Fuel cells are the dream power source for vehicles: they can use hydrogen and oxygen as fuel and oxidizer, respectively, and produce only electricity and water (plus a little heat). Compared to battery-powered electric vehicles, hydrogen-powered fuel cell vehicles offer higher energy density, which leads to greater range and lower weight. Sure, they have their downsides—such as requiring a complete hydrogen infrastructure à la oil pipelines and fueling stations—but batteries vs. fuel cells is a debate for another day (and story).
The first hydrogen fuel cell vehicle (General Motors/Chevy Electrovan) was created in 1966. Researchers have been developing proton exchange membrane (PEM) fuel cells for past 15 years. So why don’t we see any in cars on the road? In a word: catalysts. Despite intense development, catalysts used in PEM fuel cells haven’t reached the levels of performance, lifetime, or cost to be commercially viable. In a recent issue of Nature, Mark Debe, senior scientist in the Fuel Cell Components Program at 3M, summed up the recent progress and prospects for fuel cell catalysts, including potential manufacturing issues.
First off, what is a catalyst? How does a fuel cell even work? What is the air-speed velocity of an unladen swallow? (African or European?) One question at a time, please.
In brief, a fuel cell directly converts the chemical energy locked in a fuel (like hydrogen) into electricity though a reaction with an oxidizer (typically, oxygen). All fuel cells consist of an anode, cathode, and electrolyte, which classifies the type of fuel cell (for example, in a PEM fuel cell, the PEM is the electrolyte) and allows the charges to move between the anode and cathode.
In the case of hydrogen and oxygen in a PEM fuel cell, hydrogen is split on the anode side into protons and electrons. The protons travel through the membrane electrolyte while the electrons move through an external circuit—generating an electrical current—to the cathode, where oxygen molecules react with the arriving protons and electrons to create water.
Each individual fuel cell generates only a small amount of electricity (less than a volt), so the overall “fuel cell” is actually a stack of a couple hundred cells. Each cell, or membrane electrode assembly, is comprised of the two electrodes (anode and cathode) sandwiching the PEM, surrounded by porous gas diffusion layers that bring the fuel and air in and water out.
The overall reaction occurring in a fuel cell is the same as when you burn hydrogen: hydrogen plus oxygen produces water and energy. In both cases, the energy of the system must reach a certain activation level before the reaction will proceed. In the case of combustion, this is done with an ignition source such as a high-temperature spark. PEM fuel cells, on the other hand, operate at much lower temperatures. This is where the catalyst comes in: it effectively lowers the activation energy by increasing the reaction rate without being consumed in the process. (By contrast, solid oxide fuel cells operate at much higher temperatures and therefore don’t need a catalyst).
The most effective catalysts in hydrogen fuel cells use platinum for both the anode and cathode. Here is the problem (one of the problems, at least): platinum is expensive. Right now, the cost is over $1400 per troy ounce, just under that of gold. Most catalyst research focuses on how to use less platinum (or none at all) while simultaneously increasing performance and durability.
With all this in mind, let’s take a look at where we are now.
You may not realize it, but we actually do have some cars running on hydrogen on the road. The US Department of Energy (DOE) teamed up with a couple major car manufacturers (Ford, Hyundai, Kia, Daimler, and GM) to test a total of almost 200 vehicles. According to DOE reports, these test fleets used at least 0.4 milligrams of platinum per square centimeter on the cathode alone. The goal for 2017 is to use 0.125 milligrams per square centimeter of platinum group metals (which includes ruthenium, rhodium, palladium, osmium, iridium, and platinum) total between the anode and cathode. In a fuel-cell assembly rated at eight kilowatts per gram of platinum, this works out to eight grams total per vehicle—close to what is used in current internal combustion engines (in the catalytic converter).
According to a DOE technical plan, as of 2011 we’ve reached a power density of about 5.3 kilowatts per gram of PGM, and 0.15 milligrams of PGM per square centimeter—nearly there. However, the stability of the catalyst isn’t yet where we need it, limiting the lifespan to below the 5,000 hour target (corresponding to about 150,000 miles).
There are two conventional platinum-based catalyst approaches. The first uses “Pt blacks,” which are extremely small platinum particles that absorb light very well and appear black, with high surface-to-volume ratios—ideal for a catalyst, where the reaction activity occurs on the surface. The second involves platinum nanoparticles spread onto larger carbon black particles. However, both of these approaches would require far too much (expensive) platinum to reach the performance and durability goals necessary for use in commercially viable fuel cells.
Faced with the difficult task of improving catalyst performance but at the same time using an equal or less amount of platinum, researchers decided to simply design new catalyst nanoparticles.
New catalyst designs
Debe classified the new designs for platinum-based catalysts into four categories. The first, extended surface area catalysts, is fairly self-explanatory. By increasing the surface area, such as by applying thin films on particles or using a porous film, these catalysts can increase reaction activity while using less platinum in the process.
The most promising approach in this category appears to be nanostructured thin-film (NSTF) catalysts. In these, a thin film of a platinum alloy coats a tiny, thin layer of crystalline, organic whiskers. Each whisker is less than a micrometer tall and over 2,000 times thinner than a human hair. Since NSTF catalysts are so thin, the volume is low, resulting in a high surface-area-to-volume ratio. In addition, the organic whiskers are not conductive, preventing any corrosive electrical currents.
The second category involves platinum or platinum-alloy nanoparticles on low-aspect-ratio carbon black or oxide support particles. This is similar to the conventional approach using platinum nanoparticles, except now the size and shape of the nanoparticles are controlled to increase reaction activity and reduce the amount of platinum. The sizes are on the order of nanometers, and the shapes include octahedra, cubes, and more exotic shapes like truncated octahedrons.
Another promising approach in this category uses core-shell nanoparticles (think of a hollow ball). In these, the amount of platinum is reduced significantly since it is removed from the core, and the reaction activity can be increased by filling the core with a material that optimizes properties of the surface platinum layer. Core materials include palladium and palladium alloys with cobalt, iron, iridium, and gold, as well as alloys of other metals like gold and nickel. There are some issues to overcome with these catalysts, though. The performance in actual fuel cells wasn’t as high as in laboratory tests, and researchers need to develop a scalable manufacturing process capable of generating the particles without leaving a pinhole in the platinum layer (to protect the core from leaching).
Other categories of new platinum-based catalysts include nanoparticles on high-aspect-ratio supports (like carbon fiber or nanotubes) and unsupported nanoparticles (such as lone platinum nanotubes or nanoparticles). No specific catalyst designs in either of these have demonstrated particularly high performance yet, however.
What about avoiding platinum—and its high costs—altogether? Researchers have investigated catalysts using palladium and its alloys, but the performance can barely reach that of conventional platinum-based catalysts. Plus, the price isn’t that much cheaper.
Recently, according to Debe, catalysts avoiding precious metals altogether—using metals such as cobalt and iron—have demonstrated huge performance improvements. For example, an iron-based cathode catalyst reached about a tenth of the current density of platinum-based cathodes. However, the lifetime of such catalysts appears to be shorter at the voltage potentials necessary for use in vehicle fuel cells, so this area still needs some work.
Given all of these different approaches under development, which are the most promising? Several of the concepts mentioned above—in particular, the NSTF and shape/size-controlled nanoparticle catalysts—appear to perform at levels necessary to meet DOE targets. Even commercially available platinum-alloy/carbon catalysts come close, although the durability isn’t yet where it needs to be.
Of course, even the most promising catalyst technology still has to be manufactured. Up until now, the quantity of catalysts required for the small number of test vehicles (less than 200 for the DOE studies) didn’t pose much of a challenge. DOE cost targets are based on half a million fuel-cell vehicles produced per year, and the numbers start to get much bigger if the technology is to spread into the larger world market.
To demonstrate the scales involved in manufacturing fuel-cell catalysts for a large number of vehicles, Debe ran some numbers. Producing fifteen million vehicles (ten percent of the global market in 2030) would require 4.5 billion individual fuel cells if each stack contained 300 cells (each about 300 square centimeters in area). Given a production line operating at full capacity, this requires around 11,700 individual cells per minute (worldwide). Cars are produced around one per minute in each production line, so to match this, 20 fuel-cell lines would have to each produce 10 fuel cells a second.
What about the catalyst, which we’ve spent so much time talking about? At the target area density for platinum of 0.1 milligrams per square centimeter, the electrodes will be less than two micrometers thick—meaning precision coating methods. To produce the number of fuel cells needed, the production lines will have to run at 20 meters per minute. This would require one and a half kilograms of platinum per hour, or nearly $1.7 million worth of platinum in a day. Every day, per manufacturing line. This may seem high, but it is similar to the cost of platinum group metals already used in cars—that is what the target density is based on.
According to Debe, these manufacturing requirements will lead to a catalyst-coating approach similar to that already used to produce most multi-layer optical-film-coated glasses: all-dry vacuum coating. In that sector, manufacturers are already making 250 million square meters of glass per year—far more than the 135 million square meters of catalyst than would be needed.
Debe believes that, based on current progress, catalyst performance will peak in a few years well above the DOE goals for 2017. In fact, he argues that improving performance shouldn’t be the primary goal of researchers at this point—catalysts are already where they need to be. Instead, research should focus on creating catalysts that not only hit the durability and power targets, but can be manufactured at high volumes.
According to the article, there is cause for optimism. Recent developments in new kinds of catalysts offer higher performance without reducing the lifetime or increasing the cost. However, it will still be a few years before any of the newer concepts can be incorporated into realistic fuel cells, with the issue of high-volume manufacturing looming in the approaching horizon.