Tuning an ionic-electronic mixed conductor NdBa0.5Sr0.5Co1.5Fe0.5O5+δ for electrolyte functions of advanced fuel cells

An ionic-conducting electrolyte mainly governs the solid oxide fuel cell performance. In this work, a mixed conductor NdBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ was tuned as an electrolyte via compositing with a proton conductor BaZr_0.3Ce_0.6Y_0.1O_{3- δ} (BZCY), which realizes an ionic conductivity of 0.16 S cm^?1 at 550 °C along with fuel cell power density of 470 mW cm^?2. The 10 wt.% proton conducting BZCY can not only effectively block the electronic conductivity of NBSCF, but also greatly improve its ionic conductivity and the corresponding device's power output. The interfacial conduction could take a crucial role in the ion transporting process of BZCY-NBSCF composite. These interfaces or nanoscale grain boundaries formed amongst two phases keep excellent capability for designing and creating high performance electrochemical devices along with high-power density.     

A solid oxide carbon fuel cell operating on pomelo peel char with high power output

The pomelo peel char (PC) was prepared and used as fuel for solid oxide electrolyte direct carbon fuel cells with nickel‐yttrium stabilized zirconia anode, thin‐film YSZ electrolyte, and La_0.8Sr_0.2MnO₃ cathode. The power densities of fuel cells operating on PC and catalyst‐loaded PC (PCC) fuels achieved 309 and 518 mW cm^?2 at 850°C, respectively, which are among the highest power densities reported in the literature on DCFCs. The PC exhibited superior gasification reactivity than coal char due to its unique reticulated foam carbon structure with a homogeneously distributed inherent catalyst. The stability tests at a current density of 50 mA cm^?2 and 825°C indicate that the cell using PC fuel operated in a more stable manner than that using PCC, and the fuel availabilities for PC and PCC were 47.25% and 34.71%, respectively. The results suggest that PC is a promising solid carbonaceous fuel for solid oxide electrolyte direct carbon fuel cells based on its adequate gasification reactivity and high compatibility with the fuel cells.     

The semiconductor SrFe0.2Ti0.8O3-δ-ZnO heterostructure electrolyte fuel cells

Highly ion-conducting properties in heterostructure composites and semiconductors have drawn significant attention in recent years for developing new electrolytes in low-temperature solid oxide fuel cells (LT-SOFCs). In this work, a new semiconductor heterostructure composite SrFe_0.2Ti_0.8O_3-δ (SFT)-ZnO consisting of p-type SFT and n-type ZnO is proposed and evaluated as an electrolyte in LT-SOFCs. Electrochemical studies reveal that the prepared SFT-ZnO is a mixed ion-electron conductor possessing a high ionic conductivity of 0.21 S cm⁻¹ at 520 °C and the assembled SFT-ZnO fuel cell can achieve a favorable peak power output of 650 mW cm⁻² along with high open-circuit voltage (OCV) of 1.06 V at 520 °C. By referring the semiconductor conduction types and energy band parameters of SFT and ZnO, a p-n bulk-heterojunction effect is proposed to describe the electronic blocking and ionic promotion processes of SFT-ZnO electrolyte in a fuel cell. Our work suggests a new insight into the design of effective LT-SOFC electrolytes by using semiconductor heterostructure material.     

High‐performance solid PEO/PPC/LLTO‐nanowires polymer composite electrolyte for solid‐state lithium battery

Solid polymer composite electrolyte (SPCE) with good safety, easy processability, and high ionic conductivity was a promising solution to achieve the development of advanced solid‐state lithium battery. Herein, through electrospinning and subsequent calcination, the Li_0.33La_0.557TiO₃ nanowires (LLTO‐NWs) with high ionic conductivity were synthesized. They were utilized to prepare polymer composite electrolytes which were composed of poly (ethylene oxide) (PEO), poly (propylene carbonate) (PPC), lithium bis (fluorosulfonyl)imide (LiTFSI), and LLTO‐NWs. Their structures, thermal properties, ionic conductivities, ion transference number, electrochemical stability window, as well as their compatibility with lithium metal, were studied. The results displayed that the maximum ionic conductivities of SPCE containing 8 wt.% LLTO‐NWs were 5.66 × 10^?5 S cm^?1 and 4.72 × 10^?4 S cm^?1 at room temperature and 60°C, respectively. The solid‐state LiFePO₄/Li cells assembled with this novel SPCE exhibited an initial reversible discharge capacity of 135 mAh g^?1 and good cycling stability at a charge/discharge current density of 0.5 C at 60°C.     

Compositing protonic conductor BaZr0.5Y0.5O3 (BZY) with triple conductor BaCo0.4Fe0.4Zr0.1Y0.1O3-δ (BCFZY) as electrolyte for advanced solid oxide fuel cell

BaZr_0.5Y_0.5O₃(BZY) electrolyte exhibited enormous potential for low temperature solid oxide fuel cell (SOFC) due to its proton dominant mobile carriers rather than oxygen ions during the electrochemical process. In order to enhance the ionic conductivity, triple conductor BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_3-δ (BCFZY) was composited with proton conductor BZY to form semiconductor-ionic conductor composite (SIM), which was applied as electrolyte to fabricate symmetrical fuel cell. The microstructural and electrochemical properties for BZY, BCFZY and BZY-BCFZY composites were studied. After optimizing the weight ratio of composite content, the ionic conductivity and electronic conductivity reached an equilibrium state to obtain the maximum cell performance, namely the highest output of 902.5 mW cm^?2 and an open circuit voltage (OCV) of 1.043 V at 550 °C. The BZY-BCFZY cell also presented decent power output at low temperature, a power density of 265.625 mW cm^?2 was received even at 500 °C, demonstrating that the BZY-BCFZY composite was a potential electrolyte for low temperature SOFCs.     

Semiconductor Fe-doped SrTiO3-δ perovskite electrolyte for low-temperature solid oxide fuel cell (LT-SOFC) operating below 520 °C

High-temperature operation of solid oxide fuel cells causes several degradation and material issues. Lowering the operating temperature results in reduced fuel cell performance primarily due to the limited ionic conductivity of the electrolyte. Here we introduce the Fe-doped SrTiO_3-δ (SFT) pure perovskite material as an electrolyte, which shows good ionic conduction even at lower temperatures, but has low electronic conduction avoiding short-circuiting. Fuel cell fabricated using this electrolyte exhibits a maximum power density of 540 mW/cm² at 520 °C with Ni-NCAL electrodes. It was found that the Fe-doping into the SrTiO_3-δ facilitates the creation of oxygen vacancies enhancing ionic conductivity and transport of oxygen ions. Such high performance can be attributed to band-bending at the interface of electrolyte/electrode, which suppresses electron flow, but enhances ionic flow.     

Developing cuprospinel CuFe2O4–ZnO semiconductor heterostructure as a proton conducting electrolyte for advanced fuel cells

The interfacial properties of electrolyte materials have a crucial impact on the ionic conductivity of solid batteries and solid oxide fuel cells. Here we construct cuprospinel CuFe₂O₄ (CFO)–ZnO composite as a functional electrolyte for fuel cell device. In an optimal composition of 0.3CFO-0.7ZnO electrolyte fuel cell, the maximum power output of 675 mW cm^?2 is obtained at 550 °C. The electrical properties and electrochemical performance are strongly dependent on the ratios between CFO and ZnO in CFO-ZnO composite. Notably, surprising fuel cell performance with high ionic conductivity is attained by constructing this p-type CFO composited with n-type ZnO. Proton conduction was further verified experimentally. The interfacial ionic conduction pathway between the two constituent phases plays a vital role to enhance the proton conductivity, and the bulk p-n heterojunction can block internal electronic pass. An excellent current and power densities of CFO-ZnO composite are observed along with a high conductivity of 0.35 S·cm-1 at 550 °C. This work opens a new perspective for the semiconductor materials that can widely be developed for electrolytes, based on their tunable band structure.     

Stability evaluation and quantitative analysis of filmy cathode in solid oxide fuel cells under operating conditions

For interface-modified cathodes via the infiltration method, the quantitative analysis of degradation mechanism in solid oxide fuel cells (SOFCs) is the key to optimizing cell stability. Here, we prepare the anode-supported SOFC with a multifunction layer (MFL) cathode-infiltrated filmy La_0.6Sr_0.4CoO_3-δ (LSC), which provides a peak power density as high as 1.1 W cm^?2 at 750 °C, and outstanding durability with slight voltage loss under 0.5 A cm^?2 and 750 °C for over 1000 h. According to collected data from electrochemical impedance spectroscopy (EIS) at different operating times, the distribution of relaxation time (DRT) and equivalent circuit model (ECM) methods are applied to quantify the contribution of different electrode processes to the whole voltage degradation during the cell operation. The result shows that oxygen ionic transport and charge transfer at the MFL/electrolyte interface dominate almost all voltage degradation (95.31%), while the oxygen surface exchange and oxygen ionic bulk diffusion in the cathode just contributes 1.82%. Microtopographic characterization provides that the filmy morphology of LSC cathode remains intact, but the Sr element enriches at the MFL/electrolyte interface. Therefore, the exacerbation of oxygen ionic transport and charge transfer at the MFL/electrolyte interface due to Sr enrichment is identified as the predominant degradation mechanism during long-term galvanostatic operation.     

Doped ceria electrolyte rich in oxygen vacancies for boosting the fuel cell performance of LT-CFCs

Multifunctional semiconductor CeO₂ is used as an electrolyte for a fuel cell application, delivering meaningful power density and better OCV. Further, surface doping of Al was employed to attain the CeAlO₂ (ADC) electrolyte with enriched O-vacancies surface layer enabling high ionic conduction and excellent power density of 1020 mW/cm² at 520 °C. It is noticed that surface doping entailed the band alignment between CeO₂ and ADC due to the difference in Fermi level establishing space charge region, which further constitutes built-in field enhancing the charge transportation and minimizing the e-conduction. Furthermore, the theoretical calculation was performed to assist the formation of O-vacancies in the ADC structure. These findings suggest surface doping is the best approach to attain excellent performance and designing new electrolytes and electrodes for advanced low-temperature ceramic fuel cell technology.     

Interface engineering of bi-layer semiconductor SrCoSnO3-δ-CeO2-δ heterojunction electrolyte for boosting the electrochemical performance of low-temperature ceramic fuel cell

A comparative study is performed to investigate the electrochemical performance of the low-temperature ceramic fuel cells (CFCs) utilizing two different novel electrolytes. First, a perovskite semiconductor SrCo_0.3Sn_0.7O_3-δ was used as an electrolyte in CFCs due to its modest ionic conductivity (0.1 S/cm) and demonstrated an acceptable power density of 360 mW/cm² at 520 °C. The performance of the cell was primarily limited due to the moderate ionic transport in the electrolyte. In order to improve the ionic conductivity, a new strategy of using a novel bi-layer electrolyte concept consist of SrCo_0.3Sn_0.7O_3-δ and CeO_2-δ in CFCs. These bi-layers of two electrolytes have successfully established heterojunction which considerably improved the ionic conductivity (0.2 S/cm) and enhance the open-circuit voltage of the cell from 0.98 V to 1.001 V. Moreover, the CFCs utilizing bi-layer electrolyte have produced a remarkable power density of 672 mW/cm² at 520 °C. This enhancement of ionic conduction, power density and blockage of electron conduction in the bi-layer electrolyte was studied via band alignment mechanism based on proposed p-n heterojunction. Our work presents a promising methodology for developing advanced low-temperature CFC electrolytes.     

Stability considerations of semiconductor ionic fuel cells from a LNCA (LiNi0.8Co0.15Al0.05O2-δ) and Sm-doped ceria (SDC; Ce0.8Sm0.2O2-δ) composite electrolyte

Recent advances in composite materials, especially semiconductor materials incorporating ionic conductor materials, have led to significant improvements in the performance of low-temperature fuel cells. In this paper, we present a semiconductor LNCA (LiNi_0.8Co_0.15Al_0.05O_2-δ) which is often used as electrode material and ionic Sm-doped ceria (SDC; Ce_0.8Sm_0.2O_2-δ) composite electrolyte, sandwiched between LNCA thin-layer electrodes in a configuration of Ni-LNCA/SDC-LNCA/LNCA-Ni. The incorporation of the semiconductor LNCA into the SDC electrolyte with optimized weight ratios resulted in a significant power improvement, from 345 mW cm^?2 with a pure SDC electrolyte to 995 mW cm^?2 with the ionic-semiconductor SDC-LNCA one where the corresponding ionic conductivity reaches 0.255 S cm^?1 at 550 °C. Interestingly, the coexistence of ionic and electron conduction in the SDC-LNCA membrane displayed not any electronic short-circuiting but enhanced the device power outputs. This study demonstrates a new fuel cell working principle and simplifies technologies of applying functional ionic-semiconductor membranes and symmetrical electrodes to replace conventional electrolyte and electrochemical technologies for a new generation of fuel cells, different from the conventional complex anode, electrolyte, and cathode configuration.     

Designing Gadolinium-doped ceria electrolyte for low temperature electrochemical energy conversion

Reducing the operational temperature of solid oxide fuel cells (SOFC) is vital to improving their durability and lifetime. However, a traditional SOFC suffers from high ohmic and polarization losses at low temperatures, leading to poor performance. Gadolinium-doped ceria is the best ionic conductor for SOFC at lower temperatures. The present work envisages the GDC as an electrolyte for applying low-temperature solid oxide fuel cells (LT-SOFCs). So, in this regard, herein, GDC is synthesized through a wet chemical co-precipitation technique as a functional electrolyte layer fixed between two symmetrical porous electrodes NCAL (Ni_0.8Co_0.15Al_0.05LiO₂). Due to the improved surface properties of the synthesized GDC, particles perform better than commercially available GDC. The synthesized GDC electrolyte shows an impressive fuel cell performance of 569 mW/cm² and a high ionic conductivity of 0.1 S/cm at a shallow temperature of 450 °C. Moreover, the fuel cell device utilizing the synthesized GDC remained stable for 150 h of operation at a high current density of 110 mA/cm² at 450 °C. The high conduction mechanism has been proposed in detail. The results show that excellent fuel cell performance, high ionic conductivity, and better stability can be achieved at exceptionally low enough temperatures. Also, the proposed work suggests that new electrolytes can be designed for developing advanced low-temperature fuel cell technology.     

The composite electrolyte with an insulation Sm2O3 and semiconductor NiO for advanced fuel cells

Novel Sm₂O₃?NiO composite was prepared as the functional electrolyte for the first time. The total electrical conductivity of Sm₂O₃?NiO is 0.38 S cm^?1 in H₂/air condition at 550 °C. High performance, e.g. 718 mW cm^?2, was achieved using Sm₂O₃?NiO composite as an electrolyte of solid oxide fuel cells operated at 550 °C. The electrical properties and electrochemical performance are strongly depended on Sm₂O₃ and NiO constituent phase of the compositions. Notably, surprisingly high ionic conductivity and fuel cell performance are achieved using the composite system constituting with insulating Sm₂O₃ and intrinsic p-type conductive NiO with a low conductivity of 4 × 10^?3 S cm^?1. The interfacial ionic conduction between two phases is a dominating factor giving rise to significantly enhanced proton conduction. Fuel cell performance and further ionic conduction mechanisms are under investigation.     

Fine interface contact and high activity of nickel-based anode modified by gadolinium for hydrogen and methane fuel

In this work, gadolinium is used to modify nickel catalyst, which can improve the properties of nickel oxide particle and inhibit its sintering and grain growth. Interface contact between nickel catalyst and YSZ is significantly improved and fine anode microstructure can be obtained when gadolinium is used to modify Ni-YSZ anode. Fine interface contact of GdNi-YSZ anode can accelerate charge transfer process and steam formation process, which leads to high activity for electrochemical oxidation of hydrogen and low impedance resistance. The remarkable characteristic of GdNi-YSZ anode cell is that the cell performance for humidified methane fuel is greatly improved due to the high anode activity for methane reforming and electrochemical oxidation of hydrogen. The maximum power density of GdNi-YSZ anode cell with humidified methane as fuel can reach 1.59 W/cm² at 800 °C and 0.46 W/cm² at 650 °C. High performance of GdNi-YSZ anode cell with humidified methane as fuel leads to much H₂O produced during the electrochemical oxidation process, which can depress carbon deposition and improve the cell stability for humidified methane fuel.     

Advanced fuel cell based on semiconductor perovskite La–BaZrYO3-δ as an electrolyte material operating at low temperature 550 °C

As an electrolyte, enough ionic conductivity, either proton (H⁺) or oxide (O²⁻) conduction, has demanded the better performance of low-temperature (especially below 550 °C) solid oxide fuel cell (LT-SOFCs). Notably, that either conductivity, higher performance, reliability, or higher cost is hampering the LT-SOFC marketing. In our current subject, we report the La-doped BZY proton conductor as an electrolyte has exhibited high ionic conductivity of 0.15 S/cm with a higher performance of 0.78 W/cm² at 550 °C. Also, the performance of LBZY is superior to the un-doped BZY electrolyte. Such high performance mainly ascribed due to the doping of La into BZY. Besides, the mechanism for high ion conductivity is explained. This work manifests that using the LBZY semiconductor perovskite as an electrolyte is more suitable for fuel cell technology.     

CeO2 coated NaFeO2 proton-conducting electrolyte for solid oxide fuel cell

Nowadays, the low-temperature operation has become an inevitable trend for the development of SOFCs. Transition metal layered oxides are considered as promising electrolyte materials for low-temperature solid oxide fuel cells (LT-SOFCs). In this work, we report the CeO₂ coated NaFeO₂ as an electrolyte material for LT-SOFC. The study results revealed that the piling of CeO₂ significantly influenced the open-circuit voltage (OCV) as well as the power output of the fuel cells. In comparison with pure NaFeO₂, the denser structure of CeO₂ coated NaFeO₂ leads to higher OCV (1.06 V, 550 °C). The electrochemical impedance spectrum (EIS) fitted results showed that NaFeO₂–CeO₂ composites possessed higher ionic boundary conductivity. This is because that the hetero-interfaces between NaFeO₂ and CeO₂ provide fast ion conducting path. The high ionic conductivity of CeO₂ coated NaFeO₂ lead to admirable fuel cell power output of 727 mW cm^?2 at 550 °C.     

Electrochemical investigations of cobalt-free perovskite cathode material for intermediate temperature solid oxide fuel cell

A novel cobalt-free perovskite zinc-doped lanthanum strontium iron oxide (La_0.8Sr_0.2Zn_xFe_1?xO_3?δ, LSZF, x = 0.1–0.3) is synthesized and evaluated as cathode material for intermediate temperature solid oxide fuel cell (IT-SOFC) with samarium doped ceria (SDC) electrolyte. LSZF cathode at x = 0.2 composition demonstrates the remarkable electrochemical activity at intermediate temperature (550 °C): such as, high electrical conductivity (13.63 S cm^?1), excellent thermal stability with SDC electrolyte (12.10 μK^?1), high surface area (4.52 m² g^?1), extremely reduced area specific resistance (0.69 Ω cm^?2) and low activation energy (0.117 eV). Furthermore, single fuel cells are fabricated using LSZF as a cathode, which exhibits the excellent performance by achieving the high power density of 409 mW cm^?2 under natural gas as a fuel and ambient air as an oxidant at 550 °C with good stability over 10 h. All experimental results indicate that the LSZF is a promising cathode material for natural gas based intermediate temperature fuel cell applications.     

High-performance La0.5(Ba0.75Ca0.25)0.5Co0.8Fe0.2O3-δ cathode for proton-conducting solid oxide fuel cells

La_0.5(Ba_0.75Ca_0.25)_0.5Co_0.8Fe_0.2O_3-δ, a simple perovskite cathode material with high electrical conductivity (940 S cm^?1 at 600 °C) and impressive surface catalytic activity, was prepared and used in proton-conducting solid oxide fuel cells. As its thermal expansion coefficient is higher than that of the electrolyte material BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3-δ, they were combined and used as a composite cathode. The crystal structure, chemical compatibility, electrical conductivity, cell performance, and the oxygen reduction reaction of the cathode material were explored, and we found that the single fuel cell developed with the composite cathode achieved excellent electrochemical performance, with both a low polarization resistance and high peak power density (0.044 Ω cm² and 1102 mW cm^?2 at 750 °C, respectively). Outstanding stability was also achieved, as indicated by a long-term 100-h test. Additionally, the rate-limiting steps of the oxygen reduction reaction were the oxygen adsorption, dissociation, and diffusion processes.     

Energy band modulation of Mg-doped ZnO electrolyte for low-temperature advanced fuel cells

Mg-doped ZnO was developed as an ion conductor electrolyte that achieves an ionic conductivity of 0.24 S cm^?1 at 550 °C. An open-circuit voltage of 1.12 V and a maximum power density of 859 mW cm^?2 was achieved by using 5% Mg-doped ZnO having the bandgap of 3.241 eV and Fermi level value of ?3.43 eV. X-ray photoelectron spectroscopy studies showed that the improved performance was due to the high concentration of oxygen vacancies. UV diffuse reflectance and UV photoelectron spectroscopy analysis confirmed that Mg doping can modulate the energy band structure of ZnO to improve its electrochemical properties. In addition, 5% Mg-doped ZnO can form an optimal Schottky junction with the metal Ni generated by the reduction of the anode to prevent the cell from short-circuiting and accelerate the ion transport rate between the anode and electrolyte interface. This study provides a new idea for the application of wide bandgap semiconductor energy band modulation to low temperature advanced fuel cell electrolyte materials.     

Interfacial ionic transport in natural palygorskite-Na0.60CoO2 nanocomposite mineral materials

Natural clay minerals have been applied to various energy fields due to their low cost, good stability, and abundance of resources, but few of them have been investigated for fuel cell application. Proton ceramic fuel cell, one of the most promising sustainable-energy-conversion technologies, has attracted a lot of attentions because it possesses high ionic conductivity at relatively low temperatures (<600 °C). However, the characteristics of expensive construction cost and high temperature operation hinder the development and industrialization of fuel cell. In this paper, natural clay minerals palygorskite (PAL) and Na_0.60CoO₂ (NCO) nanosheets composite were synthesized via an electrostatic adsorption method, and used as the electrolyte of fuel cell. NCO/PAL demonstrated a high conductivity of 0.27 S?cm^?1 at 550 °C and the maximum power output of 641 mW cm^?2 was achieved for NCO/PAL cell. The mechanism of proton transport at the interface of NCO/PAL semiconductor-insulator composite was discussed.