Category Archives: Hydrogen

A more accurate permeation tester

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There are two ASTM-approved methods for measuring the gas permeability of a material. The equipment is very similar, and REB Research makes equipment for either. In one of these methods (described in detail here) you measure the rate of pressure rise in a small volume.This method is ideal for high permeation rate materials. It’s fast, reliable, and as a bonus, allows you to infer diffusivity and solubility as well, based on the permeation and breakthrough time.

Exploded view of the permeation cell.

For slower permeation materials, I’ve found you are better off with the other method: using a flow of sampling gas (helium typically, though argon can be used as well) and a gas-sampling gas chromatograph. We sell the cells for this, though not the gas chromatograph. For my own work, I use helium as the carrier gas and sampling gas, along with a GC with a 1 cc sampling loop (a coil of stainless steel tube), and an automatic, gas-operated valve, called a sampling valve. I use a VECO ionization detector since it provides the greatest sensitivity differentiating hydrogen from helium.

When doing an experiment, the permeate gas is put into the upper chamber. That’s typically hydrogen for my experiments. The sampling gas (helium in my setup) is made to flow past the lower chamber at a fixed, flow rate, 20 sccm or less. The sampling gas then flows to the sampling loop of the GC, and from there up the hood. Every 20 minutes or so, the sampling valve switches, sending the sampling gas directly out the hood. When the valve switches, the carrier gas (helium) now passes through the sampling loop on its way to the column. This sends the 1 cc of sample directly to the GC column as a single “injection”. The GC column separates the various gases in the sample and determines the components and the concentration of each. From the helium flow rate, and the argon concentration in it, I determine the permeation rate and, from that, the permeability of the material.

As an example, let’s assume that the sample gas flow is 20 sccm, as in the diagram above, and that the GC determines the H2 concentration to be 1 ppm. The permeation rate is thus 20 x 10-6 std cc/minute, or 3.33 x 10-7 std cc/s. The permeability is now calculated from the permeation area (12.56 cm2 for the cells I make), from the material thickness, and from the upstream pressure. Typically, one measures the thickness in cm, and the pressure in cm of Hg so that 1 atm is 76cm Hg. The result is that permeability is determined in a unit called barrer. Continuing the example above, if the upstream hydrogen is 15 psig, that’s 2 atmospheres absolute or or 152 cm Hg. Lets say that the material is a polymer of thickness is 0.3 cm; we thus conclude that the permeability is 0.524 x 10-10 scc/cm/s/cm2/cmHg = 0.524 barrer.

This method is capable of measuring permeabilities lower than the previous method, easily lower than 1 barrer, because the results are not fogged by small air leaks or degassing from the membrane material. Leaks of oxygen, and nitrogen show up on the GC output as peaks that are distinct from the permeate peak, hydrogen or whatever you’re studying as a permeate gas. Another plus of this method is that you can measure the permeability of multiple gas species simultaneously, a useful feature when evaluating gas separation polymers. If this type of approach seems attractive, you can build a cell like this yourself, or buy one from us. Send us an email to reb@rebresearch.com, or give us a call at 248-545-0155.

Robert Buxbaum, April 27, 2022.

Low temperature hydrogen removal

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Platinum catalysts can be very effective at removing hydrogen from air. Platinum promotes the irreversible reaction of hydrogen with oxygen to make water: H2 + 1/2 O2 –> H2O, a reaction that can take off, at great rates, even at temperatures well below freezing. In the 1800s, when platinum was cheap, platinum powder was used to light town-gas, gas street lamps. In those days, street lamps were not fueled by methane, ‘natural gas’, but by ‘town gas’, a mix of hydrogen and carbon monoxide and many impurities like H2S. It was made by reacting coal and steam in a gas plant, and it is a testament to the catalytic power of Pt that it could light this town gas. These impurities are catalytic poisons. When exposed to any catalyst, including platinum, the catalyst looses it’s power to. This is especially true at low temperatures where product water condenses, and this too poisons the catalytic surface.

Nowadays, platinum is expensive and platinum catalysts are no longer made of Pt powder, but rather by coating a thin layer of Pt metal on a high surface area substrate like alumina, ceria, or activated carbon. At higher temperatures, this distribution of Pt improves the reaction rate per gram Pt. Unfortunately, at low temperatures, the substrate seems to be part of the poisoning problem. I think I’ve found a partial way around it though.

My company, REB Research, sells Pt catalysts for hydrogen removal use down to about 0°C, 32°F. For those needing lower temperature hydrogen removal, we offer a palladium-hydrocarbon getter that continues to work down to -30°C and works both in air and in the absence of air. It’s pretty good, but poisons more readily than Pt does when exposed to H2S. For years, I had wanted to develop a version of the platinum catalyst that works well down to -30°C or so, and ideally that worked both in air and without air. I got to do some of this development work during the COVID downtime year.

My current approach is to add a small amount of teflon and other hydrophobic materials. My theory is that normal Pt catalysts form water so readily that the water coats the catalytic surface and substrate pores, choking the catalyst from contact with oxygen or hydrogen. My thought of why our Pd-organic works better than Pt is that it’s part because Pd is a slower water former, and in part because the organic compounds prevent water condensation. If so, teflon + Pt should be more active than uncoated Pt catalyst. And it is so.

Think of this in terms of the  Van der Waals equation of state:{\displaystyle \left(p+{\frac {a}{V_{m}^{2}}}\right)\left(V_{m}-b\right)=RT}

where V_{m} is molar volume. The substance-specific constants a and b can be understood as an attraction force between molecules and a molecular volume respectively. Alternately, they can be calculated from the critical temperature and pressure as

{\displaystyle a={\frac {27(RT_{c})^{2}}{64p_{c}}}}{\displaystyle b={\frac {RT_{c}}{8p_{c}}}.}

Now, I’m going to assume that the effect of a hydrophobic surface near the Pt is to reduce the effective value of a. This is to say that water molecules still attract as before, but there are fewer water molecules around. I’ll assume that b remains the same. Thus the ratio of Tc and Pc remains the same but the values drop by a factor of related to the decrease in water density. If we imagine the use of enough teflon to decrease he number of water molecules by 60%, that would be enough to reduce the critical temperature by 60%. That is, from 647 K (374 °C) to 359 K, or -14°C. This might be enough to allow Pt catalysts to be used for H2 removal from the gas within a nuclear wast casket. I’m into nuclear, both because of its clean power density and its space density. As for nuclear waste, you need these caskets.

I’ve begun to test of my theory by making hydrogen removal catalyst that use both platinum and palladium along with unsaturated hydrocarbons. I find it works far better than the palladium-hydrocarbon getter, at least at room temperature. I find it works well even when the catalyst is completely soaked in water, but the real experiments are yet to come — how does this work in the cold. Originally I planned to use a freezer for these tests, but I now have a better method: wait for winter and use God’s giant freezer.

Robert E. Buxbaum October 20, 2021. I did a fuller treatment of the thermo above, a few weeks back.

Automobile power 2021: Batteries vs gasoline and hydrogen

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It’s been a while since I did an assessment of hydrogen and batteries for automobile propulsion, and while some basics have not changed, the price and durability of batteries has improved, the price of gasoline has doubled, and the first commercial fuel cell cars have appeared in the USA. The net result (see details below) is that I find the cost of ownership for a gasoline and a battery car is now about the same, depending on usage and location, and that hydrogen, while still more pricey, is close to being a practical option.

EV Chargers. They look so much cooler than gasoline hoses, and the price per mile is about the same.

Lithium battery costs are now about $150/kwh. That’s $10,000 for a 70 kWh battery. That’s about 1/5 the price of a Tesla Model 3. The reliability that Tesla claims is 200,000 miles or more, but that’s with slow charging. For mostly fast charging, Car and Driver’s expectation is 120,000 miles. That’s just about the average life-span of a car these days.

The cost of the battery and possible replacement adds to the cost of the vehicle, but electricity is far cheaper than gasoline, per mile. The price of gasoline has doubled to, currently, $3.50 per gallon. A typical car will get about 24 mpg, and that means a current operation cost of 14.6¢/mile. That’s about $1,460/year for someone who drives 10,000 miles per year. I’ll add about $150 for oil and filter changes, and figure that operating a gas-powered car engine costs about $1,610 per year.

If you charge at home, your electricity costs, on average, 14¢/kWh. This is a bargain compared to gasoline since electricity is made from coal and nuclear, mostly, and is subsidized while gasoline is taxed. At level 2 charging stations, where most people charge, electricity costs about 50¢/kWh. This is three times the cost of home electricity, but it still translates to only about $32 for a fill-up that take 3 hours. According to “Inside EVs”, in moderate temperatures, a Tesla Model 3 uses 14.59 kWh/100 km with range-efficient driving. This translates to 11.7¢ per mile, or $1170/year, assuming 10,000 miles of moderate temperature driving. If you live in moderate climates: Californian, Texas or Florida, an electric car is cheaper to operate than a gasoline car. In cold weather gasoline power still makes sense since a battery-electric car uses battery power for heat, while a gasoline powered car uses waste heat from the engine.

Battery cars are still somewhat of more expensive than the equivalent gasoline car, but not that much. In a sense you can add $400/year for the extra cost of the Tesla above, but that just raises the effective operating cost to about $1,570/year, about the same as for the gasoline car. On the other hand, many folks drive less than 50 miles per day and can charge at home each night. This saves most of the electric cost. In sum, I find that EVs have hit a tipping point, and Tesla lead the way.

Now to consider hydrogen. When most people think hydrogen, they think H2 fuel, and a PEM fuel cell car. The problem here is that hydrogen is expensive, and PEM FCs aren’t particularly efficient. Hydrogen costs about $10/kg at a typical fueling station and, with PEM, that 1 kg of hydrogen takes you only about 25 miles. The net result is that the combination hydrogen + PEM results in a driving cost of about 40¢/mile, or about three times the price of gasoline. But Toyota has proposed two better options. The fist is a PEM hybrid, the hydrogen Prius. It’s for the commuter who drives less than about 40 miles per day. It has a 10kWh battery, far cheaper than the Tesla above, but enough for the daily commute. He or she would use charge at home at night, and use hydrogen fuel only when going on longer trips. If there are few long trips, you come out way ahead.

Toyota 2021 Mirai, hydrogen powered vehicle

Toyota also claims to have a hydrogen powered Corolla or debut in 2023. This car will have a standard engine, and I would expect (hope) will drive also — preferably — on hythane, a mix of hydrogen and methane. Hythane is much cheaper per volume, and more energy dense, see my analysis. While Toyota has not said that their Corolla would run on hythane, it is supposed to have an internal combustion engine, and that suggests that hythane will work in it.

A more advanced option for Toyota or any other car/truck manufacturer would be to design to use solid oxide fuel cells, SOFCs, either with hydrogen or hythane. SOFCs are significantly more efficient than PEM, and they are capable of burning hythane, and to some extent natural gas too. Hythane is not particularly available, but it could be. Any station that currently sells natural gas could sell hythane. As for delivery to the station, natural gas lines already exist underground, and the station would just blend in hydrogen, produced at the station by electrolysis, or delivered. Hythane can also be made locally from sewer gas methane, and wind-power hydrogen. Yet another SOFC option is to start with natural gas and convert some of the natural gas to hydrogen on-board using left-over heat from the SOFC. I’ve a patent for this process.

Speaking of supply network, I should mention the brown outs we’ve been having in Detroit. Electric cars are part of the stress to the electric grid, but I believe that, with intelligent charging (and discharging) the concern is more than manageable. The driver who goes 10,000 miles per year only adds about 2,350 kWh/year of extra electric demand. This is a small fraction of the demand of a typical home, 12,154 kWh/year.It’s manageable. Then again, hythane adds no demand to the electric grid and the charge time is quicker — virtually instantaneous.

Robert Buxbaum, September 3, 2021

A useful chart, added September 20, 2021. Battery prices are likely to keep falling.

Branson’s virgin spaceplane in context.

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Virgin Galactic Unity 22, landing.

Branson’s Virgin Space Ship (VSS) Unity was cheered as a revolutionary milestone today (July 10) after taking Branson, three friends and two pilots on a three minute ride to the edge of space, an altitude of 53.5 miles or 283,000 feet. I’d like to put that achievement into contest, both with previous space planes, like the Concorde and X-15 (the 1960s space plane), and also in context with the offerings of Elon Musk’s Space-X and Bezos’s, Blue Horizon.

To start with, the VSS Unity launched from a sub-sonic mother ship, as the X-15 had before it. This saves a lot in fuel weight and safety equipment, but it makes scale up problematic. In this case, the mother-ship was named Eve. Unity launched from Eve at 46,000 feet, about 9 miles up, and at Mach 0.5; it took Eve nearly 90 minutes to get to altitude and position. It was only after separation, that Unity began a one minute, 3 G rocket burn that brought it to its top speed, Mach 3, at about 16 miles up. What followed was a 3 minute, unpowered glide to 53.5 miles and down. Everyone seems to have enjoyed the three minutes of weightlessness, and it should be remembered that there is a lot of difference between Mach 3 and orbital speed, Mach 31. Also there is a lot of difference between a sub-orbital and orbital.

Concorde SST landing in Farnborough.

By comparison, consider the Concorde SSTs that first flew in 1976. It reached about 2/3 the speed of Unity, Mach 2.1, but carried 120 commercial passengers. It took off from the ground and maintained this speed for 4500 miles, going from London to Houston in 4.5 hours. While the Concorde only reached an altitude of 60,000 feet, it is far more impressive going at Mach 2.1 for 4.5 hours than going at Mach 3 for three minutes. And there is a lot of difference between 120 passengers and 4. There is also the advantage of taking off from the ground. A three minute ride in a space plane should not require a 90 minute ascent on a mother ship.

X-15 landing, 1962.

Next consider the X-15 rocket plane of the 1960s. This was a test platform devoted to engine and maneuverability tests; it turns out that maneuverability is very difficult. The X-15 hit a maximum altitude of 354,200 ft, 67 miles, and a maximum speed of Mach 6.72, or 4520 mph. That’s significantly higher than Branson’s VSS, and double the maximum speed. As an aside, the X-15 project involved the development of a new nickel alloy that I use today, Inconel X-750. I use this as a support for my hydrogen membranes. If any new materials were developed for VSS, none were mentioned.

The Air Force’s X-37B Orbital Test Vehicle at Kennedy Space Center, May 7, 2017.

Continuing with the history of NASA’s X-program, we move to the X-41, a air-breathing scramjet of the 1980s and 90s. It reached 95,000 feet, and a maximum speed of Mach 9.64. That’s about three times as fast as Virgin’s VSS. The current X-plane is called X-37B, it is a rocket-plane like the X-15 and VSS, but faster and maneuverable at high speed and altitude. It’s the heart of Trump’s new, US Space force. In several tests over the past 5 years, it has hit orbital speed, 17,426 mph, Mach 31, and orbital altitudes, about 100 miles, after being launched by a Atlas V or a Falcon 9 booster. The details are classified. Apparently it has maneuverability. While the X-37B is unmanned, a larger, manned version, is being built, the X-37C. It is supposed to carry as many as six.

Reaching orbital speed or Mach 31 implies roughly 100 times as much kinetic energy per mass as reaching the Mach 3.1 of Virgin’s VSS. In this sense, the space shuttle, and the current X-plane are 100 times more impressive than Virgin’s VSS. There is also a lot to be said for maneuverability and for a longer flight duration– more than a few minutes. Not that I require Branson to beat NASA’s current offerings, but I anyone claiming cutting edge genius and visionary status should at least beat NASA’s offerings of the 1960s, and the Concorde planes of 1976.

Bezos’s Blue Origin, and the New Shepard launcher.

And that bring’s us to the current batch of non-governmental, space cadets. Elon Musk stands out to me as a head above the rest, at least. Eight years ago, his Grasshopper rocket premiered the first practical, example of vertical take off and landing booster. Today, his Falcon 9 boosters send packages into earth orbit, and beyond, launching Israel’s moon project, as one example. That implies speeds of Mach 31 and higher, at least at the payload. It’s impressive, even compared to X-37, very impressive.

Bezos’ offering, the Blue Origin Shepherd, seems to me like a poor imitation of the SpaceX Falcon. Like Falcon, it’s a reusable, vertical takeoff and landing platform, that launches directly from earth, and like Falcon it carries a usable payload, but it only reaches speeds of Mach 3 and altitudes about 65 miles. Besides, the capsule lands by way of parachutes, not using wings like the space shuttle, or the X-37B, and there is no reusable booster like Falcon. Blue Origin started carrying payloads only in 2019, five yers after SpaceX. There is nothing here that’s cutting edge, IMHO, and I don’t imagine it will be cheaper either.

Branson has something that the other rocket men do not have, quite: a compelling look: personal marketing, a personal story, and a political slant that the press loves and I find hypocritical and hokey. The press, and our politicians, managed to present this flight as more than an energy wasting, joy ride for rich folks. Instead, this is accepted as Branson’s personal fight against climate change. Presented this way, it should qualify as a tax-dodge. I don’t see it getting folks to stop polluting and commit to small cars, but the press is impressed, or claims to be. The powers have committed themselves to this type of Tartuffe, and the press goes along. You’d think that, before giving Branson public adoration for his technology or environmentalism, he should have cutting technology and have been required to save energy, or pollute less. At least beat the specs of the X-15. Just my opinion.

Robert Buxbaum, July 12, 2021

Adding H2 to an engine improves mpg, lowers pollution.

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I month ago, I wrote to endorse hythane, a mix of natural gas (methane) and 20-40% hydrogen. This mix is ideal for mobile use in solid oxide fuel cell vehicles, and not bad with normal IC engines. I’d now like to write about the advantages of an on-broad hydrogen generator to allow adjustable composition fuel mixes.

A problem you may have noticed with normal car engines is that a high hp engine will get lower miles per gallon, especially when you’re driving slow. That seems very strange; why should a bigger engine use more gas than a dinky engine, and why should you get lower mpg when you drive slow. The drag force on a vehicle is proportional to speed squared. You’d expect better milage at low speeds– something that textbooks claim you will see, counter to experience.

Behind these two problems are issues of fuel combustion range and pollution. You can solve both issues with hydrogen. With normal gasoline or Diesel engines, you get more or less the same amount of air per engine rotation at all rpm speeds, but the amount of air is much higher for big engines. There is a relatively small range of fuel-air mixes that will burn, and an even smaller range that will burn at low pollution. You have to add at least the minimal fuel per rotation to allow the engine to fire. For most driving that’s the amount the carburetor delivers. Because of gearing, your rpm is about the same at all speeds, you use almost the same rate of fuel at all speeds, with more fuel used in big engines. A gas engine can run lean, but normally speaking it doesn’t run at all any leaner than about 1.6 times the stoichiometric air-to-fuel mix. This is called a lambda of 1.6. Adding hydrogen extends the possible lambda range, as shown below for a natural gas – fired engine.

Engine efficiency when fueled with natural gas plus hydrogen as a function of hydrogen amount and lambda, the ratio of air to stoichiometric air.

The more hydrogen in the mix the wider the range, and the less pollution generally. Pure hydrogen burns at ten times stoichiometric air, a lambda of ten. There is no measurable pollution there, because there is no carbon to form CO, and temperature is so low that you don’t form NOx. But the energy output per rotation is low (there is not much energy in a volume of hydrogen) and hydrogen is more expensive than gasoline or natural gas on an energy basis. Using just a little hydrogen to run an engine at low load may make sense, but the ideal mix of hydrogen and ng fuel will change depending on engine load. At high load, you probably want to use no hydrogen in the mix.

As it happens virtually all of most people’s driving is at low load. The only time when you use the full horse-power is when you accelerate on a highway. An ideal operation for a methane-fueled car would add hydrogen to the carburetor intake at about 1/10 stoichiometric when the car idles, turning down the hydrogen mix as the load increases. REB Research makes hydrogen generators based on methanol reforming, but we’ve yet to fit one to a car. Other people have shown that adding hydrogen does improve mpg.

Carburetor Image from a course “Farm Power”. See link here. Adding hydrogen means you could use less gas.

Adding hydrogen plus excess air means there is less pollution. There is virtually no CO at idle because there is virtually no carbon, and even at load because combustion is more efficient. The extra air means that combustion is cooler, and thus you get no NOx or unburned HCs, even without a catalytic converter. Hydrogen is found to improve combustion speed and extent. A month ago, I’d applied for a grant to develop a hydrogen generator particularly suited to methane engines. Sorry to say, the DoT rejected my proposal.

Robert Buxbaum June 24, 2021

Brown’s gas for small scale oxygen production.

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Some years ago I wrote a largely negative review of Brown’s gas, but the COVID crisis in India makes me want to reconsider. Browns gas can provide a simple source of oxygen for those who are in need. First, an explanation, Browns gas is a two-to-one mix of hydrogen and oxygen; it’s what you get when you do electrolysis of water without any internal separator. Any source of DC electricity will do, e.g. the alternator of a car or a trickle charger of the sort folks buy for their car batteries, and almost any electrode will do too (I’d suggest stainless steel). You can generate pressure just by restricting flow from the electrolysis vessel, and it can be a reasonable source of small-scale oxygen or hydrogen. The reaction is:

H2O –> H2 + 1/2 O2.

The problem with Brown’s gas is that it is explosive, more explosive than hydrogen itself, so you have to handle it with care; avoid sparks until you separate the H2 from the O2. Even the unseparated mix has found some uses, e.g. as a welding gas, or for putting in cars to avoid misfires, increase milage, and decrease pollution. I think that methanol reforming is a better source of automotive hydrogen: hydrogen is a lot safer than this hydrogen-oxygen mix.

Browns gas to oxygen for those who need it.

The mix is a lot less dangerous if you separate the oxygen from the hydrogen with a membrane, as I show in the figure. at right. If you do this it’s a reasonable wy to make oxygen for patients who need oxygen. The electrolysis cell can be a sealed bottle with water and the electrodes; add a flow restriction as shown to create the hydrogen pressure that drives the separation. The power can be an automotive trickle charger. You can get this sort of membranes from REB Research, here and many other suppliers. REB provide consulting services if you like.

In a pinch, you don’t even need the membrane, by the way. You can rely on your lungs to make the separation. A warning, though, the mix is dangerous. Avoid all sparks. Also, don’t put salt into the water. You can can put in some baking soda or lye to speed the electrolysis, but If you put salt in, you’ll find you don’t make oxygen, but will instead make chlorine. And chlorine is deadly. If you’re not sure, smell the gas. If it smells acrid, don’t use it. This is the chlorine-forming reaction.

2NaCl + 2 H2O –> H2 + Cl2 + 2NaOH

Ideally you should vent the hydrogen stream out the window, but for short term, emergency use, the hydrogen can be vented into your home. Don’t do this if anyone smokes (not that anyone should smoke about someone on oxygen). This is a semi-patentable design, but I’m giving it away; not everything that can be patented should be.

Robert Buxbaum, May 13, 2021.

Hythane and fuel cells to power buses and trains.

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Fuel cells are highly efficient and hardly polluting. They have a long history of use in space, and as a power source for submarines. They are beginning to appear powering city buses and intercity trains, at least in Europe, but not so much in the US or Canada. The business case for fuel cells is that they provide clean electric power to the train or bus, without the need for overhead wires. Avoiding wires helps make up for the high cost of hydrogen as a fuel. The reluctance to switch to fuel cells is the US is due to the longer distances that must be covered. The very low volumetric energy density of hydrogen means you need many filling stations with hydrogen fuel cells, and many fill ups per trip.

Energy density CNG, hydrogen, hythane.

On a mass-basis, hydrogen is energy dense, with 1 kg providing the same energy as 2-3 kg of gasoline. The problem with hydrogen (aside from the cost) is that its mass density is very low, less than 50g/liter, even at high pressure. This is terribly un-dense on a volume basis. It would take 20 liters of high pressure hydrogen (about 5 gallons) to take a car or bus as far as with one gallon of gasoline. Even with a huge tank of high pressure hydrogen, 150 gallons or so, a cross country trip would require some 12 fill ups, one every 250 miles, and this is an annoyance, besides being an infrastructure problem.

Then there is cost. In California, hydrogen costs far more than gasoline, between $12 and $15 per kg. That’s ten times as expensive as gasoline on a weight basis and 4 times as expensive on an energy basis. What’s needed is a cheaper, more energy-dense version of hydrogen, ideally one that can be used in both fuel cells and IC engines, and the version I’d like to suggest is hythane, a mix of methane (natural gas) and 20-30% hydrogen.

Hythane dispenser

Hythane has about 3 times the volumetric energy density of hydrogen, and about 1/3 the price. It makes less CO and CO2 pollution because there is far less carbon. On an energy basis, hythane costs just slightly more than gasoline, and requires less infrastructure. Natural gas is cheap and available, delivered by pipeline, without the need for hydrogen delivery trucks. Because hythane has about three times the volumetric energy density of hydrogen, the tank described above, that would give a 250 mile ride with hydrogen, would give 750 miles with hythane. This means a lot fewer fueling stations are needed, and a lot fewer forced stops. As a bonus, hythane can be used in (some) IC engines as well as in fuel cells.

Hydrogen for hythane-automotive use can be made on site, by electrolysis of water. Because there is relatively little hydrogen in the mix, only 25% by volume, or 8% on an energy basis, there is relatively little burden on the electric grid, and fueling will be a lot faster than with battery chargers. Hythane is already in use in buses in China and Canada. These are normal combustion buses but hythane works even better — more efficiently — with fuel cells (solid oxide fuel cells) and thus hythane provides a path to efficiency and greater fuel cell use.

Hythane bus, Montreal.

Natural gas does not work as well in fuel cells; it requires a pre-reformer to make some H2, and even then tends to coke. To be used in most fuel cells, the methane has to be converted, at lest partially into hydrogen and this takes heat energy and water.

CH4 + H2O + energy –> 3H2 + CO

Reforming is a lot easier with hythane; it can be done within the fuel cell. Within a SOFC, the hydrogen combustion, H2 + 1/2 O2 –> H2O, provides heat and water that helps feed the reforming reaction and helps prevent coking. Long term, fuel cells will likely dominate the energy future, but for now it’s nice to have a fuel that will work well in normal IC engines too.

Robert Buxbaum, April 28, 2021

Blue diamonds, natural and CVD.

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The hope diamond resides in the Smithsonian. It really is a deep blue. It has about 5 ppm boron.

If you’ve ever seen the Hope Dimond, or a picture of it, you’ll notice a most remarkable thing: it is deep blue. While most diamonds are clear, or perhaps grey, a very few are colored. Color in diamonds is generally caused by impurities, in the case of blue diamonds, boron. The Hope diamond has about 5 ppm boron, making it a p-semiconductor. Most blue diamonds, even those just as blue, have less boron. As it turns out one of the major uses of my hydrogen purifiers hydrogen these days is in the manufacture of gem -quality, and semiconductor diamonds, some blue and some other colors. So I thought I’d write about diamonds, colored and not, natural and CVD. It’s interesting and a sort of plug for my company, REB Research.

To start off, natural diamond are formed, over centuries by the effect of high temperature and pressure on a mix of carbon and a natural catalyst mineral, Kimberlite. Diamonds formed this way are generally cubic, relatively clear, and inert, hard, highly heat conductive, and completely non-conducting of electricity. Some man made diamonds are made this way too, using high pressure presses, but gem-quality and semiconductor diamonds are generally made by chemical vapor deposition, CVD. Colored diamonds are made this way too. They have all the properties of clear diamonds, but they have controlled additions and imperfections. Add enough boron, 1000 ppm for example, and the diamond and the resulting blue diamond can conduct electricity fairly readily.

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Seeds of natural diamond are placed in a diamond growth chamber and heated to about 1000°C in the presence of ionized, pure methane and hydrogen.

While natural diamond are sometimes used for technical applications, e.g. grind wheels, most technical-use diamonds are man-made by CVD, but the results tend to come out yellow. This was especially true in the early days of manufacture. CVD tends to make large, flat diamonds. This is very useful for heat sinks, and for diamond knives and manufacturers of these were among my first customers. To get a clear color, or to get high-quality colored diamonds, you need a mix of high purity methane and high purity hydrogen, and you need to avoid impurities of silica and the like from the diamond chamber. CVD is also used to make blue-conductive diamonds that can be used as semiconductors or electrodes. The process is show in the gif above from “brilliantearth”.

Multicolored diamonds made by CVD with many different dopants and treatments.

To make a CVD diamond, you place 15 to 30 seed- diamonds into a vacuum growth chamber with a flow of methane and hydrogen in ratio of 1:100 about. You heat the gas to about 1000°C (900-1200°C) , while ionizing the gas using microwaves or a hot wire. The diamonds grow epitaxially over the course of several days or weeks. Ionized hydrogen keeps the surface active, while preventing it from becoming carbonized — turning to graphite. If there isn’t enough hydrogen, you get grey, weak diamonds. If the gas isn’t pure, you get inclusions that make them appear yellow or brown. Nitrogen-impure diamonds are n-semiconductors, with a band gap greater than with boron-blue diamonds, 0.5-1 volts more. Because of this difference, nitrogen-impure diamonds absorb blue or green light, making them appear yellow, while blue diamonds absorb red light, making them blue. (This is different from the reason the sky is blue, explained here.) The difference in energy, also makes yellow diamonds poor electrical conductors. Natural, nitrogen-impure diamonds fluoresce blue or green, as one might expect, but yellow diamonds made by CVD fluoresce at longer wavelengths, reddish (I don’t know why).

The blue moon diamond, it is about as blue as the hope diamond though it has only 0.36 ppm of boron.

To make a higher-quality, yellow, n-type CVD diamonds, use very pure hydrogen. Bright yellow and green color is added by use of ppm-quantities of sulfur or phosphorus. Radiation damage also can be used to add color. Some CVD diamond makers use heat treatment to modify the color and reduce the amount of red fluorescence. CVD pink and purple diamonds are made by hydrogen doping, perhaps followed by heat treatment. The details are proprietary secrets.


Orange-red phosphorescence in the blue moon diamond.

Two major differences help experts distinguish between natural and man-made diamonds. One of these is the fluorescence, Most natural diamonds don’t fluoresce at all, and the ones that do (about 25%) fluoresce blue or green. Almost all CVD diamonds fluoresce orange-red because of nitrogen impurities that absorb blue lights. If you use very pure, nitrogen-free hydrogen, you get clear diamonds avoid much of the fluorescence and yellow. That’s why diamond folks come to us for hydrogen purifiers (and generators). There is a problem with blue diamonds, in that both natural and CVD-absorb and emit red light (that’s why they appear blue). Fortunately for diamond dealers, there is a slight difference in the red emission spectrum between natural and CVD blue diamonds. The natural ones show a mix of red and blue-green. Synthetic diamonds glow only red, typically at 660 nm.

Blue diamonds would be expected to fluoresce red, but instead they show a delayed red fluorescence called phosphorescence. That is to say, when exposed to light, they glow red and continue to glow for 10-30 seconds after the light is turned off. The decay time varies quite a lot, presumably due to differences in the n and p sites.

Natural diamond photographed between polarizers show patterns that radiate from impurities.

Natural and CVD also look different when placed between crossed polarizers. Natural diamonds show multiple direction stress bands, as at left, often radiating from inclusions. CVD diamonds show fine-grained patterns or none at all (they are not made under stress), and man-made, compression diamonds show an X-pattern that matches the press-design, or no pattern at all. If you are interested in hydrogen purifiers, or pure hydrogen generators, for this or any other purposes, please consider REB Research. If you are interested in buying a CVD diamond, there are many for sale, even from deBeers.

Robert Buxbaum, October 19, 2020. The Hope diamond was worn by three French kings, by at least one British king, and by Miss Piggy. A CVD version can be worn by you.

A hydrogen permeation tester

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Over the years I’ve done a fair amount of research on hydrogen permeation in metals — this is the process of the gas dissolving in the metal and diffusing to the other side. I’ve described some of that, but never the devices that measure the permeation rate. Besides, my company, REB Research, sells permeation testing devices, though they are not listed on our site. We recently shipped one designed to test hydrogen permeation through plastics for use in light weight hydrogen tanks, for operation at temperatures from -40°C to 85°C. Shortly thereafter we got another order for a permeation tester. With all the orders, I thought I’d describe the device a bit — this is the device for low permeation materials. We have a similar, but less complex design for high permeation rate material.

Shown below is the central part of the device. It is a small volume that can be connected to a high vacuum, or disconnected by a valve. There is an accurate pressure sensor, accurate to 0.01 Torr, and so configured that you do not get H2 + O2 reactions (something that would severely throw off results). There is also a chamber for holding a membrane so one side is help in vacuum, in connection to the gauge, and the other is exposed to hydrogen, or other gas at pressures up to 100 psig (∆P =115 psia). I’d tested to 200 psig, but currently feel like sticking to 100 psig or less. This device gives amazingly fast readings for plastics with permeabilities as low as 0.01 Barrer.

REB Research hydrogen permeation tester cell with valve and pressure sensor.

REB Research hydrogen permeation tester cell with valve and pressure sensor.

To control the temperature in this range of interest, the core device shown in the picture is put inside an environmental chamber, set up as shown below, with he control box outside the chamber. I include a nitrogen flush device as a safety measure so that any hydrogen that leaks from the high pressure chamber will not build up to reach explosive limits within the environmental chamber. If this device is used to measure permeation of a non-flammable gas, you won’t need to flush the environmental chamber.

I suggest one set up the vacuum pump right next to the entrance of the chamber; in the case of the chamber provided, that’s on the left as shown with the hydrogen tank and a nitrogen tank to the left of the pump. I’ve decided to provide a pressure sensor for the N2 (nitrogen) and a solenoidal shutoff valve for the H2 (hydrogen) line. These work together as a safety feature for long experiments. Their purpose is to automatically turn off the hydrogen if the nitrogen runs out. The nitrogen flush part of this process is a small gauge copper line that goes from the sensor into the environmental chamber with a small, N2 flow bleed valve at the end. I suggest setting the N2 pressure to 25-35 psig. This should give a good inert flow into the environmental chamber. You’ll want a nitrogen flush, even for short experiments, and most experiments will be short. You may not need an automatic N2 sensor, but you’ll be able to do this visually.

Basic setup for REB permeation tester and environmental chamber

Basic setup for REB permeation tester and environmental chamber

I shipped the permeation cell comes with some test, rubbery plastic. I’d recommend the customer leave it in for now, so he/she can use it for some basic testing. For actual experiments, you replace mutest plastic with the sample you want to check. Connect the permeation cell as shown above, using VCR gaskets (included), and connect the far end to the multi-temperature vacuum hose, provided. Do this outside of the chamber first, as a preliminary test to see if everything is working.

For a first test live the connections to the high pressure top section unconnected. The pressure then will be 1 atm, and the chamber will be full of air. eave the top, Connect the power to the vacuum pressure gauge reader and connect the gauge reader to the gauge head. Open the valve and turn on the pump. If there are no leaks the pressure should fall precipitously, and you should see little to no vapor coming out the out port on the vacuum pump. If there is vapor, you’ve got a leak, and you should find it; perhaps you didn’t tighten a VCR connection, or you didn’t do a good job with the vacuum hose. When things are going well, you should see the pressure drop to the single-digit, milliTorr range. If you close the valve, you’ll see the pressure rise in the gauge. This is mostly water and air degassing from the plastic sample. After 30 minutes, the rate of degassing should slow and you should be able to measure the rate of gas permeation in the polymer. With my test plastic, it took a minute or so for the pressure to rise by 10 milliTorr after I closed the valve.

If you like, you can now repeat this preliminary experiment with hydrogen connect the hydrogen line to one of the two ports on the top of the permeation cell and connect the other port to the rest of the copper tubing. Attach the H2 bleed restrictor (provided) at the end of this tubing. Now turn on the H2 pressure to some reasonable value — 45 psig, say. With 45 psi (3 barg upstream) you will have a ∆P of 60 psia or 4 atm across the membrane; vacuum equals -15 psig. Repeat the experiment above; pump everything down, close the valve and note that the pressure rises faster. The restrictor allows you to maintain a H2 pressure with a small, cleansing flow of gas through the cell.

If you like to do these experiments with a computer record, this might be a good time to connect your computer to the vacuum reader/ controller, and to the thermocouple, and to the N2 pressure sensor. 

Here’s how I calculate the permeability of the test polymer from the time it takes for a pressure rise assuming air as the permeating gas. The volume of the vacuumed out area after the valve is 32 cc; there is an open area in the cell of 13.0 cm2 and, as it happens, the  thickness of the test plastic is 2 mm. To calculate the permeation rate, measure the time to rise 10 millitorr. Next calculate the millitorr per hour: that’s 360 divided by the time to rise ten milliTorr. To calculate ncc/day, multiply the millitorr/hour by 24 and by the volume of the chamber, 32 cc, and divide by 760,000, the number of milliTorr in an atmosphere. I found that, for air permeation at ∆P = one atm, I was getting 1 minute per milliTorr, which translates to about 0.5 ncc/day of permeation through my test polymer sheet. To find the specific permeability in cc.mm/m2.day.atm, I multiply this last number by the thickness of the plastic (2 mm in this case), divide by the area, 0.0013 m2, and divide by ∆P, 1 atm, for this first test. Calculated this way, I got an air permeance of 771 cc.mm/m2.day.atm.

The complete setup for permeation testing.

The complete setup for permeation testing.

Now repeat the experiment with hydrogen and your own plastic. Disconnect the cell from both the vacuum line and from the hydrogen in line. Open the cell; take out my test plastic and replace it with your own sample, 1.87” diameter, or so. Replace the gasket, or reuse it. Center the top on the bottom and retighten the bolts. I used 25 Nt-m of torque, but part of that was using a very soft rubbery plastic. You might want to use a little more — perhaps 40-50 Nt-m. Seal everything up. Check that it is leak tight, and you are good to go.

The experimental method is the same as before and the only signficant change when working with hydrogen, besides the need for a nitrogen flush, is that you should multiply the time to reach 10 milliTorr by the square-root of seven, 2.646. Alternatively, you can multiply the calculated permeability by 0.378. The pressure sensor provided measures heat transfer and hydrogen is a better heat transfer material than nitrogen by a factor of √7. The vacuum gauge is thus more sensitive to H2 than to N2. When the gauge says that a pressure change of 10 milliTorr has occurred, in actuality, it’s only 3.78 milliTorr.  The pressure gauge reads 3.78 milliTorr oh hydrogen as 10 milliTorr.

You can speed experiments by a factor of ten, by testing the time to rise 1 millitorr instead of ten. At these low pressures, the gauge I provided reads in hundredths of a milliTorr. Alternately, for higher permeation plastics (or metals) you want to test the time to rise 100 milliTorr or more, otherwise the experiment is over too fast. Even at a ten millTorr change, this device gives good accuracy in under 1 hour with even the most permeation-resistant polymers.

Dr. Robert E. Buxbaum, March 27, 2019; If you’d like one of these, just ask. Here’s a link to our web site, REB Research,

A logic joke, and an engineering joke.

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The following is an oldish logic joke. I used it to explain a conclusion I’d come to, and I got just a blank stare and a confused giggle, so here goes:

Three logicians walk into a bar. The barman asks: “Do all of you want the daily special?” The first logician says, “I don’t know.” The second says, “I don’t know.” The third says, “yes.”

The point of the joke was that, in several situations, depending on who you ask, “I don’t know” can be a very meaningful answer. Similarly, “I’m not sure.”  While I’m at it, here’s an engineering education joke, it’s based on the same logic, here applied:

A team of student engineers builds an airplane and wheel it out before the faculty. “We’ve designed this plane”, they explain, “based on the principles and methods you taught us. “We’ve checked our calculations rigorously, and we’re sure we’ve missed nothing. “Now. it would be a great honor to us if you would join us on its maiden flight.”

At this point, some of the professors turn white, and all of them provide various excuses for why they can’t go just now. But there is one exception, the dean of engineering smiles broadly, compliments the students, and says he’ll be happy to fly. He gets onboard the plane seating himself in the front of the plane, right behind the pilot. After strapping himself in, a reporter from the student paper comes along and asks why he alone is willing to take this ride; “Why you and no one else?” The engineering dean explains, “You see, son, I have an advantage over the other professors: Not only did I teach many of you, fine students, but I taught many of them as well.” “I know this plane is safe: There is no way it will leave the ground.”heredity cartoon

Robert Buxbaum, November 2i, 2018.  And one last. I used to teach at Michigan State University. They are fine students.