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Sophisticated shape-controlled design is yielding ever more active nanocatalysts [Also see Report by Zhang et al. ] Around 360 BCE, in his work Timaeus , the Greek philosopher Plato elaborated on the four elements as the basic components of our cosmos: earth, water, air, and fire. He argued that each element consists of small, highly symmetric corpuscles—the cube for earth, the tetrahedron for fire, the icosahedron for water, and the octahedron for air. The faces of the latter three corpuscles consist of equilateral triangles, which—according to Plato—allows air, water, and fire to interconvert. Plato would likely be thrilled to learn that, as recently confirmed by Huang et al. ( 1 ), nanoscale Pt-Ni octahedra are the catalytically most active known material for converting air (molecular oxygen) into water and fire (thermal energy). On page 412 of this issue, Zhang et al. ( 2 ) show that octahedral and cubic hollow shells of just a few atomic Pt layers are also versatile catalysts, with the octahedral shells particularly active for oxygen reduction. Such tiny metallic octahedra may one day become the building blocks of electrodes for electrochemical energy conversion.

Proton exchange membrane fuel cells have been recently developed at an increasing pace as clean energy conversion devices for stationary and transport sector applications. High platinum cathode loadings contribute significantly to costs. This is why improved catalyst and support materials as well as catalyst layer design are critically needed. Recent advances in nanotechnologies and material sciences have led to the discoveries of several highly promising families of materials. These include platinum-based alloys with shape-selected nanostructures, platinum-group-metal-free catalysts such as metal-nitrogen-doped carbon materials and modification of the carbon support to control surface properties and ionomer/catalyst interactions. Furthermore, the development of advanced characterization techniques allows a deeper understanding of the catalyst evolution under different conditions. This review focuses on all these recent developments and it closes with a discussion of future research directions in the field. The high platinum loadings at the cathodes of proton exchange membrane fuel cells significantly contribute to the cost of these clean energy conversion devices. Here, the authors critically review and discuss recent developments on low- and non-platinum-based cathode catalysts and catalyst layers.

The electrochemical conversion of 5-Hydroxymethylfurfural, especially its reduction, is an attractive green production pathway for carbonaceous e-chemicals. We demonstrate the reduction of 5-Hydroxymethylfurfural to 5-Methylfurfurylalcohol under strongly alkaline reaction environments over oxide-derived Cu bimetallic electrocatalysts. We investigate whether and how the surface catalysis of the MO x phases tune the catalytic selectivity of oxide-derived Cu with respect to the 2-electron hydrogenation to 2.5-Bishydroxymethylfuran and the (2 + 2)-electron hydrogenation/hydrogenolysis to 5-Methylfurfurylalcohol. We provide evidence for a kinetic competition between the evolution of H 2 and the 2-electron hydrogenolysis of 2.5-Bishydroxymethylfuran to 5-Methylfurfurylalcohol and discuss its mechanistic implications. Finally, we demonstrate that the ability to conduct 5-Hydroxymethylfurfural reduction to 5-Methylfurfurylalcohol in alkaline conditions over oxide-derived Cu/MO x Cu foam electrodes enable an efficiently operating alkaline exchange membranes electrolyzer, in which the cathodic 5-Hydroxymethylfurfural valorization is coupled to either alkaline oxygen evolution anode or to oxidative 5-Hydroxymethylfurfural valorization. Electrochemical conversion of biomass emerges as a green alternative to high pressure, high temperature and hazardous equivalents from classical organic chemistry. Here, the authors show that the electrochemical hydrogenolysis can prevail over the hydrogenation under strong alkaline conditions.

Designing active and stable electrocatalysts with economic efficiency for acidic oxygen evolution reaction is essential for developing proton exchange membrane water electrolyzers. Herein, we report on a cobalt oxide incorporated with iridium single atoms (Ir-Co 3 O 4 ), prepared by a mechanochemical approach. Operando X-ray absorption spectroscopy reveals that Ir atoms are partially oxidized to active Ir >4+ during the reaction, meanwhile Ir and Co atoms with their bridged electrophilic O ligands acting as active sites, are jointly responsible for the enhanced performance. Theoretical calculations further disclose the isolated Ir atoms can effectively boost the electronic conductivity and optimize the energy barrier. As a result, Ir-Co 3 O 4 exhibits significantly higher mass activity and turnover frequency than those of benchmark IrO 2 in acidic conditions. Moreover, the catalyst preparation can be easily scaled up to gram-level per batch. The present approach highlights the concept of constructing single noble metal atoms incorporated cost-effective metal oxides catalysts for practical applications. Designing active, stable and cost-effective catalysts for the acidic oxygen evolution reaction remains a challenge. Here, the authors report iridium single atoms incorporated cobalt oxides, showing distinctly enhanced performance in the acid.

Coupled tandem electrolyzer concepts have been predicted to offer kinetic benefits to sluggish catalytic reactions thanks to their flexibility of reaction environments in each cell. Here we design, assemble, test, and analyze the first complete low-temperature, neutral-pH, cathode precious metal-free tandem CO 2 electrolyzer cell chain. The tandem system couples an Ag-free CO 2 -to-CO 2 /CO electrolyzer (cell-1) to a CO 2 /CO-to-C 2+ product electrolyzer (cell-2). Cell-1 and cell-2 incorporate selective Ni-N-C-based and Cu-based Gas Diffusion Cathodes, respectively, and operate at sustainable neutral pH conditions. Using our tandem cell system, we report strongly enhanced rates for the production of ethylene (by 50%) and alcohols (by 100%) and a sharply increased C 2+ energy efficiency (by 100%) at current densities of up to 700 mA cm −2 compared to the single CO 2 -to-C 2+ electrolyzer cell system approach. This study demonstrates that coupled tandem electrolyzer cell systems can offer kinetic and practical energetic benefits over single-cell designs for the production of value-added C 2+ chemicals and fuels directly from CO 2 feeds without intermediate separation or purification. Tandem concepts for electroreduction of CO 2 offer a valuable toolkit to tackle sluggish reaction kinetics and raise the production of e-chemicals. Here, the authors report a cascade system with two coupled electrolyzers using Ni-N-C and Cu-based catalysts for enhanced CO 2 to multi-carbon conversion

Exploring an active and cost-effective electrocatalyst alternative to carbon-supported platinum nanoparticles for alkaline hydrogen evolution reaction (HER) have remained elusive to date. Here, we report a catalyst based on platinum single atoms (SAs) doped into the hetero-interfaced Ru/RuO 2 support (referred to as Pt-Ru/RuO 2 ), which features a low HER overpotential, an excellent stability and a distinctly enhanced cost-based activity compared to commercial Pt/C and Ru/C in 1 M KOH. Advanced physico-chemical characterizations disclose that the sluggish water dissociation is accelerated by RuO 2 while Pt SAs and the metallic Ru facilitate the subsequent H* combination. Theoretical calculations correlate with the experimental findings. Furthermore, Pt-Ru/RuO 2 only requires 1.90 V to reach 1 A cm −2 and delivers a high price activity in the anion exchange membrane water electrolyzer, outperforming the benchmark Pt/C. This research offers a feasible guidance for developing the noble metal-based catalysts with high performance and low cost toward practical H 2 production. Exploring an active and cost-effective catalyst for alkaline hydrogen evolution reaction remains elusive to date. Here, the authors report the platinum single-atoms doped ruthenium/ruthenium oxides showing distinctly enhanced catalytic performance.

The reduction of Pt content in the cathode for proton exchange membrane fuel cells is highly desirable to lower their costs. However, lowering the Pt loading of the cathodic electrode leads to high voltage losses. These voltage losses are known to originate from the mass transport resistance of O 2 through the platinum–ionomer interface, the location of the Pt particle with respect to the carbon support and the supports’ structures. In this study, we present a new Pt catalyst/support design that substantially reduces local oxygen-related mass transport resistance. The use of chemically modified carbon supports with tailored porosity enabled controlled deposition of Pt nanoparticles on the outer and inner surface of the support particles. This resulted in an unprecedented uniform coverage of the ionomer over the high surface-area carbon supports, especially under dry operating conditions. Consequently, the present catalyst design exhibits previously unachieved fuel cell power densities in addition to high stability under voltage cycling. Thanks to the Coulombic interaction between the ionomer and N groups on the carbon support, homogeneous ionomer distribution and reproducibility during ink manufacturing process is ensured. Reducing Pt content in cathodes for proton exchange membrane fuel cells is crucial to lower costs but results in high voltage losses. A Pt catalyst/support design that substantially reduces local oxygen-related mass transport resistance is reported.

Cu oxides catalyze the electrochemical carbon dioxide reduction reaction (CO2RR) to hydrocarbons and oxygenates with favorable selectivity. Among them, the shape-controlled Cu oxide cubes have been most widely studied. In contrast, we report on novel 2-dimensional (2D) Cu(II) oxide nanosheet (CuO NS) catalysts with high C 2+ products, selectivities (> 400 mA cm −2 ) in gas diffusion electrodes (GDE) at industrially relevant currents and neutral pH. Under applied bias, the (001)-orientated CuO NS slowly evolve into highly branched, metallic Cu 0 dendrites that appear as a general dominant morphology under electrolyte flow conditions, as attested by operando X-ray absorption spectroscopy and in situ electrochemical transmission electron microscopy (TEM). Millisecond-resolved differential electrochemical mass spectrometry (DEMS) track a previously unavailable set of product onset potentials. While the close mechanistic relation between CO and C 2 H 4 was thereby confirmed, the DEMS data help uncover an unexpected mechanistic link between CH 4 and ethanol. We demonstrate evidence that adsorbed methyl species, *CH 3 , serve as common intermediates of both CH 3 H and CH 3 CH 2 OH and possibly of other CH 3 -R products via a previously overlooked pathway at (110) steps adjacent to (100) terraces at larger overpotentials. Our mechanistic conclusions challenge and refine our current mechanistic understanding of the CO 2 electrolysis on Cu catalysts. Copper oxides (CuO) can selectively catalyze the electrochemical reduction of CO 2 to hydrocarbons and oxygenates. Here, the authors study the activity and morphological evolution of 2D CuO nanosheets under applied electrode potentials to conclude the primacy of dendritic shapes and involvement of a new coupling pathway.

Direct electrochemical reduction of CO 2 to fuels and chemicals using renewable electricity has attracted significant attention partly due to the fundamental challenges related to reactivity and selectivity, and partly due to its importance for industrial CO 2 -consuming gas diffusion cathodes. Here, we present advances in the understanding of trends in the CO 2 to CO electrocatalysis of metal- and nitrogen-doped porous carbons containing catalytically active M–N x moieties (M = Mn, Fe, Co, Ni, Cu). We investigate their intrinsic catalytic reactivity, CO turnover frequencies, CO faradaic efficiencies and demonstrate that Fe–N–C and especially Ni–N–C catalysts rival Au- and Ag-based catalysts. We model the catalytically active M–N x moieties using density functional theory and correlate the theoretical binding energies with the experiments to give reactivity-selectivity descriptors. This gives an atomic-scale mechanistic understanding of potential-dependent CO and hydrocarbon selectivity from the M–N x moieties and it provides predictive guidelines for the rational design of selective carbon-based CO 2 reduction catalysts. Inexpensive and selective electrocatalysts for CO 2 reduction hold promise for sustainable fuel production. Here, the authors report N-coordinated, non-noble metal-doped porous carbons as efficient and selective electrocatalysts for CO 2 to CO conversion.

Unlike energy efficiency and selectivity challenges, the kinetic effects of impure or intentionally mixed CO2 feeds on the catalytic reactivity of the direct electrochemical CO2 reduction reaction (CO2RR) have been poorly studied. Given that industrial CO2 feeds are often contaminated with CO, a closer investigation of the CO2RR under CO2/CO co-feed conditions is warranted. Here, we report mechanistic insights into the CO2RR reactivity of CO2/CO co-feeds on Cu-based nanocatalysts. Kinetic isotope-labelling experiments—performed in an operando differential electrochemical mass spectrometry capillary flow cell with millisecond time resolution—showed an unexpected enhanced production of C2H4, with a yield increase of almost 50%, from a cross-coupled 12CO2–13CO reactive pathway. The results suggest the absence of site competition between CO2 and CO molecules on the reactive surface at the reactant-specific sites. The practical significance of sustained local interfacial CO partial pressures under CO2 depletion is demonstrated by metallic/non-metallic Cu/Ni–N-doped carbon tandem catalysts. Our findings show the mechanistic origin of improved C2 product formation under co-feeding, but also highlight technological opportunities of impure CO2/CO process feeds for H2O/CO2 co-electrolysers.