I. Garbayo

Full ceramic micro solid oxide fuel cells: towards more reliable MEMS power generators operating at high temperatures

Batteries, with a limited capacity, have dominated the power supply of portable devices for decades. Recently, the emergence of new types of highly efficient miniaturized power generators like micro fuel cells has opened up alternatives for continuous operation on the basis of unlimited fuel feeding. This work addresses for the first time the development of a full ceramic micro solid oxide fuel cell fabricated in silicon technology. This full-ceramic device represents a new generation of miniaturized power generators able to operate at high temperatures, and therefore able to work with a hydrocarbon fuel supply. Dense yttria-stabilized zirconia free-standing large-area membranes on micromachined silicon were used as the electrolyte. Thin-film porous electrodes of La0.6Sr0.4CoO3−δ and gadolinia-doped ceria were employed as cathode and anode materials, respectively. The electrochemical performance of all the components was evaluated by partial characterization using symmetrical cells, yielding excellent performance for the electrolyte (area specific resistance of 0.15 Ω cm2 at temperatures as low as 450 °C) and the electrodes (area specific resistance of the cathode and anode below 0.3 Ω cm2 at 700 °C). A micro solid oxide fuel cell with an active area of 2 mm2 and less than 1 micrometer in thickness was characterized under fuel cell conditions, using hydrogen as a fuel and air as an oxidant. A maximum power density of 100 mW cm−2 and 2 mW per single membrane was generated at 750 °C, having an open circuit voltage of 1.05 V. Impedance spectroscopy of the all-ceramic membrane showed a total area-specific resistance of ∼3.5 Ω cm2.

Micro-solid state energy conversion membranes: influence of doping and strain on oxygen ion transport and near order for electrolytes

Enhancing the efficiency of micro-energy conversion devices through engineering of the structure–transport relationship is a prerequisite toward next generation micro-electrolysers and fuel cells. Here, oxygen ion conducting free-standing thin films are key elements, forming buckled and strained membranes for gas exchange and energy conversion. The electro-chemo-mechanics of free-standing membranes vs. substrate-supported films are investigated as model device structures to study the factors influencing the ionic transport, and answer the fundamental question: how strongly does solid solution doping vs. lattice strain affect the defect-induced oxygen ion transport in buckled electrolyte membrane films for solid state micro-devices? Importantly, we demonstrate that tuning the electro-chemo-mechanics of doped ceria films can influence the ionic transport through the effect of opposed strained volumes altering the clustering of oxygen vacancy defects. Strain is studied by comparing flat substrate-supported films to compressive buckled membrane devices and observing subsequent changes in atomistic near order via Raman spectroscopy. The buckling resulted in a significant increase of the activation energy for ionic transport, greater than classical extrinsic doping. The power of electro-chemo-mechanic engineering of ceramic films is demonstrated in finding the best strategy for optimizing ionic conduction and thereby enhancing future performance in thin film electrolytes for micro-energy conversion devices.

Monolithic micro fuel cells as integrated power sources in MEMS

The increasing interest in fully integrated electronic systems for portable applications has originated new research developments in micropower sources. In this context, micro fuel cells arise as a promising solution for the power supply of portable microsystems [1,2]. The main issue in order to use such devices for this purpose is to be able to fabricate them by using a monolithic architecture compatible with the standard microfabrication processes. In order to do this for PEM type fuel cells, one of the first topics to cover is the fabrication of the membrane with microfabrication compatible processes. This paper explains the results achieved with a first hybrid demonstrator of a direct methanol micro fuel cell (µDMFC) and two different approaches for new monolithic micro fuel cells.

Fabrication and characterization of yttria-stabilized zirconia membranes for micro solid oxide fuel cells

The present study is devoted to analyze the compatibility of yttria-stabilized zirconia thin films prepared by pulsed laser deposition technique for developing new silicon-based micro devices for micro solid oxide fuel cells applications. Yttriastabilized zirconia free-standing membranes with thicknesses from 60 to 240 nm and surface areas between 50x50 μm2 and 820x820 μm2 were fabricated on micromachined Si/SiO2/Si3N4 substrates. Deposition process was optimized for deposition temperatures from 200ºC to 800ºC. A complete mechanical study comprising thermomechanical stability, residual stress of the membranes and annealing treatment as well as a preliminary electrical characterization of ionic conductivity was performed in order to evaluate the best processing parameters for the yttria-stabilized zirconia membranes.

Unveiling the outstanding oxygen mass transport properties of Mn-rich perovskites in grain boundary-dominated La0.8Sr0.2(Mn1-xCox)0.85O3±δ nanostructures

Ion transport in solid-state devices is of great interest for current and future energy and information technologies. A superior enhancement of several orders of magnitude of the oxygen diffusivity has been recently reported for grain boundaries in lanthanum–strontium manganites. However, the significance and extent of this unique phenomenon are not yet established. Here, we fabricate a thin film continuous composition map of the La0.8Sr0.2(Mn1–xCox)0.85O3±δ family revealing a substantial enhancement of the grain boundary oxygen mass transport properties for the entire range of compositions. Through isotope-exchange depth profiling coupled with secondary ion mass spectroscopy, we show that this excellent performance is not directly linked to the bulk of the material but to the intrinsic nature of the grain boundary. In particular, the great increase of the oxygen diffusion in Mn-rich compositions unveils an unprecedented catalytic performance in the field of mixed ionic–electronic conductors. These results present grain boundaries engineering as a novel strategy for designing highly performing materials for solid-state ionics-based devices.

A Simple and Fast Electrochemical CO2 Sensor Based on Li7La3Zr2O12 for Environmental Monitoring

In the goal of a sustainable energy future, either the energy efficiency of renewable energy sources is increased, day-to-day energy consumption by smart electronic feedback loops is managed in a more efficient way, or contribution to atmospheric CO2 is reduced. By defining a next generation of fast-response electrochemical CO2 sensors and materials, one can contribute to local monitoring of CO2 flows from industrial plants and processes, for energy management and building control or to track climate alterations. Electrochemical Li+ -garnet-based sensors with Li7 La3 Zr2 O12 solid electrolytes can reach notable 1 min response time at lowered operation temperatures to track 400-4000 ppm levels of CO2 when compared with state-of-the-art NASICON-based sensors. By using principles of redefining the electrode electrochemistry, it is demonstrated that Li6.75 La3 Zr1.75 Ta0.25 O12 can be used to alter its classic use as energy-storage function to gain additional functions such as CO2 tracking.

Fabrication and characterization of a fuel flexible micro-reformer fully integrated in silicon for micro-solid oxide fuel cell applications

A novel design of a fuel-flexible micro-reactor for hydrogen generation from ethanol and methane is proposed in this work. The micro-reactor is fully fabricated with mainstream MEMS technology and consists of an array of more than 20000 through-silicon vertically aligned micro-channels per cm2 of 50 μm in diameter. Due to this unique configuration, the micro-reformer presents a total surface per projected area of 16 cm2/cm2 and per volume of 320 cm2/cm3. The active surface of the micro-reformer, i.e. the walls of the micro-channels, is homogenously coated with a thin film of Rh- Pd/CeO2 catalyst. Excellent steam reforming of ethanol and dry reforming of methane are presented with hydrogen production rates above 3 mL/min·cm2 and hydrogen selectivity of ca. 50% on a dry basis at operations conditions suitable for application in micro-solid oxide fuel cells (micro-SOFCs), i.e. 700-800ºC and fuel flows of 0.02 mLL/min for ethanol and 36 mLG/min for methane (corresponding to a system able to produce one electrical watt).

Engineering mass transport properties in oxide ionic and mixed ionic-electronic thin film ceramic conductors for energy applications

Abstract New emerging disciplines such as Nanoionics and Iontronics are dealing with the exploitation of mesoscopic size effects in materials, which become visible (if not predominant) when downsizing the system to the nanoscale. Driven by the worldwide standardisation of thin film deposition techniques, the access to radically different properties than those found in the bulk macroscopic systems can be accomplished. This opens up promising approaches for the development of advanced micro-devices, by taking advantage of the nanostructural deviations found in nanometre-sized, interface-dominated materials compared to the “ideal” relaxed structure of the bulk. A completely new set of functionalities can be explored, with implications in many different fields such as energy conversion and storage, or information technologies. This manuscript reviews the strategies, employed and foreseen, for engineering mass transport properties in thin film ceramics, with the focus in oxide ionic and mixed ionic-electronic conductors and their application in micro power sources.

Micro solid oxide fuel cells: a new generation of micro-power sources for portable applications

Portable electronic devices are already an indispensable part of our daily life; and their increasing number and demand for higher performance is becoming a challenge for the research community. In particular, a major concern is the way to efficiently power these energy-demanding devices, assuring long grid independency with high efficiency, sustainability and cheap production. In this context, technologies beyond Li-ion are receiving increasing attention, among which the development of micro solid oxide fuel cells (μSOFC) stands out. In particular, μSOFC provides a high energy density, high efficiency and opens the possibility to the use of different fuels, such as hydrocarbons. Yet, its high operating temperature has typically hindered its application as miniaturized portable device. Recent advances have however set a completely new range of lower operating temperatures, i.e. 350-450°C, as compared to the typical <900°C needed for classical bulk SOFC systems. In this work, a comprehensive review of the status of the technology is presented. The main achievements, as well as the most important challenges still pending are discussed, regarding (i.) the cell design and microfabrication, and (ii.) the integration of functional electrolyte and electrode materials. To conclude, the different strategies foreseen for a wide deployment of the technology as new portable power source are underlined.