Amyloid‐Templated Palladium Nanoparticles for Water Purification by Electroreduction

Abstract Electrocatalysis offers great promise for water purification but is limited by low active area and high uncontrollability of electrocatalysts. To overcome these constraints, we propose hybrid bulk electrodes by synthesizing and binding a Pd nanocatalyst (nano‐Pd) to the electrodes via amyloid fibrils (AFs). The AFs template is effective for controlling the nucleation, growth, and assembly of nano‐Pd on the electrode. In addition, the three‐dimensional hierarchically porous nanostructure of AFs is beneficial for loading high‐density nano‐Pd with a large active area. The novel hybrid cathodes exhibit superior electroreduction performance for the detoxification of hexavalent chromium (Cr6+), 4‐chlorophenol, and trichloroacetic acid in wastewater and drinking water. This study provides a proof‐of‐concept design of an AFs‐templated nano‐Pd‐based hybrid electrode, which constitutes a paradigm shift in electrocatalytic water purification, and broadens the horizon of its potential engineered applications.


Analysis and characterization
Morphology of the electrodes was observed by atomic force microscopy (AFM, MultiMode 8, Bruker, U.S.A.) in soft tapping mode. Prior to AFM measurements, a solution of AFs-Pd was prepared as mentioned above, and an aliquot of the solution was deposited on freshly-cleaved mica sheets. After 2 min, the mica sheets were rinsed by DI water and dried by a gentle flow of compressed air.
Transmission electron microscopy (TEM), scanning TEM (STEM), and high-resolution TEM (HRTEM) images of the electrodes were obtained at accelerating voltages of 80 kV and 200 kV, respectively, with a TEM FEI Talos F200X (U.S.A.) equipped with a field emission gun and large-collection angle energy-dispersive X-ray spectroscopy (EDX) detector (Super-X, Bruker, U.S.A.). AFs were removed from the sample because they would be damaged by the 200-kV accelerating voltage required to gain sufficient spatial resolution to resolve the atomic planes of Pd in HRTEM imaging. The chemical maps were performed at 80 kV to precisely avoid this type of damage, and thus allow visualizing AFs, and at the same time increase the yield in X-rays compared to 200-kV electrons, but then the spatial resolution was not sufficient for resolving the atomic planes of the Pd particles. A diluted solution of AFs-Pd (4 μL) was dripped on a 2-nm carbon-coated copper grid (Quantifoil, Germany) and let for drying in the air for a couple of minutes before TEM measurements.
The structure and properties of the electrodes were characterized by scanning electron microscopy (SEM, LEO 1530, Zeiss, Germany) at an accelerating voltage of 3 kV. Prior to the measurements, small pieces of the electrodes were mounted to aluminium stubs by conductive carbon paste. The elemental composition of the electrodes was determined by energy-dispersive X-ray spectroscopy (EDX, Pegasus system, EDAX, U.S.A.) with a Si(Li) detector connected to SEM. Fourier transform infrared spectroscopy (FTIR, 640-IR, Varian, U.S.A.) equipped with the Imaging Golden Gate Diamond ATR accessory was employed to confirm the coating of AFs on the electrodes. The samples were scanned at room temperature over 4000 cm -1 to 400 cm -1 with a resolution of 2 cm -1 .
The Pd concentration and its loading content on the cathodes were determined by inductively coupled plasma optical emission spectrometry (ICP-OES, Optima 5300 DV, PerkinElmer, U.S.A.). A UV/Vis spectrometer (Cary Series, Agilent, U.S.A.) was used in the range of 250 nm -700 nm to monitor the catalytic reaction at the absorption peak of 540 nm. The electrochemical properties, including cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), were determined by the electrochemical workstation (CHI760, Chenhua, China) and the workstation (Interface 1000, Gamry, U.S.A.), respectively. CV curves were acquired in the solution of Na2SO4 (2 mM) at a scan rate of 1 mV s -1 . EIS measurements were performed at open-circuit potential over 100 Hz -0.01 Hz with an amplitude of 5 mV. Zsimpwin software was used for EIS data-fitting. Electrochemical active surface area (ECSA) was determined according to previous studies [3] , using the reference value of capacity per unit area for smooth Pd metals/alloys (23 µF cm -2 ).

Performance tests Electrochemical reactor and flow-through experiment
Electroreduction by CP-AFs-Pd was carried out using an H-type reactor, as displayed in Figure 1b. The pre-treated Nafion membrane was vertically placed to separate the anode chamber (200 mL) and the cathode chamber (200 mL). The initial concentration of Cr 6+ was 1 ppm in the cathode chamber with Na2SO4 (2 mM) serving as a supporting electrolyte. The solution pH was adjusted to 5.0 ± 0.1 by H2SO4. The reactor was then purged with argon gas and sealed for subsequent experiments. In addition, the argon purge was continued during the whole electroreduction process. The reactor was connected to the electrochemical workstation (CHI760, Chenhua, China), while platinum sheet and Ag/AgCl served as counter and reference electrodes, respectively. Samples were taken from the cathode chamber at predetermined intervals. In the flow-through tests, TiSO-REM-AFs-Pd served as the cathode (Figures 3d,h). The synthetic wastewater was pumped into the hollow porous cathode and then recycled back to the reactor.
The energy consumption during electroreduction tests was calculated according to the previous study [3a] : [Eq. S1] where Ucell (V) is the cell voltage, I (A) the current, t (h) the reaction time and V (m 3 ) the effective volume of the electrolytic cell.   Only slight removal was found of Cr 6+ and 4-CP by adsorption on CP, CP-AFs, and CP-AFs-Pd at 0 V vs. SHE ( Figure S2).
In addition, the insignificant removal by CP and CP-AFs at -1.2 V vs. SHE was also considered to be the effect of adsorption ( Figures  3a,b). The concentration of total Cr was also decreased by 10%, as AFs tended to adsorb Cr 3+ after Cr 6+ reduction ( Figure S5a). Phenol was found to constitute the main electroreduction product of 4-CP, with a concentration of approximately 1 ppm ( Figure S5b).
To offer further mechanistic insight into H*-mediated electroreduction, tertiary butanol (TBA, 10 mM) was added to scavenge the in situ generated H*. As shown in Figure S5, adding TBA led to a 48% and 59% decline in reduction efficiency of Cr 6+ and 4-CP, respectively, confirming the essential role of H* for electroreduction. Prior to H2 desorption following Heyrovsky or Tafel steps, highly-active H* intermediates generated from water electrolysis on the CP-AFs-Pd electrode served as a robust reducing agent in electroreduction (Eqs. S2-S4). Consequently, electroreduction for Cr 6+ and 4-CP by H* can be presented in Eqs. S5 and S6. The CP-AFs-Pd also exhibited lower charge-transfer resistance than Pd-free cathodes (CP and CP-AFs; Figure S7).