One-atom-thick hexagonal boron nitride co-catalyst for enhanced oxygen evolution reactions

Developing efficient (co-)catalysts with optimized interfacial mass and charge transport properties is essential for enhanced oxygen evolution reaction (OER) via electrochemical water splitting. Here we report one-atom-thick hexagonal boron nitride (hBN) as an attractive co-catalyst with enhanced OER efficiency. Various electrocatalytic electrodes are encapsulated with centimeter-sized hBN films which are dense and impermeable so that only the hBN surfaces are directly exposed to reactive species. For example, hBN covered Ni-Fe (oxy)hydroxide anodes show an ultralow Tafel slope of ~30 mV dec−1 with improved reaction current by about 10 times, reaching ~2000 mA cm−2 (at an overpotential of ~490 mV) for over 150 h. The mass activity of hBN co-catalyst is found exceeding that of commercialized catalysts by up to five orders of magnitude. Using isotope experiments and simulations, we attribute the results to the adsorption of oxygen-containing intermediates at the insulating co-catalyst, where localized electrons facilitate the deprotonation processes at electrodes. Little impedance to electron transfer is observed from hBN film encapsulation due to its ultimate thickness. Therefore, our work also offers insights into mechanisms of interfacial reactions at the very first atomic layer of electrodes.

As shown in Supplementary Fig. 1, XPS (Supplementary Fig. 1a, b) analysis prove the B-N chemical bonding structures and UV-visible spectra (Supplementary Fig. 1c) proved that the band gap is consistent with the monolayer hBN reported in the literature 2 .Other characterizations, transmission electron microscope (TEM) and atomic force microscope (AFM) images consistently show that the as-grown hBN is indeed monolayer (Supplementary Fig. 1d-f).The HAADF-STEM (Supplementary Fig. 1g, h) clearly distinguish the B and N atoms in the hexagonal lattice, with higher intensity of N atoms (blue) than that of B atoms (cyan).Fast Fourier transform spots from the whole image (inset of Supplementary Fig. 1g) demonstrate hexagonal spots, assuring the hexagonal structure of the sample.The STEM-EELS spectrum (Supplementary Fig. 1i) also shows two peaks around 198.8 and 410.1 eV, corresponding to K-shell ionization edge of B and N atoms, respectively 1,3 .All these experimental evidences support the monolayer hBN crystal nature of our films.

Ion permeation measurements
To further investigate the impermeability of our CVD hBN crystals, we measured ion transport through the CVD hBN membrane.Schematic of our experimental set-up is shown in Supplementary Fig. 2. The CVD hBN membrane is transferred to a SiN chip with a 2 μm diameter hole, and the SiN chip is mounted in the middle of two reservoirs.Ag/AgCl electrodes are placed inside each reservoir to measure ionic current.No detectable ionic current is obtained within our measurement limit (~5 pA), indicating that the permeability of K + and OH -through the CVD hBN membrane is <10 -11 S. That indicates a membrane porosity <10 -6 , which value is consistent with that estimated in gas permeation experiments.

Gas permeation measurements
To investigate whether our hBN membrane is dense and continuous, we measured helium gas (He) transport through hBN membranes.Schematic of our experimental set-up is shown in Supplementary Fig. 3a.In brief, chemical vapor deposition (CVD) hBN membranes were suspended on an aperture drilled on silicon substrates and were sandwiched between two He leak tight vacuum chambers (leak rate <10 -14 mol s -1 ), following the established methods reported in ref. 4. One chamber is filled with helium gas at pressure PHe = 1 bar, while the other chamber is kept at vacuum and is connected to a He leak detector (Leybold Quadro Dry).Due to the small kinetic diameter of He gases (kinetic diameter 2.6 Å), its permeation can be used to detect angstromscale defects in membranes 5 .As shown in Supplementary Fig. 3b, however, the permeability of helium through the CVD hBN membrane is under our detection limit (<10 -14 mol s -1 ).In a parallel experiment, we measured the helium permeation through the aperture without hBN coverage, which is about 10 -8 mol s -1 .Within our measurement accuracy limit, we estimate a membrane porosity <10 -6 , or 1 nm 2 defective area per micron meter square.As shown in Supplementary Fig. 5a to 5c, X-ray photoelectron spectroscopy (XPS) results confirm the presence of Ni, Fe, and O elements.Specifically, the Ni 2p spectrum shows two peaks that can be assigned to Ni 2+ , with Ni 2p3/2 at 855.4 eV and Ni 2p1/2 at 873.1 eV.The Fe 2p3/2 at 711.5 eV can be assigned to Fe 3+ 6 .The O 1s characteristic peak at 531.0 and 531.9 eV can be attributed to lattice oxygen and hydroxy groups 7 .(X-ray Diffraction) XRD characterization shows no diffraction signals, indicating that the NiFeOxHy layer is likely to be amorphous.Scanning electron microscopy shows that the obtained NiFeOxHy layer is uniform with no visible dis-continuity.Energy dispersive spectrum (EDS) elemental analysis confirms that the Ni, Fe and O elements are uniformly distributed, with a 3:1 atomic ratio between Ni and Fe elements (Supplementary Fig. 5d-f The XPS peak attributed to oxygen (O) demonstrates substantial disparity between its profiles before and after hBN encapsulation (Supplementary Fig. 6c).The hBN/NiFeOxHy shows three O 1s characteristic peaks at 530.9, 531.9, and 533.5 eV.The 530.9 eV peak can be ascribed to lattice oxygen.The peak features positioned at 531.9 and 533.5 eV can be attributed to OH groups and H2O adsorbed on the surface, respectively 6,7 .The ECSA of the hBN/NiFeOxHy is 3.08 cm 2 , and its JECSA is 60 mA cm -2 @1.53 V which is an order of magnitude higher than other NiFeOxHy catalysts in literature 7,8,9 .

Supplementary
EIS was measured in 1M KOH (pH = 13.65).In Supplementary Fig. 8c, the Nyquist plot is fitted using Randles equivalent circuit model.The charge transfer resistance of hBN/NiFeOxHy catalyst is found to be 0.4 Ω cm -2 , which is comparable to that of NiFeOxHy reported literatures 10,11 .This is also consistent with our conclusion that the presence of hBN introduces negligible interlayer charge transfer impedance.

Isotope labelling experiments
NiFeOxHy were labeled with 18 O-isotopes by using H2 18 O solutions for electrochemically deposition.Afterward, the 18 O-labeled catalysts were rinsed with H2 16 O for serval times to remove the remaining H2 18 O.The Raman peaks of the 18 O-labeled hBN/NiFeOxHy shifts to lower wavenumbers as compared to that of 16 O-labeled hBN/NiFeOxHy (Supplementary Fig. 9a), because of the impact of oxygen mass on the vibration mode 12 .This result suggests the successful fabrications of 18 O labelled hBN/NiFeOxHy samples.
The OER performance of 18 O-labeled hBN/NiFeOxHy was measured using an analogous method as described in main texts.To analyze their gas products, OER reactions were performed in a closed electrolytic cell at 1.72 V versus RHE.Gas product was transferred from the chamber to our Gas Chromatography-Mass Spectrometry using an injection needle.Signals of three possible products were monitored: 16 O 16 O, 16 O 18 O, and 18 O 18 O.If lattice oxygen oxidation mechanism (LOM) dominate, 18 O element is expected to be found in the gas products.However, this expectation is found against our experiments, where no 18 O element higher than nature abundance was detected within our detection limit (Supplementary Fig. 9b).

Figure 3 .
Helium transport through hBN membranes.(a) Schematics of gas permeation measurements set-up.Black circles represent rubber O-rings for sealing.(b) Leak rate (i.e.He permeation rate) as a function of time.We fill the top chamber with the He gas at the time point marked as "Helium on", and pumped out the He gas at the time point marked as "Helium off".Supplementary Figure 4. Schematic diagram of wet transfer method.Characterization of NiFeOxHy catalytic layer Supplementary Figure 5. Characterization of the electrochemically deposited NiFeOxHy catalytic layer.(a) to (c) X-ray photoelectron spectroscopy of Ni 2p, Fe 2p and O 1s spectrum of NiFeOxHy, respectively.(d) Scanning electron microscope image of the NiFeOxHy layer.(e) to (f) Energy dispersive spectrum for the elemental distribution in the NiFeOxHy layer.(g) Optical image of NiFeOxHy on Au contact.The NiFeOxHy layer thickness is about 60 nm.
).All these results indicate that the materials prepared are amorphous NiFeOxHy.Supplementary Figure 6.X-ray spectrum characterizations of hBN, NiFeOxHy and hBN/NiFeOxHy heterostructures.(a) Ni 2p, (b) Fe 2p and (c) O 1s spectrum of NiFeOxHy with and without hBN encapsulation.(d) B 1s and (e) N 1s spectrum of hBN before and after assembly on NiFeOxHy layers.No peak shift is observed in all cases, indicating the absence of bonding between hBN and NiFeOxHy layers

Figure 7 .
Stability of hBN/NiFeOxHy heterogeneous electrodes.(a,b) XPS spectra and (c) electron microscope characterization of hBN/NiFeOxHy before and after OER.No detectable peak shift or visible damage of hBN layer is observed.Supplementary Figure 8.The electrochemical active surface area (ECSA) and EIS measurements.(a) Cyclic Voltammetry (CV) curves for hBN/NiFeOxHy carried out in non-faradic regions at different scan rates in 1M KOH.(b) The Cdl calculations.(c) Electrochemical impedance spectra (EIS) for hBN/NiFeOxHy.