Will the Fuel Cell have a Second Life?

The fuel cell enjoyed the height of popularity in the1990s when scientists and stock promoters envisioned a world run on clean and inexhaustible resource – hydrogen. They predicted cars running on fuel cells and households generating electricity from back-yard fuel cells. Improvements in stack design during that time led to increased power densities and lower costs. The stock prices skyrocketed and promoters got blinded. High manufacturing costs, marginal performance and short service life stood in the way of turning the hydrogen dream into reality. Hype and investment funding has since moderated and we hope that a more sensible approach will eventually find the proper use for the fuel cell.

Before resetting expectations, it had been said that the fuel cell is as revolutionary in transforming the world as the microprocessor had been. Experts uttered further that using an inexhaustible source of fuel, hydrogen, would improve the quality of life, and environmental concerns of burning fossil fuels would be solved forever. From 1999 through 2001, over 2,000 organizations were actively involved in fuel cell development and four of the largest public fuel cell companies in North American raised more than a billion dollars in public stock offerings. What went wrong? Is burning hydrogen instead of fossil fuel a misconception? Let’s look at this closer.

A fuel cell is twice as efficient to convert carbon fuel to electricity than combustion does. Hydrogen, the simplest element consisting of one proton and one electron, is plentiful and exceptionally clean as a fuel. Hydrogen makes up 90 percent of the universe and is the third most abundant element on the earth’s surface. Such a wealth of fuel would provide an almost unlimited pool of energy at relatively low cost, but there is a hitch.

The fuel cell core (stack) that converts oxygen and hydrogen to electricity is expensive to build and hydrogen is more costly to produce than gasoline in terms of net calorific value (NCV). Some say that hydrogen is nearly energy neutral, meaning that it takes as much energy to produce as it delivers at the end destination. Unless the energy source used to produce hydrogen comes from a renewable source, hydrogen will not solve the energy issue, nor will it reduce the carbon footprint.

Hydrogen is not a source of energy per se but represents a medium to transport and store it. When envisioning “burning an endless supply of hydrogen,” we must first produce the resource. Hydrogen is not abundantly available in the earth ready to use, as oil and natural gas is and needs other energies to make it into a usable product, similar to electricity to charge a battery. If electricity produces hydrogen, then this energy source should come from a renewable resource. This is not always the case and much comes is derived from burning coal, oil and natural gas.

Fossil fuel lends itself well to produce hydrogen, however, converting this valuable fuel to hydrogen does not make much sense when considering that the process does not add much value. Seen from this angle, hydrogen cannot compete with fossil fuel pumped “free” from the earth as a gift to humanity.

Fuel storage is a further disadvantage. Liquid hydrogen has a low energy density and the volumetric storage in terms of energy is about five times less than petrol products. In liquid form, hydrogen needs extensive insulation for cold keeping, and in gaseous form pressurized hydrogen requires heavy storage tanks.

A reformer would allow the use of methanol, propane, butane and natural gas, however, when converting these fossil fuels into pure hydrogen, some leftover carbon is being released. Although 90 percent less potent than what comes from the tailpipe of a car, carrying a reformer adds to vehicle weight and increases cost. In addition, reformers are known to be sluggish and the net benefit of the fuel cell over the internal combustion engine diminishes.

Sir William Grove, a Welsh judge and gentleman scientist developed the fuel cell concept in 1839, but the invention never took off. This was in part due to the rapidly advancing internal combustion engine that promised better early results. It was not until the second half of the 20th century that the fuel cell was put to practical use during the Gemini space program in the 1960s. NASA preferred this clean power source to nuclear or solar energy and the alkaline fuel cell system that was chosen generated electricity and produced the drinking water for the astronauts.

High material cost made the fuel cell prohibitive for commercial use at that time. This did not hinder Dr. Karl Kordesch, the co-inventor of the alkaline battery, from converting his car to an alkaline fuel cell in the early 1970s. Kordesch drove his car for many years in Ohio, USA. He placed the hydrogen tank on the roof and utilized the trunk for the fuel cell as well as back-up batteries. According to Kordesch, there was “enough room for four people and a dog.”

A fuel cell is an electrochemical device that combines hydrogen fuel with oxygen to produce electricity, heat and water. The fuel cell is similar to a battery in that an electrochemical reaction takes place as long as fuel is available. The hydrogen fuel is stored in a pressurized container and oxygen is taken from the air. Because of the absence of a burning process, there are no harmful emissions, and the only byproduct is fresh water. The water emitted from the proton exchange membrane fuel cell (PEMFC) is so pure that visitors of Vancouver’s Ballard Power Systems were being served hot tea made from this clean water.

Fundamentally, a fuel cell is electrolysis in reverse, using two electrodes separated by an electrolyte. The anode (negative electrode) gets the hydrogen and the cathode (positive electrode) the oxygen. A catalyst at the anode separates hydrogen into positively charged hydrogen ions and electrons, the oxygen is ionized and migrates across the electrolyte to the anodic compartment where it combines with hydrogen. A single fuel cell produces 0.6 to 0.8V under load. To obtain higher voltages, several cells are connected in series.

As there are different battery chemistries, so also are there several fuel cell systems to choose from. In Table 1, we summarize the advantages and limitations of the most common fuel cells. The developments refer to the time of writing.

Fuel Cell in a Vehicle

Fuel cells for automotive use the Proton Exchange Membrane, or PEM for short. PEM uses a polymer electrolyte and is one of the furthest developed and most commonly used fuel cell systems today. The PEM system allows compact design and achieves a high energy to weight ratio. Another advantage is the relatively quick start-up when applying hydrogen. The stack runs at a moderate temperature of about 80°C (176°F) and has a 50-percent efficiency. (In comparison, the internal compaction motor is only about 25 percent efficient).

The limitations of the PEM fuel cell are high manufacturing costs and complex water management systems. The stack contains hydrogen, oxygen and water and if dry, water must be added to get the system going; too much water causes flooding. The system requires pure hydrogen; lower fuel grades can cause decomposition of the membrane. Testing and repairing a stack is difficult and this becomes apparent when realizing that a 150V, 50kW stack to power a car requires 250 cells.

Extreme operating temperatures are a further challenge. Freezing water can damage the stack and the manufacturer recommends heating elements to prevent ice formation. When cold, the start-up is slow and at first the performance is poor. Excessive heat can also cause damage and controlling the operating temperatures, as well as supplying enough oxygen requires compressors, pumps and other accessories that consume about 30 percent of the energy generated.

If operated in a vehicle, the PEMFC stack has an estimated service life of 2000-4000 hours. Start and stop conditions, induce drying and wetting, contribute to membrane stress. Running continuously, the stationary stack is good for about 40,000 hours. Stack replacement is a major expense.

Even with these limitations, the fuel cell as propulsion system is in many ways superior to batteries. It reduces the need to carry large batteries, a necessity with a vehicle propelled by batteries alone. Figure 2 illustrates the practical travel range of a vehicle powered by a fuel cell compared to lead acid, NiMH or Li-ion batteries. One can clearly note that lead and nickel-based batteries gets too heavy when increasing the size to enable larger driving distances. In this respect, the fuel cell enjoys similar qualities to the IC engine in that it can conquer large distances with only the extra weight of fuel.

Figure 2: Driving range as a function of energy storage.

The logarithmical curves of battery power places limitations in terms of size and weight when increasing the distances between charge. In comparison, the fuel cell and IC engine share a linear progression.

Note: 35 MPa hydrogen tank refers to 5,000 psi pressure; 70 MPa is 10,000 psi

Courtesy of International Journal of Hydrogen Energy

Although the fuel cell assumes the duty of the IC engine in a vehicle, poor response time and a weak power band make onboard batteries necessary. In this respect, the FC car resembles an electric vehicle with an onboard power aggregate to keep the batteries charged. The battery is the master and the fuel cell becomes the slave. On start-up, the vehicle relies 100 percent on the battery and the fuel cell only begins contributing after reaching the steady state in 5-30 seconds. During the warm-up period, the battery must also deliver power to activate the air compressor and pumps. When warm, the FC provides enough power for cruising, and when accelerating or climbing hills both FC and battery provide power. During breaking, the kinetic energy is being returned to charge the battery.

The FC of a mid-sized car generates around 85kW, or 114hp, and the power couples to an electric motor of similar capacity. The onboard battery has a capacity of around 18kW and provide throttle response and power assist when passing vehicles or climbing hills. The battery serves a buffer similar to the HEV and does not get stressed by repeated deep cycling and fast charging, as is the case with the EV.

Hydrogen costs about twice as much as gasoline but the high efficiency of the FC compared to the IC engine in converting fuel to energy gives the same net effect on the pocket book except less greenhouse gases and reduced pollution.

Hydrogen is commonly derived from natural gas and we ask why not burning natural gas directly in the IC engine instead of converting it to hydrogen through a reformer and then transforming it to electricity in a fuel cell to feed the electric motors? The answer is efficiency. Burning natural gas in a combustion turbine to produce electricity has an efficiency factor of only 26-32 percent, while using a FC is 35-50 percent efficient. We must keep in mind that the machinery required with the clean FC is far more expensive and requires added maintenance than simply using a burning process.

We have no hydrogen infrastructure and building it is prohibitive. Refueling stations reforming natural gas to hydrogen to support 2,300 vehicles cost over $2 million to build. In comparison, a charging outlet for the EV is less the $1000, but the refill time would be longer than with the FC. Meanwhile, we have plenty of gas stations that offer a quick fill-up of cheap fuel.

Durability and cost are other concerns with the fuel cell and here we have seen some encouraging improvements. The service life of a FC in car driven in normal traffic conditions has doubled from 1,000 hours to 2,000 hours. The target for 2015 is 5,000 hours, or the full life of a vehicle driving 240,000 km (150,000 miles). Another challenge is cost. The fuel cell costs substantially more than an IC engine and until mass-produced, pricing for a cost comparisons is impractical to make. As a simple guideline, the FC vehicles will be more expensive than plug-in hybrids, and the plug-in hybrid cost more than a regular gasoline powered car.

It is conceivable that the fuel cell will never become the engine of choice that experts had hoped and there could be similarities with the failed attempt to fly airplanes on a steam engine in the mid 1800s. It is, however, everyone’s desire that the fuel cell will succeed, and taxpayers may one day have to pay to open the markets similar to subsidizing the electric car. It is also conceivable that governments might in the future mandate the use of fuel cells for environmental reasons. Fuel cells could also become the energy source of choice once the supply of fossil fuel gets dangerously low. Meanwhile, we hope that the development of the fuel cell will continue and become a replacement for the polluting internal combustion engine.