The difference between the hydrogen and fossil fuel as an energy carrier is that the former does not occur naturally in molecular form in the Earth crust or atmosphere whereas the latter one does and no fuel production is necessary. However, its burning causes pollutants to be emitted to the environment. For the hydrogen economy to be cost effective, one needs to produce it entirely using renewable sources. Otherwise, it will be possible only to centralise the greenhouse gases emission sources as power plants rather than progressively limiting their operation.
MacGHyver project aims at production of green hydrogen, it means hydrogen of electrolysis origin. This way, no greenhouse gases are evolved in the process. It requires just water, electrolyte (to ensure ionic conductivity) and, of course, electrical energy as it is not spontaneous. Theoretically, the voltage of water electrolysis is 1.23 V (irrespectively of the solution pH), however, the thermodynamic barriers impose the applied working voltage to be considerably higher for the process to occur. The solution pH governs the electrochemistry behind the process: in acidic and alkaline environments different water ions contribute to the water splitting reaction (according to availability of reacting species). The electrode reactions are therefore as follows:
|
In acidic environment:
|
Cathode (-): 2H+
+ 2e–→ H2 Anode (+): 2 H2O
→ O2 + 4 H+ + 4 e–
|
|
In alkaline
environment:
|
Cathode (-): 2H2O
+ 2e–→ H2 + 2OH– Anode (+): 4 OH–
→ O2 + 2 H2O + 4 e– |
Of course, the solution pH can be in between these limiting conditions and in that case the reactions on each electrode are mixed. Although the above formulas look easy and not complicated, the real processes involve complex mechanisms and occur with tremendous overpotential. Overpotential is an additional potential which needs to be applied for the reaction to take place with noticeable rate. It is currently, and has been for centuries, the bottleneck of this technology which hindered its widespread exploitation. However, although this still requires deep research, it is worth a thing.
The production of hydrogen at the cathode and oxygen at the anode occurs via mechanisms which involve adsorption step being a rate-limiting step of electrochemical reaction. The key in the process is that the adsorption strength (energy) cannot be too weak nor too high. This energy should be just in the middle, so both counter processes occur simultaneously. This is conveniently plotted on the volcano plot, where the most active elements are located on the top (or bottom, depending on the representation)1. Of course, there are platinum group elements, which are located in the peak. Unfortunately, application of these is limited due to the low abundance and therefore high price. Nevertheless, they enable very efficient operation of water electrolysers, considerably reducing the overvoltage. The operation principle of these catalysts was a mystery for many years, however, now it is known that they play a critical role in reaction mechanism. The answer why platinum is a good catalyst is that it both dissolves and liberates oxygen and hydrogen efficiently.
The application of catalyst materials in electrode material composition is the solution for overvoltage reduction. The role of catalyst is lowering of activation energy. This way, the water electrolysis can occur at voltage, which is closer to the theoretical one. According to the formula for electrical energy (voltage*current), the same amount of hydrogen can be produced with less electrical energy consumption, which means cheaper.
Alternatively, one can use non-noble metal catalysts to aim into the same effect, but in much more affordable way2. Numerous chemicals and species have been put forward, such as: metal oxides, metal sulphides, doped or undoped nanoporous carbons and others. In MacGHyver, a lot of attention is put on their exploitation. The most challenging is to find a catalyst which ensures attractive chemical reaction rates and retains its activity throughout many cycles.
Along with the
production of hydrogen on the cathode, another useful product can be produced simultaneously
at the anode. Although oxygen is valuable in many chemical and biochemical
processes, it exists naturally in the atmosphere and its direct capture is more
economic. Instead, a formation of another oxygenated product, hydrogen peroxide
is of interest3. Hydrogen peroxide has many applications in chemical
industry. Some of them are:
The most important application from the point of view of MacGHyver project is the last item from the list. As full project title states it is Microfluidic Wastewater Treatment and Creation of Green Hydrogen. The traditional production of hydrogen peroxide involves so called anthraquinone process, which emits considerable amount of pollution and is exceptionally energy demanding. Therefore, its alternative production in electrolysis is especially attractive.
Hydrogen peroxide can be produced electrochemically in two processes: it can be a product of water oxidation (two-electron water oxidation) or a product of oxygen reduction reaction (also two-electron pathway).
Alternative water oxidation: 2H2O → H2O2 + 2H+ + 2e−
Oxygen reduction reaction: O2 + 2H+ + 2e− → H2O2
References:
1. Sarah M. Stratton, Shengjie Zhang, Matthew M. Montemore. Addressing complexity in catalyst design: From volcanos and scaling to more sophisticated design strategies. Surface Science Reports, 100597 (2023).
2. Peitao Zhao, Yanling Zhao, Huagen Liang, Xueping Song, Bo Yu, Fang Liu, Art J. Ragauskas, Cuiping Wang. Novel waste-derived non-noble metal catalysts to accelerate acidic and alkaline hydrogen evolution reaction. Chemical Engineering Journal 466, 143140, 2023.
3. Sotirios Mavrikis, Samuel C. Perry, Pui Ki Leung, Ling Wang, and Carlos Ponce de León. Recent Advances in Electrochemical Water Oxidation to Produce Hydrogen Peroxide: A Mechanistic Perspective. ACS Sustainable Chemistry & Engineering 9 (1), 76-91, 2021.