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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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Rechargeable PEM fuel cell with hydrogen storage polymer

Energy-converting devices such as fuel cells are among the most efficient and clean alternative energy-producing sources. They have the potential to replace fossil-fuel-based power generators. More specifically, proton exchange membrane fuel cells (PEMFCs) are promising energy conversion devices for residential, transportation and portable applications owing to their high power density and efficiency at low operating temperatures (ca. 60–80 °C). For the complete approach, with electrolytic hydrogen renewable sources, PEM fuel cells can become one of the cleanest energy carriers. This is because water is the final product of such energy conversion systems. Currently, Nafion™ membranes are regularly used as hydrogen barriers in these fuel cells.

A Proton exchange membrane

Sufficient hydrogen gas supply is crucial for practical application of the PEMFC systems. Currently, expensive high-pressure tanks (70 MPa) are state-of-the-art for hydrogen storage. Besides cost, there are other drawbacks such as portability and safety. In order to address these issues, alternative hydrogen storage materials have been extensively investigated. For example, metal hydrides and organic hydride materials, can fix and release hydrogen via covalent bonding.

Now, Dr. Junpei Miyake and colleagues of the University of Yamanashi, Japan, have proposed an “all-polymer” rechargeable PEMFC system (RCFC). The work has been published in Nature Communications Chemistry. Their strategy was to apply a hydrogen-storage polymer (HSP) sheet (a solid-state organic hydride) as a hydrogen-storage medium inside the fuel cell. With this approach, the issues like toxicity, flammability and volatility as well as concerns related to other components such as the fuel reservoir, feed pump and vaporizer were solved. The HSP structure is based on fluorenol/fluorenone groups that take over hydrogen-storage functionality.

In order to test the performance of their HSP-based rechargeable fuel cell, the scientists attached the HSP sheet of the membrane electrode to the catalyst layer of the anode. At the same time, the cathode side was operated as in a regular PEMFC. The researchers reported that an iridium catalyst has been applied to the inside of the HSP sheet to improve the hydrogen-releasing and fixing processes.

Fuel cell operation, cycle performance and durability were tested using cycles of 6 periodic steps. At first, hydrogen was infused into HSP sheet for 2 h, followed by nitrogen gas flushing to remove hydrogen from the anode. Then, heating of the cell up to 80°C to initiated hydrogen release from the HSP sheet. Together with oxygen gas supplied to the cathode side the fuel cell produced constant electrical current.

The team demonstrated that their HSP sheet released 20%, 33%, 51%, or 96% of the total fixed hydrogen gas in 20, 30, 60, or 360 min, respectively. The temperature was 80°C in the presence of the iridium catalyst. Also, the iridium catalyst could absorb up to 58 mol% hydrogen, which was considerably lower than that stored in the HSP. The maximum operation time was approximately 10.2 s / mgHSP (ca. 509 s for 50 mg of HSP) at a constant current density of 1 mA / cm2. The RCFCs reached cycleability of least 50 cycles. In addition, the utilization of a gas impermeable sulfonated poly-phenylene membrane (SPP-QP, another type of PEM) turned out to be a good strategy to enhance the opration time of the RCFC.

The advantageous features of the reported RCFC system include better safety, easier handling and lower weight. These are perfect for example in mobile application such as fuel cell vehicles. However, for the improvement of the RCFC performance, hydrogen storage capacity and kinetics (H2-releasing/fixing reactions) as well as catalyst stability need further improvements.

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Reverse electrodialysis using Nafion™ membranes to produce renewable energy

In the order to address the global need for renewable and clean energy sources, salinity-gradient energy harvested by reverse electrodialysis (RED) is attracting significant interest in recent years. In addition, brine solution coming from seawater desalination is currently considered as a waste; however thanks to its high salinity it can be exploited as a valuable resource to feed RED. RED is an engineered adaptation of nature’s osmotic energy production where ions flow pass the cell membrane in order to produce the universal biological currency ATP. This energy is also harvested by the RED technology.

Now, more than ever there is need for sustainable and environmentally friendly technological solutions in order to keep up with ever growing demand for clean water and energy. The traditional linear way “produce and throw away” does no longer serve the society anymore and the new approach of circular economy has take a place, where any waste can be considered as a valuable resource for another process. In this respect, reverse electrodialysis is a promising electromembrane-based technology to generate power from concentrated solutions by harvesting the Gibbs free energy of mixing the solutions with different salinity. In particular, brine solutions produced in desalination plants, which is currently considered as a waste, can be used as concentrated streams in RED stack.

Avci et al. of the University of Calabria, Italy, have recently published their solution for brine disposal using RED-stack. They have realized that in order to maximize generated power, the high permselectivity and ion conductivity of membrane components in RED are essential. Although Nafion™ membranes are among the most prominent commercial cation exchange membrane solutions for electrochemical applications, no study has been done in its utilization toward RED processes. This was the first reported RED stack using Nafion™ membranes.

A typical RED unit is similar to an electrodialysis (ED) unit, which is a commercialized technology. ED uses a feed solution and the electrical energy, while producing concentrate and dilute, separately. On the other side, RED uses concentrated and dilute solutions that are mixed together in a controlled manner in order to produce spontaneously electrical energy. In a RED stack, repeating cells comprised of alternating cation and anion exchange membranes that are selective for anions and cations. The salinity gradient over each ion exchange membrane creates a voltage difference which is the driving force for the process. The ion exchange membranes are one of the most important components of a RED stack.

The performance of Nafion™ membranes (Nafion™ 117 and Nafion™ 115) have been evaluated under a high salinity gradient conditions for the possible application in RED. In order to simulate the natural environments of RED operation, NaCl solution as well as multicomponent NaCl + MgCl2 have been tested.

Gross power density under high salinity gradient and the effect of Mg2+ on the efficiency in energy conversion have been evaluated in single cell RED using Nafion™ 117, Nafion™ 115, CMX and Fuji-CEM-80050 as cation exchange membranes. Two commercial cation exchange membranes – CMX and Fuji-CEM 80050, frequently used for RED applications, have served as benchmark.

The results show that under the condition of 0.5 M / 4.0 M NaCl solutions, the highest Pd,max was achieved using Nafion™ membrane. This result is attributed to their outstanding permselectivity compared to other CEMs. In the presence of Mg2+ ions, Pd,max reduction of 17 and 20% for Nafion™ 115 and Nafion™ 117 were recorded, respectively. Both membranes maintained their low resistance; however a loss in permselectivity was measured under this condition. Even though, it was reported that Nafion™ membranes outperformed other commercial membranes such as CMX and Fuji-CEM-80050 for RED application.

(Photo: Wikipedia)

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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)