Proton-Conducting Ceramic Fuel Cells Research Articles

Additive Manufacturing of Single-Layer Protonic Ceramic Fuel Cells

Single-layer proton conducting ceramic fuel cells (PCFCs) were fabricated through a hybrid of extrusion-based 3D printing utilizing BaZr0.4Ce0.4Y0.1Yb0.1O3-δ (BZCYYb) as an ionic conductor, and BaCo0.4Fe0.4Zr0.1Y0.1O3-δ (BCFZY) as the electrode material. The rheological properties of the printable pastes were investigated in detail. The 3D printed single-layer cell produced a maximum power density of 283 mW/cm2 at 550oC. The performance was mainly limited due to limited surface area on both sides of the cell. The performance improved by 17% (331 mW/cm2) when porous layers were added on both sides of the cell. In addition, cells with traditional 3-layer structure were fabricated using NiO-BZCYYb as anode, BZCYYb as electrolyte, and BCFZY-BZCYYb as cathode. The cell produced a maximum power density of 426 mW/cm2 at 650OC. The performance improved by 120% (938 mW/cm2) when a composite of BZCYYb and a eutectic mixture of Li2CO3, Na2CO3 and K2CO3 was used as electrolyte.

Surfacial proton conducting CeO2 nanosheets

Proton conducting ceramic fuel cells (PCFCs) have been attracting much more attentions due to is affordable ionic conductivity lower operating temperatures (<600 °C). CeO2 nanosheets expose mainly to the (111) plane exhibit a super proton conductivity of 0.3 S cm−1 and a maximum power density of 948 mWcm−2 at 550 °C. In these two morphologies of CeO2, the difference in Ce3+/Ce4+ ratios and the formation of oxygen vacancies on the surface are mainly responsible for proton transport. Theoretical calculation proves that protons can move more easily on the CeO2 (111) facet with smaller binding energy (0.14 eV) and lower energy barrier for H–O formation (1.7 eV) than on other facets, also superior proton conduction for the nanosheet morphology over the nano-particles. This work has developed a morphological methodology for high proton surface conduction to design highly efficient ionic transport for advanced CFCs and electrochemical devices.

(Invited) Mechanism of Cathodic Reaction in Proton-Conducting Ceramic Fuel Cells Investigated By Patterned Model Electrodes

Proton-conducting ceramic fuel cell (PCFCs) is one type of solid oxide fuel cell using a solid-state proton-conductor, typically BaZrO3-based and BaCeO3-based ceramics, as an electrolyte. PCFC is expected to operate at intermediate temperatures with high energy conversion efficiency. One of technological challenges for realization of PCFC is the development of high performace cathode. Cathodic reaction in PCFC is electrochemical reduction of oxygen gas foriming water vapor. Since protons, electrons, and oxygen gas molcules exist in the electrolyte, electrode and gas phase, respectively, the electrode reaciton takes place at triple phase boundaries (TPBs) in principle. On the other hand, the reaction site can be expanded from TPBs to double phase boundaries (DPBs), when a mixed ionic-electronic conductor (MIEC) is used. In such an MIEC electrode, the dominant reaction sites are believed to be DPBs because of their large area. So far, some oxides are reported to exhibit mixed protonic-electronic conductor, but no clear experimental evidences have been provided to show how significantly TPB/DPB reactions contribute to the total reaction.Recently we proposed new and original model electrodes, which can be schematically illustrated in Fig. 1, to investigate a gas reaction on a solid electrolyte. These so-called "patterned thin film electrodes" are kinds of thin film electrodes. However, in contrast to a conventional thin film electrode, the electrode/electrolyte contact area was limitted by inserting a slitted insulating layer between the electrode and the electrolyte, and threfore our model electrodes can behave as a columar electrode. In the electrode of Fig. 1(a), the electrode reaction proceeds only through DPBs, and the reaction current gradually decreased with increasing the distance from the electrode/electrolyte interface depending on proton diffusion in the electrode layer. On the other hand, in the electrode of Fig. 1(b), a part of the electrode film was removed from the electrode of Fig.1(a) to introduce TPBs. In this case, the electrode reaction takes place not only through DPBs but also through TPBs. Then, by comparing the electrode performances between these novel model electrodes, the contribution of TPB/DPB reactions can be separately evaluated.We applied these model electrodes to investigate the mechanism of the PCFC cathodic reactions. (La,Sr)CoO3-d was chosen as a cathode material, and the model electrodes were fabricated on a Ba(Zr,Yb)O3-d electrolyte. On the lateral and opposite sides of the electrolyte, porous Pt and Pd electrode was set as a reference and a counter electrodes. DC polarization and electrochemical impedance spectroscopy measurements were performed with the model electrodes at 773-973 K. As results, it was found TPBs are the dominant reaction site for (La,Sr)CoO3-d PCFC cathode, although DPB reaction slightly contributes to the total reaction. More detailed disucssion will be given in the presentation based on the results by operando X-ray absorption measurements. Figure 1

Enabling Carbon-Neutral Operation of Low-Temperature Proton-Conducting Ceramic Fuel Cells Supported by Ni-(Rh, Co) Bimetallic Catalysts

Enabling Carbon-Neutral Operation of Low-Temperature Proton-Conducting Ceramic Fuel Cells Supported by Ni-(Rh, Co) Bimetallic Catalysts

Carbon-Neutral Conversion of Methane Supported by Ni-(Rh, Co) Bimetallic Catalysts for Low-Temperature Proton-Conducting Ceramic Fuel Cells

Carbon-Neutral Conversion of Methane Supported by Ni-(Rh, Co) Bimetallic Catalysts for Low-Temperature Proton-Conducting Ceramic Fuel Cells

Boosting Proton-Conducting Ceramic Electrochemical Performance Through Mixed Ionic and Electronic Conducting Cathode Functional Layers

Proton conducting ceramic fuel cells (PCFCs) are a promising new technology for clean energy generation and green fuels production. PCFCs show favorable performance, but still lag behind their higher-temperature solid-oxide fuel cell predecessors. This work sets out to tailor the cathode-electrolyte interface in PCFCs by increasing the oxygen reduction reaction (ORR) activity and oxygen transference to bolster electrochemical performance and durability.The cathode-electrolyte interface is a major source of voltage loss during PCFC operation and is also suspected to be a primary contributor to cell-level degradation. Both oxygen reduction (ORR) and H2O formation must be simultaneously facilitated in PCFC cathodes. This contrasts with SOFC cathodes, where only oxygen reduction is required since water formation occurs separately at the anode. The dual reaction processes occurring in PCFC cathodes can lead to competitive interference between water adsorption/desorption and ORR activity. We hypothesize that placing materials with high ORR activity and oxygen-ion conduction at the electrolyte-cathode interface may help alleviate polarization loss.To explore this hypothesis, Er0.4Bi1.6O3 (ESB), Dy0.08W0.04Bi0.88O1.56 (DWSB), Gd0.1Ce0.9O1.95 (GDC10) and SmCoO3 were fabricated into the cathode functional layers (CFLs) in this study. As the addition of an interlayer to the cathode-electrolyte interface can inadvertently effect the morphology (e.g. roughness, porosity) and mechanical integrity of the interface, control cells with a porous BaCe0.4Zr0.4Y0.1Yb0.1O3- δ BCZYYb cathode functional layer were also fabricated and tested. Button cells with the CFLs were fabricated through ultrasonic spray coating of thin layers of the CFL material onto a precursor BCZYYb electrolyte and co-fired. The cathode is then brush painted onto the resulting half-cell and sintered. Preliminary results, shown in Figure 1a, demonstrate that adding an ESB CFL can increase performance by over 40%. While GDC-based CFLs have led to significant increases in PCFC durability, displayed in Figure 1b, underscoring the promise of this technique. Figure 1

Proton-conducting Ceramic Fuel Cells for Co-Producing Electricity and Fuel – A Design for Manufacture and Assembly (DFMA) Analysis

This work develops preliminary design for manufacture and assembly (DFMA) models of a novel, proton-conducting, anode-supported ceramic electrochemical cell for co-producing electricity and fuels simultaneously. These DFMA models can then be used to estimated future, expected costs incorporating the novel cell design under various manufacturing production scenarios. The cell electrochemically coverts oxygen (O2) in air and methane (CH4) fuel (the primary constituent of natural gas) into electricity, water, and higher-order hydrocarbons, such as ethane (C2H6). To do this, the cell integrates a methane coupling catalyst into the anode and thereby co-produces ethane (C2H6) and hydrogen (H2) at the anode. The ethane can then be converted into other fuels. The cell is part of a joint R&D effort by Argonne National Laboratory (ANL) and the Illinois Institute of Technology (IIT). The cell’s current technology readiness level (TRL) is about a one (1), i.e. laboratory-scale demonstration of the fundamental concept.Approach:Gaia Energy Research Institute (Gaia) collaborates closely with ANL and IIT to develop the DFMA models for this novel cell design. Gaia works closely with ANL and IIT to identify and analyze key electrochemical cell engineering performance data. Gaia then develops and deploys custom DFMA computer models and data sets. DFMA models center around the electrolyte-electrode assembly (EEA), or the ‘heart’ of the electrochemical cell, where the anode and cathode half-reactions occur. Gaia first develops a specific physical embodiment of the novel cell, then the cell’s bill of materials (BOM), and finally cost estimates for the electrochemical cell, fuel cell stack, and full-scale stationary fuel cell system. Gaia follows this methodology:1. Develop a specific design that physically embodies the EEA concept;2. Develop the EEA’s BOM to specify quantities of components and materials used;3. Identify, for all key materials, price quotes from one or more manufacturers;4. Calculate the minimum costs for the EEA based on materials costs alone;5. Ascertain the EEA’s primary cost drivers;6. Quantify the uninstalled capital costs for a full-size fuel cell stack and stationary fuel cell system incorporating the novel cell design.With the exception of the EEA, other parts of the fuel cell, stack, and system were assumed to be somewhat similar in design to more traditional stationary solid oxide fuel cell (SOFC) systems. Capital costs for the novel stack design were scaled linearly with stack power density. This work estimates stack and system costs for a 100 kWe net electric stack mass-produced at a rate of 50,000 systems per year.Results:Model results indicate that the fuel cell stack subsystem will cost about $478/kW and the entire fuel cell system will cost about $597/kW, including all BOP components. These stack costs are roughly 74% higher than a ‘plain vanilla’ SOFC subsystem, analyzed by the author for DOE in a prior analysis. These system costs are roughly 48% higher than the same ‘plain vanilla’ SOFC system previously analyzed. At the same time, these cost estimates are well below the DOE ARPA-E program goal of < $2,000/kW.Gaia observed an order of magnitude difference in price quotes for several of the specialty ceramic materials investigated. Price quotes from Praxair Specialty Ceramics were approximately double those of Trans-Tech Inc. The reasons cited for the extreme price differential were different manufacturing methods, specifically a chemical co-processing approach for Praxair and a solid-state material manufacturing process (SSMP) for Trans-Tech Inc. The pros and cons of each manufacturing approach are discussed in greater detail in this talk. This analysis assumes that SSMP is used with no degradation in cell performance and bases subsequent cost estimates solely on prices from Trans-Tech Inc., and not Praxair, at ANL’s request.The primary cost driver for the EEA appears to be the BZY-NiO anode substrate. The BZY and NiO anode substrate material is estimated to cost either 2.88 cents/cm2 [based on price quotes for BZY from TransTech Inc.] or 16.40 cents/cm2 [based on price quotes from Praxair Specialty Ceramics]. If the lower BZY cost estimate is taken based on price quotes from Trans-Tech Inc., 52% of the cost derives from the BZY. If the higher BZY cost estimate is taken based on price quotes from Praxair Specialty Ceramics Inc., 92% of the cost derives from the BZY. The secondary cost driver for the EEA appears to be the anode catalyst. The catalyst material is estimated to cost 2.45 cents/cm2. 94% of its cost is due to the material cost of the platinum. The EEA is estimated to cost 5.56 cents/cm2 or $278/kW of gross electric power from the cell or stack.

Roles of Interface and Surface of Electrode Catalysts in Ammonia Electrochemical Synthesis with Proton-Conducting Ceramic Fuel Cells

Investigation of energy carriers is of critical importance for chemical energy storage and transport technologies in combination with the large-scale utilization of renewable energy. Ammonia is a promising energy carrier in terms of carbon-free fuel, high energy density, and easy liquefaction (1).Electrochemical approaches can provide a more efficient reaction process and allow more flexible operations for ammonia production, compared with the industrial Haber–Bosch process. In this study, we investigated direct electrochemical synthesis of ammonia using proton-conducting ceramic fuel cells (PCFCs) operating at high temperatures around 773 K. A ceramic proton conductor, BaCe1-xYxO3 (BCY), was used for an electrolyte membrane.In our previous studies, metal catalysts of Ru and Fe were investigated with a variety of electrode structures such as metal nanoparticle (2, 3) and cermet electrode (4). In terms of metal nanoparticle catalyst, Ru-doped BCY (BCYR) was examined (2,3). The ammonia formation rate was not so high with BCYR (ca. 10-11 mol/s cm2). To improve the ammonia formation rate, Ru-BCY and Fe-BCY (or K-Al-Fe-BCY) cermet electrodes were investigated. When pure N2 was supplied to the cathode, low ammonia formation rate was observed (ca. 10-11 mol/s cm2). When a N2/H2 gaseous mixture was supplied to the cathode, higher ammonia formation rate was observed; ammonia formation rate with Fe-BCY electrode was increased up to ca. 10-10 mol/s cm2 with an increase in cathodic polarization. In addition, our recent study showed a significant improvement of ammonia formation rate with pure Fe electrode (ca. 10-8 mol/s cm2) (5). Considering the above, the roles of interface and surface of electrode catalysts on ammonia electrochemical synthesis should be investigated for clarifying the reaction mechanism and developing effective cell design. In this study, we conducted the deuterium isotope analysis for ammonia electrochemical synthesis to investigate the contributions of interface between electrode and electrolyte, i.e., triple-phase boundary, and electrode surface for ammonia formation processes as well as relevant kinetic measurements of ammonia formation on Fe catalyst (Fig. 1a).The cell configuration was Pt|BCY|Fe-BCY for a cermet electrode. Also, Pt|BCY|Fe cell with pure iron electrode was fabricated to investigate the ammonia formation mechanism. Ammonia formation rate was measured with the configuration of Pt, H2 H2O-Ar |BCY|Fe, H2-N2. Ammonia formed in the cathode was trapped in an H2SO4 solution. Then, the obtained solution was analyzed by ion chromatography. The ammonia formation was also observed using an FTIR spectrometer with a multiple refraction cell to perform the deuterium isotope analysis.Significantly high ammonia formation rate was observed using the Pt|BCY|Fe cell: 1~6x10-8 mol/s cm2 at -1.5 V and 623 K with changing a flow rate in the cathode, which is the best performance for ammonia electrochemical formation under the H2 flow condition. This suggests that Fe surface may play a dominant role in ammonia formation rather than the triple-phase boundary between Fe and BCY. Deuterium isotope analysis was carried out with Pt, D2-H2O-Ar|BCY|Fe, H2-N2. Observed FTIR spectra showed the formation of NH3 and NH2D under the condition (Fig. 1b). The absorbance of NH2D was very weak, which suggested that most of hydrogen atoms in the ammonia molecule originated from H2 in the cathode. In the case of Pt, H2-H2O-Ar|BCY|Fe, D2-N2, ND3 was observed as a main product under the D2 flow condition in the cathode. Considering the above, the electrochemical promotion of catalyst (EPOC) for ammonia formation will be induced via promotion of N2 dissociation on Fe. N2 dissociation is the rate determining step in the ammonia formation reaction. Possible mechanism for the promotion of N2 dissociation is electron back donation with an increase in cathodic polarization. This mechanism gives a hint for designing an electrochemical reactor to promote efficient ammonia formation. Acknowledgements This work was supported by CREST, Japan Science and Technology Agency (JPMJCR1441). References D. Miura, T. Tezuka, Energy, 68(15), 428-436 (2014).J. Otomo, N. Noda, F. Kosaka ECS Trans., 68, 2663-2670 (2015).F. Kosaka, T. Nakamura, J. Otomo, J. Electrochem. Soc., 164, F1323-F1330 (2017).F. Kosaka, T. Nakamura, A. Oikawa, J. Otomo. ACS Sustainable Chem. Eng., 5(11), 10439–10446 (2017).J. Otomo, et al., 435c, 2019 AIChE Annual Meeting, Orlando, USA. Figure 1

Effect of Elemental Substitution in (Ba,Sr)6RE2Co4O15−BaCe0.5Pr0.3Y0.2O3-δ for Cathodes of Proton-Conducting Ceramic Fuel Cells

Solid oxide fuel cells are promising energy conversion devices because of high efficiency and high fuel flexibility, but their high-operating temperature (700-900 ºC) causes problems such as fast degradation of components and long start-up/shut-down time. To overcome these problems, low- temperature operation is preferable. Proton-conducting ceramic fuel cells (PCFCs) are promising energy conversion devices for intermediate-temperature operation, especially below 600 ºC. This is because the activation energy of proton migration in proton-conducting oxides, such as rare earth-doped barium cerate, is lower than that of the oxide ion. At the current state, however, the performance of PCFCs is mainly limited by the cathode materials.Recently, we have reported that the composite of Ba4Sr2Sm2Co4O15 (BSSC4224)–BaCe0.5Pr0.3Y0.2O3 (BCPY) showed higher activity below 600 ºC in 3% humidified synthetic air, as compared with high-performance cathodes reported1). In this composite, BSSC4224 mainly served as an active site for the dissociative absorption of oxygen. In this study, then, the effect of the elemental substitution, especially the Ba/Sr ratio and rare-earth elements, in (Ba,Sr)6 RE 2Co4O15, on the ASR was studied. Symmetric cells consisting of the BaCe0.8Y0.2O3 (BCY) disk electrolyte and Ba5SrRE 2Co4O15 (RE =Nd, La, Eu, Sm, Pr, and Gd), Ba4Sr2Gd2Co4O15 or Ba5SrGd2Co4O15 (BSGC5124)–BCPY electrodes were fabricated. The electrode performance was measured in 3% humidified synthetic air (21% O2–79% N2) at 450–700 ºC by impedance spectroscopy.In impedance analysis, BSGC5124 had the lowest ASR below 550 ºC among Ba5SrRE 2Co4O15 electrodes. Then, the composite of BSGC5124–BCPY was prepared and its performance was compared with that of BSSC4224–BCPY composite. We found that BSGC5124–BCPY has comparable area-specific resistance and activation energy to BSSC4224–BCPY although BSGC5124 itself had much lower area-specific resistance and activation energy than BSSC4224. Therefore, we concluded that the dissociative adsorption of oxygen on the BSGC5124 phase is not the rate limiting step for the oxygen reduction reaction in this composite system. [1] T. Matsui, K. Manriki, K. Miyazaki, H. Muroyama, K. Eguchi, J. Mat er. Chem. A, 6, 14188 (2018).

Proton-conducting ceramic fuel cells: Scale up and stack integration

We present our efforts to scale up proton-conducting ceramic fuel cells (PCFCs) from the button-cell level into small, multi-cell stacks. While recent advancements with lab-scale PCFCs are encouraging, there are few reports of scaling PCFC technology to the stack level. The compatibility of protonic-ceramic materials with stack-packaging materials - metallic interconnects, current collectors, glass-ceramic sealants, gaskets - has not been demonstrated. Here we show that through tuning of materials, fabrication procedures, and operating conditions, protonic-ceramic fuel cell stacks can achieve reasonable performance and low degradation.The MEA is based around barium cerate-zirconate perovskites. Better long-term stack durability is found with BaCe0.4Zr0.4Y0.1Yb0.1O3−δ (BCZYYb) electrolyte. The anode support is a nickel-electrolyte composite, while the cathode is BaCo0.4Fe0.4Zr0.1Y0.1O3−δ (BCFZY). Planar MEAs reach 5 cm2 in active area, and are packaged within ferritic-steel interconnects and macor frames to form multi-cell stacks. Our three-cell stack demonstrates encouraging performance, reaching 0.69 and 0.47 W cm−2 under H2 and CH4 fuels, respectively, at 600 ∘C. A gadolinium-doped ceria cathode-electrolyte interlayer reduces degradation rates to 1.5% kh−1 at 0.1 A cm−2 and 3.3% kh−1 at 0.4 A cm−2 at 550 ∘C. No chromium transport is observed. Degradation mechanisms and the role of the GDC interlayer are postulated.

Characterizing Stack Degradation in Proton-Conducting Ceramic Fuel Cells and Electrolyzers

We present our efforts to scale-up proton-conducting ceramic fuel cells (PCFC) for integration into small multi-cell stacks. Proton-conducting ceramics are an exciting new class of materials that are now emerging from research laboratories to address societal challenges in electricity generation, energy storage, and fuels synthesis. While there are many technologies under investigation, intermediate temperature proton-conducting fuel cells (PCFC) have been drawing attention due to their high performance at low temperatures in fuel cell mode (up to ~2 Wcm2 at 600 ºC) and their high efficiency (90 – 98 % Faradic efficiency) when operated in electrolysis mode. However despite the impressive achievements at lab-scale PCFC technology limited investments have been made on the scaling up process due to the challenges that the PCFC fabrication procedure represents. In fact, there are no stack demonstrations reported to date in the archival literature. In this presentation we will review our efforts at Colorado School of Mines to increase the size of PCFCs beyond the button-cell level, and to integrate these cells into multi-cell stack assemblies. The stability of our PCFC stack reached the degradation rates of 3.5% and 2.1% per 1000 hours, respectively, under fuel cell and electrolysis modes.An illustration of our stack design is shown in Figure 1, and includes a photograph of a fully assembled three-cell stack after performance testing. The design is centered on three repeating components: the protonic-ceramic electrolysis cells, the composite-ceramic frames in which the cells are bonded, and the thin metallic interconnect / bipolar plates that conduct electricity between adjacent cells. The cells feature a dense, 20 μm-thick BaCe0.4Zr0.4Y0.1Yb0.1O3- d (BCZYYb4411) electrolyte on a porous Ni-BCZYYb4411 anode fabricated through reactive-sintering methods. The cathode is a novel BaCo0.4Fe0.4Zr0.1Y0.1O3-δ (BCFZY) atop of the BCZYYb4411 electrolyte with the maximum active area of ~ 7cm2 per single unit cell. Reactive gases are fed through fuel and oxidizer ports machined into the frame. The metallic interconnect is a ferritic steel developed for intermediate-temperature solid-oxide fuel cells and electrolyzers, and is compression sealed to the frame. Metallic meshes (not shown) electrically connect the electrodes to the interconnects.High protonic-ceramic cell performance operating at 550 0C has been previously demonstrated at Colorado School of Mines. While the lower operating temperature should present benefits for the long-term stability of the stack it also brings questions regarding the behavior of stack materials such as the interconnects which represent a source of degradation in performance depending on the composition of the scale that is grown on them. In an effort to better understand the root causes behind performance degradation processes in our protonic-ceramic stacks, we have integrated numerous voltage-taps throughout the stack shown in Figure 1. In this presentation, we will review our results for long-term protonic-ceramic stack operation under both fuel cell and electrolysis mode, discuss after-test interconnects analysis showing high-iron-content scale and present strategies for reducing degradation rates and extending stack life. Figure 1

Cerium and Gadolinium co-doped perovskite oxide for a protonic ceramic fuel cell cathode

Proton-conducting ceramic fuel cells (PCFCs) show unique advantages over oxygen-ionic-conducting counterparts at intermediate and low temperatures, which have attracted significant attention worldwide in recent years. A critical issue for world-spread applications is inadequate performance due to the lack of appropriate cathodes for PCFCs. Here, cubic perovskite BaCo0.4Fe0.5-xCe0.1RexO3-δ (ReY, Gd x = 0, 0.1) materials are successfully synthesized and evaluated as the cathode in the PCFCs based on BaCe0.7Zr0.1Y0.1Yb0.1O3-δ as an electrolyte material. The Y- or Gd-doped perovskite BaCo0.4Fe0.4Ce0.1Y0.1O3-δ (BCFCeY), BaCo0.4Fe0.4Ce0.1Gd0.1O3-δ (BCFCeG) exhibit larger lattice parameters, and higher electrocatalytic activity comparing to their parent oxide BaCo0.4Fe0.5Ce0.1O3-δ (BCFCe). Among them, BCFCeG shows the best performance. The single cell with BCFCeG as the cathode material exhibits an interfacial polarization resistance as low as 0.12 Ω cm2 and delivers a promising peak power density of 504 mW cm−2 at 600 °C, while the BCFCe-based cell achieves only 437 mW cm−2 at 600 °C. The X-ray diffraction (XRD) results show good chemical compatibility for BCFCeG below 1050 °C. Moreover, it shows favorable stability in CO2-containing environment. This work confirms that BCFCeG could be a promising cathode for PCFCs.

High-Efficiency Direct Ammonia Fuel Cells Based on BaZr0.1Ce0.7Y0.2O3- δ /Pd Oxide-Metal Junctions.

A direct ammonia‐type intermediate temperature fuel cell is examined by means of a hydrogen membrane fuel cell (HMFC) comprising 1‐µm‐thick BaZr0.1Ce0.7Y0.2O3− δ (BZCY) thin‐film electrolyte and Pd solid anode. It generates the maximum power density of 0.58 W cm−2 at 600 °C with ammonia fuels, and this value is found to be three times larger than the champion data of the recently reported direct ammonia‐type proton‐conducting ceramic fuel cells (PCFCs). AC impedance spectroscopy is performed to determine the interfacial polarization resistances, disclosing that the anodic overpotentials of HMFCs are at least one order of magnitude smaller than those of anode‐supported PCFC under relatively high DC outputs. The anode reactions are driven by the oxidation of monoatomic hydrogen dissolving at the BZCY/Pd solid–solid interface, mediated via proton transfer from Pd to BZCY. The electrochemical analysis reveals that the BZCY/Pd junction forms Ohmic contact without growth of wide depletion layer and thus facilitates the proton transfer reactions because the interfacial region beneath Pd electrode can accommodate amounts of protonic defects as well as the bulk of BZCY due to the small depletion of holes under hole–proton thermodynamic equilibrium.

High Efficiency Direct Ammonia Type Fuel Cells based on BaZr x Ce0.8-X Y0.2O3/Pd Oxide-Metal Junctions

Recently, ammonia has attracted a considerable interest as a prospective carbon-free fuel source for fuel cells due to the low cost, ease in liquefaction and transportation at ambient temperatures, and high volumetric energy density. The proton conducting ceramic fuel cells (PCFCs) using proton-conducting oxides, such as BaZr x Ce0.8-x Y0.2O3- d , as an electrolyte is promising as a direct ammonia type fuel cells. The water formation in PCFCs is mainly progressive at the cathode side, unlike solid oxide fuel cells (SOFCs), and therefore, the contamination of the ammonia fuels into the exhaust water is precluded, thereby eliminating re-purification of water exhaust and simplifying total systems. Hydrogen membrane fuel cell (HMFC), which was firstly proposed by Ito et al., consists of an ultrathin proton conducting ceramic electrolyte supported on a dense hydrogen permeable metal anode. The cell facilitates efficient power generation, giving 900 mW cm-2 at 400°C, since reduced electrolyte thickness can significantly decreases the ohmic loss at lower temperatures. This unique cell configuration could be advantageous for ammonia fuel cells because 1) the NO x formation at the anode is perfectly prevented because NH3 gases are isolated from the oxide electrolytes by Pd solid anode and 2) Pd is expected to capture efficiently the hydrogen gases produced by the pyrolysis of ammonia due to the large ahydrogen sorption ability. Herein, we demonstrated the high efficiency power generation of the direct ammonia type HMFC due to the hydrogen pumping effects realized by BaZr0.1Ce0.7Y0.2O3- d (BZCY) and Pd heterointerface. Ohmic characteristics of the heterojunctions, altogether with hydrogen solubility of Pd, can facilitate the hole proton exchange between Pd and BZCY regardless of the lowered hydrogen concnetartion in NH3-fed anodes. The cell gained the maximum power density of 0.52 W cm-2 at 600°C, which was much higher than the champion data of ammonia PCFCs.

Relationship between Electrochemical Properties and Electrolyte Partial Conductivities of Proton-Conducting Ceramic Fuel Cells

Proton ceramic fuel cells (PCFCs) have attracted attentions because of the high potential to enhance the electrical efficiency of SOFCs, in which proton-conducting ceramic is used as electrolyte instead of the yttria-stabilized zirconia (YSZ) which is the most common electrolyte material of SOFCs. In previous study, we measured the P O2 dependence of total electrical conductivities of the proton-conducting solid oxides, BaZr0.1Ce0.7Y0.1X0.1O3- d (X = Ga, Sc, In, Yb, Gd), which were divided into partial conductivities of the respective charge-carriers, such as electron, ion, and hole, by using the difference in P O2dependence of the charge-carriers. In this study, we have investigated the electrochemical properties of the PCFCs with the various partial conductivities in the electrolytes. The electrochemical properties were found to have large dependences on operation condition. When ASR and external current density were given constant values of 0.383 ohm cm2 and 0.25 A cm-2, respectively, an influence of the dopant species on the electrical efficiencies η xwas found to have the following sequential order: η x=SC > η x=In > η x=Ga > η x=Y > η x=Gd When the external current density was given as a variable, increasing the external current density will result in the increase of the voltage drop and the decrease of a ratio of leakage current to external current. This suggests that according to the partial conductivities the external current density has an optimum value maximizing the electrical efficiency. Acknowledgements This research is supported by The Japan Science and Technology Agency (JST) through its “Center of Innovation Science and Technology based Radical Innovation and Entrepreneurship Program (COI Program)."

Methane-Fueled Proton-Conducting Ceramic Fuel Cell Stacks

In this paper, we present our efforts on scale up of proton-conducting ceramic fuel cells (PCFCs) from button cells to small stacks. The PCFCs are based on BaCe0.2Zr0.6Y0.2O3-d (BCZY26) electrolyte. Anode-supported membrane-electrode assemblies are fabricated using conventional ceramic-processing techniques, and reach 40 mm in diameter. Multi-cell PCFC stacks are found to generate reasonable power densities at intermediate temperature under direct methane-steam fuel feeds, reaching 258 mW / cm2 at 0.75 V and 550 ºC.

Relationship between Electrochemical Properties and Electrolyte Partial Conductivities of Proton-Conducting Ceramic Fuel Cells

The electrochemical properties of the proton-conducting ceramic fuel cells (PCFCs) with BaZr0.1Ce0.7Y0.1X0.1O3- d (BZCYX, X = Ga, Sc, In, Yb, or Gd) electrolytes have been investigated. BZCYX materials were found to have various partial conductivities of charge-carriers such as ion, hole, and electron. The electrochemical properties exhibited strong dependences on operation conditions. When ASR and external current density were fixed at 0.4 Ω cm2 and 0.25 A cm-2, respectively, the electrical efficiency, η(X), was found to have the following sequential order: η(Sc) > η(In) > η(Ga) > η(Yb) > η(Gd). On the other hand, when ASR was not fixed but the thickness of the electrolyte was fixed at 25 μm, large variations appeared in the leakage current of the cells with the BZCYX electrolytes. The sequential order of the electrical efficiency with the fixed thickness was different from that with the fixed ASR as described in the above inequality expression, and depends on the operating temperature. The ratios of the leakage current with X = Yb or Gd were higher than those with X = Ga, Sc, or In. These high ratios were found to cause the serious drop in the electrical efficiency at an external current density of 0.25 A cm-2. We have successfully found out the candidates for the X element in BZCYX, by which high-efficient power generation would be expected.

High Efficiency Hydrogen Membrane Fuel Cells with BaCe0.8Y0.2O3-δ Electrolyte Thin Films and Pd1-xAgx Solid Anodes

Fuel cell performances at intermediate temperature 600°C were examined for the hydrogen membrane fuel cell (HMFC) designed by rf sputter deposition of a BaCe0.8Y0.2O3-δ thin film electrolyte (1 μm thickness) on a Pd1-xAgx (x = 0, 0.2 and 0.4) solid anode. The maximum power density reached to 1.2 W cm−2 at 600°C with the Pd0.8Ag0.2 anodes, and this value was much higher than the champion data of the recent anode-supported proton-conducting ceramic fuel cells (PCFCs). By electrochemical impedance analysis, it was demonstrated that the hydrogen permeability was crucial to the anode polarization losses of the HMFC because the charge transfer resistances and concentration overpotentials at BaCe0.8Y0.2O3-δ/Pd oxide-metal interfaces are simultaneously improved with the alloys having higher hydrogen permeability.

Proton uptake into the protonic cathode material BaCo0.4Fe0.4Zr0.2O3-δ and comparison to protonic electrolyte materials

Proton uptake in mixed-conducting cathode materials is of particular interest as it allows the oxygen reduction reaction to proceed via the bulk path in proton conducting ceramic fuel cells (PCFC). This work investigates the proton concentration of BaCo0.4Fe0.4Zr0.2O3−δ (BCFZr) and the predominant proton uptake reactions using thermogravimetry. Based on the obtained proton concentrations increasing from 0.5mol% at 400°C to 1.5mol% at 200°C, the bulk path is expected to be active for BCFZr. The variation of proton concentrations with the concentration of electronic defects indicates nonideal behavior with detrimental interactions between protons and (trapped) holes. The comparison of BCFZr with other materials emphasizes that several factors determine the proton concentration such as oxide ion basicity, B-site cations and their oxidation state and B-O covalency.

Thermal Evolution of BaO-CuO Flux as Sintering Aid for Proton Conducting Ceramic Fuel Cells

The eutectic melt of BaO-CuO flux is known to be a potential sintering aid for Ba(Zr,Y)O₃ (BZY) electrolyte for proton-conducting ceramic fuel cells (PCFCs). A density of BZY higher than 97% of theoretical density can be achieved via sintering at 1300℃ for 2 h using a flux composed of 28 mol% BaO and 72 ㏖% CuO. In the present study, chemical and structural evolution of BaO-CuO flux throughout the sintering process was investigated. An intermediate holding step at 1100℃ leads to formation of various impurity compounds such as BaCuO 1.977 , Ba 0.92 Cu 1.06 O 2.28 and Cu 16 O 14.15 , which exhibit significantly larger unit cell volumes than the matrix. The presence of such secondary compounds with large lattice mismatch can potentially lead to mechanical failure. On the other hand, direct heating to the final sintering temperature produced CuO and Cu₂O as secondary phases, whose unit cell volumes are close to that of the matrix. Therefore, the final composition of the flux is strongly affected by the thermal history, and a proper sintering schedule should be used to obtain the desired properties of the final product.