Hydrogen through photocatalysis

The startup Yellow SiC from Berlin is working on an innovative technology that does not require electrolyzers to produce green hydrogen. Yellow cells made of silicon carbide generate hydrogen directly on their surface. They utilize a broader spectrum of sunlight in this than other solar cells. Depending on the location, the production of “golden hydrogen” could be significantly cheaper than the production of green hydrogen from solar power. In this way, 6 cents per kWh could be achieved, even at our latitudes.

The H₂ color palette is already quite extensive to the point of being dizzying. Now another variant is being added with “golden hydrogen.” At least that’s what Yellow SiC from Berlin have named their future product. The startup has developed its own technology: The innovative HydroSiC-Zelle splits water into hydrogen and oxygen in a one-step process. In this direct photocatalysis (more precisely photochemical water splitting), high-purity silicon carbide (3C-SiC) serves as a catalyst.

The 3C refers to the cubic crystal structure. This semiconductor material is yellow, which explains the name of the young company. Its solar cell does not require any electrical cables and consists solely of a 3C-SiC plate surrounded by water, which is exposed to sunlight from one side. This multi-junction solar cell utilizes a broader spectrum of sunlight than other solar cells – and that increases the efficiency. So there is no need for an electrolyzer. Instead, solar cells on the roof could produce green hydrogen directly.

First pilot plant in Osnabrück
There are currently various prototypes that show that photocatalysis works with the test material. “At the moment, we are mainly working on improving the material properties,” states cofounder and managing director Dr. Christopher Höfener. For a pilot plant, the electrodes that are currently produced on a laboratory scale still need to be produced in larger quantities. “We are now working on scaling up the manufacturing processes for the electrodes,” says the physicist and mechanical engineer.

Prototype cell: Photocatalysis with the test material works. Source: Yellow SiC

The young company has ambitious goals: As early as 2025, Yellow SiC wants to produce the golden hydrogen in a first pilot plant for H₂ generation on the site of a paper mill in Osnabrück, Niedersachsen. “Electrolysis for H₂ production is a thing of the past,” former investment banker Höfener is convinced. In his view, a two-stage procedure is simply too inefficient. This conviction is also underpinned by his financial commitment: Of the more than 8 million euros of the development costs raised, a significant part comes from his own assets.

Cofounder and CTO of the company Prof. Siegmund Greulich-Weber acquired his expertise in silicon carbide at the University of Paderborn. He has been working full-time for the startup for several years and leads the development team in Berlin.

The cost per kg of H₂ ultimately depends on the efficiency of the material achieved:

“In contrast to silicon, where there is a physical limit for efficiency of around 33 percent, this limit for doped 3C-SiC silicon carbide is 63 percent – and thus achieves almost twice as many percentage points in the conversion,” says Greulich-Weber. He and his team assume that efficiencies of around 25 percent can be achieved for this so-termed solar-to-hydrogen process.

Hydrogen for 2 cents per kwh
The cost of conventional green hydrogen from electrolysis is currently between 4.5 and 6.7 US dollars per kilogram, or around 16 US cents per kWh. The young company forecasts the price of its product at 0.75 to 2 US dollars per kg – depending on the solar radiation per square meter. That would be less than a third of the current costs. This would result in costs of around 6 euro cents per kWh at our latitudes in Central Europe or 2 cents per kWh in North Africa. “Converted to the cost per kg of hydrogen, this amounts to 2 euros or 0.75 euro,” calculates Höfener.

The main advantage of the new technology lies in the lower costs and the smaller space requirement compared to a combination of photovoltaics and electrolysis. “The cost advantage arises because everything occurs in one step” stresses Höfener. This eliminates the high investment costs for the electrolyzer and the efficiency losses caused by the electrolysis process.

The next step for Yellow SiC is the demonstration of a prototype that shows the achievable efficiency on an area of a few square meters. Another important technical task is further optimization of the material. For this purpose, the electrodes produced will now be characterized using various methods.

2,000 °C and the highest purity
There are, however, still a few hurdles to overcome: Because the production of the material requires very high temperatures of around 2,000 °C – which entails a whole series of technical challenges. In addition, the process may only be carried out under the highest level of purity. More precisely: Out of one million atoms, only a maximum of one atom may be a foreign atom (1 ppm), is the company’s own stipulation. At the same time, attention must be paid to the correct doping and surface structure of the material.

That is a complex process. “The challenges are somewhat less when the material is used in electrolysis,” compares Höfener. Yellow SiC has already achieved interesting results in the tests: This way, the company was able to replace platinum and iridium in PEM electrolysis.

Another challenge is the search for skilled workers, because the team intends to continue to grow. Needed are mainly engineers, physicists and chemists, especially with experience in the fields of PEM/AEM electrolysis and the production of high-temperature ceramics. All in all, therefore very specialist topics. Berlin-Adlershof, where the startup is located, offers with partners from industry and research such as Helmholtz-Zentrum Berlin (HZB) an excellent environment for this.

Prototype cell, Source: Yellow SiC

The technology strengthens decentralized H₂ production and thus fits in with the energy transition and the millions of photovoltaic systems on the roofs of private homes. However, the first applications will be for the chemical industry and steelworks, thinks Höfener. The company, founded in 2020, will start looking for new investors again at the end of the year.

At the beginning of last year, Conenergy from Essen already invested: “We were very impressed by the product, business idea and management of the company,” chairman Niels Ellwanger explains the decision of Conenergy to acquire a stake in the Berlin-based company. The task now is to bring further venture capitalists on board. For the new pilot plant, about 10 million euros of fresh money must come in.