Research Engineer, SUNCAT Center for Interface Science and Catalysis
- Nitride or Oxynitride? Elucidating the Composition-Activity Relationships in Molybdenum Nitride Electrocatalysts for the Oxygen Reduction Reaction CHEMISTRY OF MATERIALS 2020; 32 (7): 2946–60
- In Situ X-Ray Absorption Spectroscopy Disentangles the Roles of Copper and Silver in a Bimetallic Catalyst for the Oxygen Reduction Reaction CHEMISTRY OF MATERIALS 2020; 32 (5): 1819–27
Understanding the Origin of Highly Selective CO2 Electroreduction to CO on Ni, N-doped Carbon Catalysts.
Angewandte Chemie (International ed. in English)
Ni,N-doped carbon catalysts have shown promising catalytic performance for CO 2 electroreduction (CO 2 R) to CO; this activity has been attributed to the presence of nitrogen-coordinated, single metal atom active sites. However, experimentally confirming Ni-N bonding and correlating CO 2 reduction (CO 2 R) activity to these species has remained a fundamental challenge. We synthesized polyacrylonitrile-derived Ni, N-doped carbon electrocatalysts (Ni-PACN) with a range of pyrolysis temperatures and Ni loadings and correlated their electrochemical activity with extensive physiochemical characterization to rigorously address the origin of activity in these materials. We found that the CO- 2 R to CO partial current density increased with increased Ni content before plateauing at 2 wt% which suggests a dispersed Ni active site. These dispersed active sites were investigated by hard and soft x-ray spectroscopy, which revealed that pyrrolic nitrogen ligands selectively bind Ni atoms in a distorted square-planar geometry that strongly resembles the active sites of molecular metal-porphyrin catalysts.
View details for DOI 10.1002/anie.201912857
View details for PubMedID 31919948
- Ternary Ni-Co-Fe oxyhydroxide oxygen evolution catalysts: Intrinsic activity trends, electrical conductivity, and electronic band structure NANO RESEARCH 2019; 12 (9): 2288–95
- Earth-Abundant Oxygen Electrocatalysts for Alkaline Anion-Exchange-Membrane Water Electrolysis: Effects of Catalyst Conductivity and Comparison with Performance in Three-Electrode Cells ACS CATALYSIS 2019; 9 (1): 7–15
Operando X-Ray Absorption Spectroscopy Shows Iron Oxidation Is Concurrent with Oxygen Evolution in Cobalt-Iron (Oxy)hydroxide Electrocatalysts
ANGEWANDTE CHEMIE-INTERNATIONAL EDITION
2018; 57 (39): 12840–44
Iron cations are essential for the high activity of nickel and cobalt-based (oxy)hydroxides for the oxygen evolution reaction, but the role of iron in the catalytic mechanism remains under active investigation. Operando X-ray absorption spectroscopy and density functional theory calculations are used to demonstrate partial Fe oxidation and a shortening of the Fe-O bond length during oxygen evolution on Co(Fe)Ox Hy . Cobalt oxidation during oxygen evolution is only observed in the absence of iron. These results demonstrate a different mechanism for water oxidation in the presence and absence of iron and support the hypothesis that oxidized iron species are involved in water-oxidation catalysis on Co(Fe)Ox Hy .
View details for DOI 10.1002/anie.201808818
View details for Web of Science ID 000444941600041
View details for PubMedID 30112793
- Transition-Metal-Incorporated Aluminum Oxide Thin Films: Toward Electronic Structure Design in Amorphous Mixed-Metal Oxides JOURNAL OF PHYSICAL CHEMISTRY C 2018; 122 (25): 13691–704
- The role of Cr doping in Ni-Fe oxide/(oxy)hydroxide electrocatalysts for oxygen evolution ELECTROCHIMICA ACTA 2018; 265: 10–18
Morphology Dynamics of Single-Layered Ni(OH)(2)/NiOOH Nanosheets and Subsequent Fe Incorporation Studied by &ITin Situ&IT Electrochemical Atomic Force Microscopy
2017; 17 (11): 6922–26
Nickel (oxy)hydroxide-based (NiOxHy) materials are widely used for energy storage and conversion devices. Understanding dynamic processes at the solid-liquid interface of nickel (oxy)hydroxide is important to improve reaction kinetics and efficiencies. In this study, in situ electrochemical atomic force microscopy (EC-AFM) was used to directly investigate dynamic changes of single-layered Ni(OH)2 nanosheets during electrochemistry measurements. Reconstruction of Ni(OH)2 nanosheets, along with insertion of ions from the electrolyte, results in an increase of the volume by 56% and redox capacity by 300%. We also directly observe Fe cations adsorb and integrate heterogeneously into or onto the nanosheets as a function of applied potential, further increasing apparent volume. Our findings are important for the fundamental understanding of NiOxHy-based supercapacitors and oxygen-evolution catalysts, illustrating the dynamic nature of Ni-based nanostructures under electrochemical conditions.
View details for DOI 10.1021/acs.nanolett.7b03313
View details for Web of Science ID 000415029000062
View details for PubMedID 28991484
Reactive Fe-Sites in Ni/Fe (Oxy)hydroxide Are Responsible for Exceptional Oxygen Electrocatalysis Activity
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
2017; 139 (33): 11361–64
Fe is a critical component of record-activity Ni/Fe (oxy)hydroxide (Ni(Fe)OxHy) oxygen evolution reaction (OER) catalysts, yet its precise role remains unclear. We report evidence for different types of Fe species within Ni(Fe)OxHy- those that are rapidly incorporated into the Ni oxyhydroxide from Fe cations in solution (and that are likely at edges or defects) and are responsible for the enhanced OER activity, and those substituting for bulk Ni that modulate the observed Ni voltammetry. These results suggest that the exceptional OER activity of Ni(Fe)OxHy does not depend on Fe in the bulk or on average electrochemical properties of the Ni cations measured by voltammetry, and instead emphasize the role of the local structure.
View details for DOI 10.1021/jacs.7b07117
View details for Web of Science ID 000408519600014
View details for PubMedID 28789520
- Influence of Electrolyte Cations on Ni(Fe)OOH Catalyzed Oxygen Evolution Reaction CHEMISTRY OF MATERIALS 2017; 29 (11): 4761–67
- Measurement Techniques for the Study of Thin Film Heterogeneous Water Oxidation Electrocatalysts CHEMISTRY OF MATERIALS 2017; 29 (1): 120–40
- Fe (Oxy)hydroxide Oxygen Evolution Reaction Electrocatalysis: Intrinsic Activity and the Roles of Electrical Conductivity, Substrate, and Dissolution CHEMISTRY OF MATERIALS 2015; 27 (23): 8011–20
- Oxygen Evolution Reaction Electrocatalysis on Transition Metal Oxides and (Oxy)hydroxides: Activity Trends and Design Principles CHEMISTRY OF MATERIALS 2015; 27 (22): 7549–58
Revised Oxygen Evolution Reaction Activity Trends for First-Row Transition-Metal (Oxy)hydroxides in Alkaline Media
JOURNAL OF PHYSICAL CHEMISTRY LETTERS
2015; 6 (18): 3737–42
First-row transition-metal oxides and (oxy)hydroxides catalyze the oxygen evolution reaction (OER) in alkaline media. Understanding the intrinsic catalytic activity provides insight into improved catalyst design. Experimental and computationally predicted activity trends, however, have varied substantially. Here we describe a new OER activity trend for nominally oxyhydroxide thin films of Ni(Fe)O(x)H(y) > Co(Fe)O(x)H(y) > FeO(x)H(y)-AuO(x) > FeO(x)H(y) > CoO(x)H(y) > NiO(x)H(y) > MnO(x)H(y). This intrinsic trend has been previously obscured by electrolyte impurities, potential-dependent electrical conductivity, and difficulty in correcting for surface-area or mass-loading differences. A quartz-crystal microbalance was used to monitor mass in situ and X-ray photoelectron spectroscopy to measure composition and impurity levels. These new results provide a basis for comparison to theory and help guide the design of improved catalyst systems.
View details for DOI 10.1021/acs.jpclett.5b01650
View details for Web of Science ID 000361858800036
View details for PubMedID 26722749
Cobalt-Iron (Oxy)hydroxide Oxygen Evolution Electrocatalysts: The Role of Structure and Composition on Activity, Stability, and Mechanism
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
2015; 137 (10): 3638–48
Cobalt oxides and (oxy)hydroxides have been widely studied as electrocatalysts for the oxygen evolution reaction (OER). For related Ni-based materials, the addition of Fe dramatically enhances OER activity. The role of Fe in Co-based materials is not well-documented. We show that the intrinsic OER activity of Co(1-x)Fe(x)(OOH) is ∼100-fold higher for x ≈ 0.6-0.7 than for x = 0 on a per-metal turnover frequency basis. Fe-free CoOOH absorbs Fe from electrolyte impurities if the electrolyte is not rigorously purified. Fe incorporation and increased activity correlate with an anodic shift in the nominally Co(2+/3+) redox wave, indicating strong electronic interactions between the two elements and likely substitutional doping of Fe for Co. In situ electrical measurements show that Co(1-x)Fe(x)(OOH) is conductive under OER conditions (∼0.7-4 mS cm(-1) at ∼300 mV overpotential), but that FeOOH is an insulator with measurable conductivity (2.2 × 10(-2) mS cm(-1)) only at high overpotentials >400 mV. The apparent OER activity of FeOOH is thus limited by low conductivity. Microbalance measurements show that films with x ≥ 0.54 (i.e., Fe-rich) dissolve in 1 M KOH electrolyte under OER conditions. For x < 0.54, the films appear chemically stable, but the OER activity decreases by 16-62% over 2 h, likely due to conversion into denser, oxide-like phases. We thus hypothesize that Fe is the most-active site in the catalyst, while CoOOH primarily provides a conductive, high-surface area, chemically stabilizing host. These results are important as Fe-containing Co- and Ni-(oxy)hydroxides are the fastest OER catalysts known.
View details for DOI 10.1021/jacs.5b00281
View details for Web of Science ID 000351420800034
View details for PubMedID 25700234
Contributions to activity enhancement via Fe incorporation in Ni-(oxy) hydroxide/borate catalysts for near-neutral pH oxygen evolution
2015; 51 (25): 5261–63
Ni-borate materials are oxygen evolution catalysts that operate at near-neutral pH and have been found previously to improve due to structural changes induced via anodic conditioning. We find that this increased activity after conditioning at 0.856 V vs. SCE, as measured on a turn-over frequency basis (TOF) at 400 mV overpotential (TOF = 0.38 s(-1)), accompanies significant Fe incorporation (14%). Films conditioned in Fe-free electrolyte exhibit ∼10 times lower activity (TOF = 0.03 s(-1)). By co-depositing Fe-Ni we demonstrate high activity without conditioning (TOF = 0.24 s(-1)) which improves further with shortened (∼30 min) conditioning (TOF = 1.4 s(-1)).
View details for DOI 10.1039/c4cc08670h
View details for Web of Science ID 000351393600012
View details for PubMedID 25579228