This is the submitted version of the journal article: Wang, Xiang; Han, Xu; Du, Ruifeng; [et al.]. «Cobalt molybdenum nitride-based nanosheets for seawater splitting». ACS Applied Materials and Interfaces, Vol. 14, issue 37 (Sep. 2022), p. 41924-41933. DOI 10.1021/acsami.2c09272 This version is available at https://ddd.uab.cat/record/271943 under the terms of the license Page 1 of 18 ACS Applied Materials & Interfaces 1 2 3 4 Cobalt Molybdenum Nitride-Based Nanosheets for Seawater Splitting 5 6 Xiang Wang,[a,b] Xu Han,[c] Ruifeng Du,[a,b] Congcong Xing,[a,d] Xueqiang Qi,[e] Zhifu Liang,[a,c] Pablo 7 Guardia,[a] Jordi Arbiol,[c,f] Andreu Cabot,*[a,f] Junshan Li*[g] 8 9 aCatalonia Institute for Energy Research (IREC), Sant Adrià de Besòs, 08930 Barcelona, Spain 10 bDepartament d’Enginyeria Electrònica i Biomèdica, Universitat de Barcelona, 08028, Barcelona, 11 12 Catalonia, Spain 13 c 14 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, 15 Bellaterra, 08193 Barcelona, Catalonia, Spain 16 dInstitute of Energy Technologies, Department of Chemical Engineering and Barcelona Research 17 18 Center in Multiscale Science and Engineering, Universitat Politècnica de Catalunya, EEBE, 08019, 19 20 Barcelona, Catalonia, Spain 21 e College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China. 22 23 f ICREA Pg. Lluis Companys, 08010 Barcelona, Catalonia, Spain 24 25 g Institute of Advanced Study, Chengdu University, Chengdu 610106, China 26 27 * E-mails: A. Cabot: acabot@irec.cat; J. Li: lijunshan@cdu.edu.cn. 28 29 Abstract 30 31 The development of cost-effective bifunctional catalysts for water electrolysis is both a crucial 32 33 necessity and an exciting scientific challenge. Herein a simple approach based on a MOF sacrificial 34 35 template to preparing cobalt molybdenum nitride supported on nitrogen-doped carbon nanosheets 36 37 is reported. The porous structure of produced composite enables fast reaction kinetics, enhanced 38 39 stability and high corrosion resistance in critical seawater conditions. The cobalt molybdenum 40 nitride-based electrocatalyst is tested toward both OER and HER half-reactions using seawater 41 42 electrolyte, providing excellent performances that are rationalized using density functional theory 43 44 (DFT). Subsequently, the nitride composite is tested as a bifunctional catalyst for the overall splitting 45 46 of KOH-treated seawater from the Mediterranean Sea. The assembled system requires 47 48 overpotentials of just 1.70 V to achieve a current density of 100 mA cm01 in 1 M KOH seawater and 49 50 continuously works for over 62 h. This work demonstrates the potential of transition metal nitrides 51 for seawater splitting and represents a step forward toward the cost-effective implementation of 52 53 this technology. 54 55 Keywords: Molybdenum nitride, Cobalt nitride, Electrocatalysis, Seawater splitting, Hydrogen 56 57 58 1 59 60 ACS Paragon Plus Environment ACS Applied Materials & Interfaces Page 2 of 18 1 2 3 4 1. Introduction 5 6 Seawater is an abundant natural resource, accounting for over 96% of the planet’s total water and 7 8 presenting a fairly homogeneous geographical distribution.1,2 Thus its use as an alternative 9 10 feedstock to produce hydrogen, replacing fossil fuels, is extremely appealing.3-8 However, 11 12 implementing cost-effective seawater splitting systems remains a major challenge. The first main 13 14 limitation of the use of seawater to generate hydrogen is its high salt concentration (about 3.5 wt%) 15 and the kinetically unfavourable water oxidation to O2 (oxygen evolution reaction, OER, 4OH0 ; O2 16 17 + 2H2O + 4e0) over the chloride oxidation to chlorine (chlorine evolution reaction, CER, 2Cl0 ; Cl2 + 18 19 2e0).9-11 Besides, in alkaline media, at an onset potential of about 490 mV, Cl2 further reacts with 20 21 OH0 to form hypochlorite, which is toxic and highly corrosive. Thus, high-performance OER catalysts 22 23 with high OER activities at overpotentials well below :4.> / are required to avoid the formation of 24 25 hypochlorite.12-14 Additional challenges in the electrochemical seawater splitting are the 26 medium/long-term corrosion by chloride and the existence of ions, suspended particles, bacteria 27 28 and other microbes that can poison the electrocatalyst reducing its performance.10, 13, 15, 16, 35 29 30 Therefore, electrocatalysts with proper micro and macrostructures that facilitate the water 31 32 diffusion without being blocked are desirable. 33 34 Among possible candidates, transition-metal nitrides (TMNs) are particularly interesting 35 36 electrocatalysts for seawater splitting owing to their unique d-band electron configurations, intrinsic 37 metallic characteristics providing high electrical conductivities, good corrosion resistance and strong 38 39 mechanical properties.17, 18 Besides, the metal–nitrogen bond endows the metal nitrides with an 40 41 electron-donating character that promotes proton adsorption.19, 20 All these properties contribute 42 43 to the higher catalytic activity of the TMN compared with the corresponding transition metal. 44 45 Besides, the engineering of multimetallic nitrides and composites allows further tailoring of the d- 46 47 band position to modulate the adsorption ability of electrocatalysts and to achieve optimized 48 catalytic performance.21, 22 Nevertheless, while significant efforts have been invested into producing 49 50 efficient TMN electrocatalysts for pure water electrolysis, the performance of these catalysts in 51 52 seawater conditions has been rarely investigated. 53 54 Here we detail the synthesis of thin cobalt molybdenum nitride particles, that we denote as MoN- 55 56 Co2N, supported by nitrogen-doped carbon nanosheets (NSs) as an effective seawater 57 58 2 59 60 ACS Paragon Plus Environment Page 3 of 18 ACS Applied Materials & Interfaces 1 2 3 electrocatalyst. The nitride composite is obtained from a ZIF-67 metal-organic framework (MOF) as 4 5 a self-sacrificial template, taking advantage of its open crystalline structure, high surface area, well- 6 7 defined pores, tunable functionality and rich carbon content.23, 24 The NS morphology is selected to 8 9 shorten charge and mass transfer distance, accelerating the diffusion of reactants, and to maximize 10 11 the percentage of surface unsaturated atoms, the exposition of active sites and thus the 12 electroactive surface area.7, 25-27 This catalyst is tested toward both OER and HER using a 1 M KOH- 13 14 treated seawater electrolyte. Owing to its high performance in both half-reactions, MoN-CoxN is 15 16 used as a bifunctional catalyst for the overall splitting of natural seawater obtained from the 17 18 Mediterranean Sea and incorporating 1 M KOH. The obtained performances are compared with the 19 20 benchmark IrO2 D Pt/C pair. The excellent results obtained are rationalized using density functional 21 22 theory (DFT) calculations. 23 24 2. Results and discussion 25 26 2.1. Characterization of electrocatalysts 27 28 Cobalt molybdenum nitride supported on nitrogen-doped carbon NSs were produced in three steps, 29 30 as depicted in Figure 1. First, a cobalt-based zeolitic imidazolate framework (ZIF-67) was prepared 31 as a 3D template with high carbon content and suitable porous nanostructure.28, 29 Figure 2a displays 32 33 a representative SEM micrograph of the ZIF-67 template particles, which were characterized by a 34 35 rhombic dodecahedron geometry with an average size of 500 nm. 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 Figure 1. Schematic illustration of the synthesis of MoN-Co2N nanosheets. 51 52 In a second step, the ZIF-67 was etched and Co was partially exchanged with Mo using ammonium 53 54 molybdate both as Mo source and etching agent. In this step, ammonium molybdate hydrolizes 55 generating protons (H+) that slowly etch the ZIF-67 releasing Co2+ ions, and hydroxides (OH0) that 56 57 58 3 59 60 ACS Paragon Plus Environment ACS Applied Materials & Interfaces Page 4 of 18 1 2 3 drive the precipitation of 2-methylimidazole and molybdate and cobalt ions.30, 31 Thus, upon 4 5 hydrolysis and etching processes with ammonium molybdate, the crystalline ZIF-67 dodecahedron 6 7 particles were transformed into an amorphous and layered material, as observed in Figures 2b and 8 9 S1. 10 11 Finally, the amorphous Mo-Co precursor sample was annealed at 500 °C for 2 h under NH3/Ar 12 atmosphere. In this nitridation step, the Mo-Co precursor transformed into a nitrogen-doped 13 14 composite that conserved an ultrathin nanosheet structure (Figures 2c and S2). EDS analysis of the 15 16 annealed material revealed an overall Co:Mo molar ratio of Co/Mo = 3.2 (Figure S3). TEM images 17 18 (Figures 2d,g, S4, and S5) further confirmed the 2D ultrathin NS morphology of the annealed 19 20 material and revealed the presence of numerous holes on the NSs. Such pores can be formed by the 21 22 H2O loss from the Mo-Co precursor during the nitridation process. We foresee these holes as highly 23 convenient to promote the transport of electrolyte and overall improve the catalytic activity.32 24 25 Unlike the Mo-Co precursor, the annealed samples displayed no less than two interconnected 26 27 phases, a nitrogen-doped carbon and at least a Mo-Co-N phase (Figures 2 g, S4, and S5). The 28 29 ultrathin carbon-based layer originated from the ligand decomposition in the presence of ammonia 30 31 (Figure 2d) and can contribute to the charge transport within the electrodes.33, 34 Lying flat on this 32 33 carbon support, a uniform loading of Mo-Co-N NS-like particles was observed (Figures 2g). HRTEM 34 characterization of the Mo-Co-N particles revealed the presence of at least two phases, MoN and 35 36 Co2N (Figures 2e,f and S6). Figure 2e displays an HRTEM image of a crystal structure that matches 37 38 well with the Co2N orthorhombic phase (space group =Pmnn) with a=2.8535 Å, b=4.6056 Å, 39 40 c=4.3443 Å. Figure 2f displays an HRTEM image of a crystallographic domain matching the MoN 41 42 trigonal phase (space group =P-31m) with a=b=5.6650 Å, and c=5.5200 Å. While different phases 43 44 were identified by HRTEM analysis, STEM-EELS elemental maps displayed a uniform distribution of 45 N, Co and Mo throughout the entire Mo-Co-N particles (Figures 2h and S7). Thus, we hypothesize 46 47 that both phases, MoN and Co2N, are intimately interconnected within MoN-Co2N heterostructures. 48 49 Figure 3a shows an AFM image of the MoN-Co2N NSs, which allowed to determine the NS thickness 50 51 to be ca. 3.2 nm (Figure 3b). 52 53 54 55 56 57 58 4 59 60 ACS Paragon Plus Environment Page 5 of 18 ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment ACS Applied Materials & Interfaces Page 6 of 18 1 2 3 consistent with HRTEM results. Raman spectroscopy was used to further analyze the structural 4 5 properties of the material. The Raman spectrum of MoN-Co2N NSs showed four Raman bands, at 6 7 about 482, 512, 619 and 281Q 0,, that are assigned to the Eg, F2g, and A1g vibration modes of Co2N, 8 9 respectively (Figure 3f). 38, 39 Besides, a band at 47.Q 0, is associated with the MoN phase.40 These 10 11 bands are slightly shifted compared with literature values, which demonstrates the interaction 12 between MoN and Co2N crystal lattices. 13 14 To study the Mo precursor effect on the morphology of the final product, we replaced the 15 16 ammonium molybdate with sodium molybdate. The product obtained after the Mo introduction 17 18 was denoted as Na2MoO4-ZIF-67. XRD analysis showed Na2MoO4-ZIF-67 to display a similar crystal 19 20 structure as ZIF-67 (Figure 3d). After the nitridation step, the material obtained with an ammonium- 21 22 free Mo precursor displayed a much more compact nanocage-like morphology (Figure S9). Besides, 23 EDS analysis of this sample, denoted as Mo-Co2N, revealed a low molybdenum content (Co/Mo = 24 25 8.6). As a reference, a Mo-free CoxN was obtained by annealing the ZIF-67 in the ammonia 26 27 atmosphere, with no etching step (Figure S10). The obtained material, CoxN, also displayed a more 28 29 compact morphology than the MoN-Co2N NSs. XRD patterns showed the CoxN sample to display 30 31 intense peaks at about 44 and 51 that could be potentially assigned to Co, Co4N or other CoXN 32 33 phases (Figures 3e and S8). Taking into account the effectiveness of the nitration process observed 34 from the large amount of nitrogen observed by EELS, EDX and XPS (below), we discarded the 35 36 presence of metallic Co. Besides, the Raman spectrum of CoxN showed the vibration bands, at about 37 38 480, 510, 620, and 28.Q 0, assigned to the Eg, F2g, and A1g vibration modes of CoxN, respectively 39 40 (Figure 3f), further discarding the assignment of the XRD pattern to Co. 38, 39 41 42 The surface area and porosity of the prepared catalysts were analysed from nitrogen adsorption- 43 44 desorption isotherms. As shown in Figure 3c, MoN-Co2N exhibits a type-IV isotherm with an H3-type 45 hysteresis loop at high relative pressures (0.8–1.0 P/P0). The = 0" 0 (BET) specific 46 47 surface area of MoN-Co2N was 228 m2 g-1, well above that of CoxN (44.5 m2 g-1). The average pore 48 49 size and the cumulative pore volume of MoN-Co2N were 15.8 nm and 0.53 cm3 g0,, respectively 50 51 (Figure S11). This result revealed the existence of a large density of mesopores within the material, 52 53 related to the holey NS structure, which is highly positive to increasing electroactive surface area 54 55 and the rate of transport of reactants, including hydrion/ hydroxide ions, and products, such as 56 oxygen and hydrogen.36, 37 57 58 6 59 60 ACS Paragon Plus Environment Page 7 of 18 ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment ACS Applied Materials & Interfaces Page 8 of 18 1 2 3 2.2. Oxygen evolution reaction 4 5 The electrocatalytic performance of CoxN-, Mo-Co2N- and MoN-Co2N-based catalysts was evaluated 6 7 using a three-electrode configuration. As additional references, commercial IrO2 and Pt/C catalysts 8 9 were also tested for OER and HER reactions, respectively. 10 11 Figure 4a,b displays the OER LSV curves obtained in a 1 M KOH electrolyte. LSV curves revealed the 12 MoN-Co2N electrocatalyst to be characterized by the highest OER catalytic activities. MoN-Co2N 13 14 electrocatalyst required an overpotential of 267 mV to achieve 10 mA cm01 of current density and 15 16 309 mV to drive 100 mA cm01. These overpotentials are significantly lower than those measured 17 18 from IrO2 (323 and 428 mV), Mo-Co2N (288 and 375 mV), and CoxN (321 mV and 421). As displayed 19 20 in Figure 4c, the Tafel slope of MoN-Co2N was just 35.6 mV dec0,, well below that of Mo-Co2N (58.3 21 22 mV dec0,) and CoxN (65.7 dec-1). The small Tafel slopes obtained with the MoN-Co2N catalysts 23 demonstrate fast OER electrocatalytic kinetics. Overall, the MoN-Co2N OER performance was 24 25 significantly improved over most other TMN electrocatalysts reported to date (Figure 4d, Table S1). 26 27 Figure 4e displays the double-layer capacitances (Cdl) measured from CV data (Figure S12) for the 28 29 different catalysts, as an indication of their electrochemically active surface area (ECSA).34 The MoN- 30 31 Co2N catalyst exhibited the largest Cdl (15.01 mF cm01) among the tested catalysts, well above that 32 33 of Mo-Co2N (10.35 mF cm01), almost threefold higher than that of CoxN (5.07 mF cm01) and over 34 tenfold higher than IrO2 (1.27 mF cm01). The related MoN-Co2N ECSA was as high as 375.25 cm01 35 36 (Table S2). Nevertheless, even when normalizing the OER current density to the ECSA, the MoN- 37 38 Co2N electrocatalyst still displayed the highest OER activity values (Figure 4f). 39 40 The electron transport and transfer properties of the electrocatalysts were evaluated by EIS (Figure 41 42 3g). The MoN-Co2N catalyst exhibited the lowest charge-transfer resistance (Rct = 28.0 R'! 43 44 significantly below that of Mo-Co2N (54.6 R'! CoxN (82.8 R' and IrO2 (11.7 R'! confirming its fast 45 charge transfer rate. 46 47 The OER durability of the MoN-Co2N electrocatalyst was evaluated in a 1 M KOH electrolyte by 48 49 analyzing the variation of the LSV curve after 3,000 CV cycles. As observed in Figure 4h, the initial 50 51 and final LSV curves almost overlapped, which confirms the excellent cycling stability of the MoN- 52 53 Co2N electrocatalyst. Besides, a chronoamperometry measurement of the MoN-Co2N catalyst in 54 55 alkaline conditions showed a negligible current density decay after 100 h of continuous OER 56 operation at 267 mV (Figure 4i). 57 58 8 59 60 ACS Paragon Plus Environment Page 9 of 18 ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment ACS Applied Materials & Interfaces Page 10 of 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment Page 11 of 18 ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment ACS Applied Materials & Interfaces Page 12 of 18 1 2 3 As noted above, DFT calculations showed that the DOS of MoN-Co2N at the Fermi level was 4 5 significantly larger than that of Co2N, which involves a larger density of free electrons available for 6 7 redox reactions.46 DFT calculations also showed the d-band center of MoN-Co2N to be farther away 8 9 from the Fermi level compared with Co2N. Being the d-band center a good descriptor of the metal- 10 11 adsorbate bonding strength, the downshift of the d-band center also involves a weaker interaction 12 between adsorbed H and MoN-Co2N, thus faster desorption.20, 22 This effect is reflected in the free 13 14 energy of H adsorption %VTH*). As shown in Figure 6f, the VTH* of Co2N was 0. 91 eV, which was well 15 16 above (in absolute value) that of MoN-Co2N, 0. 1, eV. The much lower absolute value of VTH* is 17 18 consistent with the downshift of the d-band center and explains the enhanced HER activity through 19 20 the acceleration of the hydrogen desorption process.42 21 22 2.4. Seawater splitting 23 The main challenge for alkaline seawater splitting is the formation of electrode corroding 24 25 hypochlorite on the anode side, where it competes with OER.52 MoN-Co2N is an excellent candidate 26 27 electrocatalyst for seawater splitting as it requires an overpotential of only 309 mV to drive an OER 28 29 current density of 100 mA cm01 in 1 M KOH electrolyte, well below the 490 mV that activates the 30 31 formation of hypochlorite. Besides, metal nitrides present good corrosion resistance, which is a 32 33 fundamental catalyst requirement to be used for seawater splitting.17, 53 34 Owing to the excellent activity and stability of MoN-Co2N, we tested it in an alkaline saline 35 36 electrolyte (1 M KOH + 1 M NaCl aqueous electrolyte, simulating seawater) and in alkaline natural 37 38 seawater (1 M KOH seawater) after filtering the formed hydroxides (see details in the SI, Figure S15). 39 40 As displayed in Figure 7a,b, the OER and HER performance of the MoN-Co2N catalyst in 1 M KOH 41 42 seawater were below those obtained in 1 M KOH and 1 M KOH + 1 M NaCl water. The lower 43 44 electrocatalytic activity in seawater is associated with the blocking/poisoning of the catalyst sites by 45 the natural seawater impurities, e.g. ions, molecules, suspended particles, and microbes. Specifically, 46 47 in 1 M KOH seawater electrolyte, the MoN-Co2N electrocatalyst required OER overpotentials of 357 48 49 mV (100 mA cm01) and 432 mV (500 mA cm01) and HER overpotentials of 304 mV (100 mA cm01) and 50 51 451 mV (500 mA cm01). The overpotentials needed for MoN-Co2N to drive current densities of 10, 52 53 100 and 500 mA cm01 in 1 M KOH, 1 M KOH in NaCl water and 1 M KOH seawater electrolytes as an 54 55 OER and an HER electrocatalyst are summarized in Figure 7c. 56 The overall water/seawater splitting performance was investigated using a two-electrode cell that 57 58 12 59 60 ACS Paragon Plus Environment Page 13 of 18 ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment ACS Applied Materials & Interfaces Page 14 of 18 1 2 3 The measured amount matched well the theoretical prediction (see SI for details), confirming a 4 5 nearly 100% Faradaic efficiency (Figure S18b). 6 7 Finally, we examined the structural and chemical properties of the MoN-Co2N catalyst after the 8 9 long-term stability test in 1 M KOH + seawater electrolyte. High-resolution XPS data confirmed the 10 11 surface of the particles to be completely oxidized (Figure S19). Besides, TEM and HAADF-STEM 12 images of MoN-Co2N after the long-term reaction showed the material to keep a porous NS 13 14 structure, proving its high stability (Figure S20). Moreover, EELS elemental maps demonstrated the 15 16 homogeneous distribution of the different metals, Mo and Co, within each NS to be maintained and 17 18 the NS to still contain large amounts of N, although the surface of the catalyst appeared to be fully 19 20 oxidized (Figure S21). 21 22 3. Conclusion 23 24 In summary, MoN-Co2N NSs were synthesized using ZIF-67 as a self-sacrificial template through 25 26 proton etching, ion exchange, and metal nitridation processes. The porous NS structure enables an 27 28 effective proper electrolyte diffusion and accelerates the release of bubbles, which translates into 29 30 outstanding catalytic activities and stabilities even at high current density. The prepared MoN-Co2N 31 electrocatalysts possess excellent OER and HER catalytic activity, requiring an overpotential of 267 32 33 and 105 mV to drive OER and HER at 10 mA cm01 current density in 1.0 M KOH electrolyte, 34 35 respectively. As for overall water/seawater splitting, a system based on an MoN-Co2N bifunctional 36 37 electrocatalyst only required voltages of 1.63 and 1.70 V to reach a current density of 100 mA cm01 38 39 in 1 M KOH freshwater and 1 M KOH natural seawater, respectively. These overpotentials are 40 41 significantly lower than those required by the benchmark IrO2-Pt/C pair. DFT calculations revealed 42 the MoN-Co2N to be characterized by a proper d-band center that provided low OER and HER energy 43 44 barriers and a balanced intermediates adsorption strength. Overall, this work presented a novel 45 46 nitride electrocatalyst for the cost-effective and stable production of hydrogen production from 47 48 seawater. 49 50 Acknowledgements 51 52 X. W., X. H., R. D., and C. X. thank the China Scholarship Council (CSC) for the scholarship support. 53 54 ICN2 acknowledges funding from Generalitat de Catalunya 2017 SGR 327. The authors thank the 55 56 support from the Spanish MCIN project COMBENERGY (PID2019-105490RB-C32) and NANOGEN 57 58 14 59 60 ACS Paragon Plus Environment Page 15 of 18 ACS Applied Materials & Interfaces 1 2 3 (PID2020-116093RB-C43), funded by MCIN/ AEI/10.13039/501100011033/. ICN2 is supported by 4 5 the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706) and is funded by the 6 7 CERCA Programme / Generalitat de Catalunya. Part of the present work has been performed in the 8 9 framework of Universitat Autònoma de Barcelona Materials Science PhD program. ZL acknowledges 10 11 funding from MINECO SO FPI PhD grant (SEV-2013-0295-17-1). 12 13 References 14 1. 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