Non-platinum Group Metal Catalysts Research Articles

Unconventional hcp/fcc Nickel Heteronanocrystal with Asymmetric Convex Sites Boosts Hydrogen Oxidation.

Developing non-platinum group metal catalysts for the sluggish hydrogen oxidation reaction (HOR) is critical for alkaline fuel cells. To date, Ni-based materials are the most promising candidates but still suffer from insufficient performance. Herein, we report an unconventional hcp/fcc Ni (u-hcp/fcc Ni) heteronanocrystal with multiple epitaxial hcp/fcc heterointerfaces and coherent twin boundaries, generating rugged surfaces with plenty of asymmetric convex sites. Systematic analyses discover that such convex sites enable the adsorption of *H in unusual bridge positions with weakened binding energy, circumventing the over-strong *H adsorption on traditional hollow positions, and simultaneously stabilizing interfacial *H2O. It thus synergistically optimizes the HOR thermodynamic process as well as reduces the kinetic barrier of the rate-determining Volmer step. Consequently, the developed u-hcp/fcc Ni exhibits the top-rank alkaline HOR activity with a mass activity of 40.6 mA mgNi -1 (6.3 times higher than fcc Ni control) together with superior stability and high CO-tolerance. These results provide a paradigm for designing high-performance catalysts by shifting the adsorption state of intermediates through configuring surface sites.

Nanocone-Modified Surface Facilitates Gas Bubble Detachment for High-Rate Alkaline Water Splitting

Alkaline water splitting (AWS) is one of the most attractive technologies for green hydrogen production. In comparison to the acidic proton exchange membrane (PEM), AWS offers greater flexibility in using non-platinum group metal catalysts and could generate higher-purity hydrogen. However, commercial AWS systems are normally operated at a lower current density (~400 mA cm-2) than PEM electrolyzers. To rapidly generate hydrogen at an industrial scale, it is critical to boost the operating current density of AWS to a level of ~1000 mA cm-2. Nevertheless, the significant amount of gas bubbles generated during high-rate AWS could be detrimental to the process. Bubbles can block the catalysts’ active site, hinder electrolyte diffusion, and increase Ohmic resistance, thus deteriorating the reaction rate and restricting the cell voltage efficiency. The detachment of gas bubbles may also damage the catalyst layer. In this study, we demonstrate a general strategy for facilitating bubble detachment by modifying the nickel electrode surface with nickel nanocone nanostructures, which turns the surface into underwater superaerophobic. The simulation and experimental data show that bubbles take a considerably shorter time to detach from the nanocone-modified nickel foil than the unmodified foil. As a result, these bubbles also have a smaller detachment size and less chance for bubble coalescence. The nanocone-modified electrodes, including nickel foil, nickel foam, and 3D-printed nickel lattice, all show substantially reduced overpotentials at 1000 mA cm-2 compared to their pristine counterpart. The electrolyzer assembled with two nanocone-modified nickel lattice electrodes retains >95% of the performance after testing at ~900 mA cm-2 for 100 hours and the surface NC structure is also well preserved, demonstrating its excellent electrochemical and mechanical stability at high current density. Our findings offer an exciting and simple strategy for enhancing the bubble detachment and, thus, the electrode activity for high-rate AWS.

Influence of Nb Doping on the Performance of Oxygen Reduction Reaction on TiO2 in Acidic Environmental

Polymer electrolyte fuel cells (PEFCs) have attracted attention as one of the next-generation power sources that are environmentally friendly. However, the high manufacturer's costs, short service life, and insufficient cell performance are solved to realize widespread commercialization. A non-platinum group metal catalyst is relatively low in cost and highly durable. In particular, the oxides of group 4 and 5 elements were reported to show high oxygen reduction reaction (ORR) activity and durability. Their oxides are considered dominant candidates for the non-platinum group metal catalysts, although they have a fateful flaw that their ORR activity is clearly poorer than traditional platinum-supported carbon catalysts. It was reported that introducing oxygen vacancies and other transition metals into the crystal structure of 4- and 5-group metal oxides effectively enhanced the ORR activity. The active site of 4- and 5-group metal oxides is unclear despite it being well-known that the oxygen vacancies and dopant atoms are related to its ORR activity. It is now indicated that candidates of active site were the oxygen vacancies around the dopant atom and the crystal distortion. However, it is hard to separate their effects because both dopant atoms and oxygen vacancies distort the local crystal structure. On the other hand, identifying the ORR active site leads to obtaining the catalyst design with high ORR activity. We focused on the oxide nanosheet, the simplest structure formed by a six-coordinate octahedral, as the model electrode. The thickness of oxide nanosheets is about 1 nm, and forming a single layer on a substrate is relatively easy. Thus, it was expected that the electron conductivity was guaranteed enough by the tunnel effect. This study employed titanium oxide nanosheets (TiO2ns) as the model catalyst and niobium as a dopant to investigate the effect of dopant atoms on ORR activity. The ratio of Nb/Ti in the prepared model electrode was 0/1, 1/1, and 2/1. In addition, the oxygen vacancies were introduced into all prepared electrodes to study the impact of oxygen vacancies on ORR activity at each Nb/Ti ratio. The thickness of Nb-doped TiO2ns (Nb-TiO2ns) was about 1.2 nm for the 1/1 ratio and slightly thicker for the 0/1 ratio. In contrast, the thickness of the 2/1 ratio of Nb-TiO2ns was about two times thicker than the 1/1 ratio. The thicknesses of Nb-TiO2ns in each ratio were increased by the reduction treatment to introduce the oxygen vacancies. One reason for this thickness increase may be related to the phase transition into the anatase structure. ORR activity was evaluated from the on-set potential of ORR (E ORR) measured at the step scan with 3 minutes step width. E ORR was slightly elevated with Nb doping. The ion radii of Nb and Ti atoms are 3 pm different when it is assumed that both ions are six-coordinated and their valence is Nb5+ and Ti4+. These results showed that the crystal structure distortion probably has a slight but positive effect on ORR activity.After the reduction treatments, the E ORR of the 0/1 ratio of Nb-TiO2ns increased and was the same as that of the 2/1 ratio. The introduction of oxygen vacancies certainly enhanced the ORR activity of the 0/1 ratio of Nb-TiO2ns. The oxygen vacancies are thought to be necessary to enhance the ORR activity, although the reduction condition wasn’t optimized for the Nb-doped samples.From the above results, we consider that the active site is the oxygen vacancy site rather than the crystal distortion site at this time.

(Invited) Traversing Length Scales for Electrochemical Hydrogen Production and Use: Developing a Platform for Material Design and in Situ Material Characterization of Electrocatalysts

Converting chemical processing to carbon-free or to closed carbon cycle processing (e.g. carbon neutral) will require large quantities of sustainable, and likely electrically produced, fuels and processes. Hydrogen is both a chemical fuel alternative, but also an important commodity for chemical manufacturing e.g. ammonia production for fertilizers. Water electrolyzers (WE) and hydrogen fuel cells (H2FCs) for either making or converting hydrogen to electrical energy will require cheaper and more readily available catalyst materials than the current platinum or iridium based state-of-the-art. Alkaline zero-gap devices (anion exchange membrane, AEM WE or H2FCs) offer a promising landscape for earth abundant materials, however they have traditionally underperformed due to large scale component instabilities, e.g. membranes, ionomer, catalysts, etc. With recent advances in alkaline membrane technology it is imperative that commercializable catalysts are developed. Due to a variety of architectural differences, the dissimilar microenvironments and catalyst-electrolyte-gas interfaces between classical fundamental (e.g. three electrode cell measurements with a rotating disc electrode (RDE) and device studies makes it difficult to translate materials to industrial readiness. It is imperative that these trends translate from the small lab-scale (mA), an environment that engenders rapid catalyst evaluation, to industrial-relevant scales (> A) in multi-variable devices.Herein I will discuss work we have done both on developing a robust a platform for measuring performance trends across length scales and microenvironments and expanding tools for characterizing in situ material stability related primarily to WE and H2FCs. For H2FCs, I will discuss our teams’ recent work translating Ag-Pd catalysts from first principles with density function theory (DFT), to classic fundamental studies with RDEs, and finally to H2FCs with a max ~ 10 W/mgPGM on the cathode.1,2 I will discuss the thin film, ionomer-free system that extended across length scales and microenvironments and how we are currently using this platform to develop Ag-based fully non-platinum group metal catalysts. Comparing both H2FCs and WEs, I will go on to discuss one large component of performance, stability, and our ongoing efforts to develop tools for in situ characterization. On-line inductively coupled plasma mass spectrometry (on-line ICP-MS) and electrochemical mass spectrometry (EC-MS) are emerging tools for monitoring material dissolution and product production, respectively, in real time. By coupling these techniques, looking at Co-based metals we have identified degradation mechanisms as well as windows of stable hydrogen evolution and oxygen reduction catalysis in highly acidic electrolytes. A focus on catalyst activity and nature of the active site across microenvironments and in situ material stability provides insights that could enable the next generation materials and stabilization techniques, e.g. in situ, synthetic, morphology, etc. for improving overall performance across technological length scales. References 1) Zamora Zeledón, J. A.; Stevens, M.B.; Gunasooriya, G.T.K.K.; Gallo, A.; Landers, A.T.; Kreider, M.E.; Hahn, C.; Nørskov, J.K.; Jaramillo, T.F. Tuning the Electronic Structure of Ag-Pd Alloys to Enhance Performance for Alkaline Oxygen Reduction, Nature Comm., 2021, 12, 1-9.2) Douglin, J.C.; Zamora Zeledón, J. A; Keider, M. E; Singh, R.K.; Stevens, M.B. ‡ Jaramillo, T. F. ‡ Dekel, D.R. ‡ High performance anion-exchange membrane fuel cells with ionomerless cathodes employing ultra-low loading Ag-Pd alloy electrocatalysts, Nature Energy, 2023, 1-11.

Operando Raman Spectroscopy Reveals Degradation Byproducts from Ionomer Oxidation in Anion Exchange Membrane Water Electrolyzers.

This work showcases the discovery of degradation mechanisms for nonplatinum group metal catalyst (PGM free) based anion exchange membrane water electrolyzers (AEMWE) that utilize hydroxide ion conductive polymer ionomers and membranes in a zero gap configuration. An entirely unique and customized test cell was designed from the ground up for the purposes of obtaining Raman spectra during potentiostatic operation. These results represent some of the first operando Raman spectroscopy explorations into the breakdown products that are generated from high oxidative potential conditions with carbonate electrolytes. We provide a unique design and fabrication method for three-dimensional (3D) printable flow cells that enable spatially resolved Raman spectra collection from the electrode surface into the bulk electrolyte. It is proposed that the generation of breakdown products from the hydroxide-conductive ionomers and membranes originates from a multistep, free radical reaction pathway resulting in chain scission of the poly aryl backbone. This hypothesis is backed by the detection of carboxylic and aromatic functional group Raman signals from small molecules that had dissolved and diffused into the bulk electrolyte.

Stability of Ni-Fe-Layered Double Hydroxide Under Long-Term Operation in AEM Water Electrolysis.

Anion exchange membrane water electrolysis (AEMWE) is an attractive method for green hydrogen production. It allows the use of non-platinum group metal catalysts and can achieve performance comparable to proton exchange membrane water electrolyzers due to recent technological advances. While current systems already show high performances with available materials, research gaps remain in understanding electrode durability and degradation behavior. In this study, the performance and degradation tracking of a Ni3Fe-LDH-based single-cell is implemented and investigated through the correlation of electrochemical data using chemical and physical characterization methods. A performance stability of 1000h, with a degradation rate of 84µVh-1 at 1Acm-2 is achieved, presenting the Ni3Fe-LDH-based cell as a stable and cost-attractive AEMWE system. The results show that the conductivity of the formed Ni-Fe-phase is one key to obtaining high electrolyzer performance and that, despite Fe leaching, change in anion-conducting binder compound, and morphological changes inside the catalyst bulk, the Ni3Fe-LDH-based single-cells demonstrate high performance and durability. The work reveals the importance of longer stability tests and presents a holistic approach of electrochemical tracking and post-mortem analysis that offers a guideline for investigating electrode degradation behavior over extended measurement periods.

Fundamental Insight into Phenyl-Free Polynorbornene Ionomers Enables High Performance Anion Exchange Membrane Fuel Cells

Anion exchange membrane fuel cells (AEMFCs) are seen as a possible successor to proton exchange membrane fuel cell technologies. A major motivation behind AEMFC development is the potential to use less costly materials, such as non-platinum group metal catalysts, thus reducing the stack cost. Anion exchange ionomers (AEIs) are polymers that facilitate ion transport in the catalyst layer play and a critical role in the performance of fuel cells. In fact, cell performance is profoundly affected by fundamental interactions between the catalyst surface and the AEI. Two such interactions are of particular importance: phenyl adsorption on hydrogen oxidation catalysts and electrochemical oxidation of phenyl moieties on oxygen evolution catalysts. Both are detrimental to an alkaline device’s performance and durability.We compared the adsorption energy of phenyl-containing ionomers for implementing phenyl-free ionomers. Density functional theory calculations indicated that the norbornane fragment has minimal adsorption energy on Pt(111) due to the absence of aromatic electrons. A soluble quaternized polynorbornene ionomer was prepared by vinyl addition polymerization. This ionomer enables high performance in fuel cells, (peak power density > 2 W cm-2), proving the advantage of the phenyl-free structure. This study establishes the phenyl adsorption energy-electrode performance relationship, highlighting the importance of material interactions between the catalysts and ionomers.

NiFeOx and NiFeCoOx Catalysts for Anion Exchange Membrane Water Electrolysis

Hydrogen production using an Anion exchange membrane (AEM) electrolyzer allows the use of non-platinum group metal catalysts for oxygen evolution reaction (OER). Nickel and Cobalt-based oxides are active in an alkaline environment for OER and are relatively inexpensive compared to IrO2 catalysts used in Polymer electrolyte membrane (PEM) electrolysis. Mixed metal oxide catalysts NiFeOx and NiFeCoOx catalysts were synthesized by the coprecipitation method using NaOH. X-ray diffraction results showed mainly NiO diffraction peaks for the NiFeOx catalyst due to the low concentration of Fe, for the NiFeCoOx catalyst, NiCo2O4 diffraction peaks were observed. NiFeCoOx catalysts showed a higher Anion exchange membrane water electrolysis (AEMWE) performance compared to NiFeOx and commercial NiO, the highest current density at 2 V was 802 mA cm−2 at 70 °C using 1 M KOH as an electrolyte. The effect of electrolyte concentration was studied by using 0.01 M, 0.1 M and 1 M KOH concentrations in an electrolysis operation. Electrochemical Impedance spectroscopy was performed along with the equivalent circuit fitting to calculate ohmic and activation resistances, the results showed a decrease in ohmic and activation resistances with the increase in electrolyte concentration. Commercially available AEM (Fumasep FAA-3-50 and Sustainion dioxide membrane X-37-50 grade T) were tested at similar conditions and their performance was compared. EIS results showed that X-37-50 offered lower ohmic resistance than the FAA-3-50 membrane.

High-Performance Alkaline Water Electrolysis Using Anion-Exchange Membrane-Electrode Assembly with Catalyst Coated Membrane and Platinum Free Catalysts

Possibility of operation a water electrolysis technology using cost-effective components for current collectors or porous transport layers and non-platinum group metal catalysts with fast response to intermittent loads from renewable energy sources makes membrane alkaline water electrolysis (MAWE) an attractive technology for producing low cost “green” hydrogen. MAWE represents a technology combining the advantages of the already established electrolyser technologies, i.e. proton exchange membrane water electrolysis (PEMWE) and alkaline water electrolysis (AWE). It combines the relatively low-cost materials of AWE with utilisation of the ion-selective polymer electrolyte as separator of the electrode compartments which is commonly used in PEMWE. In the case of the utilisation of the ion-selective polymer electrolyte the electrodes can be pressed directly to its surface thus creating the membrane-electrode assembly (MEA). Reduction of the gap between the electrodes and separator to zero lowers the ohmic drop on the cell. This positively affects the efficiency of the water electrolysis. In MAWE the anion-selective materials are still facing some limitations in terms of poorer chemical stability in high pH environments and at elevated temperatures due to degradation of the anion exchange groups either by Hoffman elimination or nucleophilic attack by OH- ions, or lower ionic conductivity.Primarily two different MEA fabrication methods are recognized in literature [1]: catalyst-coated substrate (CCS) and catalyst-coated membrane (CCM). In the CCS method, catalyst ink, consisting of a binder/ionomer, catalyst particles and solvent, is applied to one side of the porous transport layer whereas in CCM method the catalyst ink is applied to the ion-selective membrane. The CCM approach is established in the PEMWE technology, but in MAWE the above-mentioned problem with stability of anion-selective materials limits the research.From PEMWE we know that CCM approach provides several advantages, including close contact between the catalyst layer and membrane resulting in better utilization of the catalyst compared to CCS. This advantage allows the MEA to maintain the overall performance with lower catalyst loading. Also, the catalyst layer thickness is reduced, which lowers ohmic losses on the MEA. However, the CCM method can also cause poor electrical contact between the MEA and current collector or within the catalyst layer itself.In this work, we first compare the CCS and CCM approaches and study the possibility of catalyst loading reduction. Then, with CCM we further optimize the composition of the catalyst layer, study the effect of method of preparation and compare two different membranes – non-commercial with 2 different thicknesses and commercial oneAs anion-selective material we use chloromethylated block copolymer polystyrene-ethylene-butylene-styrene (PSEBS-CM) functionalized with 1,4-diazabicyclo[2.2.2]octane (DABCO) groups. The solution of PSEBS-CM is used also as a binder of the catalyst layer. This material showed good ionic conductivity, ion exchange capacity and stability at elevated temperature. For comparison reason, Fumapem® FAA-3-50 membrane is tested. NiCo2O4 and NiFe2O4 are used as anode and cathode catalysts, respectively. Air-brush or computer controlled ultrasonic dispersion of catalyst ink are used to prepare MEA. As many operating conditions have been reported to significantly affect MAWE performance, including membrane thickness, electrolyte type, and operating temperature we decided to test the performance of the prepared CCSs and CCMs by mean of the load curves in in the range 1.5 – 2.0 V in different concentrations of KOH (1 – 15 wt.%) at 50 °C. To identify the influence of membrane thickness, we use membrane with thickness of 250 or 60 µm. To have deeper insight on the system, electrochemical impedance spectroscopy is used under the MAWE conditions at voltage of 0, 1.5 and 1.8 V in frequency range 100 kHz – 0.1 Hz with amplitude of 10 mV. Scanning electron microscopy is used to observe the morphology of the layers. The chosen CCMs undergo the stability test measured at 50 °C in 10 wt.% KOH at current density of 250 mA cm-2 for 160 hours.The obtained results show the possibility of: 1) replacing the CCS with CCM, 2) reduction of the catalyst load by 75%, 3) improvement of performance with optimized catalyst/binder ratio and 4) improvement of performance with reduction of the membranes thickness.Acknowledgments:This project has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement No 875118. This Joint Undertaking receives support from the European Union’s Horizon 2020 research and innovation programme, Hydrogen Europe and Hydrogen Europe research. Financial support by the Technology agency of the Czech Republic within the framework of the project No. TK 02030001 is gratefully acknowledged. Huah, L.B., et al., Chinese Journal of Chemical Engineering, 2020.

(Digital Presentation) Oxygen Reduction Reaction on Waste Tire Derived Carbon Material and Synthesized Non-Platinum Group Metal Catalysts in Alkaline Solution

Over the past decades, a considerable amount of work has been put into replacing expensive Pt in oxygen reduction reaction (ORR) catalysts for use in fuel cells, such as with Fe or Co and nitrogen co-doped carbon materials [1-3]. At the same time, a major environmental issue is the lack of adequate methods for repurposing or recycling waste tires. Herein, it was investigated whether waste tires could be used as a source of carbon for synthesizing ORR catalysts.To prepare the carbon support, waste tire granules were pyrolyzed at 1000 °C for three hours in Ar. This material was then used to synthesize four non-platinum-group metal (NPGM) catalysts via dry ball-milling [2] and wet synthesis [2, 3] methods. FeSO4∙7H2O or Co(NO3)2·6H2O salts and 2,2’-bipyridine were used as precursors. Wet synthesis required the precursors to be dissolved in water and mixed with the carbon material, after which the solution was dried. For the dry method, all precursors were mixed and ball-milled for one hour. The products of both synthesis methods were subsequently pyrolyzed at 800 °C for 1.5 h in Ar.All materials were characterized using HR-SEM, SEM-EDX, and nitrogen sorption methods. The materials were heterogeneous and contained additives commonly present in tires as constituents of tire filler materials or residual products of tire production (Si, S, Ca, and Zn). All materials exhibited low porosity and a rather low specific surface area (SBET = 92 to 150 m2 g-1).Electrochemical analyses were carried out in an O2 saturated 0.1M KOH solution using the rotating disk electrode method. Although the carbon support on its own did not show significant activity toward the ORR, the modified Co-N/C and Fe-N/C catalysts demonstrated substantially higher onset potentials (∆Eonset ~150 mV). The best electrochemical activity was achieved by the Co-N/C material synthesized using the wet method. These results show that waste tire-derived carbon materials can be used to successfully synthesize NPGM catalysts suitable for application in fuel cells. Acknowledgements This work was supported by PRG676 "Development of express analysis methods for micro-mesoporous materials for Estonian peat derived carbon supercapacitors" (1.01.2020- 31.12.2024), TK141 "Advanced materials and high-technology devices for sustainable energetics, sensorics and nanoelectronics" (1.01.2016−1.03.2023), and LLTKT20148 "Production of Polymer Electrolyte Membrane Fuel Cell" (10.02.2020−9.07.2021) References [1] F. Roncaroli, E. S. Dal Molin, F. A. Viva, M. M. Bruno, and E. B. Halac, Electrochimica Acta, 174, p. 66 (2015)[2] P. Teppor, R. Jäger, E. Härk, S. Sepp, M. Kook, O. Volobujeva, P. Paiste, Z. Kochovski, I. Tallo, and E. Lust, J. Electrochem. Soc., 167, 054513 (2020)[3] R. Jäger, P. E. Kasatkin, E. Härk, P. Teppor, T. Romann, R. Härmas, I. Tallo, U. Mäeorg, U. Joost, P. Paiste, K. Kirsimäe, and E. Lust, J. Electroanal. Chem., 823, p. 593 (2018)

(Digital) Oxygen Reduction Reaction on Waste Tire Derived Carbon Material and Synthesized Non-Platinum Group Metal Catalysts in Alkaline Solution

A carbon material was pyrolyzed from waste tire granules and used to synthesize non-platinum group metal (NPGM) catalysts via dry ball-milling and wet synthesis methods using Fe or Co salts and 2,2’-bipyridine as precursors. These materials were studied with HR-SEM, SEM-EDX, and N2 sorption analysis methods as well as the rotating disk electrode method in alkaline solution (0.1 M KOH). All materials exhibited a heterogeneous morphology as well as low porosity and low specific surface area (92 < S BET < 150 m2 g-1). Although the carbon support itself did not show significant activity toward the oxygen reduction reaction (E onset = -0.24 V vs Hg/HgO), the modified NPGM materials had a considerably higher onset potential (∆E onset ~150 mV). It was shown that active NPGM catalysts can be synthesized from waste tire derived carbon materials.

Assessing the Oxidative Stability of Anion Exchange Membranes in Oxygen Saturated Aqueous Alkaline Solutions

While anion exchange membrane water electrolyzers show promise as a source of green hydrogen using low-temperatures and non-platinum group metal catalysts, many concerns must be addressed. A primary challenge for the development of high-performance anion exchange membrane water electrolyzers is the fabrication of a stable membrane that will be able to survive long-term stability test while maintaining high anion conductivity, which is a necessity for a durable water electrolyzer. This method will present a standardized protocol that can be used by researchers to assess the quality of their AEM materials and be able to provide insight into how materials may be degrading and how to improve the quality of AEMs. Using Mohr or Volhard titration to measure ion-exchange capacity, EIS to determine the ionic conductivity, NMR and FTIR to understand the extent of membrane degradation over time, and mechanical analyzers to determine the strength of their AEMs, this standardized protocol will guide researchers to determining and improving upon the long-term durability and performance of their AEM materials.

Catalytic activity of Ni3Mo surfaces for hydrogen evolution reaction: A density functional theory approach

Ni3Mo alloys are promising non-platinum group metal catalyst candidates for hydrogen evolution reactions in alkaline solution. The Volmer step for the hydrogen evolution reaction in alkaline medium was examined using density functional theory (DFT). We examined hydrogen adsorption on Ni3Mo surfaces [(001), (020), (100), and (101)]. Ni3Mo(101) showed the fastest dissociation of water in the first step of the HER among the investigated Ni3Mo surfaces. Hydrogen atom chemisorption was a key reaction that determines HER performance; the adsorption free energies revealed that Ni3Mo(101) has a higher electrocatalytic activity than the other surfaces of Ni3Mo. Our work provides insight into the excellent HER catalytic performance of Ni3Mo in alkaline solution and is expected to inform the design of efficient binary non-PGM catalyst for the HER.

Highly Active Fe-N/C Oxygen Electrocatalysts Based on Silicon Carbide Derived Carbon

In this work oxygen reduction catalysts were synthesized by modifying silicon carbide derived carbon with Fe and N through ball milling. The presence of a process control agent and grinding ball diameter in the milling step were varied in order to study their effect on both the physical and electrochemical characteristics of the catalysts prepared. It was established that the ball diameter and process control agent can noticeably impact the surface morphology, particle size distribution, and specific surface area of the materials. Despite these differences, similar high half-wave potential values of 0.85 V vs RHE in alkaline media were established for all the catalysts. However, the electrochemical performance of the materials varied considerably in a PEMFC experiment. More specifically, particle size distribution and specific surface area appear to be important parameters when studying non-platinum group metal catalysts in single cell setups.

The non-precious metal ORR catalysts for the anion exchange membrane fuel cells application: A numerical simulation and experimental study

Alkaline anion exchange membrane fuel cells (AEMFCs) are attracting more and more attention due to the advantages of using non-platinum-group (NPG) metal catalysts and less expensive metal hardware at the high pH conditions. However, the studies of electrodes with the non-precious metal are still less and the performance of the AEMFC operated with the NPG metal catalysts need to improve. In this work, based on AEMFCs operated with the commercial non-precious metal ORR catalysts (Acta 4020), a two dimensional, two-phase flow and steady-state agglomerates model is developed, and the effects of operational conditions of the relative humidity and the structure of the catalyst layer on fuel cell performance are numerically studied and analyzed. The results demonstrate that the relative humidity directly impacts the water distribution and transport in the MEA, and the low relative humidity in the cathode can increase the water back diffusion from the anode to the cathode and improve the fuel cell performance. An increase in the catalyst loading has been found to have a positive effects on the fuel cell performance, but the improvement is limited when the catalyst loading increases to a certain value. In addition, the increase in the mass ratio of catalyst to ionomer results in a decrease in the thickness of the ionomer film, but the excessive mass ratio of the catalyst to the ionomer also leads to a decrease in ionic conductivity, thereby deteriorating the performance of fuel cell. At last, operating with the optimized conditions from the model, the AEMFC realized a good fuel cell performance, and the peak power density reached 566 mW cm−2 and 326 mW cm−2 for H2/O2 and H2/Air (CO2-free) at 60 °C, respectively, and the results are higher than those reported in references.

Active site manipulation in MoS2 cluster electrocatalysts by transition metal doping.

The development of non-platinum group metal catalysts for the hydrogen evolution reaction (HER) in water electrolyser devices is essential for their widespread and sustainable deployment. In recent years, molybdenum disulfide (MoS2) catalysts have received significant attention as they not only exhibit good electrocatalytic HER activity but also, crucially, acid-stability. However, further performance enhancement is required for these materials to be competitive with Pt and to that end transition metal doping of MoS2 has been explored as a route to further increasing its catalytic activity. In this work, cluster beam deposition was employed to produce controlled cobalt-doped MoS2 clusters (MoS2-Co). We demonstrate that, in contrast to previous observations of performance enhancement in MoS2 resulting from nickel doping (MoS2-Ni), the introduction of Co has a detrimental effect on HER activity. The contrasting behaviours of Ni and Co doping are rationalized by density functional theory (DFT) calculations, which suggest that HER-active surface vacancies are deactivated by combination with Co dopant atoms, whilst their activity is retained, or even partially enhanced, by combination with Ni dopant atoms. Furthermore, the adatom dopant-vacancy combination kinetics appear to be more than three orders of magnitude faster in MoS2-Co than for MoS2-Ni. These findings highlight a fundamental difference in the influence of transition metal dopants on the HER performance of MoS2 electrocatalysts and stress the importance of considering surface atomic defects when predicting their behaviour.

Quaternized cellulose and graphene oxide crosslinked polyphenylene oxide based anion exchange membrane

Anion exchange membrane fuel cells (AEMFCs) have captivated vast interest due to non-platinum group metal catalysts and fuel flexibility. One of the major shortcomings of AEMFCs, however, is the lack of a stable and high anion conducting membrane. This study introduces a new strategy for fabrication of high conducting anion exchange membrane (AEM) using a hybrid nanocomposite of graphene oxide (GO), cellulose, and poly(phenylene oxide) (PPO), which are functionalized with 1,4-diazabicyclo[2.2.2]octane. The compositional ratio of GO/cellulose/PPO was optimized with respect to ionic conductivity, water uptake, swelling ratio, and mechanical properties. The membrane at GO/cellulose/PPO weight ratio of 1/1/100 displayed an impressive hydroxyl conductivity of ∼114 mS/cm at 25 °C and ∼215 mS/cm at 80 °C, which is considerably higher than the highest value reported. Further, the hybrid composite membranes were mechanically stable even when operating at high temperature (80 °C). The result indicates that the introduction of quaternized GO and cellulose into a polymer matrix is a promising approach for designing high performance AEMs.

Metal organic framework-modified nitrogen-doped graphene oxygen reduction reaction catalyst synthesized by nanoscale high-energy wet ball-milling structural and electrochemical characterization

Abstract

In Situ X-Ray Absorption Spectroscopy Investigation of Silver-Copper Nanoparticles for the Oxygen Reduction Reaction

The electrochemical conversion between oxygen and water is a key process gating the wider deployment of a variety of renewable energy technologies. In particular, the slow kinetics and limited selection of suitable catalysts for the oxygen reduction reaction (ORR) are main impediments to the expansion of critical clean-energy technologies such as hydrogen fuel-cell vehicles or grid-scale energy storage. While Pt-based catalysts are the conventional choice for acidic electrolytes, favorable oxygen reduction kinetics and stability considerations that occur in alkaline electrolytes provide possibility for using non-platinum group metal catalysts, including silver. In particular, bimetallic or alloyed systems of silver and copper have been predicted by theory to be active for the ORR [1]. While these two metals have a large miscibility gap in the bulk, it has been shown that small nanoparticles can support intermixing of the silver and copper in a stable or metastable phase [2]. In addition, we have shown that co-sputtering silver and copper as both thin films and nanoparticles results in ORR catalytic activity that surpasses that of either metal on its own. In this work we investigate these bimetallic catalysts using in situ x-ray absorption spectroscopy (XAS) in order to help understand the activity enhancements seen when combining copper and silver for the ORR.[1] K. Shin, D.H. Kim, and H.M. Lee. ChemSusChem (2013), 6: 1044-1049.[2] P. Grammatikopoulos, J. Kioseoglou, A. Galea, J. Vernieres, M. Benelmekki, R.E. Diaz, and M. Sowwan. Nanoscale (2016), 8: 9780-9790.

Toward Proper Understanding of the Nature of the Active Sites in Non-PGM Catalysts and the Redox Mechanism for ORR

Developing efficient catalysts based on inexpensive and abundant materials for the sluggish oxygen reduction reaction (ORR) constitutes one of the grand challenges in the fabrication of commercially viable fuel cell devices and metal–air batteries for future energy applications. Despite recent advances in developing active and durable non-platinum group metal catalysts as promising alternatives to Pt, a less empirical synthesis approach is still missing due to the lack of proper understanding of the mechanistic origin of the ORR and the underlying surface properties under working conditions that govern catalytic activity. Here, by employing in situ x-ray absorption spectroscopy (XAS) together with complementary characterization techniques such as Mössbauer spectroscopy etc., we propose that the distorted Fe2+-N4-Cx moiety, in which the central Fe2+ ion is located out of the N4-plane and at low spin state without the fifth axial ligand, is the active site responsible for the high ORR activity of pyrolyzed Fe-based catalysts. The local structure of this active site under ex situ conditions is drastically different from that obtained under working conditions, and is thus disconnected from the ORR activity. In situ XAS studies further reveal that the ORR is mediated by the reversible Fe2+/3+ redox transitions with potential-dependent population of active sites.1 Accordingly, the high ORR activity of the distorted Fe2+-N4-Cx moiety is attributable to the out-of-plane Fe displacement with low spin state, which leads to high Fe2+/3+ redox transition potential that minimizes the site-blocking effect at the targeted potential, as well as appropriate number of eg-electrons that favors O2 adsorption. Figure 1