pubs.acs.org/acscatalysis Research Article Highly Durable Nanoporous Cu2−xS Films for Efficient Hydrogen Evolution Electrocatalysis under Mild pH Conditions Roser Fernández-Climent, Jesús Redondo, Miguel García-Tecedor, Maria Chiara Spadaro, Junnan Li, Daniel Chartrand, Frederik Schiller, Jhon Pazos, Mikel F. Hurtado, Victor de la Peña O’Shea, Nikolay Kornienko, Jordi Arbiol, Sara Barja,* Camilo A. Mesa,* and Sixto Giménez* Cite This: ACS Catal. 2023, 13, 10457−10467 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: Copper-based hydrogen evolution electrocatalysts are promising materials to scale-up hydrogen production due to their reported high current densities; however, electrode durability remains a challenge. Here, we report a facile, cost-effective, and scalable synthetic route to produce Cu2−xS electrocatalysts, exhibiting hydrogen evolution rates that increase for ∼1 month of operation. Our Cu2−xS electrodes reach a state-of-the-art performance of ∼400 mA cm−2 at −1 V vs RHE under mild conditions (pH 8.6), with almost 100% Faradaic efficiency for hydrogen evolution. The rise in current density was found to scale with the electrode electrochemically active surface area. The increased performance of our Cu2−xS electrodes correlates with a decrease in the Tafel slope, while analyses by X-ray photoemission spectroscopy, operando X-ray diffraction, and in situ spectroelectrochemistry cooperatively revealed the Cu-centered nature of the catalytically active species. These results allowed us to increase fundamental understanding of heterogeneous electrocatalyst transformation and consequent structure−activity relationship. This facile synthesis of highly durable and efficient Cu2−xS electrocatalysts enables the development of competitive electrodes for hydrogen evolution under mild pH conditions. KEYWORDS: green hydrogen, hydrogen evolution reaction, electrocatalysis, Cu-based electrodes, operando ECSA increase, mechanistic analysis 1. INTRODUCTION to other transition metal catalysts toward HER.15−17 Out- Global energy demand is expected to rise around 30% by 2040 standing η values, as low as ∼50 mV at 10 mA cm−2, have been according to the International Energy Agency (IEA).1 reported when Cu chalcogenides are heterostructured and Transforming renewable electricity into green hydrogen tested under extreme pH conditions (see Table S1 for (H2), via the water splitting process, has emerged as a electrode benchmarking).18 To reduce manufacturing costs promising energy vector to respond to this increasing energy and to simplify electrode architectures, non-heterostructured demand and to decarbonize transportation, heating, and fine Cu2S electrodes are appealing to optimize. For example, Baht chemicals sectors.2,3 Current technologies to split water use and Nagaraja reported a η ∼330 mV at 10 mA cm−2 for a extreme pH conditions causing environmental pollution and hydrothermally grown Cu2S cathode in 1 M KOH and η ∼312 handling hazards. Particularly, Pt-based cathodes, which mV at 10 mA cm−2 when using 0.5 M H2SO4.19 However, remain the state-of-the-art catalyst for the hydrogen evolution these Cu2S electrocatalysts are measured under extreme pH reaction (HER), are only operative in acidic media among conditions, i.e., highly acidic or alkaline electrolytes, thus other limitations such as scarcity and high costs.4 Thus, increasing the maintenance costs and the potential risks of scaling-up the HER toward the gigawatt-scale requires highly environmental pollution when scaling-up H2 production. efficient and durable electrocatalysts produced from earth- Consequently, efficient HER under mild pH conditions is abundant materials and with low-cost, environmental friendly manufacturing techniques, and being operative under mild pH conditions. Received: April 12, 2023 In this context, electrocatalysts based on non-critical Revised: June 30, 2023 materials are attracting increasing attention to replace Pt and Published: July 26, 2023 Ru to scale-up green H2 production.5−14 In particular, Cu chalcogenide and oxide electrocatalysts are interesting due to their remarkably low overpotentials (η) and stability compared © 2023 The Authors. Published by American Chemical Society https://doi.org/10.1021/acscatal.3c01673 10457 ACS Catal. 2023, 13, 10457−10467 ACS Catalysis pubs.acs.org/acscatalysis Research Article key to achieve a clean and low-risk energy transition through activity is mainly driven by catalytic centers located at the Cu both water and seawater splitting technologies. However, species at the surface. scarce studies of efficient HER on Cu2S electrodes in close to neutral conditions are reported. 2. RESULTS AND DISCUSSION Scaling-up green H2 production does not only require 2.1. Synthesis and Structural Characterization. The efficient electrodes but also require durable electrodes. synthesis of Cu2−xS was carried out on a copper substrate upon Continuous operation reported for Cu-based electrocatalysts modification of a procedure reported elsewhere.37,38 First, a ranges from a few hours up to several days.12,18−27 To the best copper foil substrate was cleaned and electropolished (Figure of our knowledge, the most durable Cu2S-containing electro- S1). The pre-treated copper substrate was drop-casted with a catalyst was reported by Bae et al., with a Cu2S-Mo2S electrode Na2S4 solution forming a homogeneous layer of copper sulfide in continuous operation for 10 days in 1 M H2SO4.18 However, (Cu2−xS) on the substrate’s surface followed by abundant non-heterostructured Cu2S electrodes measured in a mild rinsing with deionized H2O. The catalyst mass loading was electrolyte (1 M phosphate buffer, pH ∼7) have been estimated as ∼0.400 mg cm−2. A more detailed description of measured only for 10 h.28 It is therefore apparent that the the synthesis can be found in the Experimental Section. development of platinum group metal (PGM)-free, low-cost, The morphology and structure of the as-synthesized and highly efficient HER electrocatalysts still remains a electrodes were investigated taking advantage of different challenge. characterization techniques. Figure 2a as well as Figures S2 and In this study, we report a cost-effective, facile method to S3 show scanning electron microscopy (SEM), atomic force synthetize a Cu2−xS electrocatalyst with η values as low as 130 microscopy images (AFM), and energy-dispersive spectrosco- mV at 10 mA cm−2 and 332 mV at 50 mA cm−2 under mild py (EDS) chemical composition (Figure S2b) of the Cu2−xS conditions (pH 8.6). These values compare to the lowest film exhibiting a thickness of ∼6 μm (Figure S2c,d) featuring a reported η for Cu2−xS-derived catalysts operating at extreme highly porous structure characteristic of foamy materials, in pH, preserving Faradaic efficiencies (FE) near 100% for HER agreement with previous reports.39,40 This is in contrast to the during the tested time. Strikingly, our Cu2−xS electrocatalyst flat surface of electropolished copper foil before the synthesis, exhibits a remarkable durability of almost 1 month, which, to as shown in Figure S2e. The EDS analysis of the film confirms the best of our knowledge, is the longest reported to date for the composition and stoichiometry of Cu2S (Figure S2b and this material at such high current densities. This new Table S2). High-resolution transmission electron microscopy benchmark durability achieved is superior compared to that (HRTEM) analysis reveals a high-temperature Cu1.8S digenite reported for highly competitive noble metal electrodes, such as structure (space group, s.g. 224) where the family of (211) RuP nanoparticles (∼200 h, i.e., over a week of operation),29 planes could be identified, from a measured interplanar setting a technical breakthrough and a great step toward distance of 0.243 nm (HRTEM image and FFT analysis in scalability for industrial renewable electrocatalytic hydrogen Figure 2b). Additionally, the chemical composition of the production. Several copper sulfide-derived electrocatalysts electrodes was confirmed by electron energy loss spectroscopy previously reported and collected in Table S1 are compared (EELS) in scanning transmission electron microscopy in terms of performance (overpotential) and durability in (STEM) mode. STEM-EELS spectrum image composition Figure 1. Crucially, we carried out a comprehensive structural maps are shown in Figure 2c. In particular, the S L-edge at 165 eV (green), C K-edge at 284 eV (blue), and Cu L-edge at 931 eV (red) were used. Note that C is only present in the TEM grid and not in the electrocatalyst. Furthermore, XRD of the as-synthesized Cu2−xS electrocatalyst showed peaks at 2θ values of 27.84, 32.23, 46.23, and 54.9° that could be indexed to (111), (200), (220), and (311) planes of the Cu1.8S cubic phase, respectively (PDF no. 24-61).41,42 Two additional minor peaks are observed at 2θ values of 36.5 and 42.4° and indexed to the (111) and (200) crystal planes, respectively (JCPDS no. 05-0667).43 These peaks are assigned to the formation of superficial Cu2O after the synthesis. The characteristic peaks for metallic Cu (PDF #04-0836) are also visible in the diffractogram, with a preferred orientation (111) (Figure 2d). Finally, UV−vis−NIR characterization shows a broad absorption spectrum up to ∼1300 nm and a second NIR Figure 1. Comparative overpotentials at 10 mA cm−2 (η10), 50 mA band that is tentatively assigned to a localized surface plasmon cm−2 (η50), and 100 mA cm−2 (η100), durability time benchmark for resonance due to the Cu deficiency in the material (Figure copper sulfide electrocatalysts in Table S1, and our Cu2−xS cathode in S2).44,45 this present work for acidic,12,18,19,30,31 neutral,28,32,33 and basic media.19−22,26,27,34−36 Chemical analysis of the surface of the Cu2−xS electrode was carried out by XPS. Figure 2e and Figure 2f show the representative Cu LMM and S 2p spectra of the Cu2−xS and chemical characterization of the electrodes before, during, electrodes (blue solid line), respectively. The reference spectra and after the HER test, taking advantage of the combination of of metallic Cu are shown in red dotted line. The surface of the operando spectroelectrochemistry (SEC), X-ray diffraction electrodes was gently sputtered with Ar+ ions in the analysis (XRD), and X-ray photoemission spectroscopy (XPS). chamber prior to the XPS characterization to remove adsorbed Analysis of Cu2−xS electrodes simultaneously to the electro- species from air exposure after the synthesis (see the chemical tests unveils for the first time that the catalytic Experimental Section and Figure S4). The XPS data unveils 10458 https://doi.org/10.1021/acscatal.3c01673 ACS Catal. 2023, 13, 10457−10467 ACS Catalysis pubs.acs.org/acscatalysis Research Article Figure 2. Structural characterization of the Cu2−xS electrodes. (a) Top-view SEM image, (b) HRTEM image together with the indexed power spectrum and a detailed image of the Cu2−xS structure, and (c) STEM-EELS analysis (bottom left). High-angle annular dark-field (HAADF) STEM general image of the nanostructured Cu1.8S (top left). EELS compositional maps showing the different elemental distributions: C (blue), S (green), and Cu (red) (right). Individual elemental maps for C (blue), S (green), and Cu (red). (d) XRD reference patterns for Cu1.8S (light gray) and Cu (black) and diffractograms for the electropolished copper foil (red) before the synthesis and the as-synthesized Cu2−xS on the copper substrate (blue). (e) XPS analysis for Cu LMM and (f) S 2p spectra of the Cu2−xS electrodes. Reference spectra measured on a metallic Cu substrate are shown in red dotted lines. a Cu2−xS stoichiometry of the electrodes.46 The Cu LMM 3a), during the operation period. Along the durability test, the Auger emission lineshape (Figure 2e) is compatible with the CA measurement was systematically interrupted to investigate presence of Cu(I) or Cu(II) chemical states,47 with the the nature of the increase in J by performing complementary absence of metallic Cu. The Cu 2p spectral region of the fresh electrochemical measurements. Linear sweep voltammetries electrode (Figure S4) reveals a well-defined 2p2/3 component (LSVs) in Figure 3b show the increasing J, at matched with a binding energy at 932.7 eV. The absence of satellite potentials, with time as indicated by the gray arrow, following peaks at ∼943.6 eV allows us to exclude the presence of Cu(II) the increase observed in the CA (Figure 3a). This enhanced species.47,48 The S 2p spectrum (Figure 2f) shows a strong performance was also observed by an anodic shift in the doublet peak with a S 2p3/2 binding energy at 162.2 eV. The overpotential to reach −10 mA·cm−2 (geometric area), from observed binding energy is attributed to the S2− species approximately −530 mV to a benchmark value of approx- bonded with Cu+ to form Cu2−xS. The presence of a broader imately −130 mV (Figure 3b, inset). It is apparent from these and less intense feature at 167 eV may be caused by the results that our Cu2−xS electrode requires ∼60 mV less presence of residual SOx species on the catalyst surface overpotential compared to a previously reported Cu2−xS oxidized in air. cathode for HER in the KPi electrolyte at pH 7.28 Blank 2.2. Electrocatalytic Performance. The functional LSVs performed on the bare Cu substrate in comparison with evaluation of our Cu2−xS electrocatalyst for H2 evolution was the Cu2−xS electrocatalyst (Figure S6) do not reveal a assessed by chronoamperometric (CA) measurements per- significant HER activity, clearly showing that the HER catalytic formed at −1 V vs RHE and simultaneous measurements of activity of the electrode arises from the Cu2−xS material. the FE. Remarkably, our Cu2−xS electrocatalyst exhibits a Furthermore, the here investigated Cu2−xS electrodes were record-breaking durability of ∼1 month of continuous tested in a two-electrode configuration reaching ∼15 mA cm−2 operation in a three-electrode configuration (Figure 3a). To with a remarkable durability of 65 h (Figure S7). Compared to ensure that the catalyst is highly efficient to evolve hydrogen, other Cu 2−xS and Cu2−x S-derived electrocatalysts, our not only at the beginning but also during days of continuous electrode exhibits among the lowest overpotential values operation, subsequent gas chromatography (GC) measure- obtained to date for electrocatalysts based on Cu2−xS (Figure ments were carried out to verify that the FE remains close to 1 and Table S1 for a state-of-the-art comparison).19,49 Note 100% over time (Figure S5). This striking durability that most of the works included in Table S1 were tested under outperforms the best Cu2−xS and Cu2−xS-derived electro- extreme pH conditions (highly acidic or alkaline), in which catalyst reported to date by nine times20 (see Figure 1 and faster reaction kinetics are expected. However, our motivation Table S1 for reported durability benchmark) as well as highly herein is to focus on moderate pH values to overcome the competent noble metal electrodes, such as RuP nanoparticles problems derived from the use of highly acidic or alkaline (∼200 h, i.e., over a week of operation).29 Strikingly, the media accelerating the implementation of both neutral and current density (J), at −1 V vs RHE, exhibited by our Cu2−xS seawater splitting technologies. electrocatalyst increases monotonically by 8-fold, from To rationalize such an increasing performance of our Cu2‑xS approximately −50 to approximately −400 mA cm−2 (Figure electrodes shown in Figure 3a,b and considering how the 10459 https://doi.org/10.1021/acscatal.3c01673 ACS Catal. 2023, 13, 10457−10467 ACS Catalysis pubs.acs.org/acscatalysis Research Article Figure 3. Electrocatalytic performance characterization of the Cu2−xS electrodes. (a) Chronoamperometric measurement at −1 V vs RHE of the Cu2−xS catalyst for 28 days of continuous operation. The dashed gray line represents the quasi-linear increase in the catalytic current density as a function of operation time. Steady-state currents at −1.0 V vs RHE normalized by the electrochemical surface area (ECSA) are shown as light blue empty dots. (b) Linear sweep voltammograms (LSV), measured at 20 mV s−1, of the same Cu2−xS electrode as a function of operation time between day 1, i.e., freshly synthesized catalyst (darker blue), and after 28 days (lighter blue) of continuous operation. Inset: zoom between the first and the last LSV to compare the overpotential at −10 mA cm−2 (dashed red line). (c) Cathodic current densities (|J|) measured at −1.0 V vs RHE, from panel (b) (blue filled dots) compared to the ECSA increase ratio (empty green dots, RECSA) calculated using eq 1. Note that the time is in the log scale. (d) Series (RS) and charge transfer (RCT) resistances and capacitance, RS (gray dots), RCT (violet dots), and C (green dots) at the 28th day of measurement. The gray area denotes the potential region where RCT < RS. (e) Tafel plot comparing the 1st (dark red data) with the 28th (pink data) day of measurement of the steady-state catalytic currents (see Figure S14 for the corresponding steady-state LSVs) and (f) Tafel slope values as a function of operation time obtained from panel (e). All measurements were performed in 0.1 M KHCO3 (pH 8.6). electrode surface increases during electrochemistry (Figure RHE appears to be almost invariant as a function of time S8), we propose that an operando increase in the surface area is (Figure 3a, blue empty dots). The observed cathodic increase taking place. A larger surface area can facilitate the exposure of in J can be mostly attributed to a higher number of exposed more active sites improving the catalytic activity for HER while active sites as evidenced by the increase in ECSA measured by minimizing mass transport limitations.20,50,51 To test this EIS. This surface area increase allows our catalyst to reach hypothesis, the double-layer capacitance of the Cu2−xS catalyst remarkable current densities while preserving the FE toward was monitored systematically by electrochemical impedance H2. Interestingly, a similar increased HER performance was spectroscopy (EIS) during the whole operation time. Figure 3c reported by Yang and co-workers on CoMnS-based electrodes, shows that the ratio of the electrochemical surface area where the increased current density was associated with an increase (rECSA) in our Cu2−xS catalyst was determined in eq 1. ECSA increase due to a partial crystallinity loss of the (Cdl)t electrocatalyst.54 Concomitant with the noticeable visual ECSA t CS (Cdl)t increase in the surface area of our Cu2−xS electrocatalyst rECSA = = = ECSA1 (Cdl)1 (Cdl)1 during the HER (Figure S8 and Video S1), in Section 2.3, we CS (1) also discuss possible cathodic corrosion processes contributing where ECSA1 and ECSAt correspond to the electrochemical to the observed ECSA increase. surface area (ECSA), as reported elsewhere,52 of day 1 We now turn to analyze the charge transfer mechanism by (ECSA1) and that of subsequent measurements as a function means of EIS, for which we selected the set of data of day 28, of operation time (ECSAt). Cs is the specific capacitance, i.e., at the highest current density reported here (approximately which is canceled in eq 1, and similarly to the ECSA, (Cdl)1 −400 mA cm−2; Figure 3b). Figure 3d shows the evolution of and (Cdl)t correspond to the double layer capacitance of the both, series (RS) and charge transfer (RCT), resistances, fitted first day (Cdl)1 and that of subsequent measurements (Cdl)t. to Randle’s circuit (see the Supporting Information for further These Cdl data were obtained from the low-frequency discussion and Nyquist plots; Figure S11) with applied bias. capacitance plateau region (0.5−10 Hz), i.e., the Bode plot The crossover point where RCT becomes smaller than RS is in Figure S9, as previously reported.51,53 observed at approximately −0.3 V vs RHE in Figure 3d Figure 3c compares J (blue filled dots) with the increase in coinciding with the HER catalytic onset potential (see LSVs in electrochemical surface area (rECSA, see eq 1, green empty Figure 3b), indicating that at more cathodic potentials (gray dots). It is apparent from Figure 3c that rECSA increases by a shaded region in Figure 3d), the delivered current is limited by factor of ∼6.5 times from day 1 to day 28 of continuous the device design and engineering (reflected by RS), rather operation. We further use rECSA to normalize the LSVs from than by surface electrocatalysis (controlled by RCT). On the Figure 3b (see Figure S10). Upon normalization, J at −1 vs other hand, RS exhibits a constant behavior with bias but 10460 https://doi.org/10.1021/acscatal.3c01673 ACS Catal. 2023, 13, 10457−10467 ACS Catalysis pubs.acs.org/acscatalysis Research Article Figure 4. Operando and under inert atmosphere characterization. (a) Differential optical density spectra of the Cu2−xS (light and dark blue) and reference Cu foil (light and dark red) electrodes as a function of potential. For reference, Cu2−xS differential spectra were measured also in 0.1 M TBAP in acetonitrile, see further analysis and peak assignment in Figure S15. (b) Operando XRD diffractograms at different potentials from OCP to −1.0 V vs RHE. Inset: LSV; (c, d) reference and post electrochemical XPS measurements for (c) Cu LMM and (d) S 2p spectra. decreases as a function of the operation time due to the slope becomes approximately constant in Figure 3e, an increase in the ionic concentration of the solution (Figure inflection point in the currents and the ECSA ratio occurs S12),55,56 after ∼1 month of constantly adding electrolyte and higher J values and ECSA ratios are observed (Figure solution to the employed static electrochemical cell. 3c).59−61 Overall, the evolution of both ECSA and Tafel slopes Furthermore, although a small local pH variation (basification suggests a rather constant intrinsic activity of our Cu2−xS of 0.5−1 pH units at distance to the cathode <1 cm) was catalyst. detected, the pH of the bulk electrolyte was stable along the 2.3. HER Mechanistic Analysis. The evolution of the measurement. Interestingly, the extracted capacitances as a structure and composition of the Cu2−xS electrodes during the function of increasing the negative bias shown in Figure 3d HER process have been studied by combining a set of (green dots) exhibit a constant decreasing trend. This decrease complementary operando and post-mortem analysis techniques. in the capacitance at the catalytic region could partially be First, we performed operando spectroelectrochemical (SEC) related to the evolution of H2 gas since the produced bubbles analysis,62 where the UV−vis−NIR absorption spectra of our can decrease the electrochemical surface area, although it could Cu2−xS electrode were monitored while holding the electrode also be attributed to a decrease in the accumulated species at at different potentials. Figure 4a compares the differential the catalyst/electrolyte interface during the catalysis when the spectral fingerprint (in the UV−vis−NIR range) of the Cu2−xS charge transfer process is enhanced, as previously reported for electrode as a function of the applied potential (see Figures water oxidation electrocatalysts.57 Figure S13 also shows S15 and S16 for the full set of spectra). The counterpart higher extracted resistances and capacitances for the measure- spectra of the bare Cu foil are represented as reference. The ments at lower currents, i.e., between day 1 and 28, evidencing relatively flat spectra (light green and blue data) for both the enhanced charge transfer kinetics concomitant to Cu2−xS and Cu electrodes are obtained at low potentials (0.1 V decreased charge accumulation with operation time. vs RHE) where the current density is close to zero (Figure 3b). Tafel analysis was performed to gain insights into the charge These flat optical fingerprints evolve to spectra with at least transfer HER kinetics. Steady-state LSVs (Figure S14) were two peaks at ∼1080 and ∼1270 nm when applying higher performed concomitant to ECSA determination to determine potentials where HER catalytic currents take place.63−65 With the iR drop corrected Tafel slopes (Figure 3e,f).58 Initially, the the purpose of assigning the observed peaks, blank measure- electrocatalyst exhibits a Tafel slope of 270 mV dec−1, which ments were also performed in an inert electrolyte where gradually decreases during the first week of plateauing around hydrogen evolution is not expected. When the Cu2−xS ∼190 mV dec−1 until ∼1 month, indicating that the HER electrode is under equivalent applied potentials in acetonitrile kinetics of the Cu2−xS is faster after 1 month of continuous (Figure 4a, dark red spectrum), these two peaks are not operation correlating with the increase in J and the decrease in present. This observation suggests that the signals observed at η (Figure 3b and inset, respectively) as well as with the ∼1080 and ∼1270 nm correspond to protons bound to both decrease in RCT (Figure S13a). Note that by the time the Tafel Cu2−xS and electropolished Cu surfaces. The development of 10461 https://doi.org/10.1021/acscatal.3c01673 ACS Catal. 2023, 13, 10457−10467 ACS Catalysis pubs.acs.org/acscatalysis Research Article this optical fingerprint in the catalytic region allows us to assign spectra from the as-synthesized electrode (blue lines) and the peaks at ∼1080 and ∼1270 nm to species accumulating at metallic Cu (red dotted lines) from Figure 2e,f are included as the rate-determining step of the HER. Note that these signals reference. The surface of the electrodes was gently sputtered are only observed, while HER is taking place and is equivalent with Ar+ ions prior to the XPS characterization to eliminate the between both the Cu2−xS catalyst and bare Cu foil substrate remaining residues from the transfer procedure. The electrodes (see the shaded region in Figure 4a). This parallelism between after 60 min and 1 week of operation have evolved to a both Cu2−xS and bare Cu electrodes suggests that the HER reduced metallic state (Figure 4c), attributed to the increase in catalytic activity of Cu2−xS is driven by centers located at Cu species at the Cu2−xS surface. To further assign each optical the Cu0 (K.E. 918.4 eV) component in the Cu LMM spectra, signal observed in the spectra, additional experiments were simultaneous to the decrease in the Cu (I and II) (K.E. 917 performed by adding 10% volume of water to the inert eV) components observed in the as-synthesized electrodes. electrolyte. Figure S15 in SEC supplementary analysis shows Notably, a decrease in the S 2p component takes place with the UV−vis−NIR absorption spectra of Cu2−xS electrocatalyst operation time (Figure 4d), observing almost complete measured in 0.1 M KHCO3 and 0.1 M TBAP in acetonitrile depletion of S after 1 week of catalytic activity, in agreement and as reference a bare Cu electrode in 0.1 M KHCO3 under a with previously discussed operando XRD experiments. Such strong negative bias. Optical signals at ∼550 and 1370 nm are depletion of S could be associated with an initial formation of S assigned to optical absorptions from the Cu2−xS electrode in vacancies, which evolves to a dissolution of S to the electrolyte, agreement with previous reports.45,63 Additionally, a NIR as shown in Figure S19. The S 2p XPS spectrum of the fresh absorption band from free H2O (i.e., water molecules in the Cu2−xS electrode (petrol blue line; Figure 4d) shows two peaks solvent not interacting with the catalyst surface) is also located at 162.2 eV (S 2p3/2) and 163.37 eV (S 2p1/2), observed at ∼1500 nm.66 Interestingly, an optical absorption at corresponding to the spin−orbit splitting (ΔE = 1.17 eV) of ∼1200 nm emerges when the bias increases toward more negative potentials and it is discussed in the Supporting the 2p orbital. Both components contain equivalent chemical Information. information. The S 2p spectrum of the electrode after 60 min Operando XRD analysis was carried out on our Cu2−xS of operation (purple line; Figure 4d) presents a substantial electrocatalyst (Figure 4b) to unveil crystallinity changes (about 1/3) reduction of its initial intensity. The deconvoluted taking place under catalytic conditions. Figure 4b shows the analysis of the spectral features reveals the emergence of an evolution of the XRD patterns when the potential is additional component at higher binding energies (E(2p3/2) = progressively increased from open circuit potential (OCP, 162.7 eV), which adds up to approximately 38% of the total S i.e., ∼0.2 V vs RHE) to −1 V vs RHE. Under OCP conditions, signal (see Figure S20). We tentatively attribute this higher- characteristic phases of Cu, Cu2O, Cu1.8S, and CuSO4 are energy component as the fingerprint of a transient phase at the observed. The Cu2O phase disappeared between −0.4 and beginning of the electrochemical process, which precludes the −0.6 V vs RHE (green asterisk in the cathodic linear sweep total depletion of S from the electrode. All fitting parameters voltammogram, inset in Figure 4b). At more cathodic are provided in the Supporting Information. potentials (−0.8 and −1 V vs RHE), the signals related to Further post-mortem characterization by SEM, XRD, and the Cu1.8S and the secondary CuSO4 phase also vanished, related to the following cathodic wave (blue asterisk in the HRTEM after removing the electrode from the solution in air inset of Figure 4b) in the linear sweep voltammogram and the was not informative due to the formation of an extensive oxide corresponding growth of the Cu signal around −1.0 V vs RHE. layer (not shown). A more precise quantification of the evolution of the Cu peak In general, we observe that the increased HER performance was elusive since the Cu foil underneath results in the correlates with an operando ECSA increase in our catalyst. As oscillation in intensity. the HER proceeds, we detect severe changes in crystallinity Complementary, XPS measurements were performed to and a progressive S leaching process leading to a highly porous probe the chemical state and composition of the surface of the and active catalytic micro/nanostructure, as shown schemati- Cu2−xS electrodes immediately after HER. The electrochemical cally in Figure 5. The increase in porosity is consistent with an experiments and emersion of the sample were carried out in a increase in the ECSA leading to a higher density of exposed glovebox under a N2 (99.999%) inert atmosphere. Then, the active sites that are found to be centered at Cu species. sample transfer to the XPS setup was conducted using a portable glovebox under N2, which was adapted to be directly attached to the ultra-high vacuum (UHV, P < 10−9 mbar) system. Copper sulfide compounds are known to readily oxidize upon air exposure.67,68 Transfer and post-mortem characterization in air could mislead the interpretation of the actual composition of the surface (see Figure S17 for a comparison of air vs N2 exposed samples). We also notice the relevance of an abundant rinsing of the electrode after HER to avoid residues of the bulk electrolyte solution on the sample surface, which might inhibit the photoemitted signal from the surface of the electrode (see Figure S18 for a comparison of washed versus unwashed samples). Figure 4c and Figure 4d Figure 5. Schematic representation of the proposed changes in show, respectively, the Cu LMM Auger transition and S 2p crystallinity of our Cu2−xS electrocatalyst, concomitant to the S (gray) core level emission of the electrodes after 60 min (purple solid leaching process that leads to more catalytically active Cu species line) and 1 week (blue petroleum solid line) of operation. The (brown) to be exposed leading to faster HER. 10462 https://doi.org/10.1021/acscatal.3c01673 ACS Catal. 2023, 13, 10457−10467 ACS Catalysis pubs.acs.org/acscatalysis Research Article 3. CONCLUSIONS resolution transmission electron microscopy (HRTEM) and In summary, we report an earth-abundant Cu2−xS electro- scanning transmission electron microscopy (STEM) inves- catalyst for sustainable H2 production. A simple, fast, tigations were performed on a field emission gun FEI Tecnai environmentally friendly, and cost-effective synthetic route is F20 microscope. High-angle annular dark-field (HAADF) presented to obtain highly efficient Cu2−xS electrodes for H2 STEM was combined with electron energy loss spectroscopy production with an FE of ∼100% and current densities that (EELS) in a Tecnai microscope by using a GATAN increase from 50 to ∼400 mA·cm−2, with remarkably low QUANTUM energy filter to obtain compositional maps. overpotentials (−130 mV at 10 mA·cm−2) in a mild pH Profilometer measurements were performed on a mechanical electrolyte, for almost 1 month of continuous operation. All profilometer Veeco model Dektak 6 (see Figure S2d). these features make our electrocatalyst very competitive for its X-ray diffractograms were recorded using a Rigaku Miniflex scalability and subsequent technological deployment. We 600 (Rigaku Corporation, Tokyo, Japan) with Cu Kα radiation presented a comprehensive electrochemical, structural, and (λ = 1.5418 Å) (see Figure 2d). chemical analysis on the mechanistic pathway of the reaction Topographic images of the samples were measured by and the electrode evolution under operando conditions. Our atomic force microscopy (AFM) using an Asylum Research study unveils a direct relationship between the increase in the Cypher ES (Oxford Instruments-Asylum Research, Santa current densities and the ECSA and establishes a faster HER Barbara, USA) in tapping mode under ambient conditions kinetics with increasing operation time. SEC analysis reported with a silicon cantilever AC160TSA-R3 (Olympus, Tokyo, for the first time for copper-based films for HER, in situ XRD Japan) with spring constant k = 25.4 N/m, resonant frequency analysis, and air-free XPS characterization of the post-mortem f = 291 kHz, and a tip radius of 7 nm (see Figure S3). electrode demonstrates that the catalytically active sites are 4.3. X-ray Photoemission Spectroscopy Measure- located on Cu species. In conclusion, we believe that the Earth ments. XPS has been carried out holding the sample at abundance of the developed Cu2−xS electrocatalyst and the room temperature and illuminating it with monochromatized fast, environmentally friendly, and cost-effective synthetic Al Kα light (hν = 1486.6 eV) from a microfocus setup (SPECS strategy, combined with the excellent performance in almost Focus 600, spot size 500 μm). The excited photelectrons were neutral pH conditions and relevant guidelines for catalyst collected by a SPECS 150 hemispherical analyzer at emission design and optimization based on mechanistic analysis, may and incidence angles of 40 and 60°, respectively. The overall pave the way for the future design of highly efficient, durable, experimental resolution was extracted from Fermi edge analysis and economically competitive hydrogen evolution electro- of a reference gold sample and resulted in 0.4 eV. To average catalysts to be used in a wide range of electrosynthetic inhomogeneities of the electrodes, especially after the HER, applications. different positions of the samples were characterized by XPS. The data presented here is the result of averaging multiple spectra of different positions for each sample. 4. EXPERIMENTAL SECTION The characterized electrodes before the HER were sputtered 4.1. Electrode Preparation. 4.1.1. Substrate Pre-Treat- in UHV to remove the oxide layers that form upon exposure of ment. Prior to film preparation, copper foil substrates Cu2−xS to air67 and the adventitious C that adsorbs. A (Goodfellow, 0.2 mm thick, 99.9%) were cleaned with soap/ comparison of the Cu 2p, Cu LMM, O 1s, C 1s, and S 2p Milli-Q water, acetone, and ethanol. After drying the substrates spectra before and after the sputtering can be found in Figure under airflow, the substrates were electropolished in S4. phosphoric acid (Alfa-Aesar, 85%) potentiostatically 1 min at The data of electrodes after the HER corresponds to MiliQ- 3 V and 10 min at 2 V vs Carbon GDL (FuelCellStore, washed, N2 environment-produced samples. MiliQ water was Sigracet 39 BC) in a two-electrode configuration. Finally, the used to rinse the electrode after the HER to remove electrolyte electropolished copper foil was rinsed with abundant deionized residues.46 High purity (99.999%) N2 was used to reduce water and dried in air. unwanted oxidation from air exposure during the HER and 4.1.2. Na2S4 Solution Preparation. A Na2S4 polysulfide manipulation of the samples. The samples were kept in a solution was prepared following a procedure described in commercial glovebox during HER under a continuous flow of previous works38 mixing NaOH (Scharlab), Na2SO4 (Alfa N2, transported in a sealed N2 container, and inserted in the Aesar, 98%), and S (Sigma-Aldrich, 99.98%) stoichiometrically UHV load-lock without breaking the N2 atmosphere. A with deionized (DI) water in a three-neck round bottom flask. detailed comparison of the impact of sample rinsing and The resulting mixture was heated at 313 K with vigorous transport in N2 can be found in Figures S17 and S18, stirring and constant purified nitrogen bubbling for 3 h. respectively. Subsequently, the temperature of the solution was increased to 4.4. Electrochemical Measurements. The electrochem- 373 K until a color change from yellow to black was observed. ical performance of the electrodes was evaluated by linear 4.1.3. Electrode Synthesis. The Na2S4 solution was drop- sweep voltammetry (LSV) and chronoamperometry (CA) casted onto the pre-treated copper foil substrates. Then, the using an Autolab potentiostat/galvanostat PGSTAT302 in a electrode was rinsed with abundant DI water to remove the three-electrode homemade sealed electrochemical cell suitable excess of the polysulfide solution and dried under airflow. The for gas chromatography. Aqueous Ag/AgCl (0.3 M KCl, ALS, prepared films showed the whole surface of the substrate Japan) and a Pt mesh (Alfa Aesar) were used as reference and covered by a homogeneous black Cu2−xS thin layer. counter electrodes, respectively, and a 1 cm2 geometrical area 4.2. Characterization Techniques. SEM measurements Cu2−xS film as the working electrode. Potassium bicarbonate were performed with a JSM-7000F JEOL FEG-SEM system (KHCO3) was chosen as the electrolyte due to its moderate (Tokyo, Japan) equipped with an INCA 400 Oxford EDS pH, electrode performance (compared to the KPi electrolyte), analyzer (Oxford, U.K.) operating at 15 kV (see Figure 2a and and the potential implementation of our electrocatalysts in sea Figure S2 and Table S2 for the Cu2−xS EDS analysis). High- water reduction. All the potentials reported herein referred to 10463 https://doi.org/10.1021/acscatal.3c01673 ACS Catal. 2023, 13, 10457−10467 ACS Catalysis pubs.acs.org/acscatalysis Research Article the reversible hydrogen electrode (RHE) and were calculated monitored every 5 min, and H2(theo) corresponds to the through the Nernst equation VRHE = VAg/AgCl + 0.199 + 0.059· theoretical H2 evolved calculated by Faraday’s law: n(mol) = pH. Data was not iR corrected unless otherwise stated. LSVs jH2t/nF, where jH2 is the current density recorded during the were performed between 0.1 and −1 V vs RHE with a scan rate chronoamperometry measurement, t is the time in seconds, n of 0.02 V/s and step of 0.002 V, and CA was measured at a is the number of electrons transferred in the reaction, and F is constant voltage of −1 V vs RHE with an interval of 10 s. The the Faraday constant, 96,485.33 C mol−1. Furthermore, using irregular line obtained from the chronoamperometric measure- the same procedure, long-term H2 Faradaic efficiency was ment (Figure 3a) is due to a volume change of electrolytes determined by monitoring the evolved H2 every 6 h over 1 given the constant evaporation and reaction of it and week of reaction (Figure S5). subsequent refilling to 20 mL. Two-electrode durability tests Total reflection X-ray fluorescence (TXRF) analysis of the were also carried out to evaluate the electrocatalyst perform- electrolyte before and after the electrochemical test was carried ance in real operation conditions (Figure S7) since it has been out using a S4 TSTAR, Bruker, equipped with a 17.5 keV reported that three- and two-electrode characterization lead to Molybdenum primary excitation source (Figure S19). significant differences in stability.69 Cu2−xS was measured versus a Pt mesh at −3 V in 0.1 M KHCO3 for 65 h. Electrochemical impedance spectroscopy (EIS) measure- ■ * ASSOCIATED CONTENT sı Supporting Information ments were performed between 0.1 Hz and 0.1 MHz with 10 The Supporting Information is available free of charge at mV of amplitude perturbation. The EIS raw data were analyzed https://pubs.acs.org/doi/10.1021/acscatal.3c01673. with ZView software (Scribner Associates), fitting the raw data to an equivalent circuit model (see Figure S11, inset) for Supplementary SEM and AFM images, XPS character- extracting both capacitances and resistances. To investigate the ization, Faradaic efficiency, two-electrode test, EIS, SEC dependence between the series resistance (RS) and the data, and TXRF (PDF) concentration of the electrolyte, equivalent EIS data was Video showing the visual increase in the electrode area obtained, using fresh samples, as a function of the KHCO3 (MP4) concentration (see Figure S12, inset). Tafel analyses were performed by measuring, considering that the required experimental conditions for a valid analysis ■ AUTHOR INFORMATION Corresponding Authors were met (i.e., no Ohmic distortions, no background currents, Sara Barja − Department of Polymers and Advanced and at least one decade of linearity in the Tafel curve).70 Since Materials, Centro de Física de Materiales, University of the the Tafel slopes are overpotential-dependent, Tafel analysis Basque Country UPV/EHU, 20018 San Sebastián, Spain; must to be done with data acquired in a steady state and free of Donostia International Physics Center, 20018 San Sebastián, iR drop.58 Steady-state LSVs (ssLSVs) were built by measuring Spain; IKERBASQUE, Basque Foundation for Science, subsequent 5 min chronoamperometries and averaging the last 48009 Bilbao, Spain; Email: sara.barja@ehu.eus 60 s of the current vs time response, at each applied potential Camilo A. Mesa − Institute of Advanced Materials (INAM), (Figure S14). Then, the iR drop corrected overpotentials were Universitat Jaume I, 12006 Castelló, Spain; Research Cluster plotted vs log |J| extracting from these curves the Tafel slopes on Converging Sciences and Technologies (NBIC), (Figure 3e,f). Although the obtained Tafel slopes are higher Departamento de Ingeniería Electrónica, Universidad Central, than expected for Tafel, Heyrovsky, or Volmer mechanisms, Bogotá 110311, Colombia; orcid.org/0000-0002-8450- the observed trends are still informative for the evolution of the 2563; Email: cmesa@uji.es intrinsic activity of our catalyst. Sixto Giménez − Institute of Advanced Materials (INAM), Spectroelectrochemical (SEC) analysis was performed by Universitat Jaume I, 12006 Castelló, Spain; orcid.org/ coupling electrochemical and spectroscopic techniques. Chro- 0000-0002-4522-3174; Email: sjulia@uji.es noamperometric measurements were carried out for the copper sulfide electrocatalyst and on a bare copper substrate Authors in a three-electrode Raman electrochemical flow cell Roser Fernández-Climent − Institute of Advanced Materials (RedoxMe) at different potentials while taking optical (INAM), Universitat Jaume I, 12006 Castelló, Spain absorption spectra on a spectrophotometer UV−vis−NIR Jesús Redondo − Department of Polymers and Advanced (Perkin Elmer 1050+). To obtain additional data, the reactions Materials, Centro de Física de Materiales, University of the were carried out both in the bicarbonate electrolyte and in dry Basque Country UPV/EHU, 20018 San Sebastián, Spain; acetonitrile with and without adding water. Data was acquired Department of Surface and Plasma Science, Faculty of in diffuse reflectance mode using an integrating sphere, and the Mathematics and Physics, Charles University, 180 00 Prague data was further converted into absorbance by the Kubelka− 8, Czech Republic Munk equation. The measured spectra shown in Figure 4a are Miguel García-Tecedor − Institute of Advanced Materials presented as differential spectra (ΔOD) from the absorption (INAM), Universitat Jaume I, 12006 Castelló, Spain; data. Photoactivated Processes Unit, IMDEA Energy Institute, 4.5. Product Determination and Quantification. The Parque Tecnológico de Móstoles, 28935 Móstoles, Madrid, H2 generation was monitored by gas chromatography Spain; orcid.org/0000-0002-9664-4665 measurements coupling the sealed cell to an Agilent Micro- Maria Chiara Spadaro − Catalan Institute of Nanoscience and GC gas chromatograph. Chronoamperometric measurements Nanotechnology (ICN2) and BIST Campus UAB, Bellaterra were carried out at three different potentials (−0.5, −0.8, and 08193 Barcelona, Spain; orcid.org/0000-0002-6540- −1 V vs RHE) for half an hour each. The Faradaic efficiency 0377 (FE) was estimated through the relation FE (%) = H2(exp)/ Junnan Li − Department of Chemistry, Université de Montréal, H2(theo), where H2(exp) is the amount of evolved H2 in mol, Montréal, QC H2V 0B3, Canada 10464 https://doi.org/10.1021/acscatal.3c01673 ACS Catal. 2023, 13, 10457−10467 ACS Catalysis pubs.acs.org/acscatalysis Research Article Daniel Chartrand − Department of Chemistry, Université de Colombia through the Fondo Nacional de Financiamiento Montréal, Montréal, QC H2V 0B3, Canada para la Ciencia, la Tecnología y la Innovación “Francisco José Frederik Schiller − Centro de Física de Materiales and de Caldas”, call 848-2019. ICN2 is supported by the Severo Material Physics Center CSIC/UPV-EHU, 20018 San Ochoa program from Spanish MCIN/AEI (Grant No.: Sebastián, Spain; Donostia International Physics Center, CEX2021-001214-S) and is funded by the CERCA Pro- 20018 San Sebastián, Spain; orcid.org/0000-0003-1727- gramme/Generalitat de Catalunya. M.C.S. has received 3542 funding from the post-doctoral fellowship Juan de la Cierva Jhon Pazos − Research Cluster on Converging Sciences and Incorporation from MICINN (JCI-2019) and the Severo Technologies (NBIC), Departamento de Ingeniería Ochoa programme. S.B. acknowledges grant RYC-2017-21931 Electrónica, Universidad Central, Bogotá 110311, Colombia funded by MCIN/AEI/10.13039/501100011033 and by ESF Mikel F. Hurtado − Research Cluster on Converging Sciences Investing in Your Future, EUR2020-112066 funded by and Technologies (NBIC), Departamento de Ingeniería MCIN/AEI /10.13039/501100011033 and by European Electrónica, Universidad Central, Bogotá 110311, Colombia; Union NextGenerationEU/PRTR, and UPV/EHU project Materials Chemistry Area, Civil Engineering Department, EHUrOPE19/01. J.R. acknowledges the Czech Science Corporación Universitaria Minuto de Dios, Calle 80, Main Foundation and funding from PIF outgoing project number Sede Bogotá, Colombia. − Nanotechnology Applications 22-18079O. Area, Environmental Engineering Department, Universidad Militar Nueva Granada, Zipaquirá 110311, Colombia Victor de la Peña O’Shea − Photoactivated Processes Unit, IMDEA Energy Institute, Parque Tecnológico de Móstoles, ■ REFERENCES (1) Agency, I. E. World Energy Outlook 2018; 2018. 28935 Móstoles, Madrid, Spain; orcid.org/0000-0001- (2) Midilli, A.; Ay, M.; Dincer, I.; Rosen, M. A. 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