| Abstract | Gorivni članak je elektrokemijski uređaj koji iskorištava kemijsku energiju sadržanu u vodiku, te uz pomoć kemijskih reakcija proizvodi električnu struju. Produkti reakcija su voda i toplina. Gorivni članak sastoji se od dvije elektrode (pozitivne katode i negativne anode), na kojima se odvijaju kemijske reakcije, i elektrolita između njih. "Protone Exchange Membrane" gorivni članci posjeduju membranu odnosno elektrolit koji provodi protone. Na anodnoj strani molekula vodika(H2) se rastavlja na dva elektrona i dva protona. Elektroni preko grafitnog sloja odlaze u vanjski strujni krug, dok protoni prolaze kroz membranu prema katodnom dijelu gorivnog članka. S katodne strane se dovodi kisik, te se na katodi odvija reakcija stvaranja vode. Atomu kisika se pridodaju dva protona koja su prošla kroz membranu, te dva elektrona koja su prošla vanjskim strujnim krugom i grafitnim slojem na katodi, obavljajući pri tome koristan rad. Time se stvara jedna molekula vode. Linearna neravnotežna termodinamika opisuje transportne procese u sustavima za koje vrijede pretpostavke lokalne ravnoteže. U stanju lokalne ravnoteže termodinamičke veličine su dobro definirane veličine, a prvi i drugi zakon termodinamike vrijedi lokalno. Varijable stanja su neprekidna funkcije prostornih koordinata i vremena. Brzina nastajanja entropije σ je središnji pojam. Brzinu nastajanja entropije, po jedinici mase, se preko prvog i drugog zakona termodinamike, zakona očuvanja u aproksimaciji lokalne ravnoteže dovodi u odnos s termodinamičkim tokovima i silama. Cilj ovog diplomskog rada je dati teoretski opis rada gorivnog članka uz pomoć teorije linearne neravnotežne termodinamike. Počevši od definiranja osnovnog pojma, a to je brzina nastajanja entropije σ, te uz pomoć zakona o očuvanju mase, količine gibanja i energije dolazi se do definicije brzine nastajanja entropije pogodne za primjenu na gorivne članke, i to u obliku preko konjugiranih tokova i sila. Kao središnji dio rada, slijedila je primjena dobivenih jednadžbi na model PEM gorivnog članka, u kojem je članak podijeljen u pet podjedinica. Model se odnosi na stacionarno stanje sistema, pri čemu je uzet jednodimenzionalan slučaj što je olakšalo proračun. Za svaku podjedinicu dobijene su jednadžbe kojima možemo opisati promjene temperature, kemijskog potencijala odnosno molarnog udjela vode te električnog potencijala. |
| Abstract (english) | Fuel cell is an electrochemical device that takes advantage of the chemical energy contained in hydrogen, and with the help of chemical reactions produces electricity. The reaction products are water and heat. Fuel cell consists of two electrodes, on which a chemical reactions take place, and electrolyte between them. "Proton Exchange Membrane" fuel cells have a membrane or an electrolyte that conducts protons. On the anode side of the fuel cell, hydrogen molecule (H2) is divided into two electrons and two protons. The electrons travel through the graphite layer leaving fuel cell, and into the external circuit, while the protons are going through the membrane to the cathode. On the cathode side oxygen is being supplied, and the cathode reaction takes place to create water. Oxygen atom is added to two protons which have passed through the membrane, and two electrons that have passed over external circuit and the graphite layer to the cathode, thereby doing useful work. This creates one molecule of water. Linear irreversible thermodynamics describe the transport processes in systems for which there is assumption of local equilibrium. In the state of local equilibrium thermodynamic quantities are well dened, and so the rst and second law of thermodynamics applies locally. State variables are continuous functions of spatial coordinates and time. The entropy production σ is a central concept. The entropy production per unit mass is being, with help of the rst and second law of thermodynamics, the laws of conservation in the approximation of local equilibrium, brought in relation with thermodynamic uxes and forces. The aim of this thesis is to give a theoretical description of the fuel cell with the help of the theory of linear non-equilibrium thermodynamics. Starting from the denition of the basic concept, the entropy production σ, and with the help of the laws of conservation of mass, momentum and energy leads to the denition of the entropy production suitable for use on fuel cells, in the form of conjugated uxes and forces. As a central part of the work, there is application of obtained equations to model of PEM fuel cell, in which the cell is divided into ve subsystems. The model refers to a stationary state, and a one-dimensional case of fuell cell, which facilitated calculus. For each subsystem equations are derived, so we can describe changes in temperature, chemical potential, mole fraction of water and electric potential of fuel cell. |
