Bioinspired nickel phosphide electrocatalysts achieve breakthrough efficiency and selectivity for C3 and C4 products.

We report microcrystalline Ni3P as a noble-metal-free electrocatalyst for the H2 evolution reaction (HER) with high activity just below those of Ni5P4 and Pt, the two most efficient HER catalysts known. Ni3P has previously been dismissed for the HER, owing to its anticipated corrosion and its low activity when formed as an impurity in amorphous alloys. We observe higher activity of single-phase Ni3P crystallites than for other nickel phosphides (except Ni5P4) in acid, high corrosion tolerance in acid, and zero corrosion in alkali. We compare its electrocatalytic performance, corrosion stability, and intrinsic turnover rate to those of different transition-metal phosphides. Electrochemical studies reveal that poisoning of surface Ni sites does not block the HER, indicating P as the active site. Using density functional theory (DFT), we analyze the thermodynamic stability of Ni3P and compare it to experiments. DFT calculations predict that surface reconstruction of Ni3P (001) strongly favors P enrichment of the Ni4P4 termination and that the H adsorption energy depends strongly on the surface reconstruction, thus revealing a potential synthetic lever for tuning HER catalytic activity. A particular P-enriched reconstructed surface on Ni3P­(001) is predicted to be the most stable surface termination at intermediate P content, as well as providing the most active surface site at low overpotentials. The P adatoms present on this reconstructed surface are more active for HER at low overpotentials in comparison to any of the sites investigated on other terminations of Ni3P­(001), as they possess nearly thermoneutral H adsorption. To our knowledge this is the first time reconstructed surfaces of transition-metal phosphides have been identified as having the most active surface site, with such good agreement with the experimentally observed catalytic current onset and Tafel slope. The active site geometry achieved through reconstruction identified in this work shows great similarity to that reported for Ni2P­(0001) and Ni5P4(0001) facets, serving as a general design principle for the future development of even more active transition-metal phosphide catalysts and further climbing the volcano plot.

Surface-directed corner-sharing MnO 6 octahedra within numerous manganese oxide compounds containing Mn 3+ or Mn 4+ oxidation states show strikingly different catalytic activities for water oxidation, paradoxically poorest for Mn 4+ oxides, regardless of oxidation assay (photochemical and electrochemical). This is demonstrated herein by comparing crystalline oxides consisting of Mn 3+ (manganite, γ-MnOOH; bixbyite, Mn 2 O 3 ), Mn 4+ (pyrolusite, β-MnO 2 ) and multiple monophasic mixed-valence manganese oxides. Like all Mn 4+ oxides, pure β-MnO 2 has no detectable catalytic activity, while γ-MnOOH (tetragonally distorted Mn 3+ O 6 , D 4h symmetry) is significantly more active and Mn 2 O 3 (trigonal antiprismatic Mn 3+ O 6 , D 3d symmetry) is the most active. γ-MnOOH deactivates during catalytic turnover simultaneous with the disappearance of crystallographically defined corner-sharing Mn 3+ O 6 and the appearance of Mn 4+ . In a comparison of 2D-layered crystalline birnessites (δ-MnO 2 ), the monovalent Mn 4+ form is catalytically inert, while the hexagonal polymorph, containing few out-of-layer corner-sharing Mn 3+ O 6 , has ∼10-fold higher catalytic activity than the triclinic polymorph, containing in-plane edge-sharing Mn 3+ O 6 . These electronic and structural correlations point toward the more flexible (corner-shared) Mn 3+ O 6 sites, over more rigid (edge-shared) sites as substantially more active catalytic centers. Electrochemical measurements show and ligand field theory predicts that, among corner-shared Mn 3+ O 6 sites, those possessing D 3d ligand field symmetry have stronger covalent Mn−O bonding to the six equivalent oxygen ligands, which we ascribe as responsible for more efficient and faster electrolytic water oxidation. In contrast, D 4h Mn 3+ O 6 sites have weaker Mn−O bonding to the two axial oxygen ligands, have separated electrochemical oxidation waves for Mn and O, and are catalytically less efficient and exhibit slower catalytic turnover. By controlling the ligand field geometry and strength to oxygen ligands, we have identified the key variables for tuning water oxidation activity by manganese oxides. We apply these findings to propose a mechanism for water oxidation by the CaMn 4 O 5 catalytic site of natural photosynthesis.

Development of efficient electrocatalysts for the CO2 reduction reaction (CO2RR) to multicarbon products has been constrained by high overpotentials and poor selectivity. Here, we introduce iron phosphide (Fe2P) as an earth-abundant catalyst for the CO2RR to mainly C2–C4 products with a total CO2RR Faradaic efficiency of 53% at 0 V vs RHE. Carbon product selectivity is tuned in favor of ethylene glycol formation with increasing negative bias at the expense of C3–C4 products. Both Grand Canonical-DFT (GC-DFT) calculations and experiments reveal that *formate, not *CO, is the initial intermediate formed from surface phosphino-hydrides and that the latter form ionic hydrides at both surface phosphorus atoms (H@Ps) and P-reconstructed Fe3 hollow sites (H@P*). Binding of these surface hydrides weakens with negative bias (reactivity increases), which accounts for both the shift to C2 products over higher C–C coupling products and the increase in the H2 evolution reaction (HER) rate. GC-DFT predicts that phosphino-hydrides convert *formate to *formaldehyde, the key intermediate for C–C coupling, whereas hydrogen atoms on Fe generate tightly bound *CO via sequential PCET reactions to H2O. GC-DFT predicts the peak in CO2RR current density near −0.1 V is due to a local maximum in the binding affinity of *formate and *formaldehyde at this bias, which together with the more labile C2 product affinity, accounts for the shift to ethylene glycol and away from C3–C4 products. Consistent with these predictions, addition of exogenous CO is shown to block all carbon product formation and lower the HER rate. These results demonstrate that the formation of ionic hydrides and their binding affinity, as modulated by the applied potential, controls the carbon product distribution. This knowledge provides new insight into the influence of hydride speciation and applied bias on the chemical reaction mechanism of CO2RR that is relevant to all transition metal phosphides.

Thin-films of cubic-NiP2 and TiN layers are applied on Si for efficient and stable photocathodes.