reprint, amsmath,amssymb, aps, ]revtex4-2 Breaking the Ionic-Conductivity Barrier in Amorphous Alumina: Copper Doping-Induced Electron-Ion Coupling for Next-Generation SOFC Electrolytes

reprint, amsmath,amssymb, aps, ]revtex4-2 Breaking the Ionic-Conductivity Barrier in Amorphous Alumina: Copper Doping-Induced Electron-Ion Coupling for Next-Generation SOFC Electrolytes | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 3 March 2026 V1 Latest version Share on reprint, amsmath,amssymb, aps, ]revtex4-2 Breaking the Ionic-Conductivity Barrier in Amorphous Alumina: Copper Doping-Induced Electron-Ion Coupling for Next-Generation SOFC Electrolytes Authors : Muhammad Shahid Sharif 0009-0003-3174-9663 [email protected] , Sajid Rauf 0000-0003-2343-9334 , and Muhammad Ali Kamran Yousaf Shah Authors Info & Affiliations https://doi.org/10.22541/au.177257725.53363511/v1 135 views 67 downloads Contents Abstract Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract reprint, amsmath,amssymb, aps, ]revtex4-2 The performance of solid oxide fuel cells (SOFCs) is critically influenced by the electrolyte’s ability to achieve high ionic conductivity while maintaining electronic insulation. Traditional materials, such as yttria-stabilized zirconia (YSZ), face a trade-off between these two properties. This study introduces copper-doped amorphous alumina as a novel strategy to enhance electron-ion (E-I) coupling, thereby boosting ionic conduction via localized charge transfer. By doping amorphous alumina with copper, we create additional electronic states that facilitate charge transfer, which in turn enhances ion mobility. Notably, increasing copper concentration results in a progressive enhancement of localized charge transfer, which maximizes ionic conduction. Through systematic doping at concentrations of 2.5%, 5%, 7.5%, and 10%, we identify 10% copper as the optimal composition for achieving superior conductivity and electrochemical performance. Characterization techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) confirm the uniform incorporation of copper and the structural integrity of the doped material. The 10% Cu-doped alumina demonstrates an ionic conductivity of 0.16 S/cm and a peak power density of 770 mW/cm-2, significantly outperforming conventional SOFC electrolytes. This work underscores the potential of localized charge transfers in amorphous materials, positioning copper-doped alumina as a promising candidate for next-generation SOFCs. By enhancing ionic conduction through electron-ion coupling, this approach offers a pathway for the design of high-performance electrolytes with reduced activation energy and improved electrochemical stability. reprint, amsmath,amssymb, aps, ]revtex4-2 Breaking the Ionic-Conductivity Barrier in Amorphous Alumina: Copper Doping-Induced Electron-Ion Coupling for Next-Generation SOFC Electrolytes Sajid Rauf 1,* , Muhammad Shahid Sharif 1 , MAK Yousaf Shah 1 reprint, amsmath,amssymb, aps, ]revtex4-2 1 College of Mechatronics and Control Engineering and State Key Laboratory of Radio Frequency Heterogeneous Integration, Shenzhen University, Shenzhen 518000, China reprint, amsmath,amssymb, aps, ]revtex4-2 Correspondence to Dr. Sajid Rauf: [email protected] ; [email protected] reprint, amsmath,amssymb, aps, ]revtex4-2 Highlights 1. Copper doping induces strong electron–ion coupling in amorphous alumina. 2. 10% Cu-doped sample achieves 0.16 S cm⁻¹ ionic conductivity. 3. Peak power density reaches 770 mW cm⁻² at 550 °C. 4. Isotope and ion-filter tests confirm dominant proton conduction. Abstract: The performance of solid oxide fuel cells (SOFCs) is critically influenced by the electrolyte’s ability to achieve high ionic conductivity while maintaining electronic insulation. Traditional materials, such as yttria-stabilized zirconia (YSZ), face a trade-off between these two properties. This study introduces copper-doped amorphous alumina as a novel strategy to enhance electron-ion (E-I) coupling, thereby boosting ionic conduction via localized charge transfer. By doping amorphous alumina with copper, we create additional electronic states that facilitate charge transfer, which in turn enhances ion mobility. Notably, increasing copper concentration results in a progressive enhancement of localized charge transfer, which maximizes ionic conduction. Through systematic doping at concentrations of 2.5%, 5%, 7.5%, and 10%, we identify 10% copper as the optimal composition for achieving superior conductivity and electrochemical performance. Characterization techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) confirm the uniform incorporation of copper and the structural integrity of the doped material. The 10% Cu-doped alumina demonstrates an ionic conductivity of 0.16 S/cm and a peak power density of 770 mW/cm -2 , significantly outperforming conventional SOFC electrolytes. This work underscores the potential of localized charge transfers in amorphous materials, positioning copper-doped alumina as a promising candidate for next-generation SOFCs. By enhancing ionic conduction through electron-ion coupling, this approach offers a pathway for the design of high-performance electrolytes with reduced activation energy and improved electrochemical stability. The growing demand for sustainable energy solutions has placed solid oxide fuel cells (SOFCs) at the forefront of advanced power generation technologies. SOFCs, capable of operating at intermediate to high temperatures (600-800 °C), offer high efficiency, fuel flexibility, and compatibility with a wide range of energy sources, making them ideal for stationary power generation, transportation, and industrial processes [1, 2]. At the core of SOFCs lies the electrolyte, a critical component responsible for oxygen ion (O 2‑ ) or proton conduction, which directly influences power density, efficiency, and operational durability. Conventional SOFC electrolytes such as yttria-stabilized zirconia (YSZ) and gadolinium-doped ceria (GDC) have long been the benchmarks for oxygen-ion-conducting systems [3, 4]. While YSZ exhibits excellent ionic conductivity at high temperatures, its performance deteriorates below 600 °C due to high activation energy for oxygen ion migration. Similarly, GDC suffers from electronic leakage under reducing conditions, limiting its efficiency. For proton-conducting electrolytes, materials like barium zirconate-based perovskites (e.g., BaZr 0.9 Y 0.1 O 3 , BZY) have emerged as promising candidates due to their high proton conductivity at intermediate temperatures [5]. However, BZY materials are hindered by their low sinter ability, chemical instability in the presence of CO 2 , and challenges in achieving dense microstructures [6]. These limitations underscore a critical need for novel electrolyte materials capable of delivering high ionic conductivity, reduced activation energy, and long-term stability under practical SOFC operating conditions. Emerging research in mixed ionic and electronic conducting (MIEC) materials has provided transformative insights into overcoming these challenges [7, 8]. MIEC materials, widely used in SOFC cathodes, facilitate both oxygen ion and electron transport, significantly enhancing the oxygen reduction reaction (ORR) and reducing polarization losses. The principles underlying MIECs, such as defect engineering, enhanced ion mobility, and simultaneous charge carrier transport, offer a valuable framework for addressing the limitations of traditional electrolytes. While these principles have primarily been applied to cathodes, their extension to electrolyte design presents a novel opportunity. By leveraging localized electron-ion (E-I) coupling , where ionic and electronic charge carriers interact synergistically to enhance ion transport, it becomes possible to achieve significant performance gains in SOFC electrolytes [9, 10]. Despite its potential, E-I coupling remains underexplored in amorphous systems, which inherently possess unique properties such as enhanced defect mobility, higher ionic diffusivity, and structural flexibility. Amorphous alumina (Al 2 O 3 ) represents a compelling candidate for SOFC electrolyte applications due to its exceptional thermal stability and adaptability at intermediate temperatures [11, 12]. However, its low intrinsic ionic conductivity limits its direct application. To overcome this limitation, doping strategies can activate localized charge transfer and defect generation. Copper (Cu) doping, in particular, presents a promising pathway. Copper ions are known to introduce electronic states within the material and facilitate the formation of oxygen vacancies, which are critical for enabling efficient ionic conduction [13, 14]. The principles of MIECs, extended to amorphous alumina via copper doping, provide an opportunity to exploit localized E-I coupling to enhance ionic mobility while maintaining electronic insulation. Despite the promise of copper doping, its effects on amorphous alumina remain poorly understood, particularly in the context of SOFCs. While crystalline copper-doped materials have demonstrated improved ionic conductivity, the interplay between copper concentration, defect generation, and charge transfer in amorphous systems has not been systematically studied. Addressing these gaps requires a detailed investigation of how copper doping influences the amorphous structure, facilitates localized charge transfer, and optimizes ionic transport pathways. This study aims to systematically evaluate the effects of copper doping on amorphous alumina by varying copper concentrations (2.5%, 5%, 7.5%, and 10%) and investigating their impact on structural, electronic, and electrochemical properties. By tailoring the copper content, we aim to optimize defect chemistry, enhance localized E-I coupling, and achieve high ionic conductivity with reduced activation energy. Advanced characterization techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS), are employed to probe the structural, morphological and electronic modifications induced by copper doping. Electrochemical impedance spectroscopy (EIS) is used to evaluate ionic conductivity across a wide temperature range, and SOFC testing is performed to assess power density, activation energy, and operational stability. This work bridges the knowledge gap between crystalline and amorphous systems by demonstrating how principles derived from MIEC materials can be adapted to copper-doped amorphous alumina for SOFC applications. By combining insights from defect engineering, localized charge transfer, and structural disorder, this study highlights a new pathway for advancing SOFC electrolyte materials. The findings establish copper-doped amorphous alumina as a viable candidate for intermediate-temperature SOFCs, offering high ionic conductivity, enhanced defect mobility, and robust thermal stability, ultimately paving the way for next-generation energy devices that address the limitations of existing technologies. Materials Availability and Their Role in Synthesis All chemicals used in this study were of analytical grade and acquired from Sigma-Aldrich. The main precursors included aluminum nitrate nonahydrate (Al(NO 3 ) 3 ·9H 2 O) and copper nitrate hexahydrate (Cu(NO 3 ) 2 ·6H 2 O) . These were utilized without further purification. Deionized water served as the solvent for all preparative steps. The synthesis of pure alumina and copper-doped alumina powders was carried out via a sol-gel method. Copper was introduced into the alumina matrix at molar concentrations of 2.5%, 5%, 7.5%, and 10%. For each composition, precise amounts of copper nitrate and aluminum nitrate were dissolved separately in deionized water to ensure thorough dissolution. The solutions were then combined under magnetic stirring at 400 rpm to create a homogenous mixture at 60 °C for 2 hours. Citric acid was added in 20 weight percentage, and the mixture was continuously stirred at 90 °C for 4 hours. Once the gel formation was complete, it was dried in an oven at 120°C for 12 hours to eliminate residual water. The resulting solid was ground into a fine powder using a mortar and pestle. This powder was calcined at 700°C for 4 hours at a controlled heating rate of 5°C per minute to ensure complete transformation into an amorphous phase. The prepared compositions were labeled as Cu 0.025 AlO 3 (2.5-CuAlO 3 ), Cu 0.05 AlO 3 (5-CuAlO 3 ), Cu 0.075 AlO 3 (7.5-CuAlO 3 ), and Cu 0.1 Al 2 O 3 (10-CuAlO 3 ), corresponding to the varying copper concentrations. Fuel Cell Device Construction The fabricated fuel cells followed a three-layer structure consisting of an anode , an electrolyte , and a cathode . Nickel foam (Ni-foam) was employed as the current collector and electrode substrate for both anode and cathode due to its high surface area and conductivity. The electrodes were prepared by mixing Nickel-Cobalt-Aluminum Lithium oxide (Ni 0.8 Co 0.15 Al 0.05 LiO 2 , NCAL), a material known for its robust catalytic activity in hydrogen oxidation (HOR) and oxygen reduction reactions (ORR) [15], which was procured from Tianjin Bamo Company. A slurry of NCAL was prepared by dispersing the material in terpineol. After homogenization, the slurry was painted onto Ni-foam and dried at 90°C for 1 hour to form the electrode. The electrolyte was prepared by pressing copper-doped alumina powder into pellets using a hydraulic press. These pellets, with a thickness of approximately 510 μm, were placed between two NCAL-coated Ni-foam electrodes under a pressure of 220-250 MPa to construct the complete cell. The effective area of each cell was 0.64 cm 2 , with a total thickness of 1.5 mm. For proton conduction studies, BaZr 0.9 Y 0.1 O 3 (BZY) was introduced as an interlayer to suppress oxygen and electron conduction [16]. A five-layer configuration was adopted, denoted as Ni-NCAL/BZY/10-CuAlO 3 /BZY/NCAL-Ni. This configuration was critical in isolating proton conduction pathways. Electrochemical testing was conducted in air and hydrogen environments, with hydrogen flow rates maintained at 120 mL/min during performance evaluation. The synthesized copper-doped alumina materials underwent extensive characterization using several techniques to evaluate their structural, morphological, and electrochemical properties. X-ray diffraction (XRD) was performed to confirm the amorphous nature of the samples and ensure phase purity. The diffraction data were collected in the 2θ range of 10°-90° and analyzed using standard software. Raman spectroscopy was utilized to examine the vibrational modes of the materials. Scanning electron microscopy (SEM) was conducted to examine the surface morphology of the copper-doped alumina samples. Elemental mapping was performed via energy-dispersive X-ray spectroscopy (EDS) to confirm the uniform distribution of copper throughout the alumina matrix. Electrochemical impedance spectroscopy (EIS) was conducted using a Solartron Energy Lab XM impedance analyzer. The ionic conductivity was derived from the impedance data, and the total resistance was calculated using the formula: \begin{equation} \sigma=\frac{L}{R*A}\nonumber \\ \end{equation} where: • σ is the ionic conductivity, • L is the thickness of the electrolyte (cm), • R is the measured resistance (Ω), • A is the cross-sectional area of the sample (cm 2 ). Direct current polarization measurements were employed to evaluate the ionic conductivity using Ohm’s law: \begin{equation} \sigma=(L*I)/(A*V)\nonumber \\ \end{equation} where: I is the current (A), V is the applied voltage (V). To assess the performance of the SOFCs, current-voltage (I-V) and current-power (I-P) curves were analyzed under hydrogen-air conditions. The I-V curve was used to extract the total resistance, while the I-P curve provided insights into the power density and efficiency of ion transport. Results and discussion X-Ray Diffraction (XRD) Analysis The XRD patterns (Figure 1a) of copper-doped amorphous alumina with varying copper concentrations (2.5%, 5%, 7.5%, and 10%) reveal critical insights into the structural characteristics of the synthesized materials. The absence of sharp diffraction peaks across all compositions confirms the lack of long-range crystalline order, highlighting the successful formation of an amorphous phase, as shown in Figure 1(a) [17, 18]. Instead, a broad diffraction halo centered at approximately 2θ = 20°-40° is observed, which is a characteristic feature of amorphous materials and arises from short-range atomic ordering within the alumina network. The remarkable similarity of the diffraction profiles with increasing copper content demonstrates the robustness of the sol-gel synthesis route, ensuring homogeneous incorporation of copper while fully preserving the amorphous structure. Importantly, no diffraction peaks corresponding to crystalline copper oxides or alumina polymorphs are detected, confirming the formation of a single-phase amorphous electrolyte even at the highest copper loading. Complementary SEM analysis of the optimized 10% CuAlO 3 composition (Figure 1b-h) provides further insight into the microstructural features associated with its superior electrochemical performance. The SEM micrographs reveal a relatively dense and well-connected morphology composed of compact agglomerates with reduced porosity compared to lower Cu-doped samples. This densified yet non-crystalline microstructure is beneficial for electrolyte applications, as it promotes continuous ionic transport pathways while minimizing gas permeation through the electrolyte layer. Elemental mapping confirms a uniform spatial distribution of Cu, Al, and O throughout the matrix, indicating successful atomic-scale dispersion of copper within the amorphous alumina network without evidence of phase segregation or dopant clustering. Such homogeneous elemental distribution is consistent with the absence of crystalline secondary phases in XRD and supports the notion that copper is incorporated primarily as a network modifier rather than forming separate oxide domains. The coexistence of a fully amorphous structure with a dense and well-interconnected microstructure at 10% Cu doping highlights the critical role of copper in stabilizing the amorphous alumina matrix while inducing localized structural perturbations. These perturbations are expected to enhance defect density, oxygen vacancy formation, and localized electron–ion coupling, thereby facilitating efficient ion migration. The combination of amorphous structural flexibility (as evidenced by XRD) and optimized microstructural connectivity (as revealed by SEM) provides a favorable framework for reducing migration barriers and achieving high ionic conductivity. Collectively, these results demonstrate that the 10% CuAlO 3 electrolyte maintains excellent amorphous integrity while exhibiting a microstructure ideally suited for solid oxide fuel cell operation, underscoring its potential scalability and applicability in next-generation intermediate-temperature SOFCs. Figure 1. (a) X-ray diffraction (XRD) patterns of pure amorphous alumina and Cu-doped amorphous alumina with Cu contents of 2.5%, 5%, 7.5%, and 10%, showing a broad diffraction halo characteristic of an amorphous structure and the absence of crystalline secondary phases. (b) High-magnification SEM image of the optimized 10% CuAlO 3 electrolyte. (c) Corresponding SEM surface morphology of 10% CuAlO 3 at a different region. (d–f) Energy-dispersive X-ray spectroscopy (EDS) elemental mapping of Al, O, and Cu, respectively, for the 10% CuAlO 3 sample, confirming the homogeneous distribution of copper within the amorphous alumina matrix. (g) SEM image illustrating the dense and well-connected microstructure of 10% CuAlO 3 . (h) EDS spectrum of 10% CuAlO 3 , verifying the presence of Al, O, and Cu without detectable impurity phases. Electrochemical Analysis Fuel cell Performance The electrochemical performance of pure alumina and copper-doped amorphous alumina (CuAlO 3 ) was investigated to validate its potential as a high-performance electrolyte for low-temperature solid oxide fuel cells (LT-SOFCs) as shown in Figure 2. This study aimed to establish the correlation between copper doping, ionic conductivity, and power density, as hypothesized in the introduction. The evaluation was designed to prove that increasing copper concentrations enhances localized charge transfer and oxygen vacancy generation, thereby boosting ionic conduction and overall electrochemical efficiency. Benchmarking was performed across a temperature range of 550-370 °C for pure alumina and varying doping levels (2.5%, 5%, 7.5%, and 10% Cu) to examine the effect of structural modifications on fuel cell performance. Pure alumina , used as the baseline material, exhibited limited ionic conductivity and poor power output due to its lack of defect structures and rigid amorphous framework. At 550 °C , it delivered a modest 492 mW/cm 2 at 1.07 V , with the performance declining sharply to 353 mW/cm 2 at 520 °C and 179 mW/cm 2 at 490 °C . The absence of copper-induced oxygen vacancies and localized charge transfer pathways constrained its performance, highlighting the need for doping to enhance ionic transport. With 2.5% copper doping , the performance improved significantly, achieving 616 mW/cm 2 at 550 °C with an OCV of 1.07 V . The introduction of copper created localized electronic states and oxygen vacancies, validating the claim that doping facilitates ion mobility. At 520 °C achieving 508 mW/cm 2 and at 490 °C reaching 407 mW/cm 2 , the composition demonstrated higher ionic conduction than pure alumina, although the modest improvements suggested the need for higher copper concentrations to sustain ionic transport at lower temperatures. The 5% Cu-doped alumina exhibited further enhancement, achieving 675 mW/cm 2 at 550 °C and maintaining 561 mW/cm 2 at 520 °C , with 487 mW/cm 2 at 490 °C . This improvement underscores the direct correlation between increased copper concentration and enhanced ionic transport pathways. By generating additional oxygen vacancies and facilitating localized charge transfer, this composition aligned with the introduction’s hypothesis of boosting ionic conduction through optimized defect engineering. At 7.5% copper , the balance between defect generation and structural stability led to an impressive power density of 695 mW/cm 2 at 550 °C , with 604 mW/cm 2 at 520 °C and 495 mW/cm 2 at 490 °C . This composition demonstrated the highest efficiency across a wide temperature range, confirming that an optimal doping level creates sufficient ionic pathways without compromising the amorphous matrix’s stability. The results substantiate the claims in the abstract, validating the role of copper doping in transforming amorphous alumina into a highly efficient electrolyte. The 10% Cu-doped sample achieved the peak performance, delivering a remarkable 770 mW/cm 2 at 550 °C and maintaining 675 mW/cm 2 at 520 °C and 538 mW/cm 2 at 490 °C . These results highlight the role of higher copper concentrations in maximizing defect generation and localized charge transfer. However, the marginal improvement beyond 7.5% indicates a saturation point for defect density, where further increases in copper concentration provide diminishing returns. This finding reinforces the importance of optimizing the doping level to achieve a balance between ionic mobility and structural integrity. The observed performance trends confirm the claims presented in the introduction and abstract, demonstrating that copper doping enhances ionic conductivity and power density by facilitating localized charge transfer and oxygen vacancy generation. The ability of 10% CuAlO 3 to achieve 770 mW/cm 2 at 550 °C and maintain high performance across the temperature range validates its potential as a next-generation electrolyte material for LT-SOFCs. These findings establish a clear correlation between doping concentration, defect engineering, and electrochemical efficiency, providing a pathway for advancing solid oxide fuel cell technology. Figure 2 . Fuel cell performance measurement (I-V & I-P) of (a) I-V & I-P curve for amorphous pure alumina electrolyte operating at 460-550℃, (b) I-V & I-P curve for 2.5-CuAlO 3 electrolyte operating at 460-550℃, (c) I-V & I-P curve for 5-CuAlO 3 electrolyte operating at 460-550℃, (d) I-V & I-P curve for 7.5-CuAlO 3 electrolyte operating at 460-550℃, (e) I-V & I-P curve for 10-CuAlO 3 electrolyte operating at 460-550℃, (f) Arrhenius plot of the ionic conductivity for pure amorphous Al-oxide and different CuAl-oxide Phase calculate using I-V curve of the fuel cells at 400-550℃. reprint, amsmath,amssymb, aps, ]revtex4-2 Electrochemical impedance spectroscopy (EIS) The electrochemical impedance spectroscopy (EIS) analysis was conducted across a temperature range of 550 °C to 300 °C (Figure 3) to evaluate the ionic transport mechanisms, grain boundary contributions, and electrode polarization processes in copper-doped amorphous alumina (Cu-AlO 3 ) compositions [19]. The study aimed to establish a direct relationship between copper doping levels, resistance characteristics, and ionic conduction improvements, supporting the claims of enhanced localized charge transfer and ionic mobility presented in the abstract and introduction. The measured resistances, including Ohmic resistance (R 0 ), grain boundary resistance (R 1 ), and electrode polarization resistance (R 2 ) [20], provide detailed insights into charge transport across different doping concentrations, with total resistance (R t ) calculated as: R t = R 0 + R 1 reprint, amsmath,amssymb, aps, ]revtex4-2 For pure alumina, the highest resistances were recorded, demonstrating the limitations of the undoped material in supporting efficient ionic conduction. At 550 °C, pure alumina exhibited an R t of 0.352 Ω·cm 2 , with R 0 = 0.22 Ω·cm 2 and R 1 = 0.128 Ω·cm 2 . The high electrode polarization resistance (R 2 = 0.337 Ω·cm 2 ) indicates poor oxygen ion transport and low oxygen reduction reaction (ORR) kinetics. As the temperature decreased to 460 °C, Rt increased sharply to 3.63 Ω·cm 2 , driven by significant increases in R 1 (1.29 Ω·cm 2 ) and R 2 (2.15 Ω·cm 2 ). These results confirm the severe limitations of pure alumina in supporting ionic transport at lower temperatures, attributed to the absence of copper-induced oxygen vacancies and poor defect mobility. The incorporation of 2.5% copper significantly reduced the total resistance across all temperatures. At 550 °C, R t decreased to 0.323 Ω·cm 2 , with R 0 = 0.21 Ω·cm 2 and R 1 = 0.113 Ω·cm 2 . The reduction in R 2 to 0.337 Ω·cm 2 reflects improved oxygen reduction kinetics and charge transfer processes facilitated by copper doping. At 460 °C, R t increased moderately to 0.58 Ω·cm 2 , maintaining significantly lower values compared to pure alumina. These findings highlight the impact of copper doping in creating oxygen vacancies that enhance ionic mobility, consistent with the claims of improved defect conductivity through localized charge transfer. Further enhancement was observed with 5% CuAlO 3 , which demonstrated an R t of 0.299 Ω·cm 2 at 550 °C, with R 0 = 0.2 Ω·cm 2 and R 1 = 0.099 Ω·cm 2 . The R 2 value of 0.301 Ω·cm 2 indicates reduced electrode polarization resistance, supporting improved ion migration and surface reaction kinetics. At 460 °C, R t remained lower than both pure and 2.5% Cu-doped alumina at 0.78 Ω·cm 2 , with R 1 = 0.23 Ω·cm 2 and R 2 = 0.61 Ω·cm 2 . The improvements in ionic transport at this composition validate the hypothesis that increasing copper concentration promotes defect creation and facilitates ion hopping. The 7.5% Cu-doped composition achieved a remarkable R t of 0.233 Ω·cm 2 at 550 °C, with R 0 = 0.15 Ω·cm 2 and R 1 = 0.083 Ω·cm 2 . The lower R 2 value of 0.307 Ω·cm 2 highlights significant reductions in polarization resistance, attributed to optimized oxygen vacancy generation and enhanced charge transfer. Even at 460 °C, R t was recorded as 0.46 Ω·cm 2 , outperforming lower doping levels and demonstrating improved ionic conduction at reduced temperatures. The highest performance was observed with 10% CuAlO 3 , which exhibited the lowest R t values across all temperatures. At 550 °C, Rt was measured as 0.189 Ω·cm 2 , with R 0 = 0.12 Ω·cm 2 and R 1 = 0.069 Ω·cm 2 . The reduced R 2 = 0.301 Ω·cm 2 reflects enhanced electrode polarization kinetics and oxygen ion transport. Even at 460 °C, R t remained low at 0.33 Ω·cm 2 , maintaining superior performance relative to all other compositions. These results confirm that the highest copper concentration generates the most oxygen vacancies and defect pathways, enhancing ionic conductivity and reducing activation energy barriers. The EIS analysis validates the role of copper doping in mitigating both bulk and interfacial resistances, consistent with the introduction’s claims of enhanced localized charge transfer and defect generation. The significant reductions in R 0 , R 1 , and R 2 for the 10% Cu-doped sample demonstrate the effectiveness of copper in stabilizing the amorphous phase while creating well-distributed defect clusters. The ability of 10% Cu-AlO 3 to maintain low R t values across a wide temperature range highlights its potential as a high-performance electrolyte material for LT-SOFCs, fulfilling the need for improved ionic conduction and efficient oxygen incorporation. These findings underscore the critical role of copper doping in advancing the performance of amorphous alumina-based electrolytes. Figure 3. (a-e) Nyquist plot of electrochemical impedance spectroscopy measurements for the fuel cells using Pure Alumina, 2.5-CuAlO 3 , 5-CuAlO 3 , 7.5-CuAlO 3 , and 10-CuAlO 3 electrolytes measured in H 2 /air at the different temperature range from 300-500℃, (f) Comparison of EIS for all composition at 550℃ respectively. 4. Verification of Protonic Conduction Protonic conduction in 10-CuAlO 3 was definitively established using a set of complementary and mutually reinforcing experimental approaches designed to directly identify the dominant charge carrier. First, the isotopic effect was examined by replacing hydrogen (H 2 ) with deuterium (D 2 ) on both sides of the electrolyte [21, 22]. Because deuterium is a heavier isotope than hydrogen, deuterons (D + ) exhibit intrinsically lower mobility and slower hopping kinetics compared to protons (H + ). As a result, a clear deterioration in electrochemical performance and increased impedance were observed under deuterated atmospheres, providing direct and model-independent evidence that proton transport governs the ionic conduction behavior in 10-CuAlO 3 . Such isotope-dependent transport cannot be explained by oxygen-ion or electronic conduction, which are insensitive to hydrogen isotope substitution, thereby confirming protons as the primary charge carriers. To further corroborate this conclusion, selective ion-filter experiments were performed using well-established benchmark electrolytes. A BaZr 0.9 Y 0.1 O 3 (BZY) layer, known for its high proton conductivity and negligible oxygen-ion transport, was employed as a proton-selective filter, while yttria-stabilized zirconia (YSZ), a pure oxygen-ion conductor, was used as a proton-blocking layer. The compatibility of 10-CuAlO 3 with the BZY filter and its severe performance degradation when interfaced with YSZ provide decisive evidence that proton transport dominates the electrochemical response. Together, isotopic substitution and selective filter layers establish a comprehensive and unambiguous framework for confirming proton conduction in 10-CuAlO 3 . The experimental results supporting proton conduction in 10-CuAlO 3 are summarized in Figure 4, which presents isotope substitution, selective filter-layer tests, and the corresponding impedance responses. As shown in Figure 4 (a), a pronounced isotopic effect is observed when hydrogen is replaced by deuterium. The electrochemical impedance measured under D 2 -D 2 conditions is consistently higher than that obtained under H 2 -H 2 operation at 550 °C. This increase in impedance directly reflects the reduced mobility of deuterons (D + ) compared to protons (H + ) due to their larger mass, leading to slower hopping kinetics. Since oxygen-ion and electronic conduction mechanisms are insensitive to hydrogen isotope substitution, the clear H/D dependence provides direct and unambiguous evidence that proton transport dominates ionic conduction in 10-CuAlO 3 . Further confirmation is obtained from selective ion-filter experiments shown in Figure 4 (b) . The fuel cell without any filter layer delivers a peak power density of 770 mW cm -2 , demonstrating the intrinsic electrochemical activity of 10-CuAlO 3 . When a proton-conducting BZY filter layer is introduced, the maximum power density remains high at ~ 660 mW cm -2 , corresponding to ~ 86% retention of the original performance. This modest reduction is attributed to additional interfacial resistance and increased transport length introduced by the extra layer, rather than any disruption of the dominant charge carrier. The strong compatibility between BZY and 10-CuAlO 3 confirms that proton transport is continuous across the interface. In contrast, the insertion of a YSZ oxygen-ion-conducting filter layer results in a drastic decrease in power density to only ~ 115 mW cm -2 (≈ 15% of the original value), indicating severe suppression of ionic transport. Because YSZ effectively blocks protons while allowing O 2- transport, this sharp performance degradation demonstrates that oxygen-ion conduction does not significantly contribute to the electrochemical response of 10-CuAlO 3 . The impedance spectra shown in Figure 4(c) further substantiate these conclusions. The cell with the BZY filter exhibits only a moderate increase in impedance compared to the filter-free configuration, consistent with preserved proton transport and limited interfacial polarization. In contrast, the YSZ-filtered cell displays a substantially enlarged impedance arc, reflecting a dramatic increase in polarization resistance caused by blocked proton transfer at the electrolyte interface. The strong correlation between impedance evolution and fuel-cell performance confirms that proton transport governs both the ohmic and polarization behavior of the system. Collectively, the isotope-dependent transport behavior, selective compatibility with proton-conducting BZY, severe inhibition by oxygen-ion-conducting YSZ, and the corresponding impedance responses presented in Figure 4 provide direct, quantitative, and model-independent proof that proton conduction is the dominant charge-transport mechanism in 10-CuAlO 3 . Figure 4. Independent experimental verification of dominant proton conduction in the 10-CuAlO 3 electrolyte. (a) Electrochemical impedance spectra measured under H 2 -H 2 and D 2 -D 2 atmospheres at 550 °C, showing a clear isotopic effect with higher resistance under deuterated conditions due to the lower mobility of D + compared to H + . (b) Fuel cells based on 10-CuAlO 3 without any filter layer, with a proton-conducting BZY filter layer, and with an oxygen-ion-conducting YSZ filter layer. The peak power density reaches 770 mW cm -2 without a filter, retains sharply to ~115 mW cm -2 (≈15%) with YSZ. (c) Corresponding Nyquist plots for the three configurations, illustrating a modest increase in impedance with the BZY filter and a dramatic increase with the YSZ filter, confirming selective blocking of proton transport by YSZ. Conclusion This work demonstrates that copper doping effectively transforms amorphous alumina into a high-performance proton-conducting electrolyte for intermediate-temperature solid oxide fuel cells. Systematic variation of Cu concentration (2.5–10%) reveals a clear correlation between dopant level, defect generation, and electrochemical performance. Structural analyses confirm that the amorphous framework is preserved across all compositions, while uniform copper incorporation induces localized structural perturbations that enhance oxygen vacancy formation and electron–ion (E–I) coupling. Electrochemical results show progressive reductions in bulk and interfacial resistances with increasing Cu content, culminating in optimal performance at 10% Cu doping. The 10-CuAlO₃ electrolyte achieves a peak power density of 770 mW cm⁻² at 550 °C and an ionic conductivity of 0.16 S cm⁻¹, significantly outperforming undoped alumina and demonstrating competitive performance among low-temperature SOFC electrolytes. Isotopic substitution (H/D) and selective filter-layer experiments (BZY and YSZ) provide direct and model-independent confirmation that proton conduction dominates the transport mechanism. Overall, this study establishes copper-doped amorphous alumina as a viable next-generation electrolyte platform. By leveraging localized electron–ion coupling and defect engineering within a structurally flexible amorphous matrix, this approach provides a new pathway for designing high-conductivity, low-activation-energy electrolytes for advanced SOFC technologies. Acknowledgements: The National Natural Science Foundation of China (Grant No. 32250410309) reprint, amsmath,amssymb, aps, ]revtex4-2 Conflict of Interest: Authors declare no conflict of interest in this study. reprint, amsmath,amssymb, aps, ]revtex4-2 Data availability statement: Only data can be provided upon formal request to corresponding and leading author. reprint, amsmath,amssymb, aps, ]revtex4-2 References [1] Tarancón A. Strategies for lowering solid oxide fuel cells operating temperature. Energies. 2009;2:1130-50.[2] Yang G, Su C, Shi H, Zhu Y, Song Y, Zhou W, et al. Toward reducing the operation temperature of solid oxide fuel cells: our past 15 years of efforts in cathode development. 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Investigating the Proton Conduction in Co–Nd Co-Doped Amorphous Alumina-Based Electrolyte for Ceramic Fuel Cell. ACS Applied Energy Materials. 2025;8:17871-88. Information & Authors Information Version history V1 Version 1 03 March 2026 Copyright This work is licensed under a Non Exclusive No Reuse License. Keywords amorphous alumina e-i coupling pcfc Authors Affiliations Muhammad Shahid Sharif 0009-0003-3174-9663 [email protected] Southeast University View all articles by this author Sajid Rauf 0000-0003-2343-9334 Shenzhen University View all articles by this author Muhammad Ali Kamran Yousaf Shah Southeast University - Sipailou Campus View all articles by this author Metrics & Citations Metrics Article Usage 135 views 67 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Muhammad Shahid Sharif, Sajid Rauf, Muhammad Ali Kamran Yousaf Shah. reprint, amsmath,amssymb, aps, ]revtex4-2 Breaking the Ionic-Conductivity Barrier in Amorphous Alumina: Copper Doping-Induced Electron-Ion Coupling for Next-Generation SOFC Electrolytes. Authorea . 03 March 2026. DOI: https://doi.org/10.22541/au.177257725.53363511/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. 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