Hydrogen on the Horizon

There isn’t a fuel cell out there that doesn’t include some element or technology developed at Los Alamos. The Lab has been in the fuel-cell game for nearly 50 years, and now, as the energy sector goes increasingly green, hydrogen fuel cells are rolling out. Over 15,000 passenger vehicles powered by hydrogen fuel cells (HFCs) are already cruising the nation’s roadways, and soon, the first hydrogen-powered vehicle to be purchased by the federal government—a shuttle bus for bringing workers up the hill to Los Alamos—will arrive at the Laboratory.

“When it comes to hydrogen power, we are investing in two ways,” says Andy Erickson of the Laboratory’s Facilities and Operations Directorate. “We’re buying the state of the art today while simultaneously helping to bring about tomorrow’s state of the art.”

The U.S. has set itself the goal of reaching zero net emissions by 2050. The Laboratory shares this goal and is striving to be a sustainability leader within the Department of Energy. Erickson explains, “Los Alamos took a leadership position here in part because of our very long history with fuel-cell development, but also as a demonstration of our commitment to the zero-carbon initiative. And buses are just the start. The Lab has front-end loaders, snow plows, and other kinds of heavy equipment that we’d like to be powered by hydrogen.”

Getting the nation’s vehicles onto renewable fuels is a critical step to achieving the zero net emissions goal. So far, California is leading the way, aiming to have 1.5 million fuel-cell vehicles on its roads by 2025. But no sustainable transportation plan is complete without including heavy-duty vehicles. There are roughly four million diesel semi-trucks crisscrossing America, and even though they make up less than a quarter of all diesel vehicles, semis emit more than 80 percent of the greenhouse gases pumped into the atmosphere by diesel engines.

Hydrogen and air go in, power and water come out.

Hydrogen gas is a great alternative from an energy perspective, and it produces no greenhouse gas emissions. (However, the manufacturing of HFCs and the refining of hydrogen fuel may rely on fossil fuels for some time.) Essentially, hydrogen and air go in, and power and water come out.

But the HFCs running the Lab’s new shuttle bus and the other 15,000 vehicles already out there won’t work for semi-trucks and trains. For large loads and long trips, a different kind of fuel cell is needed. Existing HFCs are either too hot, too cold, too wet, or too dry—any of which can substantially shorten the life of the fuel cells.

For the past nine years, Los Alamos chemist and fuel-cell researcher Yu Seung Kim has been developing the goldilocks of HFCs: not too hot, not too cold, powerful enough, efficient enough, and durable enough to compete with diesel engines in the heavy-duty vehicle sector. It seems he and his team have succeeded.

High and dry

Maybe water was the problem. Water—both its absence and its presence—did seem to be causing problems. Some styles of HFC require water to function, which makes them more complicated, more expensive, and more sensitive to operating conditions. Other styles of HFC degrade in the presence of water, which limits the kinds of applications they can serve. So perhaps, thought Kim in a moment of clarity back in 2014, if the problem was water, then maybe the solution was not water. A fuel cell that is neither dependent on water nor degraded by it, would be a game changer for hydrogen power. Then he took his research in a whole new direction in pursuit of such a fuel cell.

Not all fuel cells use hydrogen as fuel, but those that do share a similar anatomy based on the need to separate protons from electrons. There are two electrodes—an anode and a cathode—that form a sandwich around a proton-conducting membrane. On the anode side, hydrogen gas enters and gets dissociated into protons and electrons. The protons then pass through the membrane to the cathode side. The electrons, however, can’t pass through the membrane because, as well as being proton conducting, the membrane is electron insulating. Therefore, the electrons go the long way around, traveling through an external circuit where their electricity goes into whatever device is being powered, before finally rejoining the protons on the cathode side. The reunited electrons and protons then reform into hydrogen, which combines with oxygen molecules from the air to create the only byproduct of HFC operation—water.

Although HFCs don’t burn fuel like internal combustion engines, they do produce heat from the electrochemical reactions at the electrodes. In the HFCs used for passenger cars, called low-temperature fuel cells, the membrane that separates the anode from the cathode needs water to be conductive to protons. If the water boils off because the temperature exceeded 100°C, the protons stop flowing, so the HFC stops working. To prevent this, current hydrogen-powered vehicles carry a heavy and expensive humidifier to keep the HFC hydrated and a large radiator to keep the temperature at an optimal 60°C to 95°C, depending on the power required. These low-temperature fuel cells aren’t tenable for heavy-duty vehicles because these vehicles need more power and produce more heat, which would make the radiator impractically large.

An alternative to low-temperature fuel cells is high-temperature proton-exchange membrane fuel cells. Unlike low-temperature HFCs, high-temperature HFCs use proton-conducting materials that are anhydrous—they don’t require water. In fact, they fail when exposed to water. Because water naturally condenses from the air, the operating temperature of these HFCs must be kept high, above 140°C, both to prevent condensation and to quickly evaporate the water that forms at the cathode as a byproduct of the cell’s operation. These HFCs work well for applications that run constantly, like off-grid stationary power, but for vehicles, which do frequent cold starts that create condensation, the water intolerance is a deal breaker.

Kim and his team had been working to find a way around the constraints of existing HFCs —always below 100°C or always above 140°C—when they started to think that water was the problem. “We realized that if we had anhydrous proton conductors that do not degrade with water,” he recalls, “we could get away with a much smaller radiator and we could get rid of the humidifier altogether.”

The fuel cell they have built bridges the temperature gap nicely: it works from 80°C to 200°C and has a cold-start capability. The key was to develop two brand-new materials.

First hurdle: membrane materials

The membrane in an HFC has one job: to conduct protons, but not electrons. That’s all. This separation creates a charge gradient, which is what drives the electrons through the external circuit to produce power. Kim’s team specifically wanted a membrane that could do this job without requiring water—as low-temperature HFCs do—or being degraded by water—as extant high-temperature HFCs are.

The membrane material used in commercial high-temperature HFCs is typically a thermally and chemically stable polymer, polybenzimidazole, that has been spiked with phosphoric acid to make it conductive to protons. The problem with using this material for automotive applications is that the phosphoric acid molecules aren’t bound very tightly, so below 140°C, any water molecules that are present will pull the acid molecules away, which over time, renders the membrane less and less proton-conductive.

The reason the acid is so easily pulled away by water is that its interaction with the polymer is via acid-base coordination. This association stems from charge differences between the proton-rich acid and the proton-accepting polymer. It’s not a strong chemical bond, just an attraction that is relatively easily disrupted. Therefore, Kim and his team wanted a membrane material that would hold its acid molecules tighter, rendering the pull of water less obtrusive.

Ion-pair coordination is another kind of chemical interaction whereby electrically charged molecules, or ions, are attracted to ions of the opposite charge. Ion pairing is not a chemical bond, but it is a considerably stronger association than acid-base coordination, so the scientists looked for materials likely to form an ion pair with phosphoric acid. After some exploration, they discovered that quaternary ammonium-polymer could be a substitute for polybenzimidazole polymer, and it nicely ion-pairs its ammonium to the biphosphate of the acid.

Now that scientists had a combination that worked, they wanted to soup it up.

In lab experiments testing the new material's performance in the presence of water, membranes made of the new material maintained their phosphoric acid content—i.e., their proton conductivity. Even more encouraging, the membranes degraded up to a thousand times more slowly than the original acid-base coordinated membranes did. Kim and his team had made a membrane that functioned regardless of the presence of water, exactly as they had set out to do—but the power production was lacking.

“The design was promising,” recalls Kim. “But we didn't have good electrode performance yet.”

So they went back to the lab.

Second hurdle: electrode efficiency

Kim suspected that the reason for the first prototype’s poor performance was a familiar pitfall known as catalyst poisoning. Both sides of an HFC have a catalyst—usually platinum or some other noble metal—that helps speed up the chemical reaction taking place. But because catalysts are so good at helping chemistry along, they are prone to interacting with other materials too. If either the membrane or the electrode binder—the material surrounding the electrode that helps conduct protons between the electrodes and the membrane—interacts with the catalyst, it can permanently reduce its catalytic ability. One symptom of a poisoned catalyst is sluggish electrochemistry, like the scientists were seeing.

They ran some tests with their new membrane material and found that, yes indeed, its phenyl group (-C₆H₅) can severely poison both the anode and cathode catalysts. This explained the poor performance of their first prototype. The scientists needed to protect their catalysts from the membrane’s poisonous phenyl group, and they decided to do that by changing the electrode binder material.

Because HFC power production relies on a steady flow of protons in one direction, from the anode to the cathode by way of the membrane, the intervening materials must all be proton conductive. So, an electrode binder material needs to not only protect the catalysts, but it also needs to be highly proton conductive.

Kim’s team learned, from a colleague in Germany, about a thermally and chemically stable polymer called PWN70, or poly(2,3,5,6-tetrafluorostyrene-4-phosphonic acid), officially. They liked it because, among other properties, it had minimal catalyst-poisoning activity. So they decided to try it.

With the new polymer and their new membrane in hand, the scientists' next step was to combine the materials in a test HFC and see how this second prototype compared to the first, and to commercially available HFCs. The team rigged the new prototype up to one of several bench-top test stations in their lab and were thrilled to see that it not only outperformed its predecessor, but it substantially outperformed the commercial HFC in terms of both power production and longevity. The performance was not diminished by the presence of water or by raising the operating temperature as high as 200°C.

Last hurdle: proton power

But the scientists didn’t stop there. Now that they had a membrane and electrode combination that seemed to work well, they wanted to soup it up. To really increase the efficiency of their HFC, the team wanted a “superacid.” Simply put, an acid is a molecule that donates protons to other molecules, and a superacid is one that does this very easily. Including a superacid in the electrode would speed the flow of protons through the cell, which would drive more electrons through the circuit, resulting in more power output.

Kim needed a superacid and knew where to find one. He returned his attention to the well-hydrated low-temperature HFCs that must stay below 100°C—the ones used for passenger cars. The membrane in those HFCs is made of a polymer material that goes by the brand name Nafion, and it is a superacid.

Nafion’s acid is sulfonic acid, which conducts protons only under hydrated conditions, so for anhydrous systems, it would seem to be a poor choice. However, the acidity of sulfonic acid is 10,000 times greater than that of the phosphonic acid in Kim’s polymer material. Although Nafion does not conduct protons under anhydrous conditions, Kim reasoned perhaps it could transfer its acidic proton to the phosphonic acid, thereby boosting the proton dissociation of the phosphonic acid. Whereas proton conduction is a sort of diffusion of excess protons throughout a material, proton transfer is more discrete: it’s a handoff from the more acidic molecule to the less acidic molecule. And it turned out Kim was right: a composite of Nafion and the new polymer, dubbed “protonated PWN70,” showed a ten-fold increase in proton conductivity compared to the original, unprotonated PWN70.

The scientists eagerly returned to their testing station and hooked up their third prototype—bearing both the new membrane and the protonated electrode—to see how the whole thing stacked up. The new cell showed considerably higher power density than its unprotonated predecessor. Maybe most exciting was when the scientists ran the prototype continuously for over a hundred days at 160°C to try and determine its life expectancy, not only did it not die, it didn’t even fade. Commercially available HFCs subjected to the same conditions began to lose voltage immediately and faded after about 60 days. The Los Alamos team’s new HFC maintained steady voltage for the entire hundred-day trial.

Fuel-cell futures

The hydrogen age is upon us. Hydrogen as a fuel offers clean energy, energy security, and climate change mitigation. But heavy-duty vehicles operate conspicuously outside the temperature range that existing HFCs serve, so to get them going on hydrogen requires a new kind of HFC. And if it can be simpler and cheaper, because of fewer and smaller parts by virtue of being water-free, then all the better. Kim is confident that his team’s new HFC will fill the bill. “And I’m not the only one,” he says happily. “The Laboratory is now working with major auto manufacturers to bring our fuel cell to the commercial market. It’s happening.”

He and his team have succeeded in making the goldilocks of HFCs that they set their sights on nearly ten years ago. It works in the sweet-spot range of 80°C to 200°C and has the cold-start capabilities needed for heavy-duty vehicles. Kim has secured patents for all the materials his team developed and is now working to see if these materials might have other uses as well. The closely related field of hydrogen-fuel refining—how the actual fuel for fuel cells gets made—is of particular interest to him.

Fulfilling the goal of zero emissions by 2050 requires full steam ahead on sustainable solutions. These solutions must be developed by the research community, deployed by the energy industry, and adopted across the country. The way forward for hydrogen power lies in understanding the specific pitfalls of existing technology and charting a path around them. That is just what Kim and his team have done.