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Smart fuel cells catalyzed by self-adjusting anodes improve water management

Hydrogen fuel cells are often regarded as a key element in the green energy transition. Their efficiency is double the thermochemical energy conversion of internal combustion engines. Hydrogen fuel cells convert the chemical energy of hydrogen and oxygen directly into electricity and water. Hence, water plays a central role in fuel cells. It supports ion transport and participates is product of the reaction itself. In an anion exchange membrane fuel cell (AEMFC), for the oxygen reduction reaction to take place, the water in the anode catalyst layer (ACL) must diffuse to the cathode catalyst layer (CCL). In summary, water management is required to remove water from the ACL for higher efficiency of hydrogen diffusion and to balance the water in the entire membrane electrode assembly (MEA).

Significant research efforts have been made to achieve conditions that is suitable for both the anode and cathode in AEMFC. Asymmetric humidification of reactant gases was proposed to be beneficial to achieve well equilibrated water balance between the two electrodes. At higher temperatures, excess anode water evaporates. It also causes deficiencies at the cathode which also requires water to function. To counteract this, a new system that controls the back pressure at the anode and cathode was introduced. However, external control mechanisms (active control) increase the complexity of the system control.

This is where a passive control system involving MEA modification comes into the picture. Moisture control in a fuel cells can be achieved by designing a suitable gas diffusion layer. Adopting different types of hydrophobic materials for the anode and hydrophilic for the cathode can improve overall fuel cell performance. Poly ethylene tetrafluoroethylene (PTFE) copolymer ion exchange membranes, such as Nafion™ have high water mobility. This property can help water back diffusion to avoid anode flooding while preventing dehydration of the cathode. Designing a gradient microstructure or ionomer content within the CCL could also be useful to improve cell performance and durability.

Recent research published in the journal Cell Reports Physical Science addresses these questions. The presented study was carried out to assess a multi-layer CCL design with the gradient capillary force which has a driving effect on water to solve the water balance problem of anodes in AEMFC. For the purpose of the study, platinum on carbon and platinum-ruthenium on carbon were selected as anode catalysts. Ruthenium increases the hydrogen oxidation reaction activity and possesses beneficial structural properties. Water management and performance of AEMFC would be influenced by the structure of the ACL.

Microstructure analysis of ACLs

ACLs composed of different layers of Pt/C and PtRu/C and a mixed version with a similar thickness of around 9-10 µm were analyzed with energy-dispersive X-ray spectroscopy (EDX).

Pt/C ACL had pores of less than 150 nm while PtRu/C catalysts pores ranged between 300-400 nm. The mixed ACL had a pore size <200 nm.

The researchers concluded that Pt/C and PtRu/C ACL had a stratified and gradient pore size distribution spanning across the anion exchange membrane and the gas diffusion layer. The mixed ACL, however, had a homogenous pore structure throughout the MEA.

Membrane electrode assembly using a polymer electrolyte membrane

Moisture adsorption and desorption behavior of ACLs

To investigate moisture adsorption and desorption, the change of the fuel cell’s moisture content was checked with regards to different levels of relative humidity.

It was observed that the moisture content level increased by up to 50% mass weight along with an increase in relative humidity from 20% to 80%.

With an extended equilibrium time for a relative humidity of 80%, the moisture content of Pt/PtRu and PtRu/Pt ACL began to decrease. This was evidence for the self-adjusting water management behavior.

Desorption at a relative humidity of 60% was done. The water content in ACL showed rapid adsorption and slow-release properties at each relative humidity setting.

Physical adjustment of water behavior was observed in PtRu/Pt ACLs. This was attributed to gradient nano-pores and promoted water transport when water was generated within the ACLs during the electrochemical reactions. It would facilitate fuel cells operation at high current density.

Fuel cell performance of ACLs

To assess the structural effect on water management during operation, fuel cell performance was investigated at different relative humidity and temperature levels.

With increasing relative humidity from 40% to 80%, an increase of the maximum power density was observed as well while the temperature remained constant at 50°C. This was due to higher ionic conductivity at high membrane hydration.

At relative humidity of 100%, a maximum power density of the Pt/PtRu MEA and the mixed MEA decreased, however. The inverted MEA version using PtRu/Pt an increase to 243 mW/cm2 was observed. This suggested that the moisture desorption ability of PtRu/Pt MEA promoted mass transfer during fuel cell operation.

At a temperature of 60°C and 100% relative humidity, the maximum power density of PtRu/Pt reached 252 mW/cm2.

A durability test was conducted for PtRu/Pt MEA. It showed that after continuous operation for more than 16 hours at 100 mA/cm2 the voltage drop was <4%.

Conclusion

It became clear from the tests that the PtRu/Pt anode catalyst layer with its homogenous layer had a better self-adjustment capability for fuel cell water management. The gradient nanopore structure of the catalyst layer made it possible to transport water through the capillary effect. Excess water at the anode could either be transported towards the cathode where it would be used for reaction or towards the gas diffusion layer for its removal prevented flooding. Moreover, this catalyst layer made from PtRu/Pt showed better performance abilities too.

At Frontis Energy we think that this could resolve the issues faced with water management in the fuel cells. Since it is a passive control system that involves modifying the design of the fuel cells internally, intricate external systems could be replaced or complemented. The study certainly helps future fuel cell automation as an interesting new aspect of fuel cell design was discovered that could make them smarter.

Reference: Self-adjusting anode catalyst layer for smart water management in anion exchange membrane fuel cells, Cell Reports Physical Science, Volume 2, Issue 3, 24 March 2021, 100377

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Light-driven process turns greenhouse gases into valuable products

Much research has been done in order to reduce the use of fossil petroleum products as fuels. In that respect syngas (synthetic gas) seems as a great opportunity for sustainable energy developments. Syngas is the mixture composed of hydrogen (H2) and carbon monoxide (CO) as its main components. It represents an important chemical feedstock used widely for industrial processes for generating chemicals and fuels:

Global use of syngas in industrial processes.

Syngas can be produced from methane (CH4) in a reforming reaction with water (H2O), oxygen (O2) or carbon dioxide (CO2). The process called methane dry reforming (MDR) can be combined with carbon dioxide:

CH4 + CO2 → 2 H2 + 2 CO

It is an environmentally friendly path, turning two greenhouse gases into a valuable chemical feedstock.

However, the MDR is process requires chemical catalysts and high temperatures in the range between 700 − 1,000°C. Usually, it suffers from coke deposition and, in consequence, catalyst deactivation.

Some chemists have recently demonstrated that light, and not heat, might be a more effective solution for this energy-hungry reaction.

The photocatalytic solution

A team of researchers at the Rice University in Houston, Texas, together with colleagues from Princeton University and the University of California have developed superior light-stimulated catalysts that can efficiently power MDR reactions without any heat input. This work has been published in the prestigious journal Nature Energy.

They have reported a highly efficient and coke-resistant plasmonic photocatalyst containing precisely one ruthenium (Ru) atom for every 99 copper (Cu) atoms. The isolated single-atom of Ru obtained on Cu antenna nanoparticles provides high catalytic activity for the MDR reaction. On the other side, Cu antennas allow strong light adsorption and under illumination and deliver hot electrons to ruthenium atoms. The researchers suggested that both, hot-carrier generation and single-atom structure are essential for excellent catalytic performance in terms of efficiency and coking resistance.

The optimal Cu-Ru ratio have been investigated in synthesized series of CuxRuy catalysts with varying molar ratios of plasmonic metal (Cu) and catalytic metal (Ru), where x,y are atomic percentage of Cu and Ru. Overall, the Cu19.8Ru0.2 was the most promising composition in terms of selectivity, stability and activity. In comparison to pure Cu nanoparticles, the Cu19.8Ru0.2 mix exhibits increased photocatalytic reaction rates (approx. 5.5 times higher) and improved stability with its performance maintained over 20 h period. Calculations showed that isolated Ru-atoms on Cu lower the activation barrier for the methane dehydrogenation step in comparison to pure Cu without promoting undesired coke formation.

In addition, the research has been supported by different methods (CO-DRIFTS with DFT) in order to unravel and prove single-atom Ru structures on Cu nanoparticles occurring in Cu19.9Ru0.1 and Cu19.8Ru0.2 compositions.

The comparison between thermocatalytic and photocatalytic activity at the same surface for MDR has also been demonstrated. The thermocatalytic reaction rate at 726°C (approx. 60 µmol CH4 / g / s) was less than 25% of photocatalytic reaction rate under white-light illumination with no external heat (approx. 275 µmol CH4 / g / s). This enhancement in the activity is attributed to the hot-carrier generated mechanism which is predominant in the photocatalytic MDR. The role of the hot-carrier is an increase in C−H activation rates on Ru as well as improved H2 desorption.

The scientists also reported the catalyst achieving a turnover frequency of 34 mol H2  / mol Ru / s and photocatalytic stability of 50 h under focused white light illumination (19.2 W / cm2) with no external heat.

As the synthesized photocatalysts is primarily based on Cu which is an abundant element, this approach provides a promising, sustainable catalyst operating at low-temperatures for MDR. This allows cheaper syngas production at higher rates, bringing us closer to a clean burning carbon fuel.

(Photo: Wikipedia)

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Faster photoelectrical hydrogen

Achieving high current densities while maintaining high energy efficiency is one of the biggest challenges in improving photoelectrochemical devices. Higher current densities accelerate the production of hydrogen and other electrochemical fuels.

Now a compact, solar-powered, hydrogen-producing device has been developed that provides the fuel at record speed. In the journal Nature Energy, the researchers around Saurabh Tembhurne describe a concept that allows capturing concentrated solar radiation (up to 474 kW/m²) by thermal integration, mass transport optimization and better electronics between the photoabsorber and the electrocatalyst.

The research group of the Swiss Federal Institute of Technology in Lausanne (EPFL) calculated the maximum increase in theoretical efficiency. Then, they experimentally verified the calculated values ​​using a photoabsorber and an iridium-ruthenium oxide-platinum based electrocatalyst. The electrocatalyst reached a current density greater than 0.88 A/cm². The calculated conversion efficiency of solar energy into hydrogen was more than 15%. The system was stable under various conditions for more than two hours. Next, the researchers want to scale their system.

The produced hydrogen can be used in fuel cells for power generation, which is why the developed system is suitable for energy storage. The hydrogen-powered generation of electricity emits only pure water. However, the clean and fast production of hydrogen is still a challenge. In the photoelectric method, materials similar to those of solar modules were used. The electrolytes were based on water in the new system, although ammonia would also be conceivable. Sunlight reaching these materials triggers a reaction in which water is split into oxygen and hydrogen. So far, however, all photoelectric methods could not be used on an industrial scale.

2 H2O → 2 H2 + O2; ∆G°’ = +237 kJ/mol (H2)

The newly developed system absorbed more than 400 times the amount of solar energy that normally shines on a given area. The researchers used high-power lamps to provide the necessary “solar energy”. Existing solar systems concentrate solar energy to a similar degree with the help of mirrors or lenses. The waste heat is used to accelerate the reaction.

The team predicts that the test equipment, with a footprint of approximately 5 cm, can produce an estimated 47 liters of hydrogen gas in six hours of sunshine. This is the highest rate per area for such solar powered electrochemical systems. At Frontis Energy we hope to be able to test and offer this system soon.

(Photo: Wikipedia)