Charge Photoaccumulation in Covalent Polymer Networks for Boosting Photocatalytic Nitrate Reduction to Ammonia

Abstract In the design of photoelectrocatalytic cells, a key element is effective photogeneration of electron‐hole pairs to drive redox activation of catalysts. Despite recent progress in photoelectrocatalysis, experimental realization of a high‐performance photocathode for multi‐electron reduction of chemicals, such as nitrate reduction to ammonia, has remained a challenge due to difficulty in obtaining efficient electrode configurations for extraction of high‐throughput electrons from absorbed photons. This work describes a new design for catalytic photoelectrodes using chromophore assembly‐functionalized covalent networks for boosting eight‐electron reduction of nitrate to ammonia. Upon sunlight irradiation, the photoelectrode stores a mass of reducing equivalents at the photoexcited chromophore assembly for multielectron reduction of a copper catalyst, enabling efficient nitrate reduction to ammonia. By introducing the new photoelectrode structure, it is demonstrated that the electronic interplay between charge photo‐accumulating assembly and multi‐electron redox catalysts can be optimized to achieve proper balance between electron transfer dynamics and thermodynamic output of photoelectrocatalytic systems.


Synthesis of the electrode substrates
The synthetic procedure for HrHE is illustrated in Figure S1.The HrH scaffold was synthesized by thermal-initiated radical copolymerization [1] of DMA (3.29 M) and sodium 4vinylbenzenesulfonate (0.17 M) in a mixed solvent of DMSO and deionized H2O (volume ratio: The procedure for PEDOT integration to generate HE is the same as that for HrHE.

Synthesis of the photocathode HrHE-CA
In-situ integration of monodispersing CA (CA) particles generates the photocathode, HrHE-CA, as illustrated in Figure S4.Before CA synthesis, the HrHE film was pre-saturated with a DMF solution of 2-aminoterephthalic acid (H2ATA) (75 mM) and ZrCl4 (50 mM) with an acidic modulator, HCl (5% v/v), for 6 hours, followed by in-situ formation of CA in HrHE networks under 120 °C for 6 hours.The photocathode, HrHE-CA, was obtained after rinsing with excess DMF and drying under vacuum for 12 hours.

Catalytic functionalization of the photocathode
The photocathode was functionalized with the Cu catalyst based on a modified photochemical approach reported previously. 14Briefly, the CA photocathode, HrHE-CA, was pre-saturated with the deposition solution containing CuSO4 (50 mM, aq.) for 6 hours, followed by photoelectrochemical deposition in the same solution for 7200 s under an applied bias at -0.3 V vs RHE.The CA photocathode, a platinum mesh and a Ag/AgCl electrode were used as the working, counter and reference electrodes, respectively, in an H-type electrolysis cell with a proton-exchange membrane (Nafion N117).Light illumination was provided by an AM 1.5, 1 Sun solar simulator (Beijing China Education Au-light Co. CEL-S500 with an AM 1.5 filter) whose intensity was calibrated using a standard Si cell.The deposition solution was degassed with Ar for at least 30 minutes prior to experimentation.After photoelectrochemical deposition, the film was rinsed with argon-degassed, deionized H2O to remove unreacted ions.

Characterizations for the covalent networks
The microstructures of the electrodes were analyzed by scanning electron microscopy (SEM, Hitachi SU8010), transmission electron microscopy (TEM, HT7700) and high-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM, JEM 2100F) with energy dispersive X-ray spectrometry (EDX).The electrodes were freeze-dried prior to those measurements.Contact angles were measured at room temperature with a DataPhysics OCA-15EC analyzer.Compression tests were performed by using a tensile tester (Instron, 3343) equipped with a 50 N-force sensor with loading velocity at 2.0 mm/min in air.

Photoelectrocatalytic experiments
The PEC experiments were performed using a typical three-electrode, H-type electrolysis cell with a proton-exchange membrane (Nafion, N117) separating cathodic and anodic compartments.In the reaction cell, the cathodic compartment contains the CA photocathode as the working electrode that is irradiated by the solar simulator (Beijing China Education Au-light Co. CEL-S500 with an AM 1.5 G filter) with light intensity calibrated by a standard Si cell.The anodic compartment was equipped with a platinum mesh and a Ag/AgCl electrode as the counter and reference electrodes, respectively.The electrolyte solution for a typical PEC experiment contains KNO3 (0.10 M) in argon-degassed acetate buffer (1.0 M, pH 4.5) with Na2SO4 (0.50 M) added as a supporting electrolyte.Potentiostats (CH Instruments, 760E, 660E or 920D) were used to collect photocurrent responses and provide biases to the cell.The recorded potential was converted to reversible hydrogen electrode (RHE) scale as follows: E (vs RHE) = E (vs Ag/AgCl) + 0.197 V + 0.059 pH.
After the PEC experiments, the gas products were analyzed by a gas chromatography system (Shimadzu Nexis GC-2030) equipped with both thermal-conductivity and flame-ionization detectors.The liquid products were analyzed by 1 H-NMR spectroscopy and spectrophotometric methods (vide infra).

Photoelectrocatalytic efficiency
The faradaic efficiency (FE) of H2, NO2 -and NH3 was calculated as follows: The yield rate of NH3 was calculated as follows: Yield rate (NH 3 ) = n(NH 3 ) t The selectivity of NH3 was calculated as follows: The external quantum efficiency (EQE) was calculated as follows:

EQE = 8× Moles of NH 3 Moles of incident photons
where F is the Faraday constant, n (x) (x: H2, NO2 -, NH3) is the moles of the product, Q is the total charge passing the electrode, and t is the photoelectrocatalysis time.Background products and currents were subtracted using the same experimental setup in the absence of the samples.

Quantification of ammonia (NH3)
The produced NH3 was quantified by using both indophenol-blue method [2] and 1 H-NMR spectroscopy.For the indophenol-blue method, an aliquot of the electrolyte was taken out from the cathodic compartment and diluted to the detectable range.Typically, 1.0 mL of the diluted electrolyte was mixed with a solution (1.0 mL) containing sodium hydroxide (1.0 M), salicylic acid (5.0 wt%) and sodium citrate (5.0 wt%).The resulting solution was added with sodium hypochlorite (0.50 mL, 0.05 M) and sodium nitroferricyanide (0.10 mL, 1.0 wt%).After sitting in the dark for 2 hours, the solution mixture was examined by UV-Vis absorption spectroscopy using a spectrophotometer (Shimadzu UV-2600i).The concentration of the produced NH3 was determined based on the absorbance at 650 nm using the calibration curve in the following figure.[4] Typically, the diluted electrolyte (125 μL) was mixed with maleic acid (125 μL, 100 μM in DMSO-d6), sulfuric acid (50 μL, 4.0 M in DMSO-d6) and DMSO-d6 (750 μL).The NH3 concentration was obtained from the characteristic peak area (relative to the internal standard) according to the calibration curve given in the following figure.In order to investigate nitrogen source for the produced NH3, 15 N isotope-labeling experiment was conducted by replacing K 14 NO3 with K 15 NO3 (98 atom% 15 N) in the electrolyte for the PEC experiments.The commercial K 15 NO3 was pre-purified to remove possible impurities of 14 NH4 + or 15 NH4 + .The cathodic electrolyte was examined by 1 H-NMR spectroscopy as detailed above to estimate the amount of 15 NH4 + .The 1 H-NMR specta for 15 NH4 + show doublet at 7.12 and 7.24 ppm, and for 14 NH4 + show triplet at 7.09, 7.18 and 7.26 ppm.The signal for the internal standard, maleic acid, appears at 6.25 ppm.
Calibration data for the indophenol-blue and 1 H-NMR methods are shown below for 14 NH4 + (a, b) and 15 NH4 + (c, d).

Quantification of nitrite (NO2 -)
The amount of produced NO2 -was determined by Griess test. [5]Two reagent solutions (denoted as A and B) were prepared before the test.For solution A, p-aminobenzene sulfonamide (1.0 g) was dissolved in H3PO4 aqueous solution (100 mL, 10 vol.%).For solution B, N-(1-Naphthyl) ethylenediamine dihydrochloride (0.10 g) was dissolved in H2O (100 mL).An aliquot of the diluted electrolyte (1.0 mL) was mixed with solutions A and B of 0.50 mL each.After sitting in the dark for 20 minutes, the solution mixture was examined by UV-Vis absorption spectroscopy using the spectrophotometer (Shimadzu UV-2600i).The following figure shows the calibration data by spectroscopic measurements for NH4 + (a,b) and NO2 -(c,d).The concentration of NO2 -was determined based on the absorbance at 540 nm using the calibration curve in the following figure (c,d).

DFT calculations
VASP code was adopted for DFT calculations for the catalytic processes. [6]In this method, Perdew-Burke-Ernzerhof functional within generalized gradient approximation [7] was used to process the exchange-correlation.The project augmented-wave pseudopotential [8] was applied with a kinetic energy cut-off at 500 eV, which was utilized to describe the expansion of the electronic eigenfunctions.The vacuum thickness was set to be 15 Å to minimize interlayer interactions. [9]The Brillouin-zone integration was sampled by a Γ-centered 2 × 2 × 1 Monkhorst-Pack k-point.All atomic positions were fully relaxed until the energy and force reached a tolerance of 1.0 × 10 -5 eV and 0.030 eV/Å, respectively.The dispersion corrected DFT-D method was employed to consider the long-range interactions.The D-band center was determined using the equation, , where ε and n d (ε) represent the energy level and DOS for the d orbitals, respectively. The disappearance of the C=C peak at 1610 cm -1 , compared with the DMA monomer (gray curve), indicates polymerization of DMA to form HrH. In addition, the amide peak at 1648 cm −1 shifts to 1616 cm -1 due to formation of hydrogen bonds in HrH.The characteristic peaks at 1010 cm −1 , 1040 cm -1 (symmetric SO2 stretching) and 2974 cm -1 (aromatic C-H stretching) confirm copolymerization of DMA and NaSS.

Figure S3. Linear sweep voltammograms for HrHE in pH 4.5 acetate buffer (0.10 M) with
Na2S2O3 (20 mM).Scan rate: 5.0 mV s -1 .The Faradaic currents come from oxidation of S2O3 2-to S4O6 2-. [12]The HrHE samples were prepared by polymerizing EDOT at 0.30 M (a) and 0.50 M (b) as a function of polymerization time.The two peaks in the spectrum of Zr 3d (c) at 182.8 and 185.1 eV are attributed to Zr 4+ 3d5/2 and 3d3/2, respectively, suggesting the formation of Zr-O bonds in the metal cluster. [13] The obtained lifetimes τ 1 , 2 , and corresponding percentages, A1, A2, are summarized in Table S8.The weighted average of lifetime () can be calculated by the formula: Quantum yields.The quantum yields were calculated using Ru(bpy)3 2+ as a reference standard according to the equation [14] : The transient absorption probed at 450 nm after excitation at 355 nm was used for calculation. ̅ is the quantum yield, ∆ is the transient absorption at 450 nm,  is the molar absorption coefficient at 450 nm, F is the fraction of light absorption at 355 nm calculated by the formula: F=1-10 -A (A is the absorbance of the samples).The quantum yields of CA-Cu and C-Cu probed at 2.0 μs, were determined to be 0.87 and 0.033, respectively.he two surrounding peaks at 1365 cm -1 and 1508 cm        2.46 [17]   2.83 [17]   ECB: conduction band edge potential.

Figure S2 .
Figure S2.Fourier-transform infrared (FTIR) spectra for DMA, NaSS monomer and HrH, showing the successful combination of NaSS and DMA in HrH.

Figure S4 .
Figure S4.Synthetic procedure for the photocathode with Cu solid catalyst (HrHE-CA-Cu).The Cu catalyst was loaded by site-selective photoelectrochemical deposition at CA in HrHE-CA photocathode (see Methods for the synthetic details).

Figure
Figure S6.X-ray photoelectron spectroscopy (XPS) spectra for CA in the CA photocathode.a, O 1s; b, N 1s; c, Zr 3d.The high-resolution spectrum of O 1s (a) displays the contributions of the O species associated with the linker units.The deconvolution revealed the prominent presence of C=O (531.9 eV) and Zr-O (530.8 eV) species.The spectrum of N 1s (b) shows the contribution of -NH2 bonded to the phenylene of the linker (-NH2, 399.1 eV) and the protonated amidogen form (-NH3 + , 400.1 eV).

Figure S8 .
Figure S8.Normalized absorption spectra of C and CA.Samples were dispersed in DMF for measurement.

Figure S9 .
Figure S9.UV-vis absorption spectra (a), Tauc plot (b) and Mott−Schottky plots (c) for PEDOT.The band gap and flat band potential of PEDOT are determined to be 1.39 eV and 0.59 V, respectively.For the Mott-Schottky measurements, the voltage range was 0.90 to 0.10 V (vs NHE) with an amplitude of 0.005 V at frequencies of 800, 900, 1000 Hz.The recorded potential was converted to normal hydrogen electrode (NHE) according to ENHE = EAg/AgCl + 0.197 V.

Figure S11 .
Figure S11.XPS spectra for HrHE-CA.The two peaks in the deconvoluted XPS spectrum of Zr 3d (a) at 182.7 and 185.1 eV are attributed to Zr 4+ 3d5/2 and 3d3/2, respectively, suggesting the formation of Zr-O bonds in the metal cluster and the assembly of chromophore.The high-resolution spectrum of N 1s (b) shows the contribution of -NH2 bonded to the phenylene of the linker (-NH2, 399.5 eV) and the protonated amidogen form (-NH3 + , 400.5 eV).

Figure S12 .
Figure S12.XPS spectra for HrHE-CA-Cu.The two peaks in the XPS spectrum of Zr 3d (a) at 182.3 and 184.6 eV are attributed to Zr 4+ 3d5/2 and 3d3/2, respectively, suggesting the deposition of Cu catalysts did not affect the chemical stability of the chromophore assembly.The XPS spectrum of N 1s (b) shows the similar peaks like HrHE-CA, -NH2 bonded to the phenylene of the linker (-NH2, 399.6 eV) and the protonated amidogen form (-NH3 + , 400.4 eV).

Figure S15. 15 N
Figure S15.15N-isotope-labelling experiment for determining the N source for the produced NH3.From the results, feeding the system with 15 NO3 -gives the same results as that with 14 NO3 -, evincing that the N source for the produced NH3 is completely from NO3 -in the electrolyte.

Figure S16 .
Figure S16.Simulated solar irradiation spectrum (AM 1.5 G) used for the PEC experiments.

Figure S17 .
Figure S17.Nanosecond transient absorption (TA) spectra for the chromophores without Cu catalyst.a,b, TA spectra following photoexcitation of CA (zirconium-coordinated 2aminoterephthalic acid) (a) and 2-aminoterephthalic acid (denoted as C) (b) at 355 nm, shown as a function of probe delay time.c,d, Time-resolved TA traces probed at 370 nm showing the ground state bleach of CA (c) and C (d). Red dot: experimental data.Black line: fitted data based on biexponential decay function.

Figure S18 .
Figure S18.Nanosecond TA spectra for the chromophores with Cu catalyst.a,b, TA spectra following photoexcitation of CA-Cu (a) and C-Cu (b) at 355 nm, shown as a function of probe delay time.c,d, Time-resolved TA traces probed at 370 nm for CA-Cu (c) and C-Cu (d).Red dot: experimental data.Black line: biexponential decay fits.Time-resolved decay kinetics.The biexponential decay model used for fitting the decay kinetics is shown below: ∆OD (t) = A1exp(-t/τ 1 ) + A2exp(-t/ 2 )

Figure S20 .
Figure S20.Linear sweep voltammograms for HE without heat-resistant components after heating at 100 °C for 24 hours in DMSO, DMF with 5% HCl or pH 7.0 phosphate buffer.Electrolyte: pH 7.0 phosphate buffer with 30 mM hydroquinone.Scan rate: 5.0 mV s -1 .

Figure S21 .
Figure S21.SEM micrographs for the freeze-dried HrH (a) and the hydrogel substrate without heatresistant components (HS) (b) under 25 °C (upper panel) and 150 °C for two days (lower panel).

Figure S22 .
Figure S22.Raman spectra for HrHE before and after heating in DMF and pH 7.0 phosphate buffer for 24 hours.The bottom gray line is the background spectrum for the covalent scaffold (HrH).
-1 correspond to the Cβ-Cβ inter-ring stretching and asymmetrical Cα=Cβ vibrations, respectively.The other peaks observed at 440 cm −1 and 987 cm −1 are assigned to C-O-C distortion and Cβ-Calkyl stretching, respectively.No obvious changes were observed for the samples after heating treatments, which indicates high structural stability of HrHE under elevated temperatures.

Figure S23 .
Figure S23.SEM images and corresponding EDX elemental mapping for the photoelectrode after long-term photoelectrocatalysis.

Figure S27 .
Figure S27.Projected density of states (PDOS) for adsorbed *NO3 -(a) and *H (b) on CA-Cu with the corresponding charge density difference configurations.Blue, red, green, brown, pink and silver spheres represent Cu, O, Zr, C, H and N atoms, respectively.Cyan and yellow regions indicate electron-donating and electron-withdrawing areas, respectively.The d-band centers for *NO3 -and *H adsorbed Cu are analyzed to be -2.04 and -2.16, respectively.

Figure S28 .
Figure S28.PDOS for adsorbed *NO3 -(a) and *H (b) on Cu catalysts with the corresponding charge density difference configurations.Energy barriers for the catalytic intermediates during NO3 -reduction to NH3 on Cu catalysts (c).Blue, red, pink and silver spheres represent Cu, O, H and N atoms, respectively.Cyan and yellow regions indicate electron-donating and electron-withdrawing areas, respectively.The d-band centers for *NO3 -and *H adsorbed Cu are analyzed to be -8.64 and -25.0 eV, respectively.

Table S2 . Band edge potentials of CA and HrHE.
The surface coverage of CA is estimated by approximation based on cell units and the amount of Zr.
b The surface coverages of Zr and Cu are estimated by inductively coupled plasma optical emission spectroscopy (ICP-OES).c The surface coverage of H2ATA is estimated by the ratio of H2ATA to Zr in a CA unit cell.