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.

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Combining theory and experiment in electrocatalysis: Insights into materials design

Ammonia is an important nutrient for the growth of plants. In industry, ammonia is produced by the energy expensive Haber–Bosch process where dihydrogen and dinitrogen form ammonia at a very high pressure and temperature. In principle one could also reduce dinitrogen upon addition of protons and electrons similar to the mechanism of ammonia production by nitrogenases. Recently, major breakthroughs have taken place in our understanding of biological fixation of dinitrogen, of molecular model systems that can reduce dinitrogen, and in the electrochemical reduction of dinitrogen at heterogeneous surfaces. Yet for efficient reduction of dinitrogen with protons and electrons major hurdles still have to be overcome. In this tutorial review we give an overview of the different catalytic systems, highlight the recent breakthroughs, pinpoint common grounds and discuss the bottlenecks and challenges in catalytic reduction of dinitrogen.

Challenges in reduction of dinitrogen by proton and electron transfer

As one of the most important chemicals and carbon-free energy carriers, ammonia (NH3) has a worldwide annual production of ∼150 million tons, and is mainly produced by the traditional high-temperature and high-pressure Haber–Bosch process which consumes massive amounts of energy. Very recently, electrocatalytic and photo(electro)catalytic reduction of N2 to NH3, which can be performed at ambient conditions using renewable energy, have received tremendous attention. The overall performance of these electrocatalytic and photo(electro)catalytic systems is largely dictated by their core components, catalysts. This perspective for the first time highlights the rational design of electrocatalysts and photo(electro)catalysts for N2 reduction to NH3 under ambient conditions. Fundamental theory of catalytic reaction pathways for the N2 reduction reaction and the corresponding material design principles are introduced first. Then, recently developed electrocatalysts and photo(electro)catalysts are summarized, with a special emphasis on the relationship between their physicochemical properties and NH3 production performance. Finally, the opportunities in this emerging research field, in particular, the strategy of combining experimental and theoretical techniques to design efficient and stable catalysts for NH3 production, are outlined.

Rational design of electrocatalysts and photo(electro)catalysts for nitrogen reduction to ammonia (NH3) under ambient conditions

Theoretical studies of the possibility of forming ammonia electrochemically at ambient temperature and pressure are presented. Density functional theory calculations were used in combination with the computational standard hydrogen electrode to calculate the free energy profile for the reduction of N(2) admolecules and N adatoms on several close-packed and stepped transition metal surfaces in contact with an acidic electrolyte. Trends in the catalytic activity were calculated for a range of transition metal surfaces and applied potentials under the assumption that the activation energy barrier scales with the free energy difference in each elementary step. The most active surfaces, on top of the volcano diagrams, are Mo, Fe, Rh, and Ru, but hydrogen gas formation will be a competing reaction reducing the faradaic efficiency for ammonia production. Since the early transition metal surfaces such as Sc, Y, Ti, and Zr bind N-adatoms more strongly than H-adatoms, a significant production of ammonia compared with hydrogen gas can be expected on those metal electrodes when a bias of -1 V to -1.5 V vs. SHE is applied. Defect-free surfaces of the early transition metals are catalytically more active than their stepped counterparts.

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A theoretical evaluation of possible transition metal electro-catalysts for N2 reduction

After carbon, hydrogen, and oxygen, nitrogen is the next most abundant element in the human body. Inorganic and organic compounds of nitrogen feature prominently in many biological and environmental, as well as industrial, processes. In nature, the inorganic compounds of nitrogen are controlled by a reaction cycle called the nitrogen cycle (Figure 1).1 The denitrification pathway in this cycle, that is, the conversion of nitrate to dinitrogen, is employed by certain bacteria to produce ATP anaerobically and gain energy for cell growth. All organisms use ammonia as one of the starting building blocks for the synthesis of amino acids, nucleotides, and many other important biological compounds. Nitric oxide is * Corresponding author. E-mail: m.koper@chem.leidenuniv.nl. † Present address: Energy research Centre of The Netherlands (ECN), P.O. Box 1, 1755 ZG Petten, The Netherlands. ‡ Present address: Research, Development & Innovation, AkzoNobel Chemicals bv, Velperweg 76, P.O. Box 9300, 6800 SB Arnhem, The Netherlands. Chem. Rev. 2009, 109, 2209–2244 2209

Nitrogen cycle electrocatalysis.

Electrochemical synthesis of ammonia at atmospheric pressure and low temperature in a solid polymer electrolyte cell

The electroreduction of CH2Cl2 at Ni, Cu, Pt and Ag electrodes in acetonitrile and (C4H9)4NI 0.1m as supporting electrolyte was studied. The half-wave potential was found to be in the range −2.2 to −2.5V vs SCE at room temperature. From the analysis of the gaseous products it was found that methane, ethylene, chloromethane, propene and butene isomers were the main products, while at silver and platinum cathodes methane was mainly produced. The effect of the potential on the current efficiency of the gaseous products was also studied. The current efficiency of the products increases at concentration levels of CH2Cl2 up to 0.2m, whereas at higher values its CE is not significantly influenced. The application of the Schultz–Flory distribution analysis to the experimental data showed that the hydrocarbons are mainly formed via polymerization of methylene radicals on the surface of Ni and Cu electrodes. At Ag and Pt electrodes the mechanism appears to be different.

Ch. Lambrou

Papers

Electrochemical synthesis of ammonia at atmospheric pressure and low temperature in a solid polymer electrolyte cell

Electrochemical reduction of dichloromethane to higher hydrocarbons

Electrochemical reduction of dichlorodifluoromethane in acetonitrile medium to useful fluorinated compounds

Electrochemical reduction of CCl2F2 on Nafion solid polymer electrolyte composite electrodes

Electrochemical reduction of dichlorodifluoromethane at a Nafion® solid polymer electrolyte cell