Clean energy: hydrogen remains far from a magic solution


Each morning at a transit facility in Canton, Ohio, more than a dozen buses pull up to a fueling station before fanning out to their routes in this city south of Cleveland.

The buses – made by El Dorado National and owned by the Stark Area Regional Transit Authority – look like any others. Yet collectively, they reflect the cutting edge of a technology that could play a key role in producing cleaner inter-city transportation.

In place of pollution-belching diesel fuel, one-fourth of the agency’s buses run on hydrogen. They emit nothing but harmless water vapor.

Hydrogen, the most abundant element in the universe, is increasingly viewed, along with electric vehicles, as one way to slow the environmentally destructive impact of the planet’s 1.2 billion vehicles, most of which burn gasoline and diesel fuel. Manufacturers of large trucks and commercial vehicles are beginning to embrace hydrogen fuel cell technologies as a way forward. So are makers of planes, trains and passenger vehicles.

Transportation is the single biggest U.S. contributor to climate change, which is why hydrogen power, in the long run, is seen as a potentially important way to help reduce carbon emissions.

To be sure, hydrogen remains far from a magic solution. For now, the hydrogen that is produced globally each year, mainly for refineries and fertilizer manufacturing, is made using natural gas or coal.

That process pollutes the air, warming the planet rather than saving it. Indeed, a new study by researchers from Cornell and Stanford universities found that most hydrogen production emits carbon dioxide, which means that hydrogen-fueled transportation cannot yet be considered clean energy.

Yet proponents of hydrogen-powered transportation say that in the long run, hydrogen production is destined to become more environmentally safe. They envision a growing use of electricity from wind and solar energy, which can separate hydrogen and oxygen in water. As such renewable forms of energy gain broader use, hydrogen production should become a cleaner and less expensive process.

Within three years, General Motors, Navistar and the trucking firm J.B. Hunt plan to build fueling stations and run hydrogen trucks on several U.S. freeways. Toyota, Kenworth and the Port of Los Angeles have begun testing hydrogen trucks to haul goods from ships to warehouses.

Volvo Trucks, Daimler Trucks AG and other manufacturers have announced partnerships, too. The companies hope to commercialize their research, offering zero-emissions trucks that save money and meet stricter pollution regulations.

In Germany, a hydrogen-powered train began operating in 2018, and more are coming. French-based Airbus, the world’s largest manufacturer of airliners, is considering hydrogen as well.

“This is about the closest I’ve seen us get so far to that real turning point,” said Shawn Litster, a professor of mechanical engineering at Carnegie Mellon University who has studied hydrogen fuel cells for nearly two decades.

Hydrogen has long been a feedstock for the production of fertilizer, steel, petroleum, concrete and chemicals. It’s also been running vehicles for years: Around 35,000 forklifts in the United States, about 4% of the nation’s total, are powered by hydrogen. Its eventual use on roadways, to haul heavy loads of cargo, could begin to replace diesel-burning polluters.

No one knows when, or even whether, hydrogen will be adopted for widespread use. Craig Scott, Toyota’s head of advanced technology in North America, says the company is perhaps two years from having a hydrogen truck ready for sale. Building more fueling stations will be crucial to widespread adoption.

Kirt Conrad, CEO of Canton’s transit authority since 2009, says other transit systems have shown so much interest in the technology that SARTA takes its buses around the country for demonstrations. Canton’s system, which bought its first three hydrogen buses in 2016, has since added 11. It’s also built a fueling station. Two California transit systems, in Oakland and Riverside County, have hydrogen buses in their fleets.

“We’ve demonstrated that our buses are reliable and cost-efficient, and as a result, we’re breaking down barriers that have slowed wider adoption of the technology,” Conrad said.

The test at the Port of Los Angeles started in April, when the first of five semis with Toyota hydrogen powertrains began hauling freight to warehouses in Ontario, California, about 60 miles away. The $82.5 million public-private project eventually will have 10 semis.

Hydrogen fuel is included in President Joe Biden’s plans to cut emissions in half by 2030. The infrastructure bill the Senate approved passed this week includes $9 billion for research to reduce the cost of making clean hydrogen, and for regional hydrogen manufacturing hubs.

The long-haul trucking industry appears to be the best bet for early adoption of hydrogen. Fuel cells, which convert hydrogen gas into electricity, provide a longer range than battery-electric trucks, fare better in cold weather and can be refueled much faster than electric batteries can be recharged. Proponents say the short refueling time for hydrogen vehicles gives them an edge over electric vehicles for use in taxis or delivery trucks, which are in constant use.

That advantage was important for London-based Green Tomato Cars, which uses 60 hydrogen fuel cell-powered Toyota Mirai cars in its 500-car zero emission fleet to transport corporate customers. Co-founder Jonny Goldstone said his drivers can travel over 300 miles (500 kilometers) on a tank and refuel in three minutes.

Because drivers’ earnings depend on fares, Goldstone said, “if they have to spend 40, 50 minutes, an hour, two hours plugging a car in in in the middle of the working day, that for them is just not acceptable.”

For now, Green Tomato is among the largest operators of hydrogen vehicles in what is still a tiny market in Europe, with about 2,000 fuel cell cars, garbage trucks and delivery vans on the roads.

About 7,500 hydrogen fuel cell cars are on the road in the U.S., mostly in California. Toyota, Honda and Hyundai produce the cars, which are priced thousands more than gasoline-powered vehicles. California has 45 public fueling stations, with more planned or under construction.

Unlike with buses and heavy trucks, experts say the future of passenger vehicles in the U.S. lies mainly with electric battery power, not hydrogen. Fully electric vehicles can travel farther than most people need to go on a relatively small battery.

And for now, hydrogen production is adding to rather than reducing pollution. The world produces about 75 million tons a year, most of it in a carbon emission-creating processes involving steam reformation of natural gas. China uses higher-polluting coal.

So-called “blue” hydrogen, made from natural gas, requires an additional step. Carbon dioxide emitted in the process is sent below the earth’s surface for storage. The Cornell and Stanford study found that manufacturing blue hydrogen emitted 20% more carbon than burning natural gas or coal for heat.

That’s why industry researchers are focused on electrolysis, which uses electricity to separate hydrogen and oxygen in water. Hydrogen mixes with oxygen in a vehicle’s fuel cell to produce power. The amount of electricity generated by wind and solar is growing worldwide, making electrolysis cleaner and cheaper, said Joe Cargnelli, director of hydrogen technologies for Cummins, which makes electrolyzers and fuel cell power systems.

Currently, it costs more to make a hydrogen truck and produce the fuel than to put a diesel-powered truck on the road. Hydrogen costs about $13 per kilogram in California, and 1 kilogram can deliver slightly more energy than a gallon of diesel fuel. By contrast, diesel fuel is only about $3.25 per gallon in the U.S.

But experts say that disparity will narrow.

“As they scale up the technology for production, the hydrogen should come down,” said Carnegie Mellon’s Litster.

While a diesel semi can cost around $150,000 depending on how it’s equipped, it’s unclear how much fuel cell trucks would cost. Nikola, a startup electric and hydrogen fuel cell truck maker, estimated last year that it would receive about $235,000 for each hydrogen semi it sells.

Clean electricity might eventually be used to make and store hydrogen at a rail yard, where it could refuel locomotives and semis, all with zero emissions.

Cummins foresees the widespread use of hydrogen in the U.S. by 2030, sped by stricter diesel emissions regulations and government zero-emissions vehicle requirements. Already, Europe has set ambitious green hydrogen targets designed to accelerate its use.

“That’s just going to blow the market open and kind of drive it,” Cargnelli said. “Then you’ll see other places like North America kind of follow suit.”

For millennia, humans have depended on the use of seas and waterways for the transportation of goods. Up until the late 18th century, the main power for ship propulsion was wind in the sails. This changed after the development of steamships powered by coal, and by half-way through the 20th century, diesel fueled ships had taken over. This development made ships go faster, but as trade grew, ships also became larger and heavier thereby dramatically increasing the cost on the environment. To date, if it were a country, the shipping industry would be the 6th largest emitter of CO2, with total emissions in the range of 900 million tons of CO2 per year.1,2 The European Commission projected an increase in global CO2 emissions from shipping of 80% between 1990 and 2020,3 and it is estimated that without action the global share of shipping’s greenhouse gas (GHG) emissions may reach 17% by 2050.4 The International Maritime Organization (IMO) has set a 50% reduction target for emissions related to maritime transport by 2050 compared to 2008.5 The shipping industry is also responsible for a large amount of air pollutants: emissions of NOx and SOx represent roughly 15% and 13% of the global total, respectively.6 A proposed solution to the NOx and SOx emission in the shipping industry is the use of liquefied natural gas (LNG). In theory, this could even lead to a reduction of CO2 emissions by 23% compared to diesel, since LNG has a higher hydrogen to carbon ratio.7 However, methane slip both on the engine level and in the production chain of the liquefied natural gas in reality nullifies this effect and no real greenhouse gas reduction is achieved through the use of LNG.8 The IMO has placed more and more stringent restrictions on the emissions of NOx and SOx from ships, and emissions control areas (ECAs) have been designated (such as the North and Baltic sea) where there are more stringent restrictions on the emissions of air pollutants. The IMO has also implemented the energy efficiency design index (EEDI), a design tool aiming at reducing the emissions of newly built ships. Seagoing ships have an operating lifetime of over 20 years, and since the EEDI only applies to newly built ships, the global fleet is not expected to comply with this index before 2040. This makes the usefulness of the EEDI highly contested.8 The reductions that can be reached with the design index are not at all ambitious in order to reach the emissions to combat climate change.6 To reduce the emissions of the shipping industry, more focus should be placed on other methods than ship design. Other pathways are the use of operational measures such as mandatory speed limits, for example, lower sailing speeds and a better communication between ship and port. These measures can effectively reduce the CO2 emissions from shipping already by 1–60% depending on how strict the conditions are set.9 Next to ship design and the operational methods, the most disruptive way to reduce the CO2 emissions from shipping is switching away from fossil fuels, towards renewable fuels, with lower net CO2-emissions.

Hydrogen is gaining a lot of attention as a clean fuel, since it can be generated from renewable energy through electrolysis. The production of hydrogen through electrolysis is an established technology, but not the current industrial standard.10 Current methods for producing hydrogen rely on the use of fossil fuels as a starting material. It is however possible to use water and excess renewable (solar or wind) energy for the production of hydrogen.11 The potential for solar energy use is huge: on the earth’s surface the incident solar energy is estimated to be 48[thin space (1/6-em)]000 TW. The shipping industry uses less than 0.5 TW year.12

An additional benefit is that both water and renewable energy are available globally (although cost of electricity can vary widely between regions), whereas fossil fuels are unevenly distributed on the globe. To use hydrogen as an on-board fuel, a number of demonstration projects have already been initiated, both using hydrogen fuel cells and by using hydrogen adopted combustion engines.

The most crucial bottleneck with hydrogen as a fuel is likely not the production or the end-point use but rather the storage, having even been called a show stopper in the past.13 By weight, hydrogen is an excellent energy carrier with a lower heating value (LHV) roughly 3 times that of diesel, 33 kW h kg−1 compared to 11.39 kW h kg−1. However, hydrogen is such a light gas that under atmospheric conditions the total energy content is only 3.06 W h L−1 whereas diesel contains 10.08 kW h L−1, roughly a difference of factor 3000. To deal with this low volumetric energy content at atmospheric conditions several technologies exist to concentrate hydrogen and make storage more efficient.

These include: compression, liquefaction and storage in physical or chemical carriers.14 The storage of hydrogen in physical/chemical carriers is an innovative way of handling hydrogen storage. The principle is that hydrogen is not stored as a pure compound, but bound to a carrier either via adsorption (for physical storage)15–17 or by chemical bounds.18–22 In this review we will not focus on the use of physical (adsorption) based hydrogen storage, but we will specifically discuss chemical carrier molecules for hydrogen. Many carriers for hydrogen have been proposed and these can be gasses (like nitrogen or CO2),23–27 liquids (e.g. toluene) or solid materials (e.g. metals or borane compounds).

Gaseous hydrogen compressed at 70 MPa, is a frequently used storage technique in the automotive industry28 and we will use this as a reference in this work. The hydrogen forms in liquid form that are considered are: liquid hydrogen, ammonia, synthetic fuels (from recycled CO2) and liquid organic hydrogen carriers (LOHC). As solid materials we consider some examples of metal hydrides (MgH2, NaAlH4, laves-type AB2 hydrides) and the borane compounds NaBH4 and NH3BH3.

Hydrogen storage for maritime applications is different and shows different challenges than hydrogen storage for stationary or automotive applications. However, the storage of hydrogen for maritime shipping is more challenging than these other cases. On the one hand, large quantities of hydrogen are stored on an isolated ship, whereas in large-scale stationary storage, there is the possibility of external energy input to either release the hydrogen from the carrier or keep it under ideal storage conditions. On the other hand, the storage on a ship is less restricted by weight and volume requirements compared to the automotive industries, and more extreme temperatures can also be used more safely for hydrogen storage on a large ship than in a much smaller passenger vehicle.

Production of hydrogen

Although the hydrogen atom is the most abundant atom in the universe, molecular hydrogen gas itself is not an earth abundant resource. Hydrogen gas has to be produced from other products.29 It is not an energy source like fossil fuels, but it functions more as an energy vector. Hydrogen is mostly produced as a chemical feedstock with an estimated global production of 70 million tons of pure hydrogen in 2019.30 There exist several methods for the production of hydrogen, but not all production methods for hydrogen can be considered green. Currently, about 96% of all hydrogen produced, is produced via reforming processes of fossil fuels, either natural gas, heavy oil and naphtha or from coal.

These processes produce large quantities of CO2,31 and so alternative methods for hydrogen production are under investigation. These methods use renewable feedstocks like water, and biomass. Different hydrogen production methods have been reviewed in depth by Nikolaidis and Poullikkas32 and the section on hydrogen production of this review only aims to provide the reader a basic understanding of hydrogen production, by steam reforming of methane and by electrolysis. These methods were selected since they are proven technologies, already performed on a MW scale. An exception to this is membraneless electrolysis, which is a new technology but is strongly linked to the maritime industry since it was originally developed as a ballast water purification system.

Reforming of methane

Steam reforming of methane is the current state-of-the-art technology for hydrogen production. Currently, about 48% of the globally produced hydrogen is produced via steam reforming. Two process steps are required to convert methane to hydrogen and CO2. The first step in the process converts the methane to hydrogen and CO, according to eqn (1).33

CH4 + H2O ⇌ CO + 3H2 ΔH = 206 kJ mol−1

In order to increase the hydrogen yield of the process, the water–gas-shift reaction occurs in the next step of this chemical process (eqn (2)).33

CO + H2O ⇌ CO2 + H2 ΔH = −41 kJ mol−1

From one molecule of methane and 2 molecules of water, a total of 4 moles of hydrogen can be generated, while seemingly only emitting one mole of CO2. However, the overall reaction for hydrogen production is endothermic, and this heat is supplied by the combustion of fossil fuels, generating an additional emission of greenhouse gasses.

A study showed that the median CO2 emission was 9 kg CO2 per kg hydrogen.34 Using the lower heating value of hydrogen, this resulted in a median emission of hydrogen of 270 g of CO2 per kW hH2 energy produced. Diesel fuels are known to emit ±3.15 g of CO2 per combusted gram of fuel.35 Using the same calculations as for hydrogen, 277 g of CO2 is emitted per kW h of diesel used. The advantage of lowering greenhouse gas emissions by using hydrogen from steam reforming is thus minimal, if no extra CO2 mitigation measures are taken to reduce the emissions of the steam reforming plant.36

CO2 emissions from steam reforming are not limited to the emissions from the chemical process; the heating of the product to high temperatures by combustion on natural gas, can contribute up to 41% of the total CO2 emissions in the process.37 The electrification of the reforming reactors is a possible new pathway that is under research to lower the emissions related to steam reforming.

Instead of combusting natural gas for heat, the heat would be supplied by electricity; this not only has the potential to reduce CO2 emissions from the combustion, but it would also allow for higher methane conversion.36
Next to electrification chemical engineers are working on the development of dry reforming of methane; this process combines two greenhouse gasses, CO2 and methane, to generate hydrogen and carbon monoxide. This reaction follows eqn (3) at temperatures above 750 °C.38

CH4 + CO2 ⇌ 2CO + 2H2 ΔH0298K = 260.5 kJ mol−1

This process provides a more sustainable alternative to steam reforming, since it turns a waste gas, carbon dioxide into syngas (carbon monoxide and hydrogen), which is a useful gas stream in the chemical industry.39 This technology is often sighted in line with bio gas reforming, to enable a more sustainable production method of the syngas. Currently, dry reforming is not only used on large scales, but also this technology does provide a way of producing H2 from natural gas or biogas.40–42 Since this process still uses fossil fuels it is less likely to be as disruptive a technology as renewable energy production and this could ease the pathway to truly renewable hydrogen.

Water electrolysis

An alternative method of hydrogen production is the electrolysis of water. There exist several different kinds of water electrolysis reactions, but all start from the same common concept that when a high enough electrical current is passed through water, the water will split into hydrogen and oxygen gas (eqn (4)).


The different electrolysis methods that are discussed here are: alkaline, polymer electrolyte membrane, solid oxide and membraneless electrolysis. Electrolysis of water can be combined with renewable energy and is seen as a method to stabilize the intermittent character of renewable energy, by converting the excess energy to hydrogen.43 

The cost for this renewable hydrogen is still higher than that of steam reforming of methane, expressed in US$2016 the price for hydrogen from natural gas could be as low as 0.91–1.69 $ kg−1, whereas the prices for renewable hydrogen could be as high as 3.56–9.08 $ kg−1 for wind energy and 3.34–17.30 $ kg−1 for solar energy.44 The availability of energy is a critical factor in the development of water splitting technology; the energy required to release one mole of hydrogen by water splitting is 7 times higher than the energy required to release a mole of hydrogen from methane.45

Alkaline electrolysis. 

Alkaline electrolysis is the state-of-the-art electrolysis technology; it consists of two electrodes that are separated by a diaphragm46 that permeates ions in water but not the evolved gasses.47,48 A schematic view is seen in Fig. 1A. When current is applied to the electrodes the protons in water are reduced to hydrogen gas on the cathode, and oxygen is evolved from the oxidation of hydroxide ions at the anode. The reactions that occur at the electrodes are as given by eqn (5) and (6):49

Cathode: 4H2O + 4e → 2H2 + 4OH(5)
Anode: 4OH → O2 + 2H2O + 4e(6)

To separate the electrodes a diaphragm is used; traditionally asbestos was used and this limited the operating temperature of alkaline electrolysers to 80 °C. Due to the health risks of asbestos this is now replaced by alternative materials. Alternative materials are also under study to increase the working temperature of these electrolysers up to 150 °C; at higher temperatures less energy is required for water electrolysis reaction.50

image file: d0ee01545h-f1.tif
Fig. 1 Schematic side-view of discussed electrolysis technology, based on ref. 58. (A) Conventional alkaline electrolysis, (B) conventional PEM, (C) solid oxide electrolysis, and (D) membraneless electrolysis.

Traditionally alkaline electrolysers are operated at near atmospheric pressures (0.1–0.6 MPa); however, systems at 70 MPa also exist, and these systems have the advantage that less energy is required for post-processing of the hydrogen for high pressure applications, but at the cost of reduced hydrogen purity.51 Still there is a high cost related to electrolysis as the most efficient electrodes are made of platinum,45 and purified water is required in the electrolysis process.52

Polymer electrolyte membrane electrolysis.

 A second type of electrolysis is polymer electrolyte membrane (PEM) electrolysis, Fig. 1B. Here, the aqueous electrolyte is replaced by a proton conducting polymer membrane, that does not permeate gasses. The reactions at the electrodes are different from those in alkaline electrolysers, since only protons and electrons can be transported between the electrodes. The reactions at the anode and cathode are given by eqn (7) and (8):43

image file: d0ee01545h-t2.tif
Cathode: 2H+ + 2e → H2(8)

Water is split into oxygen and protons and the protons then migrate through the membrane to the cathode where a reduction reaction to hydrogen gas takes place. The materials used for the cathode, anode and membrane greatly influence the efficiency and the associated cost of a PEM electrolyser. As a cathode material, platinum deposited on carbon is frequently used; for the anode, iridium or iridium–ruthenium based catalysts are selected.

For the membrane Nafion™ or other fluorinated polysulfonic acid membranes are the most likely choice.53 In practice the anode, cathode and membrane are assembled to make a compact membrane electrode assembly (MEA). By placing all components close to each other the distance the electrons and protons have to migrate is lowered and this improves the overall efficiency of the system. However, the overall efficiency of PEM electrolysers is still lower than in alkaline electrolysers, but due to the low permeability of the membrane for the gasses, PEM have reported hydrogen purities of 99.999 vol% without post-processing.43,51

Solid oxide electrolysis. 

Of all types of electrolysers discussed in the literature, the system that requires the least electrical energy requirement is the solid oxide electrolyser. These systems operate at temperatures above 800 °C and due to the high thermal energy supplied to the system, less electrical energy is required.

This is especially advantageous to the overall electrical efficiency if these high temperatures can – at least partially – be supplied by high temperature waste heat stream from e.g. industrial plants or from a geothermal source. Solid oxide electrolysers have an oxygen-ion conductive membrane and the electrochemical reactions at the electrodes are then given in eqn (9) and (10):54

Cathode: 2H2O + 4e → 2H2 + 2O2−(9)
Anode: 2O2− → O2 + 4e(10)

Just like with other electrolysers the membrane in a solid oxide electrolyser plays a crucial role. In this type of electrolyser, the most used membrane is yttria stabilized zirconia, a material that allows oxygen ions to pass through while remaining stable and electronically insulating even at higher temperatures. Solid oxide electrolyzers can be designed without the need for expensive noble metal electrodes.

As the hydrogen electrode, the typical electrode material used is nickel cermet (a composite ceramic and metallic material), these materials show a high electrical conductivity and a high activity towards the electrochemical reactions. For the oxygen anode, most often perovskite materials are used, commonly LaMnO3.55 A schematic of a solid oxide electrolyser is shown in Fig. 1.

However, solid oxide electrolysers are still in an R&D phase and face critical issues concerning thermal stability. Continued heating and cooling of the system due to load changes result in microscopic cracks in the membrane and this is detrimental to the entire system. Since load changes cause issues with operational stability of the solid oxide electrolysers, these systems are described by some authors to be incompatible with grid stabilization of renewable energy,51 since grid stabilization requires the electrolyser system to be switched on and off ad hoc.

Membraneless electrolysis.

Having been originally designed as a ballast water purification system, membraneless electrolysis is an innovative new concept for hydrogen production, that has the potential to significantly reduce the cost of electrolysis equipment due to the simple configuration of the device. The simplest setup of a membraneless electrolyser is based on flow-by electrodes; in this system the electrolyte flows parallel to the electrodes and into two different outflow channels at the end of the device, as can be seen in Fig. 1D. These systems use the Segré–Silberg effect where the gas bubbles are pinned close to the electrolyte surface where they are evolved thanks to the fluid gradient of the electrolyte.

This means that by flowing the electrolyte through the channel and into the separated outlets the hydrogen and oxygen gas streams are separated from each other by the flow of the aqueous stream. The reactions at the separated electrodes are given in eqn (11) and (12):56

Cathode: 2H2O + 2e → 2OH + H2(11)
image file: d0ee01545h-t3.tif

This system can, however, not produce hydrogen of the same purity as e.g. PEM electrolysers but the reported purity for the hydrogen stream is still as good as 99%.57 Since no membranes are involved in this system there are significant advantages over other types of electrolysers: the first advantage is the simple configuration of the device. Due to the simplicity of the membraneless system, it is also quite resilient to impurities and a wide range of operating conditions.58 Membraneless electrolysers are also known to run on tap water and even brine media.57 

Another advantage is that membraneless electrolysers can work over a broad range of the pH spectrum (0.35–13.7)59 and in a broad spectrum of electrolyte solutions.57,59 All these advantages have to be weighed against the fact that membraneless electrolysers have a lower voltage efficiency compared to other systems; this is due the larger distance between the electrodes and thus the higher internal electrical resistance of the system compared to other electrolyser systems.58 

This means that higher currents have to be used with this system to generate the same amount of hydrogen compared to other fuel cell systems. Table 1 summarizes the specifications of the different types of electrolyzers.

Table 1 Operational details of discussed electrolysis technologies

TechnologyElectricity input (kW h kgH2−1)Operating temperature (°C)Hydrogen pressure (MPa)Lifetime of stack (h)Deployment scale (kW)
Alkaline53.4a80b0.7b90[thin space (1/6-em)]000b>100[thin space (1/6-em)]000b
PEM54.6c80b1.5b50[thin space (1/6-em)]000b>100[thin space (1/6-em)]000b
SOFC36.14c>800d0.1e23[thin space (1/6-em)]000f7.5g
a Ref. 32. b Ref. 49. c Ref. 54. d Ref. 55. e Ref. 60. f Ref. 55. g Ref. 51. h Ref. 56. i Ref. 58.

Membraneless electrolysers operating with electrolytes containing salts are also able to produce acids and bases for the chemical industry, and these value added chemicals have the potential to lower the cost of hydrogen production by membraneless electrolysis.56

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