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Understanding how to tune enzymatic activity is important not only for biotechnological applications, but also to elucidate the basic principles guiding the design and optimization of biological systems in nature. So far, the Michaelis-Menten equation has provided a fundamental framework of enzymatic activity. However, there is still no concrete guideline on how the parameters should be optimized towards higher activity. Here, we demonstrate that tuning the Michaelis-Menten constant ( K m ) to the substrate concentration ( [ S ] ) enhances enzymatic activity. This guideline ( K m = [ S ] ) was obtained mathematically by assuming that thermodynamically favorable reactions have higher rate constants, and that the total driving force is fixed. Due to the generality of these thermodynamic considerations, we propose K m = [ S ] as a general concept to enhance enzymatic activity. Our bioinformatic analysis reveals that the K m and in vivo substrate concentrations are consistent across a dataset of approximately 1000 enzymes, suggesting that even natural selection follows the principle K m = [ S ] . Currently, there is no well-defined strategy to increase the activity of enzymes. Here, the authors provide mathematical evidence that adjusting the Michaelis-Menten constant to the substrate concentration maximizes enzymatic activity.

Active and stable electrocatalysts for the oxygen evolution reaction are required to produce hydrogen from water using renewable electricity. Here we report that incorporating Mn into the spinel lattice of Co 3 O 4 can extend the catalyst lifetime in acid by two orders of magnitude while maintaining the activity. The activation barrier of the obtained spinel Co 2 MnO 4 is comparable to that of state-of-the-art iridium oxides, most probably due to the ideal binding energies of the oxygen evolution reaction intermediates, as shown using density functional theory calculations. The calculations also show that the thermodynamic landscape of Co 2 MnO 4 suppresses dissolution, which results in a lifetime of over 2 months (1,500 hours) at 200 mA cm −2 geo at pH 1. As the lifetimes of other 3 d metal oxygen evolution catalysts are in the order of days and weeks, despite current densities being lower by an order of magnitude, our results are an important step towards the realization of noble-metal-free water electrolysers. Polymer electrolyte membrane water electrolysis is more efficient than its alkaline counterpart, but its implementation, in part, hinges on developing Earth-abundant catalysts that are active and stable for the oxygen evolution reaction in acid. Now, it is shown that incorporating Mn into Co 3 O 4 substantially extends the catalyst lifetime in acidic electrolyte while maintaining the activity.

SignificanceMolybdenum sulfide (MoS2) is the most studied two-dimensional (2D) material bar graphene. Current research on crystal-phase engineering focuses almost exclusively on the improvement of catalytic activity. However, the potential advantages of phase engineering toward regulation of selectivity control during multistep catalytic processes remain unexplored. Here, we report atomic-scale evidence on how metallic MoS2 shows significantly higher selectivity compared to the semiconducting phase during multielectron reduction of nitrite to nitrous oxide. Namely, a reaction intermediate specific to metallic MoS2 increases the selectivity by decoupling the proton and electron transfer steps. This has previously been shown to be a universal mechanism to enhance selectivity, and therefore, our work opens directions of the application of 2D materials toward selective electrocatalysis. Molybdenum sulfide (MoS2) is the most widely studied transition-metal dichalcogenide (TMDs) and phase engineering can markedly improve its electrocatalytic activity. However, the selectivity toward desired products remains poorly explored, limiting its application in complex chemical reactions. Here we report how phase engineering of MoS2 significantly improves the selectivity for nitrite reduction to nitrous oxide, a critical process in biological denitrification, using continuous-wave and pulsed electron paramagnetic resonance spectroscopy. We reveal that metallic 1T-MoS2 has a protonation site with a pKa of ∼5.5, where the proton is located ∼3.26 Å from redox-active Mo site. This protonation site is unique to 1T-MoS2 and induces sequential proton−electron transfer which inhibits ammonium formation while promoting nitrous oxide production, as confirmed by the pH-dependent selectivity and deuterium kinetic isotope effect. This is atomic-scale evidence of phase-dependent selectivity on MoS2, expanding the application of TMDs to selective electrocatalysis.

The selective transformation of nitrogen compounds is a foundation of the modern chemical industry. Existing thermochemical processes largely rely on fossil fuels and innovating electrocatalytic processes that could use renewable energy remains challenging. Here we report the electrochemical regulation of a nitrite reduction network using a molybdenum sulfide catalyst by modulating the thermodynamic driving force of proton and electron transfer. The strategy behind this approach is based on the theory of sequential proton–electron transfer, in which the driving force of proton and electron transfer can be optimized independently. This makes it possible to target the desired reactions with selectivities of up to 80% for NO, 61% for N 2 O, 36% for N 2 and 100% for NH 4 + , comparable to the highest values reported to date using a specific catalyst optimized for a single target product. Consistency with numerical simulation highlights that sequential proton–electron transfer can be used to rationally regulate the electrochemical nitrogen network. Selective electrocatalytic conversion of nitrogen species requires control over proton and electron transfer. Here, independent optimization of the driving force for proton transfer is realized through the use of MoS 2 phases with different p K a s, allowing high selectivity for NO, N 2 O, N 2 and NH 4 + to be achieved by varying the applied potential.

Cells harvest energy from ionic gradients by selective ion transport across membranes, and the same principle is recently being used for osmotic power generation from salinity gradients at ocean-river interfaces. Common to these ionic gradient conversions is that they require intricate nanoscale structures. Here, we show that natural submarine serpentinite-hosted hydrothermal vent (HV) precipitates are capable of converting ionic gradients into electrochemical energy by selective transport of Na + , K + , H + , and Cl - . Layered hydroxide nanocrystals are aligned radially outwards from the HV fluid channels, constituting confined nanopores that span millimeters in the HV wall. The nanopores change the surface charge depending on adsorbed ions, allowing the mineral to function as a cation- and anion-selective ion transport membrane. Our findings indicate that chemical disequilibria originating from flow and concentration gradients in geologic environments generate confined nanospaces which enable the spontaneous establishment of osmotic energy conversion. Nakamura and colleagues show selective ion transport through mineral nanochannels in submarine hydrothermal vent (HV) precipitates, enabling HVs to convert ionic gradients into electrochemical energy, similar to cellular systems

Earth-abundant, acid-stable catalysts for the oxygen evolution reaction are essential for terawatt-scale hydrogen production using proton exchange membrane (PEM) electrolysers. Here we report that optimizing the lattice oxygen structure of manganese oxide allows it to sustain the oxygen evolution reaction for over one month at 1,000 mA cm −2 in 1 M H 2 SO 4 . The lifetime enhancement was achieved by substituting pyramidal oxygen with planar oxygen, which has a stronger Mn–O bond and thus suppresses the dissolution of manganese ions. Calculations show that the lattice oxygen dissolution is the bottleneck of deactivation, and this process is less favourable by over 0.2 eV on planar oxygen compared with pyramidal oxygen. Our material shows excellent performance even in a PEM electrolyser, reaching 2,000 mA cm −2 at 2 V with durability exceeding 1,000 h at 200 mA cm −2 . This study expands the potential of Earth-abundant catalysts for PEM electrolysis, which may mitigate the reliance on iridium. Precious-metal-free catalysts for water oxidation commonly suffer from low stability in acidic electrolytes. Now, by controlling the intergrowth of the γ-MnO 2 structure, it has been possible to achieve 2 A cm −2 at 2 V and a stability of over 1,000 hours at 200 mA cm −2 in a polymer electrolyte membrane electrolyser.

Molybdenum sulfide (MoS₂) is the most widely studied transitionmetal dichalcogenide (TMDs) and phase engineering can markedly improve its electrocatalytic activity. However, the selectivity toward desired products remains poorly explored, limiting its application in complex chemical reactions. Here we report how phase engineering of MoS₂ significantly improves the selectivity for nitrite reduction to nitrous oxide, a critical process in biological denitrification, using continuous-wave and pulsed electron paramagnetic resonance spectroscopy. We reveal that metallic 1T-MoS₂ has a protonation site with a pKₐ of ∼5.5, where the proton is located ∼3.26 Å from redoxactive Mo site. This protonation site is unique to 1T-MoS₂ and induces sequential proton–electron transfer which inhibits ammonium formation while promoting nitrous oxide production, as confirmed by the pH-dependent selectivity and deuterium kinetic isotope effect. This is atomic-scale evidence of phase-dependent selectivity on MoS₂, expanding the application of TMDs to selective electrocatalysis.

Molybdenum sulfide (MoS 2 ) is the most studied two-dimensional (2D) material bar graphene. Current research on crystal-phase engineering focuses almost exclusively on the improvement of catalytic activity. However, the potential advantages of phase engineering toward regulation of selectivity control during multistep catalytic processes remain unexplored. Here, we report atomic-scale evidence on how metallic MoS 2 shows significantly higher selectivity compared to the semiconducting phase during multielectron reduction of nitrite to nitrous oxide. Namely, a reaction intermediate specific to metallic MoS 2 increases the selectivity by decoupling the proton and electron transfer steps. This has previously been shown to be a universal mechanism to enhance selectivity, and therefore, our work opens directions of the application of 2D materials toward selective electrocatalysis. Molybdenum sulfide (MoS 2 ) is the most widely studied transition-metal dichalcogenide (TMDs) and phase engineering can markedly improve its electrocatalytic activity. However, the selectivity toward desired products remains poorly explored, limiting its application in complex chemical reactions. Here we report how phase engineering of MoS 2 significantly improves the selectivity for nitrite reduction to nitrous oxide, a critical process in biological denitrification, using continuous-wave and pulsed electron paramagnetic resonance spectroscopy. We reveal that metallic 1T-MoS 2 has a protonation site with a p K a of ∼5.5, where the proton is located ∼3.26 Å from redox-active Mo site. This protonation site is unique to 1T-MoS 2 and induces sequential proton−electron transfer which inhibits ammonium formation while promoting nitrous oxide production, as confirmed by the pH-dependent selectivity and deuterium kinetic isotope effect. This is atomic-scale evidence of phase-dependent selectivity on MoS 2 , expanding the application of TMDs to selective electrocatalysis.

Design principles to improve enzymatic activity are essential to promote energy-material conversion using biological systems. For more than a century, the Michaelis-Menten equation has provided a fundamental framework of enzymatic activity. However, there is still no concrete guideline on how the parameters should be optimized to enhance enzymatic activity. Here, we demonstrate that tuning the Michaelis-Menten constant (Km) to the substrate concentration (S) maximizes enzymatic activity. This guideline (Km=S) was obtained by applying the Bronsted (Bell)-Evans-Polanyi (BEP) principle of heterogeneous catalysis to the Michaelis-Menten equation, and is robust even with mechanistic deviations such as reverse reactions and inhibition. Furthermore, Km and S are consistent to within an order of magnitude over an experimental dataset of approximately 1000 wild-type enzymes, suggesting that even natural selection follows this principle. The concept of an optimum Km offers the first quantitative guideline towards improving enzymatic activity which can be used for highthroughput enzyme screening.Competing Interest StatementThe authors have declared no competing interest.

Electrocatalytic activity is markedly influenced by the Tafel slope, which dictates the sensitivity of the catalytic current with respect to the potential. Differences in the Tafel slope between electrocatalysts have been rationalized based on redox pre-equilibria preceding the rate-determining step. However, no study has experimentally observed a non-rate-determining redox event that directly correlates with the Tafel slope. Here, we show that the Tafel slope of the oxygen evolution reaction (OER) on alpha-MnO2 is markedly influenced by a Mn(III)/Mn(II) redox process located 500 mV more negative than the OER onset potential. Upon repeated cyclic voltammetry sweeps, a correlation was observed between the Tafel slope and the peak position of a redox event, which was assigned to a Mn(III)/Mn(II) redox couple based on the potential dependence of the UV-vis spectra. Notably, the UV-vis absorption of Mn(III) did not diminish even after the OER was initiated, and another redox process was observed at the OER onset, indicating that Mn(III)/Mn(II) is not the rate-determining step for OER on alpha-MnO2 in alkaline pH. Numerical simulations using physicochemical parameters of Mn(III)/Mn(II) obtained from trumpet plot analyses reproduce the experimental Tafel plots, indicating that even non-rate-determining steps should be considered during catalyst design.