Dusan Strmcnik

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.

Multimetallic Au/FePt3 Nanoparticles as Highly Durable Electrocatalyst

We report the design and synthesis of multimetallic Au/Pt-bimetallic nanoparticles as a highly durable electrocatalyst for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells. This system was first studied on well-defined Pt and FePt thin films deposited on a Au(111) surface, which has guided the development of novel synthetic routes toward shape-controlled Au nanoparticles coated with a Pt-bimetallic alloy. It has been demonstrated that these multimetallic Au/FePt(3) nanoparticles possess both the high catalytic activity of Pt-bimetallic alloys and the superior durability of the tailored morphology and composition profile, with mass-activity enhancement of more than 1 order of magnitude over Pt catalysts. The reported synergy between well-defined surfaces and nanoparticle synthesis offers a persuasive approach toward advanced functional nanomaterials.

Improving the hydrogen oxidation reaction rate by promotion of hydroxyl adsorption.

The development of hydrogen-based energy sources as viable alternatives to fossil-fuel technologies has revolutionized clean energy production using fuel cells. However, to date, the slow rate of the hydrogen oxidation reaction (HOR) in alkaline environments has hindered advances in alkaline fuel cell systems. Here, we address this by studying the trends in the activity of the HOR in alkaline environments. We demonstrate that it can be enhanced more than fivefold compared to state-of-the-art platinum catalysts. The maximum activity is found for materials (Ir and Pt₀.₁Ru₀.₉) with an optimal balance between the active sites that are required for the adsorption/dissociation of H₂ and for the adsorption of hydroxyl species (OHad). We propose that the more oxophilic sites on Ir (defects) and PtRu material (Ru atoms) electrodes facilitate the adsorption of OHad species. Those then react with the hydrogen intermediates (Had) that are adsorbed on more noble surface sites.

Design of active and stable Co-Mo-Sx chalcogels as pH-universal catalysts for the hydrogen evolution reaction.

Three of the fundamental catalytic limitations that have plagued the electrochemical production of hydrogen for decades still remain: low efficiency, short lifetime of catalysts and a lack of low-cost materials. Here, we address these three challenges by establishing and exploring an intimate functional link between the reactivity and stability of crystalline (CoS2 and MoS2) and amorphous (CoSx and MoSx) hydrogen evolution catalysts. We propose that Co(2+) and Mo(4+) centres promote the initial discharge of water (alkaline solutions) or hydronium ions (acid solutions). We establish that although CoSx materials are more active than MoSx they are also less stable, suggesting that the active sites are defects formed after dissolution of Co and Mo cations. By combining the higher activity of CoSx building blocks with the higher stability of MoSx units into a compact and robust CoMoSx chalcogel structure, we are able to design a low-cost alternative to noble metal catalysts for efficient electrocatalytic production of hydrogen in both alkaline and acidic environments.

The role of non-covalent interactions in electrocatalytic fuel-cell reactions on platinum

The classic models of metal electrode-electrolyte interfaces generally focus on either covalent interactions between adsorbates and solid surfaces or on long-range electrolyte-metal electrostatic interactions. Here we demonstrate that these traditional models are insufficient. To understand electrocatalytic trends in the oxygen reduction reaction (ORR), the hydrogen oxidation reaction (HOR) and the oxidation of methanol on platinum surfaces in alkaline electrolytes, non-covalent interactions must be considered. We find that non-covalent interactions between hydrated alkali metal cations M(+)(H(2)O)(x) and adsorbed OH (OH(ad)) species increase in the same order as the hydration energies of the corresponding cations (Li(+) >> Na(+) > K(+) > Cs(+)) and also correspond to an increase in the concentration of OH(ad)-M(+)(H(2)O)(x) clusters at the interface. These trends are inversely proportional to the activities of the ORR, the HOR and the oxidation of methanol on platinum (Cs(+) > K(+) > Na(+) >> Li(+)), which suggests that the clusters block the platinum active sites for electrocatalytic reactions.

Enhancing the Alkaline Hydrogen Evolution Reaction Activity through the Bifunctionality of Ni(OH)2/Metal Catalysts

Active in alkaline environment: The activity of nickel, silver, and copper catalysts for the electrochemical transformation of water to molecular hydrogen in alkaline solutions was enhanced by modification of the metal surfaces by Ni(OH)(2) (see picture; I = current density and η = overpotential). The hydrogen evolution reaction rate on a Ni electrode modified by Ni(OH)(2) nanoclusters is about four times higher than on a bare Ni surface.

Enhanced electrocatalysis of the oxygen reduction reaction based on patterning of platinum surfaces with cyanide

The slow rate of the oxygen reduction reaction in the phosphoric acid fuel cell is the main factor limiting its wide application. Here, we present an approach that can be used for the rational design of cathode catalysts with potential use in phosphoric acid fuel cells, or in any environments containing strongly adsorbing tetrahedral anions. This approach is based on molecular patterning of platinum surfaces with cyanide adsorbates that can efficiently block the sites for adsorption of spectator anions while the oxygen reduction reaction proceeds unhindered. We also demonstrate that, depending on the supporting electrolyte anions and cations, on the same CN-covered Pt(111) surface, the oxygen reduction reaction activities can range from a 25-fold increase to a 50-fold decrease. This behaviour is discussed in the light of the role of covalent and non-covalent interactions in controlling the ensemble of platinum active sites required for high turn over rates of the oxygen reduction reaction.

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

Activity–Stability Trends for the Oxygen Evolution Reaction on Monometallic Oxides in Acidic Environments

In the present study, we used a surface-science approach to establish a functional link between activity and stability of monometallic oxides during the OER in acidic media. We found that the most active oxides (Au ≪ Pt < Ir < Ru ≪ Os) are, in fact, the least stable (Au ≫ Pt > Ir > Ru ≫ Os) materials. We suggest that the relationships between stability and activity are controlled by both the nobility of oxides as well as by the density of surface defects. This functionality is governed by the nature of metal cations and the potential transformation of a stable metal cation with a valence state of n = +4 to unstable metal cation with n > +4. A practical consequence of such a close relationship between activity and stability is that the best materials for the OER should balance stability and activity in such a way that the dissolution rate is neither too fast nor too slow.