While a good deal of attention is being paid to the development of battery technology, research is still continuing to find approaches for more effectively manufacturing hydrogen, which can be used in fuel cells to generate electricity. An example of how fuel cell technology is being commercialized is the recent announcement by Shell and ITM Power that the world’s largest polymer electrolyte membrane hydrogen electrolysis plant will be built at Shell’s Rheinland Refinery in Germany with a capacity of 10 megawatts (1). The plant is due to become operational in 2020. 

In a previous TLT article, a photoelectrochemical process was developed that can produce hydrogen at over 100% quantum efficiency (2). The researchers used a technique called multiple exciton generation that enables the energy of one photon to be converted into multiple electrons. 

Currently platinum-based catalysts on carbon supports are favored for splitting water into hydrogen and oxygen, but wide use of this catalyst type commercially will probably not occur due to the high cost and low supply of platinum. Shaowei Chen, faculty director for COSMOS and professor in the department of chemistry and biochemistry at the University of California at Santa Cruz in Santa Cruz, Calif., says, “Thermodynamically, protons can be reduced at the potential of 0 volt to produce hydrogen, but the kinetics of the process are slow. Catalysts are required to facilitate the reaction.”

Chen indicates that an overpotential needs to be applied to enable catalysts to split water into hydrogen and oxygen. He says, “Platinum exhibits the best performance so far with an overpotential between -30 and -40 millivolts to reach the current density of 10.0 milliamps per square centimeter. Non-platinum-based catalysts exhibit overpotentials that are typically over -100 millivolts. To obtain a negative overpotential, < -100 millivolts would be remarkable.”

In a recently published study, Chen and his colleagues found that embedding ruthenium ions in carbon nitride nanosheets led to the formation of a catalyst that was active in producing hydrogen (3). He says, “We took advantage of the fact that the nitrogen atoms present in carbon nitride act in a similar manner to pyridine, which is a good coordination agent for metal ions. The ruthenium atoms form a nanocomposite with carbon nitride leading to enhanced catalytic activity. At an overpotential of -140 millivolts, the ruthenium-carbon nitride nanocomposite generated a current density of 10.0 milliamps per square centimeter.”

Chen and his researchers have now modified the ruthenium-carbon nitride nanocomposite to further improve its performance in splitting water.

Graphene
Improved catalytic performance was realized by adding graphene to the ruthenium-carbon nitride complex. Chen says, “Electrochemical measurements of this new complex with graphene produced a current density of 10.0 milliamps per square centimeter at an overpotential of only -80 millivolts, which is an upgrade in performance.”

Based on analysis by the researchers, the graphene-carbon nitride-ruthenium nanocomposite catalyst exhibited about 80% of the performance of the current platinum/carbon catalyst and was six times better than the ruthenium-carbon nitride nanocomposite. 

The graphene-carbon nitride-ruthenium nanocomposite was prepared by blending carbon nitride nanosheets with reduced graphene oxide and reacting these two materials with ruthenium chloride in water. Atomic force microscopy analysis showed that the composite consists of sandwich-type structures with a thickness of 6.3 nanometers. 

Chen says, “The sandwich structure was probably an indication of a strong  interaction between the carbon nitride and the graphene nanosheets inducing greater charge redistribution that facilitated the splitting of water. Complexing between the ruthenium metal ions and the pyridinic nitrogens in carbon nitride established favorable conditions for proton absorption and reduction. Other favorable characteristics we attribute to this catalyst were reduced charge-transfer resistance and an increased number of catalytic active sites.”

In running the experiments, the researchers loaded the catalyst on a glassy carbon electrode and negatively charged the potential to quantify hydrogen evolution. Chen says, “We measured hydrogen generation as a function of the current detected. There is a direct relationship between current density and reaction rate.”

Figure 3, which is taken from the cover of a journal that published the research, ChemSusChem, was designed by Chen’s co-author, Yi Peng. The image was based on the ancient Chinese legend of Niulang and Zhinu. The two characters are shown meeting on a bridge consisting of the graphene-carbon nitride-ruthenium composite nanocatalyst and shows the formation of hydrogen. 


Figure 3. This cover image from the journal ChemSusChem is based on the ancient Chinese legend of Niulang and Zhinu and shows them meeting on a bridge consisting of the newly developed composite nanocatalyst that effectively splits water into hydrogen and oxygen. (Figure courtesy of the University of California at Santa Cruz.)

Chen will be evaluating other low-carbon materials besides carbon nitride for their effectiveness in producing hydrogen in the future. He says, “We found that the conductivity of carbon nitride was not that good because this material is a semi-conductor. There are other low carbon materials that we intend to evaluate with metal ions such as cobalt, copper and nickel to determine if the resulting complexes are any more effective in splitting water.”

Additional information on this research can be found in a recent article (4) or by contacting Chen at shaowei@ucsc.edu. 

REFERENCES
1. Please click here. 
2. Canter, N. (2017), “Photoelectrochemical generation of hydrogen over 100% quantum efficiency,” TLT, 73 (8), pp. 12-13.
3. Peng, Y., Lu, B., Chen, L., Wang, N., Lu, J., Ping, Y. and Chen, S. (2017), “Hydrogen evolution reaction catalyzed by ruthenium ion-complexed graphitic carbon nitride nanosheets,” J. Mater. Chem. A, 5 (34), pp. 18261-18269.
4. Peng. Y., Pan, W., Wang, N., Lu, J. and Chen, S. (2018), “Ruthenium Ion-Complexed Graphitic Carbon Nitride Nanosheets Supported on Reduced Graphene Oxide as High-Performance Catalysts for Electrochemical Hydrogen Evolution,” ChemSusChem, 11 (1), pp. 130-136.