^{{\prime }{\prime }}+{\text{V}}_{\text{o}}^{\bullet \bullet }$) + αAO_(g))²². The formation of A-site vacancies facilitates Ni diffusion into the perovskite oxide lattice as an interstitial defect ($\text{N}\text{i}\text{O}+{\text{V}}_{A-site}^{{\prime }{\prime }}+{\text{V}}_{\text{o}}^{\bullet \bullet }$ → ${\left({\text{N}\text{i}}_{\text{i}, {\text{V}}_{A-site}}\right)}^{\times }+{\text{O}}_{\text{O}}^{\times }$)^23,24. Diffused Ni is exsolved from the lattice during reduction (${{\text{N}\text{i}}_{\text{i}, {\text{V}}_{A-site}}}^{\times }+{\text{O}}_{\text{O}}^{\times }$ → ${\text{V}}_{A-site}^{{\prime }{\prime }}$ + ${\text{V}}_{\text{o}}^{\bullet \bullet }+{\text{N}\text{i}}_{\text{e}\text{x}-\text{s}\text{o}\text{l}\text{v}\text{e}\text{d}}$)^25,26. Thus, the fuel electrode with Ni-diffused BZCYYb serves as a platform for the subsequent self-assembly of Ni-Rh bimetallic catalysts. As shown in Supplementary Fig. 2, exsolved Ni particles are more evident as the sintering temperature increases due to facilitated Ba evaporation and Ni diffusion. We cosintered the fuel electrode and electrolyte at 1500 ℃ to achieve the largest grain size and sufficient densification of the electrolyte and to form exsolved Ni particles. For the self-assembly of the bimetallic catalyst, we decorated the surface of Ni-diffused BZCYYb with Rh nanoparticles through a one-step infiltration process. Since Rh is highly miscible with Ni, the infiltrated Rh spontaneously mixes with subsequently exsolved Ni during H₂ reduction (${{\text{N}\text{i}}_{\text{i}, {\text{V}}_{A-site}}}^{\times }+{\text{O}}_{\text{O}}^{\times }+{\text{R}\text{h}}_{\text{s}\text{u}\text{r}\text{f}\text{a}\text{c}\text{e}}$→ ${\text{V}}_{A-site}^{{\prime }{\prime }}+{\text{V}}_{\text{o}}^{\bullet \bullet }+({\text{N}\text{i}\text{R}\text{h})}_{\text{e}\text{x}-\text{s}\text{o}\text{v}\text{l}\text{e}\text{d}}$), resulting in a self-assembled Ni-Rh bimetallic catalyst in the fuel electrode.

Figure 1(b) and Fig. 1(c) show the SEM images of the BZCYYb catalyst support without Rh infiltration (REF cell) and with Rh infiltration (Ni-Rh cell), respectively. In the REF cell, the surface of the BZCYYb catalyst support is clean and smooth before reduction under the H₂ environment, as shown in Supplementary Fig. 3(a). However, after reduction, Ni nanoparticles (30–50 nm) are exsolved to the BZCYYb surface with a surface coverage of 11–13%. In the Ni-Rh cell, Rh nanoparticles (8–11 nm) are treated by infiltration on the BZCYYb surface before reduction, as shown in Supplementary Fig. 3(b). The EDS mapping in Fig. 1(e) shows the coexistence of Ni and Rh in the exsolved particles after reduction, confirming the formation of the bimetallic catalyst on the BZCYYb surface. Interestingly, the Ni-Rh bimetallic catalyst shows ~ 7-fold higher surface coverage of 85–87% and ~ 5-fold smaller particle size (8–11 nm) than those of exsolved Ni particles in the REF cell. The higher surface coverage with the smaller particle size is attributed to the presence of infiltrated Rh nanoparticles at the BZCYYb surface, providing additional nucleation sites for exsolution under the same amount of diffused Ni²⁷. In addition, the smaller particle size of the Ni-Rh bimetallic catalyst is attributed to the higher surface energy of Rh (2,828 mJ/m²) than Ni (2,364 mJ/m²), preserving their particle size without agglomeration^28,29. The smaller particle size maximizes the catalyst surface area and induces strong metal support interactions, increasing gas conversion and catalytic activity³⁰. The particle size of the Ni-Rh bimetallic catalyst is substantially smaller or at least comparable to recently reported values through exsolution (~ 50 nm), multistep infiltration (~ 20 nm), and atomic layer deposition (~ 10 nm), demonstrating the feasibility of our simple approach for enlarging the catalytic active sites^13,15,25.

Main 2. Performance And Electrochemical/thermochemical Analyses

We evaluate the electrochemical performances of REF and Ni-Rh cells under hydrogen (97% H₂ and 3% H₂O) and methane (H₂O/CH₄) with S/C = 2 and S/C = 1, as shown in Fig. 2, Supplementary Figs. 4–5, and Supplementary Table 1. The measured open circuit voltages (OCVs) under different partial pressures of H₂ (P_H2) are close to the theoretical values, confirming the sufficient gas tightness of the electrolyte, as shown in Supplementary Figs. 6–7 and Supplementary Table 2³¹. Under H₂ operation, the Ni-Rh cell demonstrates ~ 1.20- and ~ 1.06-fold higher maximum power densities (MPDs) of ~ 1.47 W/cm² at 650 ℃ and ~ 0.69 W/cm² at 500 ℃ than those of the REF cell (~ 1.22 W/cm² at 650 ℃ and ~ 0.65 W/cm² at 500 ℃). Figure 2(a-d) shows that the improved electrochemical performances of the Ni-Rh cell are more evident with the CH₄ fuel with a lower operation temperature. Under CH₄ operation (S/C = 2), the Ni-Rh cell exhibits ~ 1.44-fold higher MPDs at 650 ℃ (~ 0.78 W/cm² for the REF cell and ~ 1.13 W/cm² for the Ni-Rh cell, respectively), and ~ 2-fold higher MPDs at 500 ℃ (~ 0.25 W/cm² for the REF cell and ~ 0.50 W/cm² for the Ni-Rh cell, respectively). These trends are evident in the CH₄ fuel condition (S/C = 1) in Supplementary Note 1.

As shown in Fig. 2(e) and Supplementary Table 3, the Ni-Rh cell exhibits outstanding MPDs under CH₄ operation, outperforming previously reported values for PCFCs and SOFCs^{4–7,32−36}. Specifically, the Ni-Rh cell shows a particularly high MPD at low temperatures, such as ~ 0.50 W/cm² at 500 ℃ under CH₄ operation of S/C = 2. These exceptional MPDs of the Ni-Rh cell are primarily attributed to the lowest area-specific polarization resistance (ASR_electrode), corresponding to the electrode resistance; the values are exceptional relative to other reported values, as shown in Fig. 2(f). To further investigate the performance improvement in electrochemical reactions, we use electrochemical impedance spectroscopy (EIS) measurements and distributed relaxation time (DRT) analyses to deconvolute the ASR_electrode into three distinct frequency ranges—high (> 10³ Hz), medium (10–10³ Hz), and low (< 10 Hz)—corresponding to the charge transfer at the triple phase boundary (TPB) of the fuel and air electrodes, the gas adsorption process and the overall surface reactions at the electrodes, and the gas diffusion and fuel reforming in the fuel electrode, respectively.^37,38. Figure 2(g) shows the deconvoluted ASR_electrode of the REF and Ni-Rh cells at 500 ℃ under different fuel conditions according to each frequency range. At high and medium frequencies, the Ni-Rh cell exhibits slightly lower resistances than the REF cell under all fuel conditions (H₂, S/C = 2, and S/C = 1). This phenomenon is attributed to the high electrochemical activity of the Ni-Rh bimetallic catalyst for charge transfer at the TPB and the overall oxidation reactions at the fuel electrode relative to the Ni monometallic catalyst³⁹. When switching the fuel from H₂ to CH₄, the medium frequency resistances significantly increase by a similar magnitude in both REF and Ni-Rh cells due to the slow gas‒solid interaction caused by the reduced partial pressure of H₂ and the sluggish CH₄ adsorption. The low-frequency resistances for the REF cell significantly increase by sluggish gas reforming under CH₄ operation, while those for the Ni-Rh cell slightly increase. As shown in Supplementary Note 2 and Supplementary Figs. 8–11, the much smaller low-frequency resistance of the Ni-Rh cell occurs primarily due to its significantly high CH₄ conversion rate, approaching thermodynamic equilibrium under S/C = 2, and its low activation energies (~ 26.6 kJ/mol). Moreover, the Ni-Rh cell shows a larger difference in CH₄ conversion than the REF cell under S/C = 2 (high partial pressure of H₂O) rather than S/C = 1 (low partial pressure of H₂O). It reveals that the improvement in CH₄ activation with the Ni-Rh bimetallic catalyst is significantly correlated with the water–catalyst interaction as well as the CH₄–catalyst interaction. In addition to the high-water dissociation properties of Rh, the high surface coverage and maximized catalyst surface area properties with a small particle size of the Ni-Rh bimetallic catalyst further improve the water–catalyst interaction, improving the electrochemical performance of direct methane PCFCs.

Main 3. Long-term Stability And Self-carbon Cleaning Mechanism

Long-term stability is the most challenging issue for the sustainable operation of direct methane PCFCs, mostly induced by carbon-cocking, which is a byproduct of methane steam reforming. Carbon-cocking blocks the electrochemical and thermochemical reaction sites and rapidly degrades the electrochemical performance^40–43. Figure 3 presents the long-term stabilities for REF and Ni-Rh cells under the S/C = 1 condition at 500 ℃, where the carbon-cocking is thermodynamically activated primarily in the temperature range of 500–700 ℃ by methane cracking (CH₄ → 2H₂ + C) and the Boudouard reaction (2CO → CO₂ + C) (Supplementary Fig. 9)^44–46. As shown in Fig. 3(a), the REF cell shows a rapid decrease in the electrochemical performance with a degradation rate of 0.4%/h. In contrast, the Ni-Rh cell demonstrates exceptional electrochemical stability for 500 h with a degradation rate of 0.02%/h, which is ~ 20-fold lower than that of the REF cell. The high similarity in the electrochemical performance and CH₄ conversion trends substantiates the importance of maintaining the CH₄ reforming activity for sustainable operation. Postmortem analysis using energy dispersive spectroscopy (EDS) (Fig. 3(b) and Supplementary Fig. 12) and Raman spectroscopy (Supplementary Fig. 13) show that the REF cell suffers from significant carbon-cocking on the catalyst surface^12,47, as evidenced by the carbon peaks (D band (disordered carbon; 1,350 cm^− 1) and G band (graphitic carbon; 1,580 cm^− 1))^4,48. This carbon-coking deactivates the Ni surface, inhibiting the H₂ supply for the hydrogen oxidation reaction at the fuel electrode. In contrast, the Ni-Rh cell shows no evidence of carbon-cocking on the catalyst surface (Fig. 3(b)) and no carbon peaks in their spectra (Supplementary Fig. 13), verifying the high tolerance for carbon-cocking. Since the CH₄ operation of S/C = 1 at 500 ℃ is the thermodynamically favored regime for carbon-cocking (Supplementary Fig. 9), the lack of evidence of carbon-cocking may imply that the Ni-Rh bimetallic catalyst undergoes self-carbon cleaning. In addition, although nanoparticles generally lose their active sites over long-term operation due to agglomeration, anchored Ni-Rh bimetallic catalysts at the BZCYYb surface show high structural stability without evidence of agglomeration, as shown in Supplementary Fig. 14. Therefore, this finding confirms that the Ni-Rh bimetallic catalyst exhibits outstandingly robust chemical and structural stabilities under CH₄ operation without carbon-cocking and agglomeration, preserving their active sites for gas reforming.

To clarify the self-carbon cleaning mechanism on the Ni-Rh bimetallic catalyst, we perform a synchrotron-based in situ X-ray photoelectron spectroscopy (XPS) measurement. We measure the changes in the chemical natures of carbon (C-C sp³ and C-Ni), oxygen defect species (OH*, Vo, and ${\text{O}}_{\text{O}}^{\times }$), and metallic catalyst (Ni and NiO) (Supplementary Note 3) under operating conditions simulating methane steam reforming (CH₄ and H₂O) during the in situ XPS measurement by infiltrating H₂O into the pore of the fuel electrode from room temperature to a reaction temperature of 500 ℃. Detailed peak definitions are provided in Supplementary Note 3. C-C sp³ (carbon) and C-Ni (carbon precursor) peaks are deconvoluted in the C 1s photoelectron spectra to quantify the amount of cocked carbon before the reaction at room temperature (As(RT)) and after the reaction with CH₄ at 500 ℃ (CH₄(500)), as shown in Fig. 4(a-b)⁴⁸. In the REF cell, when CH₄ was fed, the relative area ratio of the C-C sp³ and C-Ni spectra substantially increase by ~ 2.6 times and ~ 1.3 times, respectively, indicating carbon-cocking during the reaction. In contrast, in the Ni-Rh cell, the C-C sp³ spectra remains almost unchanged; moreover, the C-Ni spectra disappears completely, indicating that the Ni-Rh bimetallic catalyst undergoes self-carbon cleaning. However, under a dry environment (without H₂O), as shown in Supplementary Fig. 15(a-b), the relative area ratio of C-C sp³ is 5 times higher and almost identical in the REF and Ni-Rh cells, respectively, revealing that the H₂O supply plays a critical role in self-carbon cleaning.

The self-carbon cleaning process occurs through the following pathways: 1) CO formation (C* + O* $\to$ CO* + Ni*)⁴⁹, 2) COH formation (C* + OH* $\to$ COH*)⁵⁰, 3) CHO formation (CH* + O* $\to$ CHO*)⁵¹, and 4) CHOH formation (CH* + OH* $\to$ CHOH*)⁵¹. We measure the O 1s and Ni 2p photoelectron spectra to characterize the formation of oxygen vacancies (Vo), hydroxyl groups (OH*), and oxidative species (O*), which are essential for self-carbon cleaning⁴. As shown in Fig. 4(c-d), the Ni-Rh cell forms more Vo and OH* than the REF cell, and this difference is more pronounced under a wet environment than under a dry environment (Supplementary Fig. 15(c-d) and Supplementary Table 4). This phenomenon occurs because the Rh in the Ni-Rh bimetallic catalyst improves the dissociation of H₂O*, readily forming Vo and OH*^52–54. In addition, the Ni-Rh bimetallic catalyst has a strong H₂ spillover effect, forming H* species on the BZCYYb surface^18–20. These H* species react with OH* to form oxygen vacancies by a dehydration reaction (H* + OH* $\to$ H₂O_(g) + Vo)⁵⁵ and react with lattice oxygen to form hydroxide (H* + ${\text{O}}_{\text{O}}^{\times }$ $\to$ OH*)¹ on the BZCYYb surface. The evolved Vo provides more sites for OH* formation (Vo + ${\text{O}}_{\text{O}}^{\times }$ + H₂O_(g) $\to$ 2OH*)^1,3, thereby contributing to self-carbon cleaning⁵⁶. In addition, the facilitated H₂O dissociation forms more NiO on the Ni-Rh cell, as shown in Fig. 4(e-f) and Supplementary Fig. 15(e-f). Ni, with a lower electronegativity of 1.91 than Rh (2.29), attracts the O* species from H₂O* dissociation (H₂O* $\to$ OH*+H*) and subsequent OH* dissociation (OH* $\to$ O*+ H*)^57,58. As shown in Fig. 4(g), the increase in the formation of oxygen vacancies (Vo), hydroxyl groups (OH*), and oxidative species (O*) through facilitated dissociation of H₂O on the Ni-Rh bimetallic catalyst enables self-carbon cleaning, achieving robust long-term stability. Therefore, we conclude that the readily simple and cost-effective architecturing process for the fuel electrode is a rational strategy for direct methane PCFCs with high performance and stability. Furthermore, we believe that this approach is extensively applicable to other electrochemical devices that require the direct reforming of gases, such as other hydrocarbon fuels, ammonia, and CO₂, at their electrode.

We successfully demonstrated the exceptionally high electrochemical performance and stability of direct methane PCFCs by modifying the fuel electrode with a self-assembled Ni-Rh bimetallic catalyst, which is readily fabricated by a one-step infiltration process with an extremely low amount of Rh. Changes in electrochemical performance according to different operating conditions and synchrotron-based in situ XPS chemical analysis revealed that the Ni-Rh bimetallic catalyst exhibited strong water–catalyst interactions, simultaneously leading to high CH₄ conversion and self-carbon cleaning. Our results could broaden the utilization of direct alternative fuel PCFCs, such as other hydrocarbon fuels, ammonia, and CO₂, based on their high performance and sustainability without engineering complexity. Furthermore, we expected that self-assembled bimetallic catalysts could be extensively applied to other electrochemical devices requiring enlarged catalytic active sites and robust structural stabilities.

1) Fabrication of fuel cells and methane steam reforming catalyst.

An anode-supported single cell was fabricated with a configuration of NiO-BZCYYb/BZCYYb/PBSCF. NiO-BZCYYb anode powder was prepared using a mixture of homogeneous NiO powder (Kojundo Chemical), BZCYYb4411 (BaZr_0.4Ce_0.4Y_0.1Yb_0.1O_3–δ) powder (Kceracell Co.), polymethyl methacrylate (PMMA) pore former (Grand Chemical & Material) at a weight ratio of 6:4:1 with a ball-milling process in ethanol with a dispersant (BYK-2012, BYK), polyvinyl butyral (Sigma Aldrich) binder, and dibutyl phthalate (Sigma Aldrich) plasticizer for 24 h. After ball-milling, the NiO-BZCYYb slurry was dried in a dry oven for a few hours. Dried NiO-BZCYYb powder was uniaxially pressed and presintered at 1000 ℃ for 3 h to obtain mechanical strength and porosity. The anode functional layer (AFL) and BZCYYb electrolyte were deposited by slurry spin coating. AFL slurry was prepared with NiO and BZCYYb powders in a weight ratio of 6:4, mixed with a solvent (isopropyl alcohol), dispersant (BYK-2012, BYK), and binder (ethylcellulose). The electrolyte slurry was prepared using the same components and ratio as the AFL, except for using NiO powder. The spin-coated as-prepared single cell with AFL and electrolyte was cosintered at 1500 ℃ for 5 h to densify the electrolyte and grow the grain. For the cathode, the PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ (PBSCF) powder (Kceracell Co.) was mixed with the binder ink (VEH, Fuel Cell Materials) at a weight ratio of 1:1 to fabricate porous cathode structures, and it was screen-printed onto the BZCYYb electrolyte to obtain a thickness of ∼17 µm. After screen printing, the single cell with the cathode layer was sintered at 950 ℃ for 4 h.

Ni-Rh bimetallic alloy catalysts in the fuel electrode substrate were fabricated by one-step infiltration. The precursor solution for infiltration was 0.05 molarity (mol/L), which was mixed with Rh nitrate (Rh(NO₃)₃ $\bullet x$ H₂O, Sigma‒Aldrich) and ethanol solvent. In the one-step infiltration method, the precursor solution was applied once to the porous surface of the fuel electrode substrate (NiO-BZCYYb), followed by calcination at approximately 300 ℃ for 1 hour. Through one-step infiltration, we used an extremely low amount of Rh for a cell relative to Ni (~ 224 mg/cm² for Ni and ~ 0.14 mg/cm² for Rh); thus, the price of Rh was ~ 28.5-fold lower than that of Ni (2.3 $\times$ 10^-4 $/cm² for Rh ($ 765.6/lb, Daily Metal Prices) and 6.6 $\times$ 10^-3 $/cm² for Ni ($ 13.3/lb, Daily Metal Prices)).

2) Characterization of microstructure and postmortem analysis.

The morphology and microstructure of the fuel electrode substrate were observed by field emission scanning electron microscopy (FE-SEM; Inspect F, FEI). SEM-energy dispersive X-ray spectroscopy (EDS) was performed to investigate the carbon formed on the fuel electrode surface, and surface coverage was calculated by processing software (ImageJ). The particle size and chemical composition of the catalyst formed on the surface of the fuel electrode were examined by high-resolution transmission electron microscopy (HR-TEM; NEOARM JEM-ARM 200F, JEOL) and energy dispersive X-ray spectroscopy (EDS), in which samples were prepared by a focused ion beam (FIB; crossbeam 540, ZEISS). After the long-term stability test, the XRD patterns of the fuel electrode substrate were recorded by using a D8 Advance (Bruker) using Cu Kα radiation to characterize the crystal structure. The crystal structure of the fuel electrode substrate was scanned with a step size of 0.02°/s in the 2$\theta$ range = 25 to 80°. To obtain carbon formation information on the fuel electrode substrate after the long-term stability test, Raman spectroscopy with LabRam Aramis (Horiba Jobin Yvon) was performed in the range of 1,000 to 2,000 cm^-1 with a yag laser (λ = 532 nm).

3) Electrochemical evaluation of fuel cells.

Electrochemical impedance spectra (EIS) measurement in the frequency range from 1 MHz to 0.1 Hz and current-voltage (I-V) measurements were performed using IviumStat.h (Ivium Technologies). The electrochemical evaluation at operating conditions was carried out at 650–500 ℃ under various fuels (i.e., H₂ and CH₄ with a steam to carbon ratio (S/C) of 2 and 1) and air (at a flow rate of 100 sccm). H₂ fuel operation was 100 sccm of H₂ (with 3% H₂O), and CH₄ fuel operation was 32 sccm of mixed gases (S/C = 2: 25% CH₄, 50% H₂O, and 25% Ar; S/C = 1: 25% CH₄, 25% H₂O, and 50% Ar). Dry gases (Air, H₂, CH₄, and Ar) were controlled by the mass flow controller (MFC, Bronkhorst). Steam was monitored using the saturation temperature in the humidifier to control the steam concentration in the feed gas.

4) Catalytic activity.

The product gases exiting the anode outlet at the cell station were measured to analyze the catalytic activity by gas chromatography (GC, Agilent 7890B) equipped with a dual TCD. The dual TCD was connected to two columns (HP-PLOT 5A: H₂, CO, and HP-PLOT-Q: CH₄, CO₂) that separated the mixed gases.

To confirm the correlation between catalytic activity and performance, the methane conversion (CH₄ conv.) and hydrogen production rate (H₂ prod.) were calculated through the dry gas composition from GC. The methane conversion and hydrogen production rate were expressed using Eq. 1 and Eq. 2, respectively. The activation energy by the Arrhenius equation was analyzed using Eq. 3 to confirm the effect of methane conversion and hydrogen production rate as the catalyst was applied.

${CH}_{4, conv.}=\frac{{F}_{{CH}_{4}}- {Q}_{{CH}_{4}}}{{F}_{{CH}_{4}}}\times 100 \left[\text{\%}\right]$ (Eq. 1)

${H}_{2, prod.}={Q}_{{H}_{2}} \left[\text{s}\text{c}\text{c}\text{m}\right]={Q}_{{H}_{2}}\times {10}^{-3} \left[\frac{ \text{L}}{\text{m}\text{i}\text{n}}\right]={Q}_{{H}_{2}}\times \frac{ {10}^{-3}}{22.4} \left[\frac{\text{m}\text{o}\text{l}}{\text{m}\text{i}\text{n}}\right]={Q}_{{H}_{2}}\times \frac{ {10}^{-3}}{22.4\times 60} \left[\frac{\text{m}\text{o}\text{l}}{\text{s}}\right]$ (Eq. 2)

$\text{ln}\left({CH}_{4, conv.} or {H}_{2, prod.}\right)=\left(-\frac{{E}_{a}}{RT}\right)+\text{C}\text{o}\text{n}\text{s}\text{t}$ (Eq. 3)

where ${CH}_{4, conv.}$: methane conversion [%]; ${H}_{2, prod.}$: hydrogen production rate [mol/s]; $\text{ln}\left({CH}_{4, conv.} or {H}_{2, prod.}\right)$: Arrhenius equation; ${F}_{i}$: the volumetric flow rate of gas species $i$ at the inlet [sccm]; ${Q}_{i}$: the volumetric flow rate of gas species $i$ at the outlet [sccm]; ${E}_{a}$: activation energy [kJ/mol]; $R$: universal gas constant [J/mol/K]; and $T$: temperature [K]

5) Synchrotron-based in situ HR–XPS

Two types (wet and dry) of samples were prepared to confirm the self-carbon cleaning at the fuel electrode surface during the methane reforming reaction when exposed to H₂O. Samples (REF and Ni-Rh) were exposed to 3% wet Ar gas at room temperature for one day after reduction at 650°C and were designated wet samples. The samples (REF, Ni-Rh) that were not exposed to wet conditions were designated dry samples.

In situ high-resolution X-ray photoelectron spectroscopy (HR-XPS) was measured by the 10A2 beamline in the Pohang Accelerator Laboratory (PAL). During in situ HR-XPS analysis, the fuel electrode substrates were fixed to a molybdenum holder and placed in an ultrahigh vacuum (UHV) chamber maintained at a base pressure of 5 × 10^–10 torr. The binding energies and spectral resolutions were calibrated by recording the Au 4f photoelectron peak as a reference (4f^7/2, BE = 84.0 eV). Photon energy was used at the same excitation energy (960 eV) for measurement reproducibility for all samples.

To obtain a clean surface, the samples were subjected to Ar sputtering (1500 L = $5\times {10}^{-6}$ Torr for 300 s at 1 keV), and the surfaces of the fuel electrode substrates (As(RT)) were measured before the reaction. After measurements, the surface states of the fuel electrode substrates (CH₄(500)) were measured by exposing the samples to a reaction gas (25% CH₄/Ar balance) for 3000 L ($5\times {10}^{-6}$ Torr for 600 s) at 500 ℃.

Acknowledgments

This study was supported by the Ministry of Trade, Industry and Energy (MOTIE), South Korea and Korea Institute for Advancement of Technology (KIAT), South Korea through the International Cooperative R&D program (P0021202), a National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2022R1A2C3012372, 2022R1A4A1031182, 2021K1A3A1A20002574 and 2021R1C1C2006657), and the H2KOREA funded by the Ministry of Education (2022 − 0950).

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Self-assembled Ni-Rh bimetallic catalyst for the exceptional performance and stability of direct methane protonic ceramic fuel cells

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Abstract

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Introduction

Results And Discussion

Main 1. Fuel cell structure and self-assembled Ni-Rh bimetallic catalyst

Main 2. Performance And Electrochemical/thermochemical Analyses

Main 3. Long-term Stability And Self-carbon Cleaning Mechanism

Conclusion

Methods

5) Synchrotron-based in situ HR–XPS

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