The research activity of Prof. Di Noto’s group is focused on the preparation and characterization of materials for application in the field of energy. The materials are used to assemble prototype devices which are extensively characterized under operative conditions.
The research work consists in a broad range of activities, including:
· Design of new materials starting from basic chemistry and physics concepts;
· Novel chemical synthesis and processing;
· Advanced materials characterization;
· Measurement of physical and chemical properties;
· Fabrication and evaluation of prototype devices;
· Development of a fundamental understanding of the structure-property-performance relationships of the materials.
In particular, most of the research efforts are devoted to the synthesis and the study of materials for:
· Primary and secondary batteries;
· Fuel cells;
· Electrochemical supercapacitors;
· Field-effect transistors;
· Photo-electrochemical devices.
Some of the current research activities are briefly outlined below; the publications report the details, and can be found here.
Overview of energy conversion and storage systems
The Ragone plots are useful tools to highlight the peculiar features of the various energy conversion and storage systems.
Ragone plot of different energy conversion and storage systems.
The Ragone plot reported above is interpreted as follows. Fuel cells are characterized by a very high energy density and a relatively low power density. In other words, fuel cells provide relatively little electrical power over very long periods of time. On the other hand, conventional capacitors show a very high power density and a very low energy density, as they yield large currents but are discharged very quickly. Conventional batteries fall between these two extremes.
In general, the various energy conversion and storage systems are intended for different types of applications. For instance, fuel cells are particularly suitable to provide power for portable electronic devices or electric vehicles, which must be operated for long periods of time. On the other hand, since ultracapacitors can yield intense but short “bursts” of power, they are used in momentary-load devices and to “smooth” the power provided by a source.
Primary and Secondary Batteries
Batteries are “closed” electrochemical devices capable to convert the chemical energy of their components into electrical power. Secondary batteries can be recharged, while primary batteries cannot; the research activity carried out in Prof. Di Noto’s laboratory is mainly focused on primary and secondary batteries based on alkaline or alkaline-earth metals such as Li, Mg or their alloys and other metals.
The aim is to obtain batteries with a limited environmental impact, cheap, easily processable, constituted entirely by solid-state components and which can be operated up to 200°C. Secondary batteries based on alkaline, alkaline-earth metals or their alloys can be cycled extensively. These devices are constituted by an anode and a nanocomposite cathode (this latter includes the active material, the current collector and a binder to stabilize the overall system), separated by a polymer electrolyte capable to transport the metal ions. The modern technology particularly emphasize lithium as the “active metal” owing to its small radius, high mobility and large theoretical specific capacity. However, the large reactivity of lithium hinders a full exploitation of these features. The laboratory of Prof. Di Noto pioneered the use of magnesium as an alternative active metal. Magnesium, which is characterized by a lower theoretical specific capacity with respect to lithium, allows to obtain batteries with a promising performance owing to the better management of its reactivity.
Basic operation mechanism (left) and components (right) of a battery based on alkaline or alkaline-earth metals.
Disassembled Li-ion battery prototype (left); Li-polymer prototype (right).
A fuel cell is an “open” electrochemical device converting the chemical energy of its reagents into electrical power. Fuel cells can operate as long as they are provided with reagents; for this reason, they need not to be recharged. Fuel cells are not Carnot engines. Therefore, they can reach very high energy conversion efficiencies, up to 50% or more. In comparison, internal combustion engines reach at most 25-35%. There are several families of fuel cells, characterized by a large array of different materials and operating temperatures.
The research activity carried out in Prof. Di Noto’s lab is mainly focused on proton-exchange membrane fuel cells (PEMFCs). PEMFCs operate at low temperatures (T < 130°C) and do not produce polluting agents such as NOx, SOx and fine particulate. PEMFCs are essentially two-dimensional systems, consisting of a thin proton-conducting membrane separating two porous electrodes where the electrochemical reactions involved in cell operation take place. The electrochemical reactions at low temperature take place owing to suitable electrocatalysts. These latter consist of materials which show a significant concentration of platinum-group metals such as Pt, Pd, Ru. Thus, the drawbacks of these materials are the cost, availability and durability. One very interesting recent development of the PEMFCs is the direct methanol fuel cell (DMFC).
Home-made DMFCs providing power for miniature fans.
This system, which is fuelled with a water-methanol solution, is particularly attractive owing to its high power density and simple construction lacking moving parts and complex auxiliary heat and water management systems.
The research activity carried out in the laboratory of Prof. Di Noto’s on this topic is mainly focused on the development of innovative functional materials for application in PEMFCs and DMFCs. In particular, the efforts are directed towards the synthesis and characterization of innovative systems including advanced hybrid inorganic-organic proton-conducting membranes, cheaper and more active electrocatalysts, and optimized single fuel cells.
Synthesis of materials with electric and dielectric properties
In a nutshell, modern applications in the field of energy conversion and storage (e.g., primary and secondary batteries, fuel cells, supercapacitors, etc…) are based on devices consisting of three fundamental components:
· Two electrodes, the anode and the cathode (the electrodic materials);
· An ion-conducting medium (the electrolyte);
· The dielectric separator or gasket materials, characterized by a suitable morphology, mechanical and thermal properties.
The electrodes are the place where redox reactions occur, as highlighted above. On the other hand, the ion-conducting medium and the separator materials allow the charge transfer from the anode to the cathode and prevent the electronic short-circuit of the electrodes, respectively. In batteries and fuel cells the ion-conducting material acts both as an ion transport and as a separating medium; in supercapacitors, in order to prevent both electronic and ionic conduction and to increase the electrode polarization effect, the electrodes are separated by a dielectric material with a suitable k value.
The know-how developed in the laboratory of Prof. Di Noto’s includes the synthesis of functional materials for applications in both separating media and electrodes. Very advanced procedures can be applied, including operations carried out under inert atmosphere.
Basic operation mechanism of a H2 fuel cell.
A typical synthesis carried out under an Ar, He or N2 inert atmosphere.
Functional Materials for Electrodic Separating Media
There are two main families of functional materials that can be used as electrode separating media: the Ion Conductors and the Dielectrics.
The ion conductors are for the largest part hybrid inorganic-organic solid-state polymer systems, which can be grouped into four main families:
· ORMOCERS-APE (Organically Modified Ceramics As Polymer Electrolytes). These materials are three-dimensional networks composed of organic macromolecules linked together by inorganic atoms like Si, Ti, Zr, Al and others. Inorganic salts can be dissolved in these materials to determine a tailored ion conductivity.
Structure of an Al-based ORMOCER-APE doped with LiClO4.
A typical ORMOCER-APE: poly[(oligoethylene glycol) dihydroxytitanate] (left); oscillation of the oxidation states of Titanium in the ORMOCER-APE(right).
· Z-IOPEs (Zeolitic Inorganic-Organic Polymer Electrolytes). These materials are three-dimensional networks composed of organic macromolecules linked together by bridging inorganic clusters. The latter are formed by the aggregation of two or more inorganic coordination complexes and are positively or negatively charged.
Structure of a typical Z-IOPE.
· HGEs (Hybrid inorganic-organic GEls). These materials can be considered as an upgraded version of Z-IOPEs, where the organic component is based on organic multi-functional molecules with glass-forming properties instead of simple macromolecules.
Structure of a typical HGe, using glycerol as the glass-forming multi-functional molecule (left); conformational changes of glycerol molecules forming lithium cation coordination cages (right).
· Hybrid inorganic-organic proton-conducting systems. These materials are based on a proton-conducting ionomer such as Nafion, sulfonated polyetheretherketone (SPEEK), sulfonated polysulfone (SPES) and others. The ionomer is doped with a suitable filler, which can be obtained in a variety of ways including sol-gel processes, ball milling and surface functionalization of commercial ceramic powders.
Scheme of a typical hybrid inorganic-organic proton-conducting system Nafion + nanofiller. Overall secondary structure and insets showing details of the hydrophobic (upper right) and hydrophilic domains (lower right).
Several nanoparticle materials have been taken into consideration, with a particular emphasis on silica and surface-functionalized silica, TiO2, ZrO2, WO3, Ta2O5 and others. Finally, advanced salts such as delta-MgCl2 have been synthesized to dope polymer electrolytes and allow the production of prototypes of magnesium-based secondary batteries.
Structure of the delta-MgCl2 (left); structure of the alfa-MgCl2 (right).
Development of Electrode Materials
The research activity has been mainly focused on the development of:
· anodes and cathodes for primary and secondary batteries, capacitors, sensors, actuators, photo-electrochemical devices and others;
· electrocatalysts for PEMFCs, DMFCs and electrolysers.
Both families of materials are prepared using a proprietary procedure. In summary, a suitable hybrid inorganic-organic material is synthesized by sol ? gel ? plastic transitions; in a second step, the latter undergoes a suitable pyrolysis process followed by an activation process. The final materials are characterized by the desired structure, morphology (“core-shell” nanoparticles) and electrochemical activity.
General preparation procedure yielding the precursors of the electrocatalysts.
Morphology of the final electrocatalysts: HR-SEM investigation (top and lower left) and HR-TEM picture (lower right).
Characterization of materials with electric and dielectric properties
An accurate characterization of the materials synthesized in Prof. Di Noto’s laboratory requires the use of the full array of the available facilities, which are described here. In details, the following steps are usually carried out:
I) Chemical characterization
Once a material is synthesized, its chemical composition is determined by ICP-AES spectroscopy.
II) Structure and thermomechanical transitions of materials
· The main tools used in the characterization of the materials are the vibrational spectroscopies, the NMR spectroscopy, the powder XRD and the thermomechanical analyses. The complete vibrational characterization of the materials in both the far and the medium infrared allows to obtain key insights to the primary chemical structure of the materials. In addition, the integration of the latter results with the information determined by Raman spectroscopies, NMR and XRD allows to disclose details of the secondary structure of materials. Finally, vibrational spectroscopies are very important to study the interactions in bulk materials and the structure of the solvent clusters in ion-conducting systems. These latter play a crucial role in regulating the ion conductivity mechanism of the materials.
· The thermomechanical analyses are carried out to study the thermal stability and the transitions of the materials. TG is the technique of choice to study the thermal stability of the materials. MDSC allows to identify the phase transitions of the investigated systems. DMA highlights the thermomechanical relaxations of the materials. Taken together, this info complements the results derived from vibrational spectroscopies (FT-IR and Raman) and provides a clearer picture of the correlations taking place between the structure, interactions and transition events of the materials.
III) Dielectric and Electrochemical characterization
The cornerstone of the characterization techniques carried out in Prof. Di Noto’s laboratory deals with the determination of the electrical properties of the synthesized materials. Broadband Dielectric Spectroscopy is the most powerful tool to determine the electric response of the materials. Indeed, the available state-of-the-art instruments allow to carry out studies in wide frequency and temperature ranges, from 10-4 Hz to 20 GHz and from -195 to 450°C, respectively. The detailed analysis of the electrical response of the materials reveals a wide variety of phenomena and sheds light on the relaxation events responsible of the conduction mechanism. Finally, the electronic and ionic conductivity of the investigated systems can be accurately determined allowing to identify the most suitable candidates for application in prototype devices.
Fundamentals of BDS.
Main phenomena highlighted by BDS.
The electrochemical behaviour of the synthesized materials is also studied with “ex-situ” measurements carried out using multichannel potentiostats connected to electrochemical cells, through techniques such as cyclic voltammetry or chronoamperometry. This approach is particularly useful to investigate the “ideal” performance of the electrode materials used in batteries and fuel cells, without the typical limitations set by mass transport phenomena characterizing the devices. The scientific know-how of the research group allows the application of advanced state-of-the-art procedures relying on complex rotating ring-disk electrodes for the determination of electrochemical activity, electrochemical reaction mechanisms, tolerance to contaminants, electrocatalytic activity and surface area. These studies, which are very significant for applicative purposes, are also very important from a fundamental point of view. Indeed, they allow to screen a large number of materials in order to select the best candidates to build prototype devices.
Sample Rotating-Disk Electrode measurements detailing the electrochemical performance of the electrocatalysts in the hydrogen oxidation reaction and in the oxygen reduction reaction (left); Rotating Ring-Disk Electrode measurements highlighting the selectivity of the oxygen reduction reaction (right).
Assembly and Testing of Prototypes
The final step in most of the experiments carried out in the laboratory of Prof. Di Noto is the assembly and testing of prototypes using the materials synthesized and characterized as described above. The know-how is mainly focused in the assembly and testing of primary and secondary batteries, fuel cells (both PEMFCs and DMFCs), electrolysers, electrochemical supercapacitors, field-effect transistors, sensors, actuators and photo-electrochemical devices.
Assembly and testing of primary and secondary batteries
One of the most significant issues in the assembly of primary and secondary batteries based on lithium and magnesium is the need to prevent any contamination of the materials. For this reason, “good” batteries must be assembled under an inert atmosphere.
Assembly of primary and secondary batteries under inert atmosphere.
In order to achieve meaningful results, the testing of batteries must be carried out for a very long time, using standardized protocols and suitable equipment on multiple prototypes.
Battery test station (left) and a typical array of button prototype batteries (right).
Typical charge and discharge curves of a battery.
Assembly and testing of fuel cells
The key issue in the preparation of a PEMFC or a DMFC is the proper assembly of the gas-diffusion layers, electrodes and proton-exchange membrane into a single unit, the membrane-electrode assembly (MEA).
Scheme of the structure of a MEA (left) and picture of an assembled MEA (right).
The MEA assembly procedure should be optimized for each combination of the active materials. The resulting systems are then mounted in the test station and extensively characterized under standardized conditions.
Mounting of a MEA into the test station.
Typical polarization curves of MEAs assembled with the synthesized electrocatalysts.
The research activity carried out in the laboratory of prof. Di Noto was extended also to other families of devices including electrolysers, electrochemical supercapacitors, field-effect transistors, sensors, actuators and photo-electrochemical devices.
FET sensor device based on a hybrid electroactive material. Chemical structure of the hybrid inorganic-organic electroactive polymer (upper right) and characteristic curve of the device (lower right).
Structure of a potentiometric sensor based on a Mg-polymer electrolyte.
Characteristic curve of the potentiometric sensor based on Mg-polymer electrolyte.