Prof. Antonella GlisentiDepartment of Chemical SciencesUniversity of Padova

A.A. 2015/2016 Materiali Inorganici Funzionali

Per-capita electricity consumption and Human Development Index

Electricity needs

Electricity needs

Bibliography

Fuell Cell Handbook (Seventh Edition) by EG&G Technical Services, Inc. US Dep. Of Energy – Office of Fossil Energy National Energy Technology Laboratory – Morgantown West Virginia - 2004

J. Larminie, A. Dicks; Fuel Cell Systems Explained – Wiley 2000

N.Q.Minh, T. Takahashi; Science and Technology of Ceramic Fuel Cells – Elsevier 1995

L. Carrette et al. “Fuel Cell Principles, Types, Fuels, and Applications ChemPhysChem Angew. Chem.1 (2000) 162.

A. Orera, P.R. Slater; New Chemical Systems for Solid Oxide Fuell Cells; Chem. Mater. 22 (2010) 675; and references therein

FCs: Technology overview

Introduction

Unit cells

FC stacking

FC systems

FC types

Characteristics

Applications and Status

The first FC: 1839

Sir William Grove

Fuel Cells: general considerations

Fuel cells = electrochemical devices that convert chemical energy in fuelsdirectly into electrical energy with high efficiency and low environmental impact.

Combustibile(Energia Chimica) Energia elettrica

Reazione elettrochimica

Combustione

Energia termica

Ciclo termodinamico

Energia meccanica

Generatore elettrico

Fuel cells are not limited by thermodynamic limitations ofheat engines such as the Carnot efficiency.

In addition, because combustion is avoided, fuel cellsproduce power with minimal pollutant.

Confronto tra le emissioni di impianti a celle a combustibile con impianti di potenza tradizionali.

Unlike batteries the reductant and oxidant in fuel cells must becontinuously replenished to allow continuous operation.

Appleby and Foulkes = any substance capable of chemicaloxidation that can be supplied continuously (as a fluid) can beburned galvanically as fuel at the anode of a fuel cell. Similarly,the oxidant can be any fluid that can be reduced at a sufficientrate.

Fuel Cells: general considerations

Unit cells, in which the electrochemical reactions take place

Stacks, in which individual cells are modularly combined byelectrically connecting the cells to form units with the desiredoutput capacity

Balance of plant which comprises components that providefeedstream conditioning (including a fuel processor if needed),thermal management, and electric power conditioning among otherancillary and interface functions

Fuel Cells: power systems components

Fuel is fed continuously to the anode(negative electrode) and an oxidant (oftenoxygen from air) is fed continuously to thecathode (positive electrode).

The electrochemical reactions takeplace at the electrodes to produce anelectric current.

Electrolyte layer in contact with an anode and a cathode on either side.

Porous electrodes = Catalysts for the fuel oxidation

Schematic of an Individual FC

Differences with battery:

The battery = energy storage device

Fuel cell = energy conversion device

Unit cell: the core of the fuel cell

Three-phase interface: Microscopic regions in which the electrochemical reactions take place; are found where either electrode meets the electrolyte.

The density of these regions and the nature of these interfaces play a critical role in the electrochemical performance of both liquid and solid electrolyte fuel cells:

• Liquid electrolyte fuel cells

•Solid electrolyte fuel cells

Critical aspects of Cell Components

• Solid electrolyte fuel cells: the challenge is to engineer a large number of catalyst sites into the interface that are electrically and ionically connected to the electrode and the electrolyte, respectively, and that is efficiently exposed to the reactant gases.

High-performance interface: electrode which, in the zone near the catalyst, has mixed conductivity.

Critical aspects of Cell Components

General improvements: Three-phase boundary improvement: electrolyte thickness reduction, development of electrode and electrolyte materials which broaden the temperature range, development of active materials that facilitate electrochemical reactions, MIEC.

Critical aspects of Cell Components

The performance and cost are less dependent on scale than other power technologies.

Small fuel cell plants operate nearly as efficiently as large ones, with equally low emissions, and comparable cost.

Rendimenti elettrici dei sistemi tradizionali e delle celle a combustibile

Characteristics

The electrolyte transports dissolves reactants to the electrode, conducts ionic charge between the electrodes, and completes the cell electric circuit. It also provides a physical barrier to prevent the fuel and oxidant gas streams from directly mixing.

The porous electrodes, in addition to providing a surface for electrochemical reactions to take place, are to:

1) conduct electrons away from or into the three-phase interface once they are formed and provide current collection and connection with either other cells or the load

2) ensure that reactant gases are equally distributed over the cell

3) ensure that reaction products are efficiently led away to the bulk gas phase

Other critical aspects of unit cell components

Other Characteristics

• Direct energy conversion (no combustion)

• No moving parts in the energy converter

• Quiet

• Demonstrated high availability of lower temperature units

• Siting ability

• Fuel flexibility

• Demonstrated endurance/reliability of lower temperature units

• Good performance at off-design load operation

• Modular installations to match load and increase reliability

• Size flexibility

General negative features

• Market entry cost high

• Endurance/reliability of higher temperature units not demonstrated

• Stability

• Unfamiliar technology to the power industry

• No infrastructure (for H2, as an example)

• Fuel versatility

… But …

Fuel Cell Types

Classification by the Temperature

Low temperature FCs

1) polymer electrolyte fuel cell (PEFC),

2) alkaline fuel cell (AFC)

Medium temperature FCs

3) phosphoric acid fuel cell (PAFC)

High temperature FCs

4) molten carbonate fuel cell (MCFC)

5) solid oxide fuel cell (SOFC).

Fuel Cell Types

Classification by the type of electrolyte

1) polymer electrolyte fuel cell (PEFC),

2) alkaline fuel cell (AFC)

3) phosphoric acid fuel cell (PAFC)

4) molten carbonate fuel cell (MCFC)

5) solid oxide fuel cell (SOFC).

Choice of electrolyte = the operating temperature range of the fuel cell. The operating temperature dictates:

1. the physicochemical and thermomechanical properties of materials used in the cell components

2. the degree of fuel processing required. In low-temperature FCs, all the fuel must be converted to H2 prior to entering the cell; the anode catalyst (mainly platinum) is strongly poisoned by CO. In high-temperature FCs, CO and even CH4 can be internally converted to H2 or even directly oxidized electrochemically.

Fuel Cell Types

Classification by the type of fuel

• Direct Alcohol Fuel Cells (DAFC). DAFC (or, more commonly, direct methanol fuel cells or DMFC) use alcohol without reforming. Mostly, this refers to a PEFC-type FCs mainly for portable applications.

• Direct Carbon Fuel Cells (DCFC). In direct carbon fuel cells, solid carbon (presumably a fuel derived from coal, or biomass) is used directly in the anode, without an intermediate gasification step. The thermodynamics of the reactions in a DCFC allow very high efficiency conversion.

Fuel Cell Types

Fuel Cell Systems

1. A conventional fuel (natural gas, other gaseous hydrocarbons, methanol, naphtha, or coal) is cleaned, then converted into a gas containing hydrogen.

3. Power conditioning converts the electric power from dc into regulated dc or ac for consumer use.

2. Energy conversion occurs when dc electricity is generated by means of individual fuel cells combined in stacks. A varying number of cells or stacks can be matched to a particular power application.

Fuel Cell Systems

Balance of plant (BoP): Together with the stack, the BoP forms the fuel cell system. The precise arrangement of the BoP depends heavily on the fuel cell type, the fuel choice, and the application. In addition, specific operating conditions and requirements of individual cell and stack designs determine the characteristics of the BoP.

Most fuel cell systems contain:

• Fuel preparation (removal of impurities and thermal conditioning, reforming)

• Air supply (air compressors or blowers, filters)

• Thermal management. All fuel cell systems require careful management of the fuel cell stack temperature.

• Water management

• Electric power conditioning equipment

BoP represents a significant fraction of the weight, volume, and cost of most fuel cell systems.

Fuel Cells Technology Market Report 2014

European Fuel CellsNapoli 2015

State of the artAFM PEM PAFC MCFC SOFC

Temperatura di esercizio

(K)

330-400 340-400 430-500 880-920 1000-1300

Densità di Potenza

(mW/cm2)

300-500 300-900 150-300 150 150-270

Stato tecnologia

Impianti di piccola potenza

Pre-commercialeCommerciale

Impianti dimostrativi fino a

11 MW

Impianti dimostrativi fino a 2

MW

Impianti dimostrativi fino a 2

MW

Applicazioni Trazione Spazio Militari

Generatori portatili

Impianti di potenzaTrazione

CogenerazionePotenza distribuita

Impianti di PotenzaCogenerazione

Impianti di potenzaCogenerazione

Vantaggi Densità di potenza elevata

Densità di potenza elevata

Bassi tempi di avviamento

Alta efficienzaglobale in applicazioni cogenerative Tecnologia matura

Disponibilità di calore ad alta temperaturaRendimento

elevatoPossibile uso di vari

combustibili

Disponibilità dicalore ad altatemperaturaRendimento

elevatoPossibile uso di vari

combustibili

Svantaggi Necessità di combustibile molto puro

Necessità di combustibile molto puroProblemi di

gestione dell’acqua

Necessità di combustibile molto

puro

Problemi divita e stabilitàdei materiali,Richiesto un

ricircolo di anidride carbonica

SollecitazioniTermiche elevateAlto tempo diavviamento

Sommario delle prestazioni delle celle a combustibile