On the Eve of Hydrogen Era

From hydrogen — to fuel cells

Why hydrogen is the choice? So far, the fossil fuels (coal, oil, and gas) have been used as the main sources of energy. On the fuel combustion, oxygen from air oxidizes carbon to produce carbon dioxide (CO₂) — the main greenhouse thought to be responsible for global warming in recent decades and inevitable environmental disasters in the future.

And which product, in addition to energy, results from the reaction of hydrogen with oxygen? Ordinary water, of course! Now imagine a car fueled with hydrogen — no other engine could be more environmentally benign and safe. The only, but critically essential obstacle is that hydrogen in a free state is practically unavailable in the nature. In order to switch to hydrogen energy, it is necessary, first of all, to develop efficient technologies for the large-scale hydrogen production, storage, transportation, and, second, to create a new-generation power plants fueled with hydrogen.

There is a strong belief that in the XXI century, the fossil energy-carriers (coal, oil, and gas) will be substituted for hydrogen — a new environmentally sound fuel.
For the first time, hydrogen as an energy carrier was mentioned in Jules Verne’s “The Mysterious Island”. As far back as 1874, the great French novelist came up with an exciting idea that people in the future will obtain energy from water by decomposing it into hydrogen and oxygen and then by combusting hydrogen. This fantastic idea was not so crazy as one could have thought at first blush. Time has proved the prediction to be correct: now we have cars, notebook PCs, mobile phones, and even a submarine running on the hydrogen fuel. Let us continue, to the best of our abilities, this discussion, namely, consider (not in all aspects, of course — one could not embrace the boundless) the current state of the hydrogen energy and fuel cells as its part

Let us focus on hydrogen — it is worth to note that hydrogen and a hydrogen-rich gas (i. e., synthesis gas or syngas) are widely used in various industries — chemical, oil processing, metallurgy, radio electronics and even in food industry (for example, for hydrogenation of vegetable oils to produce solid (saturated) fats and margarines).

Regarding the new applications of hydrogen, it should be noted that hydrogen (or a hydrogen-rich gas) additives to conventional transportation and gas turbine fuels allow considerable benefits. The use of such a “hydrogen-enriched” fuel increases the engine efficiency and decreases harmful emissions.

Dr. T. N. Veziroglu (USA), President of the International Association for Hydrogen Energy, a leading proponent for the hydrogen energy, suggested that in several decades mankind will turn to “hydrogen civilization”. There are good reasons to believe this suggestion to be correct. Indeed, the total hydrogen production attained ca. 50 Mt in 2000, and will increase 20 fold by 2100, according to optimistic forecasts. Here, a thoughtful reader could get embarrassed and ask — where and how they are going to take all these megatons when practically the whole of hydrogen on the planet exists in a bound state? Before answering this question, let us consider the fuel cell concept.

Fuel cells: pros and cons

A fuel cell is an electrochemical cell that converts the chemical energy of a fuel directly into the electric energy; no combustion and mechanical compression-expansion steps are involved in the process. In addition to electricity, the fuel cell generates heat.

In principle, fuel cells of various types are similarly structured. Being a galvanic cell, the fuel cell contains an electrolyte and two electrodes — anode and cathode. Electric current is generated by oxidation-reduction reactions of reagents continuously fed to the electrodes from the outside.

In a simple fuel cell, for example, with proton-exchange polymer electrolyte, pure hydrogen and oxygen are usually used. At the anode, hydrogen splits into protons and electrons. The H+ ions transfer through the electrolyte to the cathode, where they combine with oxygen to produce water, which is released outside in the form of a hot steam. Electrons transfer from the anode to the cathode through an external circuit, generating electric current.

Fuel cells demonstrate numerous attractive features: high electric generation efficiency (as compared to that of conventional electric power sources), negligible emissions, quiet operation, modular structure, etc. The main currently known impediment is one, but essential — a high cost.

Efficiency of a fuel cell is determined by the ratio of the electric power output to the net calorific value of the fuel. Theoretically, the efficiency of certain fuel cells can exceed unity, but this never occurs in reality.

So, why these two concepts — hydrogen and fuel cells — are mentioned together very often? The answer is simple — it is hydrogen that is the best fuel for fuel cells and, therewith, an environmentally-sound fuel. All other fuels are converted in fuel cells less efficiently. Therefore, hydrogen and fuel cells form an “inseparable pair” showing great promises. For the power generation issue, benefits are obvious — the consumption of fossil fuels to be converted into hydrogen for fuel-cell-power-plant applications will be far less than the fuel demand for conventional power generating systems.

Methanol to fuel a notebook computer

Fuel cells are classified by the type of electrolyte, for example, the electrolyte in the alkaline fuel cell is a solution of an alkali, in the polymer electrolyte fuel cell — a proton conducting polymer membrane. The polymer electrolyte fuel cells can operate on methanol, which is a good fuel, although less efficient than hydrogen. Methanol-fueled cells seem to be the most promising power generators for portable applications (notebooks, cameras, mobile phones, etc.)

Among other types of fuel cells are phosphoric acid fuel cells, which use phosphoric acid as electrolyte; solid oxide fuel cells with ZrO₂ based ceramic electrolyte, and, finally, molten carbonate fuel cells — the electrolyte in these fuel cells is a combination of potassium and lithium carbonates. Operating temperatures of various type fuel cells varies greatly. Polymer electrolyte fuel cells operate at 80—100° C; solid oxide fuel cells and molten carbonate fuel cells — at very high temperatures (650 1000° C).

In fact, individual fuel cell of any type produces less than 1 volt of electricity. To obtain the needed voltage, multiple cells must be assembled into a fuel cell stack. However, the fuel cell stack is still not a device that can serve as an industrial or a household power source. Such a task can be managed only with the use of an electrochemical generator comprised of a fuel cells stack and peripheral components — control unit, temperature control system, fuel processor (a reactor for fuel conversion into a hydrogen-rich gas), etc.

The efficiency of a modern fuel cell is equal to 40—60 %. Hydrogen-fuelled cells demonstrate, as mentioned above, a maximum efficiency. If methane is used as fuel, the efficiency decreases since part of energy is spent for methane conversion into a hydrogen-rich gas. If heat recovery is anticipated in the system, its efficiency increases by 20—30 %.

As a result, 70 % efficiency has been already attained — these are impressive results, aren’t they? A comparison of the fuel cell efficiency with that characteristic of other modern power installations (microturbines, internal combustion engines, gas turbines, heat power plants, diesels, etc.) clearly shows that nothing could compete with the fuel cells in the low-power range. Fuel cells ideally meet the need for decentralized and autonomous power sources, which grow in popularity, especially after the recent repetitive power plant accidents.

Where hydrogen to be recovered from?

Having considered the fuel cell advantages, let us focus on hydrogen as an energy carrier that is the most compatible with fuel cells. Since a free hydrogen is not widely available on our planet, it should be derived from an appropriate source. In principle, the approach to hydrogen production is simple — take a hydrogenated substance, apply energy (renewable one, ideally), and — obtain hydrogen at the output! There are several sources and processes to produce hydrogen. Fossil and synthetic fuels come first. Currently, 50 % of hydrogen production is from natural gas, 30 % — from oil. In addition, there are coal, biomass, water, etc.

At the next step, however, a critical issue appears concerning hydrogen storage, accumulation and refueling infrastructure. One of the methods to overcome this problem consists in “on-board” hydrogen production in a device called “fuel processor”.

The cost issue of hydrogen is very essential as yet; hydrogen is not traded on the stock market; its production is a material- and energy-consuming process. As a result, the current cost of hydrogen is high. According to the US DOE estimations, the cost of hydrogen will be $ 1.5 to $ 2.9/kg by 2010. For comparison: on a heating value basis, a kilogram of hydrogen is approximately equivalent to the US gallon (~4 l) of gasoline. Therefore, for the development of the hydrogen energy, it is critically important to find in the near future the efficient ways to produce hydrogen and synthesis-gas from cheap and abundant natural gas. (By the way, our country possesses 40 % of the world natural gas resources).

Consider a general scheme of a hydrocarbon fuel (for example, natural gas) processing for a fuel cell application. The first step — catalytic reactions of partial oxidation, or steam reforming, or autothermal reforming of a natural gas — is performed at high temperatures. This step yields synthesis-gas — a mixture of hydrogen and carbon monoxide. Synthesis-gas can be used to feed high-temperature fuel cells, since both hydrogen and carbon monoxide are rapidly oxidized at high temperatures.

Before feeding the low-temperature phosphoric acid fuel cells, carbon monoxide contained in the synthesis-gas should be reduced to 1 vol. %. Otherwise, the fuel cell will not work, since CO poisons the anode. In the case of the polymer electrolyte fuel cells operating at lower temperatures, the requirement for the hydrogen purity is the most stringent — no more than 10 molecules of CO per 1 million molecules of H₂, CO being reduced to this level by a catalytic reaction of selective CO oxidation in the presence of hydrogen.

Thus, among all fuel cell types, the high-temperature fuel cells are most tolerant to the fuel quality, demonstrate the highest electric generating efficiency and use no high-cost precious metal catalysts — that makes them the most promising for autonomous stationary power source applications in the future.

“Siberian catalysis”

Our readers have obviously noted the term “catalytic”, which was mentioned above. Indeed, all highly efficient processes for the hydrogen and synthesis-gas production from a natural gas, developed in the world, inevitably involve catalysts. It should be noted that special works on these problems started in our country 10—15 years later than abroad; nevertheless, the Russian science ranges with the latest world advances in this field. Thus, the Boreskov Institute of Catalysis, SB RAS, Novosibirsk, has developed highly efficient catalysts for the reaction of partial oxidation of methane. The catalysts consist of variously shaped honeycomb monoliths or strips made of thermostable metal alloys and ceramics. Based on these catalysts, compact reactors for the natural gas conversion have been developed, which allow a throughput of ca. 4 m³ of methane per hour per 1l of the reactor volume.

Another promising development concerns the process of methane steam reforming. Since this reaction is endothermic and proceeds at high temperatures, heat supply appears to be the limiting step of the process. To overcome this problem, a “subtle” scheme has been proposed: on one side of a metal catalyst plate, a reaction of methane oxidation is performed with heat release, on the other side — the steam reforming of methane. A metal plate readily transfers the heat; as a result, the reactor productivity considerably increases. Based on this idea, the first fuel processor for high-temperature fuel cells has been developed by the joint efforts of the researches and engineers from the Boreskov Institute of Catalysis and the Federal Nuclear Center All-Russian Scientific Research Institute of Experimental Physics (Sarov) supported by JSC MMC “Norilsk Nickel”.

Sodium borohydride as a source of hydrogen shows good promises for portable fuel cells applications. The hydrogen generation from sodium borohydride is a catalytic reaction. The Boreskov Institute of Catalysis has developed monolith and granulated catalysts for this reaction, which are highly competitive with the best foreign patterns. In cooperation with the State Research Institute of Chemistry and Engineering of Organoelement Compounds (Moscow), pilot cartridges based on these catalysts have been already produced for feeding portable fuel cells.

As mentioned above, the low-temperature fuel cells use pure hydrogen; both CO and CO₂ must be removed from the feed stream. The proposed idea is simple: use an absorbent, which absorbs CO and CO₂ in the course of the steam reforming of the hydrocarbon fuel, and obtain pure hydrogen at the outlet. If we use a pair of reactors-absorbers, one of which operates in the absorbing mode, and another — in the regeneration mode, the continuous process will be arranged. The idea has been already put into practice: indeed, the hydrogen purity is about 99 %.

Among other promising developments by the Boreskov Institute of Catalysis are catalysts for methane pyrolysis with zero CO₂ emissions, membrane reactors for natural gas oxidation by the oxygen that penetrates into a reactor from the air through a special membrane, and many others — it is difficult to mention all of them.

As it is easy to see, a number of works are performed in partnership with the product companies, research institutions, enterprises, including the Siberian ones. The Siberian Branch of the Russian Academy of Sciences may play the key role in many areas related to hydrogen energy development, including designing of the new hydrogen generating processes, production of electrochemical devices, as well as scientific support of commercial-scale processes in the hydrogen energy, prognosis activity, development and promotion of national energy-targeted programs. And, certainly, — in the training of highly skilled professionals on the basis of Novosibirsk State University. It is well believed that Siberia will continue contributing to the increase in the Russian energy power.

Not yet ocean liners, but submarines

Obviously, the hydrogen energy and the fuel cells as its integral part have already been knocking hard on the door, which is half-opened. Most likely, the development of the hydrogen energy based on the fuel cells will be among the first choice priorities of the world economy in this century.

Much has been already achieved and even more remains to be done. Switching to a new energy carrier is a thorny, long and capital-intensive way, which could hardly be passed without mistakes, but should be free from “syndromes”. Remember, how long and hard was the way to the nuclear power — even now, its fraction in the energy balance does not exceed 7 %. For a hydrogen energy progress, the joint efforts of chemists, physicists, mathematicians, material engineers, power engineers, and economists — all people on the Earth — are needed.

What has been already accomplished? Now, an installed capacity of all existing electrochemical generators is about 50 MW; about 100 companies perform demonstration tests worldwide; but no commercial-scale product is available as yet.

Meanwhile, the demand for hydrogen-fuelled power plants steadily increases. In fact, no automaker remains uninvolved in designing a fuel-cell-based car. There are not only cars, notebooks, mobile phones, but even a submarine that runs on the hydrogen fuel. Thus, the Jules Verne’s fantasy comes true in the XXI century. Information on the hydrogen energy and the fuel cells is flooding in — even experts could hardly follow all the new results, developments, and achievements. More than 10 scientific periodical journals covering all aspects of the problem are issued now, at least five international conferences are annually held, as well as numerous exhibitions; various Internet resources are available. You are free to read, analyze, and make conclusions — time will show how true was the thought of the great technocratic romantic.