Ib Chorkendorff

Alloys of platinum and early transition metals as oxygen reduction electrocatalysts

The widespread use of low-temperature polymer electrolyte membrane fuel cells for mobile applications will require significant reductions in the amount of expensive Pt contained within their cathodes, which drive the oxygen reduction reaction (ORR). Although progress has been made in this respect, further reductions through the development of more active and stable electrocatalysts are still necessary. Here we describe a new set of ORR electrocatalysts consisting of Pd or Pt alloyed with early transition metals such as Sc or Y. They were identified using density functional theory calculations as being the most stable Pt- and Pd-based binary alloys with ORR activity likely to be better than Pt. Electrochemical measurements show that the activity of polycrystalline Pt(3)Sc and Pt(3)Y electrodes is enhanced relative to pure Pt by a factor of 1.5-1.8 and 6-10, respectively, in the range 0.9-0.87 V.

Molybdenum sulfides—efficient and viable materials for electro - and photoelectrocatalytic hydrogen evolution

This perspective covers the use of molybdenum disulfide and related compounds, generally termed MoSx, as electro- or photoelectrocatalysts for the hydrogen evolution reaction (HER). State of the art solutions as well as the most illustrative results from the extensive electro- and photoelectrocatalytic literature are given. The research strategies currently employed in the field are outlined and future challenges pointed out. We suggest that the key to optimising the HER activity of MoS2 is divided into (1) increasing the catalytic activity of the active site, (2) increasing the number of active sites of the catalyst, and (3) improving the electrical contact to these sites. These postulations are substantiated by examples from the existing literature and some new results. To demonstrate the electrocatalytic properties of a highly conductive MoS2 hybrid material, we present the HER activity data for multi-wall MoS2 nanotubes on multi-wall carbon nanotubes (MWMoS2@MWCNTs). This exemplifies the typical data collected for the electrochemical HER. In addition, it demonstrates that the origin of the activity is closely related to the amount of edges in the layered MoS2. The photoelectrocatalytic HER is also discussed, based on examples from literature, with an emphasis on the use of MoSx as either (1) the co-catalyst providing the HER activity for a semiconductor, e.g. Mo3S+4on Si or (2) MoS2 as the semiconductor with an intrinsic HER activity. Finally, suggestions for future catalyst designs are given.

Combining theory and experiment in electrocatalysis: Insights into materials design

Better living through water-splitting Chemists have known how to use electricity to split water into hydrogen and oxygen for more than 200 years. Nonetheless, because the electrochemical route is inefficient, most of the hydrogen made nowadays comes from natural gas. Seh et al. review recent progress in electrocatalyst development to accelerate water-splitting, the reverse reactions that underlie fuel cells, and related oxygen, nitrogen, and carbon dioxide reductions. A unified theoretical framework highlights the need for catalyst design strategies that selectively stabilize distinct reaction intermediates relative to each other. Science, this issue p. 10.1126/science.aad4998 BACKGROUND With a rising global population, increasing energy demands, and impending climate change, major concerns have been raised over the security of our energy future. Developing sustainable, fossil-free pathways to produce fuels and chemicals of global importance could play a major role in reducing carbon dioxide emissions while providing the feedstocks needed to make the products we use on a daily basis. One prospective goal is to develop electrochemical conversion processes that can convert molecules in the atmosphere (e.g., water, carbon dioxide, and nitrogen) into higher-value products (e.g., hydrogen, hydrocarbons, oxygenates, and ammonia) by coupling to renewable energy. Electrocatalysts play a key role in these energy conversion technologies because they increase the rate, efficiency, and selectivity of the chemical transformations involved. Today’s electrocatalysts, however, are inadequate. The grand challenge is to develop advanced electrocatalysts with the enhanced performance needed to enable widespread penetration of clean energy technologies. ADVANCES Over the past decade, substantial progress has been made in understanding several key electrochemical transformations, particularly those that involve water, hydrogen, and oxygen. The combination of theoretical and experimental studies working in concert has proven to be a successful strategy in this respect, yielding a framework to understand catalytic trends that can ultimately provide rational guidance toward the development of improved catalysts. Catalyst design strategies that aim to increase the number of active sites and/or increase the intrinsic activity of each active site have been successfully developed. The field of hydrogen evolution, for example, has seen important breakthroughs over the years in the development of highly active non–precious metal catalysts in acid. Notable advancements have also been made in the design of oxygen reduction and evolution catalysts, although there remains substantial room for improvement. The combination of theory and experiment elucidates the remaining challenges in developing further improved catalysts, often involving scaling relations among reactive intermediates. This understanding serves as an initial platform to design strategies to circumvent technical obstacles, opening up opportunities and approaches to develop higher-performance electrocatalysts for a wide range of reactions. OUTLOOK A systematic framework of combining theory and experiment in electrocatalysis helps to uncover broader governing principles that can be used to understand a wide variety of electrochemical transformations. These principles can be applied to other emerging and promising clean energy reactions, including hydrogen peroxide production, carbon dioxide reduction, and nitrogen reduction, among others. Although current paradigms for catalyst development have been helpful to date, a number of challenges need to be successfully addressed in order to achieve major breakthroughs. One important frontier, for example, is the development of both experimental and computational methods that can rapidly elucidate reaction mechanisms on broad classes of materials and in a wide range of operating conditions (e.g., pH, solvent, electrolyte). Such efforts would build on current frameworks for understanding catalysis to provide the deeper insights needed to fine-tune catalyst properties in an optimal manner. The long-term goal is to continue improving the activity and selectivity of these catalysts in order to realize the prospects of using renewable energy to provide the fuels and chemicals that we need for a sustainable energy future. Electrochemical energy conversion. Schematic showing electrochemical conversion of water, carbon dioxide, and nitrogen into value-added products (e.g., hydrogen, hydrocarbons, oxygenates, and ammonia), using energy from renewable sources. The combination of theoretical and experimental studies working in concert provides us with insight into these electrochemical transformations and guides the development of the high-performance electrocatalysts needed to enable these technologies. Electrocatalysis plays a central role in clean energy conversion, enabling a number of sustainable processes for future technologies. This review discusses design strategies for state-of-the-art heterogeneous electrocatalysts and associated materials for several different electrochemical transformations involving water, hydrogen, and oxygen, using theory as a means to rationalize catalyst performance. By examining the common principles that govern catalysis for different electrochemical reactions, we describe a systematic framework that clarifies trends in catalyzing these reactions, serving as a guide to new catalyst development while highlighting key gaps that need to be addressed. We conclude by extending this framework to emerging clean energy reactions such as hydrogen peroxide production, carbon dioxide reduction, and nitrogen reduction, where the development of improved catalysts could allow for the sustainable production of a broad range of fuels and chemicals.

Layered Nanojunctions for Hydrogen‐Evolution Catalysis

The production of chemical fuels by using sunlight is an attractive and sustainable solution to the global energy and environmental problems. Photocatalytic water splitting is a promising route to capture, convert, and store solar energy in the simplest chemical compound (H2). [1] Since the initial report of a photoelectrochemical cell using Pt-TiO2 electrodes for hydrogen evolution by Fujishima and Honda in 1972, considerable studies have been focused on the development of highly efficient and stable photocatalyst powder systems, and especially on using earth-abundant semiconductors and co-factors for water splitting. In practice, the achievement of the conversion of solar energy into hydrogen necessitates the spatial integration of semiconductors and co-catalysts to form surface junctions, so as to optimize the capture of light and to promote charge separation and surface catalytic kinetics. The construction of effective surface junctions is therefore of vital importance, and not only strongly depends on the properties, such as crystal structure, band structure, and electron affinity, of both semiconductors and catalysts but also on the interface between the two materials. In photocatalysis, an ohmic contact between photocatalysts and cocatalysts can allow the prompt migration of light-induced charge, thus resulting in an efficient photocatalytic reaction. Recently, we found that graphitic carbon nitride (g-CN), a polymeric melon semiconductor with a layered structure analogous to graphite, meets the essential requirements as a sustainable solar energy transducer for water redox catalysis; these requirements include being abundant, highly-stable, and responsive to visible light. g-CN is indeed a new type of visible-light photocatalyst that contains no metals, and has a suitable electronic structure (Eg = 2.7 eV, conduction band at 0.8 V and valence band at 1.9 V vs. RHE) covering the water-splitting potentials. An improvement in the efficiency of H2 production has been demonstrated by the introduction of nanohierarchical structures into g-CN. It is noted that, like many other photocatalysts, g-CN alone shows very poor electrocatalytic activities for water splitting and relies on surface co-catalysts to activate its photocatalytic functions. The co-catalyst cooperates with the light harvester to facilitate the charge separation and increases the lifetime of the photogenerated electron/hole pair, while lowering activation barriers for H2 or O2 evolution. Thus, the use of a co-catalyst leads to an increase in overall photocatalytic performance, including activity, selectivity, and stability. Generally, the efficiency of a given photocatalytic system is dependent on the ability of the co-catalysts to support reductive and/or oxidative catalysis. In particular, the structural characteristics and intrinsic catalytic properties of a co-catalyst are important. However, the study of the structural and electronic compatibility between g-CN and co-catalysts has been limited so far. The co-catalysts used are mainly platinum group metals or their oxides, which are scarce and expensive. Photocatalytic/catalytic systems based on abundantly available materials are certainly desirable for large-scale hydrogen production for future energy production based on water and sunlight. Among various hydrogen-evolution reaction (HER) catalysts, molybdenum sulfur complexes have received a lot of attention. MoS2 was found to be a good electrocatalyst for H2 evolution, and the HER activity stemmed from the sulfur edges of the MoS2 crystal layers. [10] When grown on graphene sheets, nanostructured MoS2 exhibited excellent HER activity owing to the high exposure of the edges and the strong electronic coupling to the underlying planar support. Incomplete cubane [Mo3S4] + clusters and amorphous MoS2 are also proven HER catalysts. Some of these HER catalysts have been used in photocatalytic H2 production and they exhibited a remarkable promoting effect. MoS2 has a similar structure to graphite; it has a layered crystal structure consisting of S Mo S “sandwiches” held together by van der Waals force. The fact that g-CN and MoS2 have analogous layered structures should minimize the lattice mismatch and facilitate the planar growth of MoS2 slabs over the g-CN surface, thus constructing an organic–inorganic hybrid with graphene-like thin layered heterojunctions (Scheme 1a). Such a distinct nanoscale structure has some advantages. It can increase the accessible area around the planar interface of the MoS2 and g-CN layers and diminish the barriers for electron transport through the co-catalyst, thus facilitating fast electron transfer across the interface by the electron tunneling effect. Also, thin layers can lessen the light blocking effect of the co-catalyst, thus improving the light utilization by g-CN. Importantly, the intrinsic band structures [*] Y. Hou, J. Zhang, G. Zhang, Y. Zhu, Prof. X. Wang Research Institute of Photocatalysis, College of Chemistry and Chemical Engineering, Fuzhou University Fuzhou 350002 (China) E-mail: xcwang@fzu.edu.cn

Hydrogen evolution on nano-particulate transition metal sulfides

The hydrogen evolution reaction (HER) on carbon supported MoS2 nanoparticles is investigated and compared to findings with previously published work on Au(111) supported MoS2. An investigation into MoS2 oxidation is presented and used to quantify the surface concentration of MoS2. Other metal sulfides with morphologies similar to MoS2 such as WS2, cobalt-promoted WS2, and cobalt-promoted MoS2 were also investigated in the search for improved HER activity. Experimental findings are compared to density functional theory (DFT) calculated values for the hydrogen binding energies (deltaGH) on each system.

Understanding the electrocatalysis of oxygen reduction on platinum and its alloys

The high cost of low temperature fuel cells is to a large part dictated by the high loading of Pt required to catalyse the oxygen reduction reaction (ORR). Arguably the most viable route to decrease the Pt loading, and to hence commercialise these devices, is to improve the ORR activity of Pt by alloying it with other metals. In this perspective paper we provide an overview of the fundamentals underlying the reduction of oxygen on platinum and its alloys. We also report the ORR activity of Pt5La for the first time, which shows a 3.5- to 4.5-fold improvement in activity over Pt in the range 0.9 to 0.87 V, respectively. We employ angle resolved X-ray photoelectron spectroscopy and density functional theory calculations to understand the activity of Pt5La.

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.

Discovery of a Ni-Ga catalyst for carbon dioxide reduction to methanol

The use of methanol as a fuel and chemical feedstock could become very important in the development of a more sustainable society if methanol could be efficiently obtained from the direct reduction of CO2 using solar-generated hydrogen. If hydrogen production is to be decentralized, small-scale CO2 reduction devices are required that operate at low pressures. Here, we report the discovery of a Ni-Ga catalyst that reduces CO2 to methanol at ambient pressure. The catalyst was identified through a descriptor-based analysis of the process and the use of computational methods to identify Ni-Ga intermetallic compounds as stable candidates with good activity. We synthesized and tested a series of catalysts and found that Ni5Ga3 is particularly active and selective. Comparison with conventional Cu/ZnO/Al2O3 catalysts revealed the same or better methanol synthesis activity, as well as considerably lower production of CO. We suggest that this is a first step towards the development of small-scale low-pressure devices for CO2 reduction to methanol.

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.

Using TiO2 as a Conductive Protective Layer for Photocathodic H2 Evolution

Surface passivation is a general issue for Si-based photoelectrodes because it progressively hinders electron conduction at the semiconductor/electrolyte interface. In this work, we show that a sputtered 100 nm TiO(2) layer on top of a thin Ti metal layer may be used to protect an n(+)p Si photocathode during photocatalytic H(2) evolution. Although TiO(2) is a semiconductor, we show that it behaves like a metallic conductor would under photocathodic H(2) evolution conditions. This behavior is due to the fortunate alignment of the TiO(2) conduction band with respect to the hydrogen evolution potential, which allows it to conduct electrons from the Si while simultaneously protecting the Si from surface passivation. By using a Pt catalyst the electrode achieves an H(2) evolution onset of 520 mV vs NHE and a Tafel slope of 30 mV when illuminated by the red part (λ > 635 nm) of the AM 1.5 spectrum. The saturation photocurrent (H(2) evolution) was also significantly enhanced by the antireflective properties of the TiO(2) layer. It was shown that with proper annealing conditions these electrodes could run 72 h without significant degradation. An Fe(2+)/Fe(3+) redox couple was used to help elucidate details of the band diagram.