Fill vehicle fuel tanks with it instead of gasoline. Pipe it to homes for heating and cooking instead of natural gas and to generate electricity onsite instead of sending electricity through transmission lines. And emit only water vapor where it is used. Fuel cells that electrochemically combine hydrogen and oxygen to produce electricity and heat offer the promise of making hydrogen an ideal universal fuel. Make that an ideal energy carrier rather than a fuel, because while hydrogen does grow on trees and fall with the rain, it does not occur naturally by itself. It cannot be mined or harvested. But other energy sources can be used to make hydrogen, and then the hydrogen transported or stored for use where and when needed.

Chevrolet’s Fuel Cell Vehicle

Most hydrogen production today is by steam reforming natural gas. But natural gas is already a good fuel and one that is rapidly becoming scarcer and more expensive. It is also a fossil fuel, so the carbon dioxide released in the reformation process adds to the greenhouse effect. Hydrogen has very high energy for its weight, but very low energy for its volume, so new technology is needed to store and transport it. And fuel cell technology is still in early development, needing improvements in efficiency and durability. The challenges researchers are working on to help make a hydrogen economy a reality include:

• Fuel Cells – Improving fuel cell technology and materials needed for fuel cells.
• Production – Developing technology to efficiently and cost-effectively make hydrogen from renewable energy sources.
• Storage – Developing technology to efficiently and cost-effectively store and transport hydrogen.
• Benefits – The benefits of fuel cell techology.

Fuel cells and their ability to cleanly produce electricity from hydrogen and oxygen are what make hydrogen attractive as a "fuel" for transportation use particularly, but also as a general energy carrier for homes and other uses, and for storing and transporting otherwise intermittent renewable energy. Fuel cells function somewhat like a battery – with external fuel being supplied rather than stored electricity – to generate power by chemical reaction rather than combustion. They typically consist of numerous small cells in layers though, rather than a single large one.

Fuel Cell Components

There are several different types of fuel cells using different catalysts (chemicals, in this case probably metals, that trigger a chemical reaction without themselves being used up by it) and electrolytes (non-metallic conductors of electrical ions, classically in a solution, but for fuel cells more likely in a solid membrane). In one type, for example, however, hydrogen fed to one catalyst-containing electrode splits to a positively charge hydrogen ion (proton) and a negatively charged electron. The positive ions travel through the electrolyte to the other catalyst electrode where they combine with oxygen fed to that electrode – and electrons – to produce water and heat. The necessary electrons are drawn through an electric circuit external to the cell, creating the electrical generation. 

Fuel Cell Component Materials

The potential benefits of fuel cells are significant; however, many challenges must be overcome before fuel cell systems will be a competitive alternative for consumers. Cost, performance, and durability of fuel cell components are key areas that need to be addressed. Vehicle systems operate more efficiently at higher temperature, however, the membrane materials used in current PEM fuel cells cannot withstand these higher temperatures. The National Renewable Energy Laboratory (NREL) is developing new specialized materials that can resist high temperatures and novel methods that can reduce catalyst poisoning.

One area of research is the evaluation of inorganic solid state proton conducting systems for high temperature fuel cell membranes. The goal of this research is to acquire an improved fundamental understanding of a class of inorganic proton conductors (heteropoly acids [HPA] and their salts) that exhibit high proton conductivity at elevated temperatures (well above 100°C) and to apply that understanding to fuel cell membrane technology. The HPA exhibit proton conductivity among the highest measured in the solid state, more than an order of magnitude higher than Nafion. The ultimate goal is to develop HPA-based composite materials that can be combined with polymers and other potential supports to manufacture thin films as membrane materials for use in fuel cells.

The second area of research is the evaluation of corrosion protection of metallic bipolar plates for fuel cells. The goal of this research is to investigate and develop metal bipolar plate materials and coatings that are low-cost, lightweight, corrosion-resistant, gas impermeable, and amenable to mass manufacturing. NREL’s experience in conducting oxides, which have been used in various types of solar cells, and expertise in corrosion testing are the foundation of this effort. Based on this experience, possible suitable materials (i.e., offer appropriate corrosion protection and give high conductivity) for this application include tin oxide, indium tin oxide, and zinc oxide.

List of Hydrogen Fuel Cell Cars:

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On the one hand, hydrogen’s great asset as a renewable energy carrier is that it is storable and transportable. On the other hand, its very low natural density requires storage volumes that are impractical for vehicles and many other uses. Current practice is to compress the gas in pressurized tanks, but this still provides only limited driving range for vehicles and is bulkier than desirable for other uses as well.

Liquefying the hydrogen more than doubles the fuel density, but uses up substantial amounts of energy to lower the temperature sufficiently (-253°C at atmospheric pressure), requires expensive insulated tanks to maintain that temperature, and still falls short of desired driving range. One possible way to store hydrogen at higher density is in the spaces within the crystalline structure of metal hydrides. Heat then releases the hydrogen for use. Thus far, however, densities are still not high enough and costs are high.

Another possibility is chemically storing hydrogen in compounds that readily release their hydrogen. The reverse reactions, however, could not likely be performed on board or at the filling station, so the reaction byproduct would have to be retrieved from the vehicle and returned to the production plant for regeneration. 

Compressed Hydrogen Gas Tanks

The energy density of gaseous hydrogen can be improved by storing hydrogen at higher pressures. This higher pressure requires material and design improvements in order to ensure tank integrity. Advances in compression technologies are also required to improve efficiencies and reduce the cost of producing high-pressure hydrogen.

Carbon fiber-reinforced 5000-psi and 10,000-psi compressed hydrogen gas tanks are under development by Quantum Technologies and others. Such tanks are already in use in prototype hydrogen-powered vehicles. The inner liner of the tank is a high-molecular-weight polymer that serves as a hydrogen gas permeation barrier. A carbon fiber-epoxy resin composite shell is placed over the liner and constitutes the gas pressure load-bearing component of the tank. Finally, an outer shell is placed on the tank for impact and damage resistance. The pressure regulator for the 10,000-psi tank is located in the interior of the tank. There is also an in-tank gas temperature sensor to monitor the tank temperature during the gas-filling process when tank heating occurs.

The driving range of fuel cell vehicles with compressed hydrogen tanks depends, of course, on vehicle type, design, and the amount and pressure of stored hydrogen. By increasing the amount and pressure of hydrogen, a greater driving range can be achieved but at the expense of cost and valuable space within the vehicle. Volumetric capacity, high pressure, and cost are thus key challenges for compressed hydrogen tanks. Refueling times, compression energy penalties, and heat-management requirements during compression also need to be considered as the mass and pressure of on-board hydrogen are increased.

Issues with compressed hydrogen gas tanks revolve around high pressure, weight, volume, conformability and cost. The cost of high-pressure compressed gas tanks is essentially dictated by the cost of the carbon fiber that must be used for light-weight structural reinforcement. Efforts are underway to identify lower-cost carbon fiber that can meet the required high-pressure and safety specifications for hydrogen gas tanks. However, lower-cost carbon fibers must still be capable of meeting tank thickness constraints in order to help meet volumetric capacity targets. Thus, lowering cost without compromising weight and volume is a key challenge.

Two approaches are being pursued to increase the gravimetric and volumetric storage capacities of compressed gas tanks from their current levels. The first approach involves cryo-compressed tanks. This is based on the fact that, at fixed pressure and volume, gas tank volumetric capacity increases as the tank temperature decreases. Thus, by cooling a tank from room temperature to liquid nitrogen temperature (77°K), its volumetric capacity will increase by a factor of four, although system volumetric capacity will be less than this due to the increased volume required for the cooling system.

The second approach involves the development of conformable tanks. Present liquid gasoline tanks in vehicles are highly conformable in order to take maximum advantage of available vehicle space. Concepts for conformable tank structures are based on the location of structural supporting walls. Internal cellular-type load bearing structures may also be a possibility for greater degrees of conformability.

Compressed hydrogen tanks [5000 psi (~35 MPa) and 10,000 psi (~70 MPa)] have been certified worldwide according to ISO 11439 (Europe), NGV-2 (U.S.), and Reijikijun Betten (Iceland) standards and approved by TUV (Germany) and The High-Pressure Gas Safety Institute of Japan (KHK). Tanks have been demonstrated in several prototype fuel cell vehicles and are commercially available. Composite, 10,000-psi tanks have demonstrated a 2.35 safety factor (23,500 psi burst pressure) as required by the European Integrated Hydrogen Project specifications. 

Liquid Hydrogen Tanks

The energy density of hydrogen can be improved by storing hydrogen in a liquid state. However, the issues with LH₂ tanks are hydrogen boil-off, the energy required for hydrogen liquefaction, volume, weight, and tank cost. The energy requirement for hydrogen liquefaction is high; typically, 30% of the heating value of hydrogen is required for liquefaction. New approaches that can lower these energy requirements and thus the cost of liquefaction are needed. Hydrogen boil-off must be minimized or eliminated for cost, efficiency, and vehicle-range considerations, as well as for safety considerations when vehicles are parked in confined spaces. Insulation is required for LH₂ tanks, and this reduces system gravimetric and volumetric capacity.

Liquid hydrogen (LH₂) tanks can store more hydrogen in a given volume than compressed gas tanks. The volumetric capacity of liquid hydrogen is 0.070 kg/L, compared to 0.030 kg/L for 10,000-psi gas tanks.

Liquid tanks are being demonstrated in hydrogen-powered vehicles, and a hybrid tank concept combining both high-pressure gaseous and cryogenic storage is being studied. These hybrid (cryo-compressed tanks) insulated pressure vessels are lighter than hydrides and more compact than ambient-temperature, high-pressure vessels. Because the temperatures required are not as low as for liquid hydrogen, there is less of an energy penalty for liquefaction and less evaporative losses than for liquid hydrogen tanks.

The simplest and most common element, hydrogen is all around us, but always as a compound with other elements. To make it usable in fuel cells or otherwise provide energy, we must expend energy or modify another energy source to extract it from the fossil fuel, biomass, water, or other compound in which it is found. Nearly all hydrogen production today is by steam reformation of natural gas. This, however, releases fossil carbon dioxide in the process and trades one relatively clean fuel for another, with associated energy loss, so does little to meet national energy needs. For high purity needs, a small amount of hydrogen is produced by electrolysis, but this again is only as good as the energy source used to produce the electricity used. There are, however, many possible ways to produce hydrogen with renewable energy. Some of the most promising are the following:

Thermochemical Hydrogen

Heating biomass (or fossil fuels) with limited or no oxygen present can gasify it to a mixture of hydrogen and carbon monoxide known as synthesis gas or syngas or liquefy/pyrolyze it to a liquid known as pyrolysis oil or bio-oil. Syngas can then be catalytically converted to increase the amount of hydrogen with a "water-gas-shift reaction." Pyrolysis oil can be converted to hydrogen using steam reformation and the water-gas-shift reaction.

Electrolytic Hydrogen

Electrolysis can electrochemically split water into hydrogen and oxygen in essentially the reverse of the reaction in a fuel cell. To make sense for large-scale use, this process must use an inexpensive source of electricity. Because wind energy is currently the lowest cost renewable energy, it is the leading candidate. It is also an intermittent source that would benefit from being able to produce hydrogen when its electricity is not needed and to add fuel-cell generation when electricity demand exceeds what the wind turbines can provide. The combination also benefits because electrolyzers require direct current and wind turbine power must be converted to direct current before conversion back to alternating current suitable for the electric grid.

Electrochemical Photolytic Hydrogen

How about short-circuiting the process to have renewable energy such as solar power produce hydrogen directly? Photoelectrochemical (PEC) hydrogen production replaces one electrode of an electrolyzer with photovoltaic (PV) semiconductor material to generate the electricity needed for the water-splitting reaction. The efficiency loss of separate steps is done away with, as is the cost of the other components of a solar cell. PEC is elegantly simple, but finding PV materials both strong enough to drive the water split and stable in a liquid system presents great challenges for researchers.

Biological Photolytic Hydrogen

Another way to directly tap solar energy for hydrogen production is to take advantage of ways in which nature does so. Certain microalgae and photosynthetic bacteria do sometimes use photosynthesis to make hydrogen instead of sugar and oxygen. Among challenges here is the fact that the algal enzyme that triggers the hydrogen production is inhibited by oxygen, which of course, the organism also normally produces. Another biological research avenue is to develop microorganisms that will ferment sugars or cellulose to hydrogen instead of alcohol.