The Perfect Imperfections in Electrocatalysts.

Modern day electrochemical devices find applications in a wide range of industrial sectors, from consumer electronics, renewable energy management to pollution control by electric vehicles and reduction of greenhouse gas. There has been a surge of diverse electrochemical systems which are to be scaled up from the lab-scale to industry sectors. To achieve the targets, the electrocatalysts are continuously upgraded to meet the required device efficiency at a low cost, increased lifetime and performance. An atomic scale understanding is however important for meeting the objectives. Transitioning from the bulk to the nanoscale regime of the electrocatalysts, the existence of defects and interfaces is almost inevitable, significantly impacting (augmenting) the material properties and the catalytic performance. The intrinsic defects alter the electronic structure of the nanostructured catalysts, thereby boosting the performance of metal-ion batteries, metal-air batteries, supercapacitors, fuel cells, water electrolyzers etc. This account presents our findings on the methods to introduce measured imperfections in the nanomaterials and the impact of these atomic-scale irregularities on the activity for three major reactions, oxygen evolution reaction (OER), oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER). Grain boundary (GB) modulation of the (ABO3 )n type perovskite oxide by noble metal doping is a propitious route to enhance the OER/ORR bifunctionality for zinc-air battery (ZAB). The perovskite oxides can be tuned by calcination at different temperatures to alter the oxygen vacancy, GB fraction and overall reactivity. The oxygen defects, unsaturated coordination environment and GBs can turn a relatively less active nanostructure into an efficient redox active catalyst by imbibing plenty of electrochemically active sites. Obviously, the crystalline GB interface is a prerequisite for effective electron flow, which is also applicable for the crystalline surface oxide shell on metal alloy core of the nanoparticles (NPs). The oxygen vacancy of two-dimensional (2D) perovskite oxide can be made reversible by the A-site termination of the nanosheets, facilitating the reversible entry and exit of a secondary phase during the redox processes. In several instances, the secondary phases have been observed to introduce the right proportion of structural defects and orbital occupancies for adsorption and desorption of reaction intermediates. Also, heterogeneous interfaces can be created by wrapping the perovskite oxide with negatively charged surface by layered double hydroxide (LDH) can promote the OER process. In another approach, ion intercalation at the 2D heterointerfaces steers the interlayer spacing that can influence the mass diffusion. Similar to anion vacancy, controlled formation of the cation vacancies can be achieved by exsolving the B-site cations of perovskite oxides to surface anchored catalytically active metal/alloy NPs. In case of the alloy electrocatalysts, incomplete solid solution by two or more mutually immiscible metals results in heterogeneous alloys having differently exposed facets with complementary functionalities. From the future perspective, new categories of defect structures including the 2D empty spaces or voids leading to undercoordinated sites, the multiple interfaces in heterogeneous alloys, antisite defects between anions and cations, and the defect induced inverse charge transfer should bring new dimensionalities to this riveting area of research.

Shaping a Doped Perovskite Oxide with Measured Grain Boundary Defects to Catalyze Bifunctional Oxygen Activation for a Rechargeable Zn-Air Battery.

Symmetry broken configurations within a long-range atomic arrangement exhibit new physical properties, and distinctive strategies are needed to resuscitate the localized symmetry by introducing measured defects, which can be attractive in displaying enhanced catalytic activities for energy applications. Our hypothesis is validated by introducing lattice defects due to the strain originating from a slightly higher doped grain boundary (GB) than at the interconnected grains of perovskite oxide. When Pd is doped at the B-site of ABO3-type La0.7Sr0.3CoO3-δ, a marginally higher ionic radius of Pd4+ than Co3+ enables partial deportation of Pd4+ to the GB. Consequently, the GB unit cell is relatively expanded with a higher interplanar spacing, as observed by microscopic analysis. When the Pd concentration is increased, oxygen vacancy sites are reduced and both metallic Pd and PdOx are exsolved at the perovskite oxide surface. With the Pd/Co ratio of 0.05, the defects originating from the Pd-modulated GB can be maximized to 1.29 ± 0.21% which enhances the bifunctional O2 activation ability by lowering the combined overpotential of oxygen evolution and reduction reactions (OER/ORR) to 0.91 V, duly corroborated by computational studies. The fabricated rechargeable Zn-air battery has a specific capacity of 740 mA·h/gZn (851 mW·h/gZn) when discharge is performed at 10 mA/cm2. Galvanostatic charge-discharge cycling with a 1 h cycle time shows 60 h stable performance. The OER/ORR bifunctional activity is found to be strongly correlated to the repositioned lattice symmetry at the perovskite GB.

Charge transfer from perovskite oxide nanosheets to N-doped carbon nanotubes to promote enhanced performance of a zinc-air battery.

Room temperature engineered spatially connected p-type double perovskite oxide (BaPrMn1.75Co0.25O5+δ, BPMC) nanosheets (NSs) with n-type nitrogen-doped multi-walled carbon nanotubes (NCNTs) show significant enhancement in bifunctional oxygen electrocatalytic activity. The optimization of the donor level by charge transfer from the perovskite to NCNTs is demonstrated to be a prodigious approach to facilitate redox oxygen activation. A proof-of-concept rechargeable zinc-air battery (ZAB) with BPMC containing a 10 wt% NCNT (BPMC/NCNT-10) cathode demonstrates the highest specific discharge capacity of 789.2 mA h gZn-1 and cyclic stability for 85 h at a current density of 5 mA cm-2.

An electrochemically reversible lattice with redox active A-sites of double perovskite oxide nanosheets to reinforce oxygen electrocatalysis.

The catalyst surface undergoes reversible structural changes while influencing the rate of redox reactions, the atomistic structural details of which are often overlooked when the key focus is to enhance the catalytic activity and reaction yield. We achieve chemical synthesis of ∼5 unit cell thick double perovskite oxide nanosheets (NSs) and demonstrate their precise structural reversibility while catalyzing the successive oxygen evolution and reduction reactions (OER/ORR). 4.1 nm thick A-site ordered BaPrMn1.75Co0.25O5+δ (δ = 0.06-0.17) NSs with oxygen deficient PrO x terminated layers have flexible oxygen coordination of Pr3+ ions, which promotes the redox processes. When subjected to systematic oxidation and reduction cycles by cyclic voltammetry under small electrochemical bias, the PrO1.8 phase appears and disappears alternately at the NS surface, due to the intake and release of oxygen, respectively. The structural reversibility is attributed to the two-dimensional morphology and the A-site terminated surface with flexible anion stoichiometry. Although the underlying B-site cations are well-known active sites, this is the first demonstration of A(Pr3+)-site cations influencing the activity by reversibly altering their oxygen coordination. Higher Co-doping thwarts the NS formation, affecting the catalytic performance. The facile OER/ORR activity of the thickness-tunable NSs has larger implications as a bifunctional air-electrode material for metal-air batteries and fuel cells.

Tweaking Nickel with Minimal Silver in a Heterogeneous Alloy of Decahedral Geometry to Deliver Platinum-like Hydrogen Evolution Activity.

Five-fold intertwined Agx Ni1-x (x=0.01-0.25) heterogeneous alloy nanocrystal (NC) catalysts, prepared through unique reagent combinations, are presented. With only ca. 5 at % Ag (AgNi-5), Pt-like activity has been achieved at pH 14. To reach a current density of 10 mA cm-2 the extremely stable AgNi-5 requires an overpotential of 24.0±1.2 mV as compared to 20.1±0.8 mV for 20 % Pt/C, both with equal catalyst loading of 1.32 mg cm-2 . The turnover frequency (TOF) is as high as 2.1 H2  s-1 at 50 mV (vs. RHE). Site-specific elemental analyses show the Ag:Ni compositional variation, where the apex and edges of the decahedra are Ag-rich, thereby exposing Ni onto the faces to achieve maximum charge transport for an exceptional pH universal HER activity. DFT calculations elucidate the relative H-atom adsorption capability of the Ni centers as a function of their proximity to Ag atom.

Surface Charge Modulation of Perovskite Oxides at the Crystalline Junction with Layered Double Hydroxide for a Durable Rechargeable Zinc-Air Battery.

Perovskite oxides have emerged as promising oxygen electrocatalysts for fuel cells and batteries, yet their catalytic activity and long-term stability are under debate because of local surface alterations and instabilities under sustained oxidative potential. Interconnected particles (40 nm) of Ba0.6Sr0.4Co0.79Fe0.21O2.67 (BSCF) are decorated by 10-50 wt % Ni0.6Fe0.4(OH)x [NiFe] layered double hydroxide (LDH) sheets via polyethylenimine linkage. This composite renders modulation of surface charges through Coulombic interaction and provides a leeway for electron mobility between the two components, which bestows relief to the BSCF surface from oxidative degradation. NiFe-LDH (25 wt %) bound to BSCF (BSCF/NiFe-25) is found to be the optimized bifunctional composite after considering the total overpotential of oxygen evolution and reduction reactions. With BSCF/NiFe-25 at the air electrode of a prototype-rechargeable Zn-air battery, a low discharge-charge voltage gap (1.16 V at 10 mA cm-2), unaltered cyclic stability over 100 h, and an energy density of 776.3 mW·h·gZn-1 are achieved. BSCF/NiFe-25 outperforms BSCF and is comparable to 20% Pt/C-RuO2 cathodes in all the standard figures of merit. Our work presents a general strategy to circumvent the reconstructions of perovskite oxide surface under oxidative potentials, by creating highly active, stable, and inexpensive bifunctional composite electrocatalysts for future electrochemical energy storage and conversion devices.

Value added transformation of ubiquitous substrates into highly efficient and flexible electrodes for water splitting.

Herein, we present an innovative approach for transforming commonly available cellulose paper into a flexible and catalytic current collector for overall water splitting. A solution processed soak-and-coat method of electroless plating was used to render a piece of paper conducting by conformably depositing metallic nickel nanoparticles, while still retaining the open macroporous framework. Proof-of-concept paper-electrodes are realized by modifying nickel-paper current collector with model electrocatalysts nickel-iron oxyhydroxide and nickel-molybdenum bimetallic alloy through electrodeposition route. The paper-electrodes demonstrate exceptional activities towards oxygen evolution reaction and hydrogen evolution reaction, requiring overpotentials of 240 and 32 mV at 50 and -10 mA cm-2, respectively, even as they endure extreme mechanical stress. The generality of this approach is demonstrated by fabricating similar electrodes on cotton fabric, which also show high activity. Finally, a two-electrode paper-electrolyzer is constructed which can split water with an efficiency of 98.01%, and exhibits robust stability for more than 200 h.