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At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Rational design of platinum-group-metal-free electrocatalysts for oxygen reduction reaction

Platinum-group-metal (PGM)-free materials have been promised as potential replacement for Pt as the cathodic catalyst in proton exchange membrane fuel cells. Critical design criteria of the PGM-free catalyst reside on the high active site density to compensate its generally lower turn-over frequency and improved mass-charge transfers during the electrocatalysis. Herein, this short review summarizes the research activities in recent years from our team at Argonne National Laboratory in preparing highly active oxygen reduction reaction (ORR) catalysts using rationally designed porous organic precursors, as reported in the First Telluride Science Research Center (TSRC) Workshop on PGM-free Electrocatalysis in 2019. More recent studies by others are also discussed.

25 ENERGY STORAGE↗
DOE OSTI2023

Spectroscopic Characterization of Highly Active Fe–N–C Oxygen Reduction Catalysts and Discovery of Strong Interaction with Nafion Ionomer

Scaling up clean-energy applications necessitates the development of platinum group metal (PGM)-free fuel cell electrocatalysts with high activity, stability, and low cost. Here, X-ray absorption (XAS) at the Fe K-edge and Fe K β X-ray emission (XES) spectroscopies were used to study the electronic structure of Fe centers in highly active Fe–N–C oxygen reduction catalysts with significant commercial potential. X-ray absorption near-edge structure (XANES) analysis has shown that the majority (>95%) of Fe centers are in the Fe 3+ oxidation state, while extended X-ray absorption fine structure (EXAFS) detected a mixture of single site Fe–N 4 centers (>95%) and centers with short (~2.5 Å) Fe–Fe interactions of Fe metal and/or Fe-carbide nanoparticles (<5%) featuring the Fe o oxidation state. Surprisingly, addition of Nafion, the most widely used ionomer, resulted in pronounced changes in the XAS spectra, consistent with a strong catalyst–ionomer interaction where long Fe–Fe interactions at ~3.1 Å were shown to be a feature of Fe 3+ ions bound with the Nafion. We conclude that exposure to Nafion during the device formulation has a different effect from the aggressive acid leaching typically used in the preparation of Fe–N–C catalysts. Furthermore, it was hypothesized that the polymer interacts with single sites’ Fe 3+ centers, as well as with graphene layers protecting the Fe o nanoparticles, and extracts some Fe ions into the Nafion matrix.

25 ENERGY STORAGE↗
DOE OSTI2022

Nitrogen-Doped PtNi Catalysts on Polybenzimidazole-Functionalized Carbon Support for the Oxygen Reduction Reaction in Polymer Electrolyte Membrane Fuel Cells

PtM (M = 3d transition metals) alloys are known as the promising oxygen reduction reaction catalysts and have been considered as the replacement of pure Pt catalysts for the commercialization of proton exchange membrane fuel cells. Although great progress has been made in the past three decades, the performance and durability of PtM catalysts still face stringent challenges from practical applications. Functionalization of a catalyst carbon support with nitrogen-contained groups can add charges onto its surface, which can be utilized to build a more complete ionomer/catalyst interface, to reduce the catalyst particle size, and to improve particle size distribution. Nitriding of PtNi catalysts can effectively improve the catalyst activity and stability by the modification of lattice strain. Hereby, we propose a synergistic approach of combining polybenzimidazole-grafted Vulcan XC72 carbon as the catalyst carbon support and the nitriding of PtNi to develop PtNiN/XC72-polybenzimidazole catalysts. Such PtNiN/XC72-PBI catalysts exhibit the excellent performance of fuel cell membrane electrode assembly (i.e., mass activity, 440 mA mgPt –1 ; electrochemical surface area, 51 m 2 gPt –1 ; and rated power density, 836 mW cm –2 ) as well as promising catalyst stability. The developed PtNiN/XC72-PBI meets the US DOE 2020 targets of mass activity for the fuel cell catalysts. This work provides a novel approach and a promising pathway on the development of the catalyst using such a synergistic approach-modification of the catalyst structure by nitrogen doping and functionalization of carbon support by polybenzimidazole for both high performance and high durability.

25 ENERGY STORAGE↗
DOE OSTI2020

Oxygen Reduction Reaction Catalyzed by Carbon-Supported Platinum Few-Atom Clusters: Significant Enhancement by Doping of Atomic Cobalt

Oxygen reduction reaction (ORR) plays an important role in dictating the performance of various electrochemical energy technologies. As platinum nanoparticles have served as the catalysts of choice towards ORR, minimizing the cost of the catalysts by diminishing the platinum nanoparticle size has become a critical route to advancing the technological development. Herein, first-principle calculations show that carbon-supported Pt 9 clusters represent the threshold domain size, and the ORR activity can be significantly improved by doping of adjacent cobalt atoms. This is confirmed experimentally, where platinum and cobalt are dispersed in nitrogen-doped carbon nanowires in varied forms, single atoms, few-atom clusters, and nanoparticles, depending on the initial feeds. The sample consisting primarily of Pt 2~7 clusters doped with atomic Co species exhibits the best mass activity among the series, with a current density of 4.16 A mg Pt − 1 at +0.85 V vs. RHE that is almost 50 times higher than that of commercial Pt/C.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Improving Oxygen Reduction Performance of Surface-Layer-Controlled Pt–Ni Nano-Octahedra via Gaseous Etching

This study demonstrates an atomic composition manipulation on Pt–Ni nano-octahedra to enhance their electrocatalytic performance. By selectively extracting Ni atoms from the {111} facets of the Pt–Ni nano-octahedra using gaseous carbon monoxide at an elevated temperature, a Pt-rich shell is formed, resulting in an ~2 atomic layer Pt-skin. The surface-engineered octahedral nanocatalyst exhibits a significant enhancement in both mass activity (~1.8-fold) and specific activity (~2.2-fold) toward the oxygen reduction reaction compared with its unmodified counterpart. After 20,000 potential cycles of durability tests, the surface-etched Pt–Ni nano-octahedral sample shows a mass activity of 1.50 A/mg Pt , exceeding the initial mass activity of the unetched counterpart (1.40 A/mg Pt ) and outperforming the benchmark Pt/C (0.18 A/mg Pt ) by a factor of 8. DFT calculations predict this improvement with the Pt surface layers and support these experimental observations. Therefore, this surface-engineering protocol provides a promising strategy for developing novel electrocatalysts with improved catalytic features.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Oxygen reduction reaction catalysts prepared by platinizing thermally activated zeolitic imidazolate frameworks

In this study, highly efficient and low-cost oxygen reduction reaction (ORR) catalyst is essential to improve the cost competitiveness of the proton-exchange membrane fuel cells (PEMFCs). One potential approach of cost reduction is to apply low loading Pt over a catalytically active support made of platinum metal group free (PGM-free) material to compensate the overall ORR activity through synergistic catalysis between Pt and PGM-free support. In this report, we investigated a series of catalysts prepared by adding low-loading Pt over ORR active support prepared through thermally activated Co/Zn methyl-imidazolate framework (Co/Zn-ZIF) at various Co/Zn ratios. Catalytic activity measurement and structural characterization were performed in an attempt to gain better understanding of certain structural factors that could influence the catalytic performance.

25 ENERGY STORAGE↗
DOE OSTI2023

Non-bonding interaction of dual atom catalysts for enhanced oxygen reduction reaction

Here, we demonstrate the design of graphene-supported dual atom catalysts (DACs) for the four-electron oxygen reduction reaction (ORR), by utilizing the non-bonding interaction of counterpart metals (M) that synergistically tune the electronic properties and catalytic activity of the Fe active site in FeMN 6 -DAC and FeMN 8 -DAC systems, where M stands for Fe, Co, Ni, Cu, and Zn. More specifically, for Fe-M distances below 15Å, the non-bonding interaction is significant, making the system act as the DAC. We predicted that FeNiN 6 -DAC and FeNiN 8 -DAC exhibit a low ORR overpotential (η ORR ) of 0.28 V and 0.47 V, respectively, which are at the summits of volcano plots. This low η ORR originates from the high Bader charge transfer coupled with high spin density at the Fe site in both the FeNiN 6 -DAC and FeNiN 8 -DAC systems, which weakens the adsorption of OH* intermediate while enhancing its desorption to H 2 O. Guided by these density functional theory (DFT) computational results, we synthesized FeCoN 8 -DAC and FeNiN 8 -DAC along with N-doped graphene and confirmed their structures with scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), and electron spin resonance (ESR). We verify experimentally the catalytic activities and find that FeNiN 8 -DAC has the low experimental overpotential of 0.39 V with a Tafel slope of 47 mVdec -1 . Based on these results, we propose a DFT-guided strategy to tune the charge transfer and spin population of the active site toward designing DACs for electrochemical ORR.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Nanosized Zirconium Porphyrinic Metal–Organic Frameworks that Catalyze the Oxygen Reduction Reaction in Acid

Porphyrinic metal–organic frameworks (PMOFs) are very appealing electrocatalytic materials, in part, due to their highly porous backbone, well-defined and dispersed metal active sites, and their long-range order. Herein a series of (Co)PCN222 (PCN: porous coordination network) (nano)particles with different sizes are successfully prepared by coordination modulation synthesis. These particles exhibit stability in 0.1 M HClO 4 electrolyte with no obvious particle size or compositional changes observed after being soaked for 3 days in the electrolyte or during electrocatalysis. This long-term stability enables the in-depth investigation into the electrocatalytic oxygen reduction, and it is further demonstrated that the (Co)PCN222 particle size correlates with its catalytic activity. Of the three particle sizes evaluated (characteristic length scales of 200, 500, and 1000 nm), the smallest size demonstrates the highest mass activity while the largest size has the highest surface area normalized activity. Together these results highlight the importance of determining the structural stability of framework catalysts and provide insights into the important roles of particle size, opening new avenues to investigate and improve the electrocatalytic performance of this class of framework material.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Single-phase Ru 1- x-y Mn x Co y O 2 nanoparticles as highly effective oxygen reduction electrocatalysts in alkaline media with enhanced stability and fuel-tolerance

Here, a series of single-phase binary and ternary ruthenium and transition metal oxide nanoparticles supported on Vulcan - Ru 1-x MnxO2/C, Ru1-xCoxO 2 /C, Ru 1-x-y MnxCo y O 2 /C, Ru 1-x Fe x O 2 /C, Ru 1-x Ni x O 2 /C and Ru 1-x V x O 2 /C, as well as RuO 2 /C, MnO 2 /C, Co 3 O 4 /C and Mn 3-x Co x O 4 /C - were synthesized via a facile method and studied for their oxygen reduction reaction (ORR) activity in alkaline media. Single-phase Ru 1-x Mn x O 2 /C, Ru 1-x-y Mn x Co y O 2 /C and Ru 1-x Co x O 2 /C catalysts significantly boosted ORR kinetics in alkaline media. Moreover, they were H 2 and methanol tolerant, and exhibited long term stability due to their single-phase structure. Ru 0.85 Mn 0.15 O 2 /C stood out as the most active catalyst among them. A volcano relationship of the ORR activity of these binary and ternary metal oxide catalysts vs. the O adsorption energy was found, with the maximum activity observed for the Ru 1-x Mn x O 2 /C, Ru 1-x-y Mn x Co y O 2 /C and Ru 1-x Co x O 2 /C catalysts due to their optimal O binding energy. These materials have potential applications as highly active ORR catalysts in alkaline fuel cells and metal-air batteries.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Identification of Active Sites of Pure and Nitrogen-Doped Carbon Materials for Oxygen Reduction Reaction Using Constant-Potential Calculations

Nitrogen-doped carbon materials are promising metal-free catalysts for the electrochemical oxygen reduction reaction (ORR). A better theoretical understanding on the nature of the active site(s) would help further optimization of their ORR activity. Although quantum mechanical calculations have been widely employed to elucidate the active sites over various catalysts, these calculations are typically done assuming constantcharge conditions rather than the experimentally relevant constant-potential conditions. In this study, we employ the double-reference method to simulate the energetics of the ORR over pure and N-doped carbon materials under constant-potential conditions. We demonstrate that constant-potential calculations enable more accurate theoretical predictions, comparing well with existing experiments. Our key findings are (1) the zigzag edge of pure graphite is highly active for ORR, (2) the pyridinic N-doped armchair edge is highly active for ORR in alkaline media but not in acid, and (3) graphitic N can donate electrons to pyridinic N to enhance the ORR activity. Furthermore, these fundamental insights provide guidelines for the design of better carbonbased ORR catalysts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Shallow Rate-Redox Potential Scaling in Aqueous Molecular Oxygen Reduction Electrocatalysis Across a Family of Iron Macrocycles

Rate-overpotential scaling relationships have been employed widely to understand trends in oxygen reduction reaction (ORR) electrocatalysis by dissolved metal macrocycles in organic electrolytes. Similar scaling relationships remain unknown for surface-adsorbed ORR electrocatalysts in the acidic aqueous environments germane to proton-exchange membrane (PEM) fuel cells. Herein, we examine ORR catalysis in aqueous perchloric acid media for a structurally diverse array of iron macrocycle complexes adsorbed on Vulcan carbon black. The macrocycles encompass Fe– N 4 , Fe–N 2 N' 2 and Fe–N x C 4-x motifs bearing pyrrolic, pyridinic, and N-heterocyclic carbene (NHC) moieties in the primary ligation sphere, giving rise to a 670 mV range in Fe(III/II) redox potentials, E Fe(III/II) . Experimental Tafel data in the micropolarization regime were extrapolated to the E Fe(III/II) to furnish estimated per-site-normalized current density (j per-site ) values that span ~4.6 orders of magnitude across the family of compounds. Despite the structural diversity of this family of compounds, extrapolated j per-site values correlate with the Fe(III/II) redox potentials in a roughly log-linear fashion with a shallow scaling factor of approximately 145 mV/decade. Further, these findings highlight that negative shifts in E Fe(III/II) lead to diminishing returns in catalytic rate promotion and suggest that changes to the primary ligating environment in a macrocycle are insufficient to break fundamental rate-potential scaling relationships in aqueous ORR catalysis. Together these studies motivate the further development of higher-potential iron complexes that employ motifs beyond the equatorial ligation plane to enhance ORR catalysis.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Strain-Modulated Platinum–Palladium Nanowires for Oxygen Reduction Reaction

Electrocatalytic activity of alloy nanocatalytsts can be manipulated effectively by tuning their physical properties (ensemble, geometric, and ligand effects) to afford optimal surface structure and compositions for proton exchange membrane fuel cell (PEMFC) application. Herein, highly catalytic platinum-palladium nanowires (Pt n Pd 100-n NWs) with a subtle lattice strain and Boerdijk-Coxeter helix type morphology are synthesized through a surfactant-free, thermal single phase solvent method. X-ray diffraction results show that Pt n Pd 100-n NWs are exposed through the (111) facets and their shrinking or expanding lattice parameters can be modulated by the alloy compositions. Electrochemical results reveal that their high catalytic activity correlates with the lattice shrinking, facets, and bimetallic compositions, showing higher activity when the ratio of Pt and Pd is similar to ~78:22, which is further supported by DFT results. Compared to the nanoparticle type platinum-palladium alloyed catalysts with similar metal compositions (Pt n Pd 100-n NPs), the Pt n Pd 100-n NWs exhibit significantly improved electrocatalytic activity and stability for the oxygen reduction reaction. Our findings open new strategies to design the highly active and stable alloy nanocatalysts with controllable compositions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

PdPt-TiO 2 nanowires: correlating composition, electronic effects and O-vacancies with activities towards water splitting and oxygen reduction

Here we report the optimization of both the support and the active phase of PdPt NPs supported on TiO 2 nanowires to obtain highly active electro/photocatalysts for the oxygen reduction and water splitting reactions. This system displayed strong metal-support interactions, high concentration of oxygen vacancies, and PdPt NP were ~2 nm in size. By optimizing the loading of PdPt, both the photo- and electrocatalytic activities were improved compared to commercial materials. Interestingly, a volcano plot was obtained from the activity and the PdPt composition, and the Pd 0.22 Pt 0.78 -TiO 2 /C sample afforded the optimal performance. For instance, the amount of hydrogen produced from water splitting was 11.6 mmol/g catalyst . For the ORR, the activity was similar to a commercial Pt catalyst, but a lower E onset (0.87 V RHE vs w 0.95 V RHE ) was detected. The variations in the activities with the composition correlated well with the variations in the electronic effects and the concentration of oxygen vacancies.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Platinum group metal-free Fe-based (Fe-N-C) oxygen reduction electrocatalysts for direct alcohol fuel cells

In direct alcohol fuel cells (DAFCs), the oxidation of alcohols happens at the expenses of the oxygen reduction reaction at the cathode. DAFCs' cathodes use a significant amount of platinum that is an expensive critical raw material. Moreover, platinum oxidizes alcohols, a fact that combined with alcohol crossover, decreases significantly the performance of the cells. The use of Fe-based (Fe-N-C) platinum group metal–free (PGM-free) cathodes is a convenient strategy to overcome these limitations. This review analyzes the application of PGM-free cathodes to DAFCs. The discussion focuses on acidic systems and covers the following subjects: (i) the breakdown of DAFC potential in its components, (ii) the analysis of the advantages from the use of the PGM-free cathode, and (iii) a review of the performance and durability of DAFCs. Finally, the review closes with a view of the authors of the future perspective for the research.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Topological Bismuth (1 1̅ 0) Facet for Efficient Oxygen Reduction Cathode in Fuel Cells

The performance of proton-exchange membrane fuel cells is strongly dependent on the efficiency of the electrochemical oxygen reduction reaction (ORR). Here, we report both two-dimensional (4 monolayer, ML) and three-dimensional Bi (16 ML) (11̅0) facets deliver a strong correlation between nontrivial topological surface states and low electrochemical overpotential toward ORR. Both topological Bi (11̅0) film and slab have two slightly gapped (9–45 meV) Dirac cones with the invariant Z 2 =1, derived from the parity products for the occupied states at four time-reversal invariant momentum points. By contrast, the Bi (110) film is stabilized as an ordinary insulator due to atomic buckling. The much lower thermodynamic ORR overpotential (η = 0.58 V) and higher H 2 O selectivity by four steps of H + /e – transfer are obtained on the topological Bi (11̅0) surface compared with the nontopological (110) facet (η = 0.84 V). This promotional effect can be attributed to the accumulation of topological surface bands (Bi 6p orbitals) near the Fermi energy, which provide stronger electronic transport channels along the surface Bi–Bi and Bi–O bonds. Finally, a 16 ML Bi (11̅0) slab is also constructed to simulate the topological (11̅0) surface of three-dimensional bulk Bi as a topological crystalline insulator (TCI). Furthermore, the consistent topological properties and slightly lower thermodynamic ORR overpotential (η = 0.53 V) are also observed on the thicker slab.

25 ENERGY STORAGE↗
DOE OSTI2022

New challenges in oxygen reduction catalysis: a consortium retrospective to inform future research

In this perspective, we highlight results of a research consortium devoted to advancing understanding of oxygen reduction reaction (ORR) catalysis as a means to inform fuel cell science. We demonstrate how targeted collaborations between different institutions from academic, national lab, and industry backgrounds and different scientific disciplines like theory, experiment, and characterization can yield unique insights into fuel cell catalysts. We comment on such insights into material designs for platinum-group-metal alloys, transition metal oxides, and non-traditional materials including metal–organic frameworks; systems that have served as the foundational building blocks for our consortium. We also motivate a renewed focus on catalyst durability in light of emerging technological requirements and paths forward in understanding in situ and operando electrochemical stability. Lastly, we describe new frontiers ORR research can take and how emerging artificial intelligence tools can assist researchers in capturing data, selecting new experiments, and guiding characterization to accelerate the design and discovery of fuel cell catalysts. A main goal of sharing this perspective is to discuss the rationale for our future research plans based on our consortium work. However, we also hope to illustrate both the potential impact of a collaborative strategy with the hopes of inspiring a higher degree of Industry-Academia-National Laboratory collaboration and encourage other centers and consortiums to distill and share their findings in a similar perspective-type article. Together we hope to enable the fuel cell research community to engage in a discussion of strategies for research and accelerated development of catalysts with improved activity and stability.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Modulating Electronic Structure of Atomically Dispersed Nickel Sites through Boron and Nitrogen Dual Coordination Boosts Oxygen Reduction

Abstract Atomically dispersed 3D transitional metal active sites with nitrogen coordination anchored on carbon support have emerged as a kind of promising electrocatalyst toward oxygen reduction reaction (ORR) in the field of fuel cells and metal–air cells. However, it is still a challenge to accurately modulate the coordination structure of single‐atom metal sites, especially first‐shell coordination, as well as identify the relationship between the geometric/electronic structure and ORR performance. Herein, a carbon‐supported single‐atom nickel catalyst is fabricated with boron and nitrogen dual coordination (denoted as Ni‐B/N‐C). The hard X‐ray absorption spectrum result reveals that atomically dispersed Ni active sites are coordinated with one B atom and three N atoms in the first shell (denoted as Ni‐B 1 N 3 ). The Ni‐B/N‐C catalyst exhibits a half‐wave potential ( E 1/2 ) of 0.87 V versus RHE, along with a distinguished long‐term durability in alkaline media, which is superior to commercial Pt/C. Density functional theory calculations indicate that the Ni‐B 1 N 3 active sites are more favorable for the adsorption of ORR intermediates relative to Ni‐N 4 , leading to the reduction of thermodynamic barrier and the acceleration of reaction kinetics, which accounts for the increased intrinsic activity.

Chemistry↗
DOE OSTI2023

Enhancing the activity of Fe-N-C oxygen reduction reaction electrocatalysts by high-throughput exploration of synthesis parameters

Here, the most active class of platinum group metal-free (PGM-free) oxygen reduction reaction (ORR) electrocatalysts in acidic electrolytes are those synthesized by heat treatment of iron, carbon, nitrogen precursors (Fe-N-C). Due to the large number of possible precursor compounds, a small fraction of the synthesis variable space has been explored. Correlation of synthesis variables with Fe speciation and ORR activity has been limited. In this work, an automation platform and a multi-port ball-milling were utilized to evaluate the effects of synthesis variables, such as identity of iron precursor, iron loading, and carbon and nitrogen sources on the ORR activity of iron -nitrogen-carbon catalysts in acidic electrolyte. The ORR activity is correlated with catalyst Fe speciation determined using Fe K-edge X-ray absorption spectroscopy (XAFS).

30 DIRECT ENERGY CONVERSION↗