Lone Bech

Bioinspired molecular co-catalysts bonded to a silicon photocathode for solar hydrogen evolution

The production of fuels from sunlight represents one of the main challenges in the development of a sustainable energy system. Hydrogen is the simplest fuel to produce and although platinum and other noble metals are efficient catalysts for photoelectrochemical hydrogen evolution, earth-abundant alternatives are needed for large-scale use. We show that bioinspired molecular clusters based on molybdenum and sulphur evolve hydrogen at rates comparable to that of platinum. The incomplete cubane-like clusters (Mo(3)S(4)) efficiently catalyse the evolution of hydrogen when coupled to a p-type Si semiconductor that harvests red photons in the solar spectrum. The current densities at the reversible potential match the requirement of a photoelectrochemical hydrogen production system with a solar-to-hydrogen efficiency in excess of 10%. The experimental observations are supported by density functional theory calculations of the Mo(3)S(4) clusters adsorbed on the hydrogen-terminated Si(100) surface, providing insights into the nature of the active site.

Tuning the Activity of Pt(111) for Oxygen Electroreduction by Subsurface Alloying

To enable the development of low temperature fuel cells, significant improvements are required to the efficiency of the Pt electrocatalysts at the cathode, where oxygen reduction takes place. Herein, we study the effect of subsurface solute metals on the reactivity of Pt, using a Cu/Pt(111) near-surface alloy. Our investigations incorporate electrochemical measurements, ultrahigh vacuum experiments, and density functional theory. Changes to the OH binding energy, ΔE(OH), were monitored in situ and adjusted continuously through the subsurface Cu coverage. The incorporation of submonolayer quantities of Cu into Pt(111) resulted in an 8-fold improvement in oxygen reduction activity. The most optimal catalyst for oxygen reduction has an ΔE(OH) ≈ 0.1 eV weaker than that of pure Pt, validating earlier theoretical predictions.

Design of an Active Site towards Optimal Electrocatalysis: Overlayers, Surface Alloys and Near-Surface Alloys of Cu/Pt(111)**

Therefore,thedesign of the appropriate active site is crucial to obtain highcatalytic activity, especially where multi-functionality isneeded. However, the control of a given surface on anatom-by-atom basis is particularly challenging.The electrochemical oxidation of CO is the prototypicalbifunctional reaction.

Oxygen Electroreduction Activity and X‐Ray Photoelectron Spectroscopy of Platinum and Early Transition Metal Alloys

The oxygen reduction reaction (ORR) was studied experimentally on sputter‐cleaned, polycrystalline electrodes of Pt3Y, Pt5Y, Pt2Y, Pt3Sc, Pt3Hf, Pt3Zr and Pt under conditions relevant for low‐temperature fuel cells. The surfaces were characterised in situ by means of electrochemical methods and ex situ under ultrahigh vacuum conditions. The ORR activity was established in an electrochemical cell containing 0.1 M HClO4 by use of a rotating ring disk electrode assembly. The surface composition was characterised before and after the electrochemical measurements by using angle‐resolved X‐ray photoelectron spectroscopy. The ORR activity of the electrodes increased in the following order: Pt2Y≈Pt3Zr≈Pt≈

Photoelectrocatalysis and electrocatalysis on silicon electrodes decorated with cubane-like clusters

The influence of the cluster-core unit in cluster-decorated p-Si on photo-electrochemical (PEC) hydrogen evolution has been investigated using a homologous series of cubane-like heterobimetallic sulfide compounds. These compounds stem from the generic cluster structure A3S4 or A3B?startSend?4 (A = W, Mo; B = Co, Cu). We find that the Mo-based (A = Mo) cluster-decorated Si photoelectrodes show higher PEC performance than otherwise equivalent W-based (A = W) cluster-decorated ones. This is consistent with higher electrocatalytic activity of the Mo-based clusters supported on n-Si when measured in the dark. The result of stability tests is that photoelectrodes decorated with clusters without Co (B ≠ Co) can exhibit promising stability, whereas clusters of the structure A3CoS4  (A = W, Mo) yield photoelectrodes that are highly unstable upon illumination. X-ray photoelectron spectroscopy (XPS) results suggest that both oxidation and material loss play a role in deactivation of the A3CoS4 materials. Additionally, we observe that the photocurrent depends linearly on the light intensity in the limiting current region, and the corresponding incident photon to current efficiency (IPCE) may reach approximately 80%. Density functional theory (DFT) calculations of the clusters adsorbed on the hydrogen-terminated Si surface are used to estimate and compare cluster adsorption energies on the surface as well as the H-binding energies, which is a descriptor for electrocatalytic activity.

Probing the crossover in CO desorption from single crystal to nanoparticulate Ru model catalysts

Using model catalysts, we demonstrate that CO desorption from Ru surfaces can be switched from that typical of single crystal surfaces to one more characteristic of supported nanoparticles. First, the CO desorption behaviour from Ru nanoparticles supported on highly oriented pyrolytic graphite was studied. Both mass-selected and thermally evaporated nanoparticles were deposited. TPD spectra from the mass-selected nanoparticles exhibit a desorption peak located around 410 K with a broad shoulder extending from around 480 K to 600 K, while spectra obtained from thermally evaporated nanoparticles exhibit a single broad feature from ∼350 K to ∼450 K. A room temperature deposited 50 Å thick Ru film displays a characteristic nanoparticle-like spectrum with a broad desorption feature at ∼420 K and a shoulder extending from ∼450 K to ∼600 K. Subsequent annealing of this film at 900 K produced a polycrystalline morphology of flat Ru(001) terraces separated by monatomic steps. The CO desorption spectrum from this surface resembles that obtained on single crystal Ru(001) with two large desorption features located at 390 K and 450 K due to molecular desorption from terrace sites, and a much smaller peak at ∼530 K due to desorption of dissociatively adsorbed CO at step sites. In a second experiment, ion sputtering was used to create surface defects on a Ru(0 1 54) single crystal surface. A gradual shift away from the desorption spectrum typical of a Ru(001) surface towards one resembling desorption from supported Ru nanoparticles was observed with increasing sputter time.

Bio-inspired co-catalysts bonded to a silicon photocathode for solar hydrogen evolution

The production of fuels directly or indirectly from sunlight represents one of the major challenges to the development of a sustainable energy system. Hydrogen is the simplest fuel to produce and while platinum and other noble metals are efficient catalysts for photoelectrochemical hydrogen evolution, earth-abundant alternatives are needed for largescale use. We show that bio-inspired molecular clusters based on molybdenum sulfides and tungsten sulfides mimic natures enzymes for hydrogen evolution, molybdenum sulfides evolve hydrogen at a slightly higher overpotential than platinum when deposited on various supports. It will be demonstrated how this overpotential can be eliminated by depositing the same type of hydrogen evolution catalyst on p-type Si which can harvest the red part of the solar spectrum. Such a system could constitute the cathode part of a tandem dream device where the red part of the spectrum is utilized for hydrogen evolution while the blue part is reserved for the more difficult oxygen evolution. The samples have been illuminated with a simulated red part of the solar spectrum i.e. long wavelength (ë > 620 nm) part of simulated AM 1.5G radiation. The current densities at the reversible potential match the requirement of a photoelectrochemical hydrogen production system with a solar-to-hydrogen efficiency in excess of 10%. The experimental observations are supported by DFT calculations of the Mo3S4 cluster adsorbed on the hydrogen-terminated silicon surface providing insights into the nature of the active site.