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.

The cobalt cubium Co4O4(OAc)4(py)4(ClO4) (1A(+)) containing the mixed valence [Co4O4](5+) core is shown by multiple spectroscopic methods to react with hydroxide (OH(-)) but not with water molecules to produce O2. The yield of reaction products is stoichiometric (>99.5%): 41A(+) + 4OH(-) → O2 + 2H2O + 41A. By contrast, the structurally homologous cubium Co4O4(trans-OAc)2(bpy)4(ClO4)3, 1B(ClO4)3, produces no O2. EPR/NMR spectroscopies show clean conversion to cubane 1A during O2 evolution with no Co(2+) or Co3O4 side products. Mass spectrometry of the reaction between isotopically labeled μ-(16)O(bridging-oxo) 1A(+) and (18)O-bicarbonate/water shows (1) no exchange of (18)O into the bridging oxos of 1A(+), and (2) (36)O2 is the major product, thus requiring two OH(-) in the reactive intermediate. DFT calculations of solvated intermediates suggest that addition of two OH(-) to 1A(+) via OH(-) insertion into Co-OAc bonds is energetically favored, followed by outer-sphere oxidation to intermediate [1A(OH)2](0). The absence of O2 production by cubium 1B(3+) indicates the reactive intermediate derived from 1A(+) requires gem-1,1-dihydoxo stereochemistry to perform O-O bond formation. Outer-sphere oxidation of this intermediate by 2 equiv of 1A(+) accounts for the final stoichiometry. Collectively, these results and recent literature (Faraday Discuss., doi:10.1039/C5FD00076A and J. Am. Chem. Soc. 2015, 137, 12865-12872) validate the [Co4O4](4+/5+) cubane core as an intrinsic catalyst for oxidation of hydroxide by an inner-sphere mechanism.

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.

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We report a soft-templating method for the synthesis of high surface area nickel phosphide catalyst (Ni2P). Ni2P exhibits a 40–50% CO2 reduction products selectivity over H2 formation at current densities ranging from 50–300 mA cm−2 in a flow cell.

The R,R and S,S enantiomers of N,N'‐bis(1‐phenylpropyl)‐2,6‐pyridinedicarboxamide, L(Et), react with Ln3+ ions (Ln = La, Eu, Gd, and Tb) to give stable [Ln((R,R)‐ and (S,S)‐L(Et))3]3+ in anhydrous acetonitrile solution, as evidenced by various spectroscopic measurements, including NMR and luminescence titrations. In addition to the characteristic Eu3+ and Tb3+ luminescence bands, the steady‐state and time‐resolved luminescence spectra of the aforementioned complexes show the residual ligand‐centered emission of the 1ππ* to 3ππ* states, indicating an incomplete intersystem crossing (ISC) transfer from the 1ππ* to 3ππ* and ligand‐to‐Ln3+ energy transfer, respectively. The high circularly polarized luminescence (CPL) activity of [Eu(L(Et))3]3+ confirms that using a single enantiomer of L(Et) induces the preferential formation of one chiral [Eu(L(Et))3]3+ complex, consistent with the [EuL3]3+ complexes formed with other ligands derived from a 2,6‐pyridine dicarboxamide moiety. Furthermore, the CPL sign patterns of complexes with (R,R) or (S,S) enantiomer of L(Et) are consistent with the CPL sign pattern of related [LnL3]3+ complexes with the (R,R) or (S,S) enantiomer of the respective ligands in this family.