Dusan Tripkovic

Enhancing Hydrogen Evolution Activity in Water Splitting by Tailoring Li+-Ni(OH)2-Pt Interfaces

Combining two different types of catalysts accelerated the hydrogen-generation step in water electrolysis. Improving the sluggish kinetics for the electrochemical reduction of water to molecular hydrogen in alkaline environments is one key to reducing the high overpotentials and associated energy losses in water-alkali and chlor-alkali electrolyzers. We found that a controlled arrangement of nanometer-scale Ni(OH)2 clusters on platinum electrode surfaces manifests a factor of 8 activity increase in catalyzing the hydrogen evolution reaction relative to state-of-the-art metal and metal-oxide catalysts. In a bifunctional effect, the edges of the Ni(OH)2 clusters promoted the dissociation of water and the production of hydrogen intermediates that then adsorbed on the nearby Pt surfaces and recombined into molecular hydrogen. The generation of these hydrogen intermediates could be further enhanced via Li+-induced destabilization of the HO–H bond, resulting in a factor of 10 total increase in activity.

Design and synthesis of bimetallic electrocatalyst with multilayered Pt-skin surfaces.

Advancement in heterogeneous catalysis relies on the capability of altering material structures at the nanoscale, and that is particularly important for the development of highly active electrocatalysts with uncompromised durability. Here, we report the design and synthesis of a Pt-bimetallic catalyst with multilayered Pt-skin surface, which shows superior electrocatalytic performance for the oxygen reduction reaction (ORR). This novel structure was first established on thin film extended surfaces with tailored composition profiles and then implemented in nanocatalysts by organic solution synthesis. Electrochemical studies for the ORR demonstrated that after prolonged exposure to reaction conditions, the Pt-bimetallic catalyst with multilayered Pt-skin surface exhibited an improvement factor of more than 1 order of magnitude in activity versus conventional Pt catalysts. The substantially enhanced catalytic activity and durability indicate great potential for improving the material properties by fine-tuning of the nanoscale architecture.

Unique electrochemical adsorption properties of Pt-skin surfaces.

PtM alloys (M = Co, Ni, Fe, etc.) have been extensively studied for their use in fuel cells, both in well-defined extended surfaces, as well as in nanoparticles. After the report about exceptional activity of Pt3Ni(111)-skin surface [1a] for the oxygen reduction reaction (ORR) a lot of efforts have been made to mimic this catalytic behavior at the nanoscale. It has been shown that a Pt3Ni(111) crystal annealed in ultrahigh vacuum (UHV) shows an oscillating segregation profile, with the outermost layer consisting of pure platinum while the second layer is enriched in nickel compared to the bulk composition. Such a surface we termed Pt skin, and owing to the presence of the non-noble metal in the subsurface layer it has altered electronic properties compared to the monometallic Pt single crystal with the same orientation. Accordingly, altered electronic properties induce a change in adsorption behavior, specifically a shift of surface-oxide formation to higher potentials. This adsorption behavior is believed to be the origin of the high activity for the ORR. On the opposite side of the potential scale, the adsorption of hydrogenated species, denoted as underpotentially adsorbed hydrogen (Hupd), is also largely affected on Pt-skin surfaces. [4] Despite numerous efforts dedicated to synthesize nanocatalysts with Pt-skin-type surfaces, it still remains a challenge to claim their existence at the nanoscale. To systematically resolve this issue, we attempt to provide fundamental insight into the adsorption properties of well-defined Pt-skin surfaces under relevant electrochemical conditions and to transfer that knowledge to corresponding nanocatalysts. For that reason, we first examine the formation and composition of Pt-skin surfaces by low-energy ion scattering (LEIS) and scanning tunneling microscopy (STM) in UHV, and second we study the composition of the surfaces in an electrochemical environment to establish their adsorption properties. We demonstrate by cyclic voltammetry that the surface coverage of Hupd on Pt skin is about half of that found on Pt(111), whereas the surface coverage of a saturated monolayer of carbon monoxide is similar for both surfaces. This is an important finding, which provides a link towards accurate determination of the electrochemically active surface area of nanoscale catalysts. The developed methodology provides additional evidence for the existence of Pt-skin surfaces on Pt-bimetallic nanocatalysts and can substantially diminish errors in the evaluation of the real surface area and catalytic activity. A thorough examination of the Pt-skin surfaces was performed in view of their importance in electrocatalysis as well as in response to recent questions and doubts in the

Unique Activity of Platinum Adislands in the CO Electrooxidation Reaction

The development of electrocatalytic materials of enhanced activity and efficiency through careful manipulation, at the atomic scale, of the catalyst surface structure has long been a goal of electrochemists. To accomplish this ambitious objective, it would be necessary both to obtain a thorough understanding of the relationship between the atomic-level surface structure and the catalytic properties and to develop techniques to synthesize and stabilize desired active sites. In this contribution, we present a combined experimental and theoretical study in which we demonstrate how this approach can be used to develop novel, platinum-based electrocatalysts for the CO electrooxidation reaction in CO(g)-saturated solution; the catalysts show activities superior to any pure-metal catalysts previously known. We use a broad spectrum of electrochemical surface science techniques to synthesize and rigorously characterize the catalysts, which are composed of adisland-covered platinum surfaces, and we show that highly undercoordinated atoms on the adislands themselves are responsible for the remarkable activity of these materials.

Selective catalysts for the hydrogen oxidation and oxygen reduction reactions by patterning of platinum with calix[4]arene molecules.

The design of new catalysts for polymer electrolyte membrane fuel cells must be guided by two equally important fundamental principles: optimization of their catalytic behaviour as well as the long-term stability of the metal catalysts and supports in hostile electrochemical environments. The methods used to improve catalytic activity are diverse, ranging from the alloying and de-alloying of platinum to the synthesis of platinum core-shell catalysts. However, methods to improve the stability of the carbon supports and catalyst nanoparticles are limited, especially during shutdown (when hydrogen is purged from the anode by air) and startup (when air is purged from the anode by hydrogen) conditions when the cathode potential can be pushed up to 1.5 V (ref. 11). Under the latter conditions, stability of the cathode materials is strongly affected (carbon oxidation reaction) by the undesired oxygen reduction reaction (ORR) on the anode side. This emphasizes the importance of designing selective anode catalysts that can efficiently suppress the ORR while fully preserving the Pt-like activity for the hydrogen oxidation reaction. Here, we demonstrate that chemically modified platinum with a self-assembled monolayer of calix[4]arene molecules meets this challenging requirement.

Functional links between Pt single crystal morphology and nanoparticles with different size and shape: the oxygen reduction reaction case

Design of active and stable Pt-based nanoscale electrocatalysts for the oxygen reduction reaction (ORR) will be the key to improving the efficiency of fuel cells that are needed to deliver reliable, affordable and environmentally friendly energy. Here, by exploring the ORR on Pt single crystals, cubo-octahedral (polyhedral) Pt NPs with different sizes (ranging from 2 to 7 nm), and 7–8 nm Pt NPs with different shapes (cubo-octahedral vs. cube vs. octahedral), we presented a surface science approach capable of rationalizing, and ultimately understanding, fundamental relationships between stability of Pt NPs and activity of the ORR in acidic media. By exploring the potential induced dissolution/re-deposition of Pt between 0.05 and 1.3 V, we found that concomitant variations in morphology of Pt(111) and Pt(100) lead to narrowing differences in activity between Pt single crystal surfaces. We also found that regardless of an initial size or shape, NPs are metastable and easily evolve to thermodynamically equilibrated shape and size with very similar activity for the ORR. We concluded that while initially clearly observed, the particle size and shape effects diminish as the particles age to the point that it may appear that the ORR depends neither on the particle size nor particle shape.

The role of anions in surface electrochemistry.

Some issues of the current state of understanding in the surface electrochemistry are discussed, with emphases on the role of specifically adsorbing anions in hydrogen adsorption and oxide formation, adsorption and ordering of molecular adsorbates and metal ions, metal deposition, restructuring and stability of surface atoms, and kinetics of electrochemical reactions.

Tailoring the Selectivity and Stability of Chemically Modified Platinum Nanocatalysts To Design Highly Durable Anodes for PEM Fuel Cells

Chemically modifying platinum with calix[4]arene yields a highly stable anode catalyst that effectively suppresses the oxidation reduction reaction without altering the maximum activity for the hydrogen oxidation reaction (see picture, Pt blue, C gray, O red, S yellow). This behavior extends from long-range-ordered stepped single-crystal surfaces to nanocatalysts.

Structural Effect in Electrocatalysis: Formic Acid Oxidation on Pt Electrodeposited on Glassy Carbon Support

The structural effect of Pt nanoparticles on formic acid oxidation was studied using Pt electrochemically deposited on glassy carbon as a model system. The morphology of Pt deposited on glassy carbon is defined by agglomerates whose number, size, and distribution depend on Pt loading and support pretreatment as revealed by atomic force microscopy characterization. A scanning tunneling microscopy analysis of the electrodes showed that an increase of Pt loading leads to an increase of Pt particles size and their coalescences. Electrochemical treatment of the support prior to Pt deposition results in a decrease of the particle size on acidic treated support and in their negligible change on alkaline treated support. The coalescences of the particles detected cause the formation of different defects. The most active are the electrodes with the smallest Pt loading, and with the support treated in acid having the lowest defected surface and the highest contribution of high coordinated (111) facets exposed to the reaction. The activity of the electrode decreases as the number of defects grows with increasing of the loading or with alkaline pretreatment of support, i.e., coalescence of the particles. The results obtained suggest that the ratio between the facets and the defect sites rather than particle size determines the rate of the formic acid oxidation.

Insight into electrocatalytic stability of low loading Pt-Bi/GC and Pt/GC clusters in formic acid oxidation

Formic acid oxidation was examined on platinum-bismuth deposits on glassy carbon substrate prepared by two-step process, i.e., electrochemical deposition of Bi followed by electrochemical deposition of Pt as described in our previous article (J Electrochem Soc 161:H547–H554, 2014). Upon treatment of as-prepared clusters by slow anodic sweep, bimetallic structure consisting of Bi core occluded by Pt and Bi-oxide was obtained and exhibited significant activity and exceptional stability in HCOOH oxidation. In order to explain such electrocatalytic stability, in this work, the electrochemical properties of Pt@Bi/GC catalyst were investigated applying same protocols in supporting electrolyte with or without HCOOH and compared with Pt/GC. The protocols comprised potentiodynamic, quasi-steady-state, and chronoamperometric measurements combined with the surface characterization by COads stripping voltammetry. Application of potential cycling at Pt@Bi/GC electrode in supporting electrolyte containing HCOOH leads to minor change in surface morphology, mildly leaching of Bi from the electrode surface, and negligible decrease in activity. On the other hand, significant Bi dissolution and considerable decrease in activity are the effects of the same treatment without HCOOH. Contrary to Pt@Bi/GC, Pt/GC electrodes subjected to the same protocols exhibit completely opposite properties being more stabile during potential cycling without HCOOH than in the presence of this acid. Exceptional stability in formic acid oxidation of Pt@Bi/GC catalyst is thus most probably the result of the combination of predominant dehydrogenation path of the reaction, suppressed Bi leaching, and compensation of dissolved Bi from the core as its source due to which surface morphology endured minor changes.