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Multifunctional iridium-based catalyst for water electrolysis and fuel cells

Most of the world’s energy needs are currently served by fossil fuels. The International Energy Agency (IEA) annual projection indicates that the global energy demand will increase twice by 2040, mostly in emerging markets and developing economies.

To meet increasing global energy demands and to replace depleting fossil fuels, policy makers believe that alternative clean and renewable energy sources are the best solution. Such renewable energy sources can be electricity for solar, wind or geothermal energy as well as hydroelectric power. The latter, however, has reached a certain degree of saturation in fully industrialized countries.

While solar and wind energy are available in most places of the world at more or less reasonable cost, their biggest disadvantage is that they are intermittent, difficult to store and transport, and difficult to tank in cars, planes and ships. Converting solar and wind energy in hydrogen gas could be an elegant way out of this dilemma as the fuel’s resource can be abundant water. Diversifying the energy mix by adding hydrogen at acceptable cost may prove more efficient with a lower environmental footprint as compared to other fuels. Hence, the interest for  water electrolysis and fuel cells  is constantly growing.

Most of today’s hydrogen is produced through steam reforming of natural gas. However, it can also be made from water electrolysis. Electrolysis is two-electrode reactions: the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode.

Fuel cells reverse the reaction and harvest electricity produced by fusing the hydrogen and oxygen atoms back together to obtain water. While there are different types of fuel cells, those commonly used with hydrogen as fuel are polymer electrolyte membrane fuel cells, or PEMFC. The PEM acronym is also often used for proton exchange membranes, which can be made of polymers, for example Nafion™.  In PEMFC, energy is liberated through the hydrogen oxidation reaction (HOR) at the anode and oxygen reduction reaction (ORR) at the cathode. To become economically feasible, there are still technical challenges of water electrolyzers and fuel cells to overcome. Some technical problems result in serious system degradation.

Water is pumped into a fuel cell where two electrodes split it into hydrogen (H2) and oxygen (O2)

A study published in Nature Communications by researcher of Technical University Berlin and the Korea Institute of Science and Technology, suggests using a novel iridium electrocatalyst with multifunctional properties and remarkable reversibility. While iridium also is precious and one of the platinum group metals, the novel Ir-catalyst was designed for the processes where electrochemical reactions change rapidly, such as the voltage reversal of water electrolysis and PEMFC systems. This would integrate the two energy conversion systems in one and therefore be a great economical benefit over existing solutions.

Challenges

Unexpectedly changing operating conditions such as a sudden shut-down of water electrolysis result in increased hydrogen electrode potentials which lead to degradation hydrogen producing electrodes.

In fuel cells, fuel starvation can occur at the anode, leading to voltage reversal. Ultimately, this causes degradation of fuel cell components such as the catalyst support, gas diffusion layer and flow field plates. It has been proposed to introduce a water oxidation catalyst to the anode of the PEMFcs in order to promote OER since it is the reaction that competes with the carbon corrosion reaction.

Design of a unique iridium-based multifunctional catalyst

For the study, a crystalline multifunctional iridium nanocatalyst has been designed considering the mentioned challenges in water electrolysis and fuel cell operation.

The reason why an iridium-based material has been selected is its remarkable OER activity as well as good HER and HOR catalytic activity. It is a superior material for anodes and cathodes in electrolyzers and for anodes of PEMFC. For comparisons, the researchers synthesized two catalysts  using the modified impregnation method: carbon-supported IrNi alloy nanoparticles with high crystallinity (IrNi/C-HT) and with low crystallinity (IrNi/C-LT).

The findings indicated that the surface of IrNi/C-HT had reversibly converted between a metallic character and an oxidic IrNiOx character. Under OER operation that is, anodic water oxidation, the crystalline nanoparticles form an atomically-thin IrNiOx layer. This oxide layer reversibly transforms into metallic iridium when returning towards more cathodic potentials. The reversal allows the catalyst to return to its high HER and HOR activity.

The experiments also revealed that the performance of IrNi/C-LT sharply decrease after carrying out the OER. The catalyst degradation was due to the irreversible destruction of the amorphous IrNiOx surface.

In situ/operando X-ray absorption near edge structure (XANES) and depth-resolving X-ray photoelectron spectroscopy (XPS) profiles, suggested that the thin layers of IrNiOx possess an increase in the number of d-band holes during OER, due to which catalyst IrNi/C-HT exhibited excellent OER activity. As expected, under HER conditions, the thin IrNiOx layer was reversibly converted to metallic surface. The mechanistic study of the reversible catalytic activity of the IrNiOx layer has been additionally analyzed by electrochemical flow-cell using inductively coupled plasma-mass spectrometry (ICP-MS). The results demonstrate that the reversible IrNiOx layers come from a dissolution and re-deposition mechanism.

In addition, the performance and catalytic reversibility of synthetized electrocatalysts were used to perform HOR and OER in a real electrochemical device and tested under fuel starvation of the PEMFC. Using voltage reversal, the fuel cell was converted into an electrolyzer.

Fuel starvation experiments were conducted in a single PEM fuel cell built using IrNi/C-HT and IrNi/C-LT as the catalytically active components in the anode catalyst layer. The initial fuel cell performance of IrNi/C-LT and -HT was lower than that of the commercial Pt/C catalyst due to the lower HOR and metal composition.

Further results demonstrate that IrNi/C-HT catalyst retained its bifunctional catalytic activity, reversibing between HER and OER in a real device. This approach promoted the reversibility of nanocatalysts, which enable a variety of electrochemical reactions and can be used as catalysts to resist the reverse voltage in fuel cells and water electrolysis systems.

At Frontis Energy, we are looking forward to adding the novel iridium catalyst to our Fuel Cell Shop as soon as it becomes available.

Photo: Iridium / Wikipedia

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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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Highly durable platinum-palladium-based alloy electrocatalyst for PEM fuel cells

To decrease the consumption of fossil fuel-derived energy for transportation, proton exchange membrane fuel cells (PEMFCs) are one of the most promising clean power sources. Their performance, however, strongly depends on the efficiency and durability of the electrocatalyst used for the hydrogen and oxygen reactions occurring at the electrodes. Noble metals such as platinum and gold are still considered as the most efficient catalysts. At the same time, their high cost and scarcity are major road blocks for scale commercialization of these energy devices.

Various solutions of catalyst design are intensively investigated in order to make this technology economically successful. Searching for high catalyst activity and durability for fuel cells is in focus of current research and development. To date, state-of-the-art electrocatalysts are based on carbon materials with varying platinum loadings.

Ultra-high active platinum group metal (PGM) alloy catalyst

Although, recent research reported ultra-high activity of some metal alloy catalysts, problems still remain. Some of these issues are related to utilization of high atomic percentages of PGM (sometimes up to 75% Pt), poor durability and performance under industrial conditions. In search for new solutions, researchers of the State University of New York at Binghamton, USA, and their collaborators reported a new design in journal Nature Communication: a highly-durable alloy catalyst was obtained by alloying platinum and palladium at less than 50% with 3d-transition metals (Cu, Ni or Co) in ternary compositions.

They addressed the problem of severe de-alloying of conventional alloy catalysts under the operating conditions, resulting in declining performance. For the first time, dynamic re-alloying as a way to self-healing catalysts under realistic operating conditions has been demonstrated to improve fuel cell durability.

Alloy combination and composition

The wet-chemical method was used for synthesis of Pt20PdnCu80−n alloy nanoparticles with the desired platinum, palladium and copper percentages. The selected set of ternary alloy nanoparticles with tunable alloy combinations and compositions, contained a total content of platinum and palladium of less than 50%, keeping it lower than current PGM-based alloy catalysts. The incorporation of palladium into platinum nanomaterials enabled a lower degree of de-alloying and therefore better stability. Additionally, palladium is a good metal partner to platinum due to their catalytic synergy and their resistance to acid corrosion.

To reduce the need for platinum and palladium core catalysts, a third, non-noble transition metal played a central role in the catalytic synergy of alloying formation. Non-noble metals such as copper, cobalt, nickel or similar were used. The platinum-palladium alloy with base metals allowed the researchers to fine tune the thermodynamic stability of the catalysts.

Morphology and phase structure

The thermochemical treatment of carbon-supported nanoparticles was crucial for the structural optimization. The metal atoms in the catalytic nanoparticles were loosely packed with an expanded lattice constant. The oxidative and reductive treatments of the platinum-palladium alloy (PGM <50%) allowed a thermodynamically stable state in terms of alloying, re-alloying and lattice strains. The re-alloying process not only homogenized the inhomogeneous composition by inter-diffusion upon calcination of nanoparticles, but also provided an effective pathway for self-healing following de-alloying.

Single face-centered cubic type structures were observed in Pt20PdnCu80–n nanoparticles (n = 20, 40, 60, 80) nanoalloys. Copper-doping of platinum-palladium alloys reduced the lattice constant effectively, as shown by high energy X-ray diffraction. Maximized compressive strain and maximized activity of the Pt20Pd20Cu60 catalyst confirmed strong correlation between the lattice constants and the oxygen reduction activity.

The researchers demonstrated that the thermodynamically-stable Pt20Pd20Cu60/C catalyst exhibited not only the largest compressive strain after 20,000 cycles, but also high activity and high durability. The discovery that the alloy catalyst remains alloyed under fuel cell operating condition is in sharp contrast to the fully de-alloyed or phase-segregated platinum skin or platinum shell catalysts reported in almost all current literature.

The significance in understanding of the thermodynamic stability of the catalyst system is a potential paradigm shift of design, preparation, and processing of alloy electrocatalysts.

(Photo: Pixabay)

 

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Making zinc-air batteries rechargeable using developed cobalt(II) oxide as a catalyst

Zinc-air batteries are a promising alternative to expensive lithium-ion batteries. Compared with lithium-ion technology, zinc-air batteries have a greater energy density, very low production cost, and superior safety. However, their fundamental inability to recharge has lowered their wide-scale adoption.

Zinc-air batteries use charged zinc particles to store large amounts of electricity at a time. When electricity is required, the charged zinc is combined with oxygen from the air (and water), releasing the stored electricity and producing zincate. This process is known as oxygen reduction reaction (ORR).

Theoretically, this zincate can again be broken down into oxygen and zinc ions by passing electricity through it. This process, in turn, is called oxygen evolution reaction (OER). Using these reactions, zinc-air batteries can be made rechargeable, competing with lithium-ion batteries.

The major challenge of the recharging process is the sluggish kinetics of the reactions which lead to poor cycle life. These batteries require a catalyst that could potentially enhance the ORR and the OER reactions, making their kinetics fast. Hence, the development of highly efficient catalysts is of paramount importance for rechargeable zinc-air batteries.

Previous studies have suggested transition-metal oxides as great bifunctional ORR / OER catalysts because of their ability to provide sites for the reversible adsorption of oxygen. But the methods involved in creating well-defined defects for reversible adsorption of oxygen in such oxides are challenging.

To investigate the use of cobalt(II) oxide nanosheets deposited on stainless steel or carbon cloth as a bifunctional catalyst, a group of researchers from different universities of China and Canada collaborated and conducted several experiments. Their research findings were published in the journal Nano Energy .

Research approach

Preparation of catalyst

Different nano-structures were prepared using simple heat treatment and electrodeposition to test them as bifunctional electrocatalysts. The type of nano-structures prepared were:

      • Cobalt hydroxide  nanosheets on steel and carbon cloth
      • Layered cobalt (II) oxide nanosheet on steel and carbon cloth
      • Cobalt (II) oxide on steel
      • Layered cobalt tetroxide nanosheet on steel

Material Characterization

To understand the characteristics of the prepared samples, various analyticaland tests were carried out:

Charging and discharging tests

Later discharge and charge cycling tests of single cells were operated by the battery testing system.

Results

The simple heat treatment strategy created oxygen vacancy sites. According to the authors, layered cobalt-oxide nano-sheets exhibited excellent bifunctional ORR / OER performance. Investigations suggested abundant oxygen vacancies and cobalt sites be the reason for enhanced ORR / OER performance. Later, the developed layered cobalt-oxide nanosheets on steel were used as an electrode in a rechargeable zinc-air flow battery and a record-breaking cycle life of over 1,000 hours with nearly unchanged voltage was observed. Galvanostatic discharging-charging cycles also demonstrated long life and high energy efficiency.

This research carried out provides a new method to design highly efficient bifunctional ORR / OER catalysts that could be used to enhance the cycle life of rechargeable zinc-air flow batteries. At Frontis Energy we are looking forward to industrial applications.

(Photo: Engineersforum)

Reference: https://doi.org/10.1016/j.nanoen.2020.105409 Wu et al., Cobalt (II) oxide nanosheets with rich oxygen vacancies as highly efficient bifunctional catalysts for ultra-stable rechargeable Zn-air flow battery, 2021