skip to main content

Title: Structural basis for differing electrocatalytic water oxidation by the cubic, layered and spinel forms of lithium cobalt oxides
The two polymorphs of lithium cobalt oxide, LiCoO 2 , present an opportunity to contrast the structural requirements for reversible charge storage (battery function) vs. catalysis of water oxidation/oxygen evolution (OER; 2H 2 O → O 2 + 4H + + 4e − ). Previously, we reported high OER electrocatalytic activity from nanocrystals of the cubic phase vs. poor activity from the layered phase – the archetypal lithium-ion battery cathode. Here we apply transmission electron microscopy, electron diffraction, voltammetry and elemental analysis under OER electrolysis conditions to show that labile Li + ions partially deintercalate from layered LiCoO 2 , initiating structural reorganization to the cubic spinel LiCo 2 O 4 , in parallel with formation of a more active catalytic phase. Comparison of cubic LiCoO 2 (50 nm) to iridium (5 nm) nanoparticles for OER catalysis (commercial benchmark for membrane-based systems) in basic and neutral electrolyte reveals excellent performance in terms of Tafel slope (48 mV dec −1 ), overpotential ( η = ∼420 mV@10 mA cm −2 at pH = 14), faradaic yield (100%) and OER stability (no loss in 14 hours). The inherent OER activity of cubic LiCoO 2 and spinel LiCo 2 O 4 is attributed more » to the presence of [Co 4 O 4 ] n+ cubane structural units, which provide lower oxidation potential to Co 4+ and lower inter-cubane hole mobility. By contrast, the layered phase, which lacks cubane units, exhibits extensive intra-planar hole delocalization which entropically hinders the four electron/hole concerted OER reaction. An essential distinguishing trait of a truly relevant catalyst is efficient continuous operation in a real electrolyzer stack. Initial trials of cubic LiCoO 2 in a solid electrolyte alkaline membrane electrolyzer indicate continuous operation for 1000 hours (without failure) at current densities up to 400 mA cm −2 and overpotential lower than proven PGM (platinum group metal) catalysts. « less
; ; ; ; ; ;
Award ID(s):
Publication Date:
Journal Name:
Energy & Environmental Science
Page Range or eLocation-ID:
184 to 192
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract This paper describes the development of mixed B-site pyrochlore Y 2 MnRuO 7 electrocatalyst for oxygen evolution reaction (OER) in acidic media, a challenge for the development of low-temperature electrolyzer for green hydrogen production. Recently, several theories have been developed to understand the reaction mechanism for OER, though there is an  uncertainty in most of the cases, due to the complex surface structures. Several key factors such as lattice oxygen, defect, electronic structure, oxidation state, hydroxyl group and conductivity were identified and shown to be important to the OER activity. The contribution of each factor to the performance howevermore »is often not well understood, limiting their impact in guiding the design of OER electrocatalysts. In this work, we showed mixed B-site pyrochlore Y 2 MnRuO 7 catalyst exhibits 14 times higher turnover frequency (TOF) than RuO 2 while maintaining a low overpotential of ~ 300 mV for the entire testing period of 24 h in acidic electrolyte. X-ray photoelectron spectroscopy (XPS) analysis reveals that this B-site mixed pyrochlore Y 2 MnRuO 7 has a higher oxidation state of Ru than those of Y 2 Ru 2 O 7 , which could be crucial for improving OER performance as the broadened and lowered Ru 4d band resulted from the B-site substitution by Mn is beneficial to the OER kinetics.« less
  2. Iridium oxide (IrO 2 ) is one of the best known electrocatalysts for the oxygen evolution reaction (OER) taking place in a strongly acidic solution. IrO 2 nanocatalysts with high activity as well as long-term catalytic stability, particularly at high current densities, are highly desirable for proton exchange membrane water electrolysis (PEM-WE). Here, we report a simple and cost-effective strategy for depositing ultrafine oxygen-defective IrO x nanoclusters (1–2 nm) on a high-surface-area, acid-stable titanium current collector (H-Ti@IrO x ), through a repeated impregnation–annealing process. The high catalytically active surface area resulting from the small size of IrO x and themore »preferable electronic structure originating from the presence of oxygen defects enable the outstanding OER performance of H-Ti@IrO x , with low overpotentials of 277 and 336 mV to deliver 10 and 200 mA cm −2 in 0.5 M H 2 SO 4 . Moreover, H-Ti@IrO x also shows an intrinsic specific activity of 0.04 mA cm catalyst −2 and superior mass activity of 1500 A g Ir −1 at an overpotential of 350 mV. Comprehensive experimental studies and density functional theory calculations confirm the important role of oxygen defects in the enhanced OER performance. Remarkably, H-Ti@IrO x can continuously catalyze the OER in 0.5 M H 2 SO 4 at 200 mA cm −2 for 130 hours with minimal degradation, and with a higher IrO x loading, it can sustain at such a high current density for over 500 hours without significant performance decay, holding substantial promise for use in PEM-WE.« less
  3. Nickel nitride (Ni 3 N) is known as one of the promising precatalysts for the electrochemical oxygen evolution reaction (OER) under alkaline conditions. Due to its relatively low oxidation resistance, Ni 3 N is electrochemically self-oxidized into nickel oxides/oxyhydroxides (electroactive sites) during the OER. However, we lack a full understanding of the effects of Ni 3 N self-oxidation and Fe impurity incorporation into Ni 3 N from electrolyte towards OER activity. Here, we report on our examination of the compositional and structural transformation of Ni 3 N precatalyst layers on Ni foams (Ni 3 N/Ni foam) during extended periods ofmore »OER testing in Fe-purified and unpurified KOH media using both a standard three-electrode cell and a flow cell, and discuss their electrocatalytic properties. After the OER tests in both KOH media, the Ni 3 N surfaces were converted into amorphous, nano-porous nickel oxide/(oxy)hydroxide surfaces. In the Fe-purified electrolyte, a decrease in OER activity was confirmed after the OER test because of the formation of pure NiOOH with low OER activity and electrical conductivity. Conversely, in the unpurified electrolyte, a continuous increase in OER activity was observed over the OER testing, which may have resulted from the Fe incorporation into the self-oxidation-formed NiOOH. Our experimental findings revealed that Fe impurities play an essential role in obtaining notable OER activity using the Ni 3 N precatalyst. Additionally, our Ni 3 N/Ni foam electrode exhibited a low OER overpotential of 262 mV to reach a geometric current density of 10 mA cm geo −2 in a flow cell with unpurified electrolyte.« less
  4. Non-noble metal based electrocatalysts for the hydrogen evolution reaction (HER) hold great potential for commercial applications. However, effective design strategies are greatly needed to manipulate the catalyst structures to achieve high activity and stability comparable to those of noble-metal based electrocatalysts. Herein, we present a facile route to synthesize layered Co 9 S 8 intercalated with Co cations (Co 2+ -Co 9 S 8 ) (with interlayer distance up to 1.08 nm) via a one-step solvothermal method. Benefiting from a large interlayer distance and efficient electron transfer between layers, the Co 2+ -Co 9 S 8 hybrid shows outstanding electrocatalyticmore »hydrogen evolution performance in an acid electrolyte. The electrocatalytic performance is even better than that of 20% Pt/C at the <−0.54 V region with an overpotential of 86 mV at a current density of 10 mA cm −2 in 0.5 mol L −1 H 2 SO 4 . More importantly, the system can maintain excellent stability for more than 12 h without obvious decay. This study not only presents a novel and efficient approach to synthesize cobalt sulfide intercalated with Co cations for stable electrocatalytic HER but also provides an avenue for the design of intercalated materials used in other energy applications.« less
  5. Energy harvesting from solar and water has created ripples in materials energy research for the last several decades, complemented by the rise of Hydrogen as a clean fuel. Among these, water electrolysis leading to generation of oxygen and hydrogen, has been one of the most promising routes towards sustainable alternative energy generation and storage, with applications ranging from metal-​air batteries, fuel cells, to solar-​to-​fuel energy conversion systems. In fact, solar water splitting is one of the most promising method to produce Hydrogen without depleting fossil-​fuel based natural resources. However, the efficiency and practical feasibility of water electrolysis is limited bymore »the anodic oxygen evolution reaction (OER)​, which is a kinetically sluggish, electron-​intensive uphill reaction. A slow OER process also slows the other half- cell reaction, i.e. the hydrogen evolution reaction (HER) at the cathode. Hence, designing efficient catalysts for OER process from earth-​abundant resources has been one of the primary concerns for advancing solar water splitting. In the Nath group we have focused on transition metal chalcogenides as efficient OER electrocatalysts. We have proposed the idea that these chalcogenides, specifically, selenides and tellurides will show much better OER catalytic activity due to increasing covalency around the catalytically active transition metal site, compared to the oxides caused by decreasing electronegativity of the anion, which in turn leads to variation of chem. potential around the transition metal center, [e.g. lowering the Ni 2+ -​-​> Ni 3+ oxidn. potential in Ni-​based catalysts where Ni 3+ is the actually catalytically active species]​. Based on such hypothesis, we have synthesized a plethora of transition metal selenides including those based on Ni, Ni-​Fe, Co, and Ni-​Co, which show high catalytic efficiency characterized by low onset potential and overpotential at 10 mA​/cm 2 [Ni 3 Se 2 - 200 - 290 mV; Co 7 Se 8 - 260 mV; FeNi 2 Se 4 -​NrGO - 170 mV (NrGO - N-​doped reduced graphene oxide)​; NiFe 2 Se 4 - 210 mV; CoNi 2 Se 4 - 190 mV; Ni 3 Te 2 - 180 mV]​.« less