What Limits the Current in a Fuel Cell?

Hydrogen reacts at the anode and releases energy. However, that doesn't mean that the reaction proceeds at an unlimited rate just because energy is liberated.

Activation energy must be involved so that the process can get over the "energy hill". If the probability of a molecule having sufficient energy is low, then the reaction won't proceed at normal speeds. This is the case for fuel cell reactions, with the exception of situations which involve high temperatures.

The 3 best known ways of dealing with slow reaction rates are: the use of catalysts, increasing the electrode area and raising the temperature.

The first two solutions can be employed to any chemical reaction. The third, however, is present in fuel cells only. When considering the following reaction

2H2 + 4OH- → 4H2O + 4e-

It can be noticed that fuel gas and OH- ions coming from the electrolyte need to be present alongside the activation energy. Additionally, the "meeting" between H2 and OH- ions has to happen on the surface of the electrode, since the generated electrons need to be removed.

The reaction that involves fuel/oxygen (gas) with the electrolyte (solid or liquid) and the electrode is commonly called the three phase contact.

The rate at which reactions occur depend on the the area of the electrode. Thus, these are usually porous. Additionally, to help the case, high temperatures and a catalyst are commonly employed together in the assembly and functioning of fuel cells.

Reference:

LARMINIE, James; DICKS, Andrew. Fuel Cell Systems Explained. 2. ed. West Sussex, England: Wiley & Sons Ltd., 2003. 418 p.

Comments

Photovoltaics: Band Diagram

In the previous post we discussed silicon, which is the most used material in photovoltaics. In this post, we introduce the band diagram, for which we will use silicon as an example. We will start our discussion of the band diagram with the Bohr model of the silicon atom. In semiconductor materials the outer shell of the atom, which is called the valence shell, is not completely filled. The outer shell of silicon contains 4 out of the possible 8 electrons, which we call valence electrons. As we discussed in the previous post, each silicon atom in a crystalline structure is bonded to four other silicon atoms. The bonds between the silicon atoms are called covalent bonds. These bonds actually consist of two valence electrons that are shared by two silicon atoms. All valence electrons are fixed in the lattice, forming covalent bonds, and are therefore immobile. However, at a temperature above absolute zero, thermal energy is supplied to these miconductor and some of the vale...

Watching videos on the Internet also harms the environment

Surprisingly, even the videos we watch on the Internet have an impact on the environment. A new report indicates that the internet is one of the top "villains" in the digital sector's carbon footprint, which now accounts for 4% of global greenhouse gas emissions. This share could double by 2025, leaving the digital sector on a par with road transport, as energy consumption in this sector is increasing at the rate of 9% per year. Published by the French research website The Shift Project, the report "Climate Crisis: The Unsustainable Use of Online Video" quantifies the impact of Internet video (VoD, "tubes", pornography, social networks and others) on the environment and the global climate. The study shows that, within all Internet data, online videos account for about 60 percent of the stream, or the largest volume of greenhouse gas emissions in the industry, with about 300 megatonnes of carbon dioxide equivalent (tCO2e). Of this total emission,...

I-V Characteristic of a Solar Cell

Now that we know a bit about the physical processes behind photovoltaic energy conversion, let’s take a look at the device that does this, a solar cell. In the next few posts, we will go deep into all the operating principles of solar cells. We will discuss the semiconductor physics and optics governing their behavior and you will really get a sense of how solar cells work. Nevertheless, let’s start by trying to understand an important aspect for any electrical component, the current-voltage or “i-V” characteristic of the solar cell. Here we see a basic solar cell schematic. There are many different components and layers and you will become familiar with the properties and purposes of all these layers throughout the blog. All you need to know for this post is that a solar cell can take energy from illumination, and convert it into electrical energy in an external circuit. This post’s objective is to answer ‘what’. You will need to go through the rest of the course to u...

The Environmental Engineering Hub

Search This Blog