Research Article ChemElectroChem doi.org/10.1002/celc.202201152 www.chemelectrochem.org Flexible and Binder-Free Iron Phosphide Electrodes Using a Three-Dimensional Support for High Hydrogen Productivity María Isabel Díez-García,*[a, b] Andrés Alberto García Blanco,[a, c] Sebastian Murcia-López,[a] Marc Botifoll,[d] Jordi Arbiol,[d, e] Mohammad Qamar,[f] and Joan Ramon Morante[a] In this work, an inexpensive and reliable microstructured electrode 100 mA · cm 2 by applying only 53 mV vs RHE at 50 °C in 0.5 M for the hydrogen evolution reaction (HER) is developed. This H2SO4, demonstrating excellent electrocatalytic activity for the HER cathode is made of Earth-abundant elements consisting of iron and outstanding stability in acidic electrolyte. Furthermore, the phosphide as an electrocatalyst and carbon felt (CF) as a flexible feasibility of these electrodes for industrial application is evaluated conductive scaffold. Its porous character and binder-free FeP using a PEM electrolyzer. The developed prototype with a cathodic coverage over the carbon fibers generate a high number of area of 1.8 cm2 shows a very promising performance, reaching accessible active sites for the reaction, achieving a high value of 14.9 mmol H2 · h 1 · cm 2 (corresponding to 800 mA · cm 2) at a the electrochemically active surface area. The electrode reaches voltage of only 2.1 V. Introduction stable electrocatalysts as well as minimum energy input to drive the electrochemical reactions (i. e., low overpotentials). Under this The deployment of carbon-free energy technologies that are scenario, in addition to the electrocatalytic activity of the competing with the price of fossil fuels, is expected to be a key material, other key parameters must also be considered such as factor in sustaining the future energy requirements of the world the electrode structure and the cell configuration. They are population. In this regard, H2 is a promising energy vector with a critical for fast electron transfer in the electrocatalyst/electrolyte high energy density that can be produced electrochemically interface, and for avoiding mass transport limitations by assuring from pure water with zero carbon emissions. The required input appropriate product (gas) release through the nano/microporous power for the water splitting reaction in an electrolyzer can be structure. In this work, we have paid attention to these design obtained from renewable sources, such as wind, solar, geo- issues through the fabrication of an inexpensive 3D electrode thermal or hydraulic energy. Additionally, the development of made of Earth-abundant elements, such as FeP and a carbona- efficient devices needs cost-effective, high productivity and ceous supporting scaffold (carbon felt). Likewise, concerning the cell configuration, a proton exchange membrane electrolyzer [a] Dr. M. I. Díez-García, Dr. A. A. García Blanco, Dr. S. Murcia-López, (PEM electrolyzer),[1–4] which presents some advantages over the Prof. J. R. Morante traditional electrolysis technologies, has been selected. Catalonia Institute for Energy Research (IREC) Commonly, Pt is one of the materials leading the bench- Jardins de les Dones de Negre 1, 08930 Sant Adrià del Besòs, Barcelona (Spain) mark electrocatalytic activity for hydrogen evolution reaction E-mail: midiez@irec.cat (HER). In fact, commercial PEM electrolyzers are based on Pt and [b] Dr. M. I. Díez-García Ir, but they are expensive and scarce elements. This motivates Current address: École Polytechnique Fédérale de Lausanne the research of other cost-effective materials made of Earth- Station 6, 1015 Lausanne (Switzerland) abundant elements with competitive catalytic activities com- E-mail: maria.diezgarcia@epfl.ch pared to precious metal-based catalysts. Some transition metal [c] Dr. A. A. García Blanco Instituto de Física Aplicada (INFAP-CONICET) compounds such as hydroxides, oxides, carbides, sulfides, Universidad Nacional de San Luis phosphides and nitrides have demonstrated promising electro- Av. Ejército de los Andes 950, San Luis 5700 (Argentina) catalytic activity for the HER.[5] Among them, transition metal [d] M. Botifoll, Prof. J. Arbiol Catalan Institute of Nanoscience and Nanotechnology (ICN2) phosphides have attracted attention because most of them CSIC and BIST, Campus UAB Bellaterra, 08193 Barcelona, Catalonia (Spain) exhibit favorable properties such as electrical and thermal [e] Prof. J. Arbiol conductivity,[6] with similar values to those expected for ICREA Pg. Lluís Companys 23, 08010 Barcelona, Catalonia (Spain) compounds with metallic character, which make them advanta- [f] Dr. M. Qamar geous against other electrocatalytic active materials. Interdisciplinary Research Center for Hydrogen and Energy Storage Therefore, metal phosphides based on Co, Mo, Ni and Fe King Fahd University of Petroleum and Minerals Dhahran 31261 (Saudi Arabia) have been widely studied for the HER.[7–10] For a high perform- Supporting information for this article is available on the WWW under ance not only the nature of the electrocatalyst is important but https://doi.org/10.1002/celc.202201152 also its interaction with the conductive scaffold. In the last years, © 2023 The Authors. ChemElectroChem published by Wiley-VCH GmbH. This new synthesis procedures that are advantageous against is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any previous general approaches, such as the trioctylphosphine medium, provided the original work is properly cited. (TOP) route, have been developed.[7,11,12] Among these materials, ChemElectroChem 2023, 10, e202201152 (1 of 8) © 2023 The Authors. ChemElectroChem published by Wiley-VCH GmbH 21960216, 2023, 17, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/celc.202201152 by Readcube (Labtiva Inc.), Wiley Online Library on [25/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Research Article ChemElectroChem doi.org/10.1002/celc.202201152 iron phosphide (FeP) is a promising candidate as Fe is one of the tantly, carbon felt can be pressed against the membrane for most abundant elements in the Earth’s crust, and its electro- zero-gap configuration in a PEM flow cell, which is facilitated by catalytic activity for the HER has been reported in different the soft and compressible nature of this substrate. electrolytes.[5,13,14] More recently, DFT studies have revealed In this work, we report on the fabrication of FeP-based appropriate Gibbs free energies of hydrogen adsorption in some cathodes using CF as conductive support. We followed a route FeP facets,[15,16] which contributes to the excellent performance of based on the direct growth of tiny FeP nanoparticles on the 3D this material.[8] Kwong et al.[17] pointed out that cationic vacancy structure of the CF that avoids the use of binders, facilitating defects (Fe vacancies) in FeP increase the HER activity by the charge transfer and favoring the exposure of a high number modulating hydrogen absorption. The stability of FeP has been of active sites.[12] Firstly, Fe2O3 is deposited on the carbon reported extensively at low current densities in acidic media,[5] microstructures and afterward converted to FeP by a reaction at while in alkaline media some reports pointed out instability relatively low temperatures in presence of NaH2PO2. The evidenced by some drop of the current with time at a current adequate electrocatalyst/scaffold configuration led to a high density of 10 mA · cm 2.[18,19] Furthermore, the synthesis of FeP via electroactive area and good electron transport that derived, the decomposition of sodium hypophosphite is simple, scalable considering also the electrocatalyst characteristics, in optimized and provides more flexibility in the use of different metal overpotentials for the HER. Then, these cathodes have been precursors. Additionally, the morphology is preserved after the tested at a high current density at 50 °C, which is close to the phosphidization step,[7,14] allowing more control over the ultimate operative temperatures used in commercial electrolyzers. The structure and the electrocatalyst/substrate configuration. This feasibility of the synthesis route and the flexibility of the CF facilitates the direct growth of FeP on the scaffolds without extra substrate facilitate the integration of the FeP-based cathode in additives that may hinder electron transport,[20,21] or block the a PEM electrolyzer under flow cell conditions. active sites[14] thereby decreasing the catalytic activity. On the other hand, the use of carbonaceous scaffolds is advisable in practical electrolyzers as they are generally highly Results and Discussion conductive and relatively cost-effective. Previously, the electro- catalytic activity of FeP deposited on carbon nanostructures[22] Physicochemical characterization of the FeP/CF electrodes and other carbon scaffolds such as carbon cloth[14,22–25] have before HER been studied. Carbon felt (CF) is composed of carbon fibers with a micrometric diameter that may be favorable for the mass The electrodes were characterized by XRD before and after the transport of the produced gases and homogenous composition heat treatment at 350 °C in presence of NaH2PO2. Figure 1a inside the pores, especially when the hydrogen production shows that, after annealing at 400 °C in air, the diffraction peaks rates are high. CF is reported as robust support for large-scale match with those of the hematite phase (JCPDS 01-072-0469). production because of its adequate electrical conductivity, The crystallite size calculated by the Scherrer equation in the macroporous structure, chemical stability, and high surface peak of the plane (1 1 0) is 22 nm. The Fe2O3/CF electrodes area.[26] In fact, because of these favorable properties, it is were converted to FeP by the reaction with PH3, generated extensively employed in redox flow battery devices.[27] Impor- in situ by thermal decomposition of the sodium hypophosphite Figure 1. (a) XRD patterns of the Fe2O3/CF (after the annealing at 400 °C in air) and the as-prepared FeP/CF electrode after phosphidization at 350 °C. The peak positions according to the JCPDS cards 078-1443 and 072-0469 for FeP and Fe2O3, respectively, are included at the bottom of the graph. (b) HRTEM image of an FeP crystal from the FeP/CF electrode after the acid etching, the magnification of the red squared area in (c) and its FFT spectrum in (d). ChemElectroChem 2023, 10, e202201152 (2 of 8) © 2023 The Authors. ChemElectroChem published by Wiley-VCH GmbH 21960216, 2023, 17, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/celc.202201152 by Readcube (Labtiva Inc.), Wiley Online Library on [25/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Research Article ChemElectroChem doi.org/10.1002/celc.202201152 at 350 °C: 2 NaH2 PO2 �H2 O ! PH3 þ Na2 HPO4 þH2 O.[28] After the scopy (EELS) analysis in Figure 2 shows a homogeneous phosphidization step, the XRD pattern confirms the trans- distribution of Fe and P throughout the particles. Oxygen was formation of Fe2O3 into the orthorhombic phase of FeP (JCPDS also detected but with very low intensity indicating that some 01-078-1443). The crystallite size decreased after phosphidiza- undesired oxygenated species can be present as impurities. tion to 11 nm, calculated from the plane (2 1 1) of FeP. It should Thus, it can be concluded that the deposited electrocatalyst is be mentioned that after the acid etching treatment, which is mostly composed of nanoparticles of FeP. performed before the electrochemical tests to remove some impurities derived from the synthesis, the XRD pattern also shows the peaks ascribed to FeP, as expected. The plane (100) Electrochemical characterization of the FeP/CF electrodes in of graphitic carbon is also present in the XRD patterns, in three-electrode configuration agreement with the spectra of the bare carbon felt (Fig- ure S1).[26] The morphology of the FeP/CF electrode was The electrocatalytic activity of the FeP/CF electrodes for the analyzed by HRSEM. Figure S2a–c show the carbon fibers HER was evaluated in 0.5 M H2SO4 and 1 M KOH. Figure 3a-b covered by the FeP nanoparticles. In general, the coalescence of show the polarization curves after IR correction. The respective these nanoparticles forming a porous layer is observed in some overpotentials for the FeP/CF electrode in the acidic electrolyte regions (see Figures S2b–c). Such morphological features give at room temperature are calculated to be 41, 55 and 97 mV to rise to a high surface area and consequently high surface produce 10, 20 and 100 mA · cm 2. As a reference test, the reaction sites while the macropores are still preserved. This is electrochemical response of the bare CF substrate using the beneficial to avoid diffusion limitations, especially for the same tantalum-based electrical contact was also measured to release of the hydrogen bubbles produced and at the same evaluate the activity of the HER in the selected potential range. time, the mass transport through the electrolyte is guaranteed. The CF was exposed to the same heat treatments as those of To get more insights into the crystalline structure of FeP the FeP/CF electrodes, but without being immersed in the iron nanocrystals, the electrodes after the acid etching were precursor solution. Figure S4 shows that the CF produces much analyzed by high-resolution transmission electron microscopy lower currents below 0 V vs RHE than the FeP/CF electrode, (HRTEM). Figure 1b–c show a nanoparticle of about 20 nm with ruling out any significant contribution of the supporting well-defined lattice fringes oriented along the [010] direction. material and the metallic contact to the HER activity. In the Figure 1d displays the Indexed Power spectrum (Fast Fourier alkaline electrolyte, the overpotentials are higher than those transform (FFT)) obtained in the red squared region of Figure 1b recorded under acidic conditions, which is a typical behavior in (and the magnification of this area in Figure 1c) that agrees the HER electrocatalysis. Water dissociation takes place before with the orthorhombic phase (PNMA space group 62) of FeP. hydrogen adsorption in alkaline media, which generally makes This crystalline phase is in agreement with the XRD results in HER reaction more sluggish[31,32] Figure 3b shows the linear Figure 1a. The [010] facet has been identified as active for the sweep voltammetry for the FeP/CF in 1 M KOH electrolyte and HER according to DFT calculations and experimental results.[29,30] the overpotentials are 63, 82 and 123 mV for producing 10, 20 Other facets were also identified in the electrode by applying and 100 mA · cm 2, respectively. The electrochemical activity for FFT (Figure S3) in agreement with the nanoparticulated mor- the HER obtained in this study is among the best reported in phology of the electrocatalyst. Electron energy-loss spectro- the literature in both electrolytes, including electrodes prepared Figure 2. EELS maps of Fe, P, O and Fe + P in the red squared area marked in the STEM-ADF image (on the left side) of an FeP nanoparticle from an acid- etched FeP/CF electrode. ChemElectroChem 2023, 10, e202201152 (3 of 8) © 2023 The Authors. ChemElectroChem published by Wiley-VCH GmbH 21960216, 2023, 17, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/celc.202201152 by Readcube (Labtiva Inc.), Wiley Online Library on [25/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Research Article ChemElectroChem doi.org/10.1002/celc.202201152 nism in both cases, being the limiting reaction step the Heyrovsky step. This reaction mechanism is the most reported for the HER catalyzed by FeP in both electrolytes,[25,33–35] except few reports that point out a Volmer-Tafel mechanism.[23,24] The FeP/CF electrodes show high exchange current densities (j0) of 1.44 and 0.98 mA · cm 2 in the acidic and alkaline electrolytes, respectively. This parameter was estimated from the intercept with the x-axis in the Tafel plots (Figure 3c) obtained from the extrapolation of the linear region. For direct deposition of FeP, the j0 recorded in this study is one of the highest values reported in the literature (see Tables S1 and S2). When the temperature is increased to 50 °C (Figure 3a) in the acidic electrolyte, the potentials after IR correction are reduced by 33, 35 and 44 mV for 10, 20 and 100 mA · cm 2 with respect to those obtained at 25 °C. This enhancement in the HER activity with the temperature can be correlated with a shift of the intercept with the x-axis toward higher values in the Tafel plot, which is linked to a higher j0 (Figure S5a). A higher exchange current density is related to improved kinetics of the HER.[36] On the contrary, in alkaline media, there is only a slight change of the overpotentials at 50 °C (Figure S5b) in compar- ison with those at room temperature (Figure 3b). This behavior may be related to some changes in the electrode upon increasing the temperature in this electrolyte. The ECSA (electrochemically active surface area) is determined to get insights into the electrode characteristics leading to high electrocatalytic activity. It can be estimated through the value of the double-layer capacitance, Cdl, (see ECSA determination section in the SI for details),[37] which can be determined from the values of the capacitive current densities as a function of the scan rate.[32] Herein, the potential limits for the cycles were set between 0.2 and 0.05 V vs Ag/AgCl in 0.5 M H2SO4 (Figure S6). From Figure 4a, Cdl was estimated to be 190 mF · cm 2 for an FeP/CF electrode. Considering a reference double-layer capacitance value of 40 μF · cm 2 for a planar surface,[29,32] the ECSA for the FeP/CF electrode can be estimated to be about 4700 cm2 per cm2 of geometric area. Remarkably, the Cdl value is 1–2 orders of magnitude higher than others reported for metal phosphides.[38] The impressive ECSA value is a result of the high porosity and thickness of the electrode and the nanoparticulated morphology of the deposited electrocatalyst. These results point out the suitability of the employed synthetic method for achieving a homogenous deposition of the FeP throughout all the carbon fibers while having wide pores for assuring good wettability of the Figure 3. Linear sweep voltammograms (negative-going scans) in (a) 0.5 M whole electrode and good hydrogen release. The linear sweep H2SO4 and (b) 1 M KOH at 25 °C (red lines) and 50 °C (blue line), and (c) Tafel voltammetry displaying the current density normalized by ECSA is plots at 25 °C in both electrolytes for FeP/CF electrodes. presented in Figure S7. Another relevant kinetic parameter for evaluating the electro- catalytic activity of the FeP/CF electrode is the turnover frequency by the direct deposition of the electrocatalysts on porous (TOF), which represents the number of product molecules substrates (see Tables S1 and S2). generated per active site (Nact) in unit time.[32] The determination The determination of the Tafel slopes helps to obtain of an accurate value of Nact is challenging and depends on the insights into the reaction mechanisms. Thus, Tafel plots in method employed.[39] One extended method is based on the Figure 3c are obtained from the cyclic voltammetries in Fig- analysis of the pair of redox peaks appearing in the cyclic ure 3a–b. At room temperature, Tafel slopes in acidic and voltammetry between 0.2 and 0.6 V vs RHE in 1 M PBS.[32] It alkaline electrolytes are estimated to be 49 and 62 mV · dec 1, should be noted that this method considers the metal sites as the respectively. Such values suggest a Volmer-Heyrovsky mecha- active sites for the reaction in transition metal-based ChemElectroChem 2023, 10, e202201152 (4 of 8) © 2023 The Authors. ChemElectroChem published by Wiley-VCH GmbH 21960216, 2023, 17, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/celc.202201152 by Readcube (Labtiva Inc.), Wiley Online Library on [25/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Research Article ChemElectroChem doi.org/10.1002/celc.202201152 Figure 5. (a) HRTEM image of an FeP crystal from the FeP/CF electrode after HER at 100 mA · cm 2 in 0.5 M H2SO4 together with the magnification of the area squared in red in (b) and the FFT spectrum in (c). (d) The EELS maps of Fe, P, O and Fe + P in the blue squared area marked in the STEM-ADF image on the left side for an FeP nanoparticle. Figure 4. (a) Capacitive currents, (ja-jc)/2, as a function of the scan rate and (b) TOF values as a function of the potential in 0.5 M H2SO4 electrolyte for an FeP/CF electrode. this period, any significant current decay was observed. Fig- ure S10 shows the XPS surveys for both electrolytes in which the corresponding signals of Fe 2p, P 2p, C 1s and O 1s can be electrocatalysts.[39] Assuming a one-electron transfer process identified. Importantly, after HER in both electrolytes, the P 2p related to the redox peaks, the number of active sites is estimated region in Figure 6a-b displays two peaks at 129.6 and 130.4 eV to be 8.1 · 1017 per geometric cm2 (Figure S8 and Eq. S2). Figure 4b that correspond to the 2p3/2 and 2p1/2 signals of phosphorous displays the TOF values for the HER calculated by Eq. S1 for an bonded to iron as iron phosphide.[14,18] Additionally, a peak near FeP/CF electrode. TOF is approximately 0.42 molecules H2 s 1 at an 133.5 eV in the P 2p region appears in both cases. This peak is overpotential of 100 mV, which is a comparable value to those correlated with PO43 that is typically detected at the surface of obtained for high-performing metal phosphides for the HER.[7,38] transition metal phosphides due to partial oxidation upon air These results support the high catalytic activity of the fabricated exposure.[13,40–42] The Fe 2p spectra in Figure S11 display two FeP electrocatalyst anchored in the 3D scaffold. peaks at 707.5 eV and 720.3 eV that correspond to the Fe 2p3/2 The FeP/CF electrodes were also characterized after 30 min and 2p1/2 doublet ascribed to iron bounded to phosphorous in at a cathodic current of around 100 mA · cm 2 (after HER). FeP,[13,18] while the other peaks are associated to iron phos- Figure 5a shows an HRTEM image of an FeP nanocrystal, and a phates (according with the presence of the intense peak of magnification of the red squared area in Figure 5b. The FFT PO43 in the P 2p region) or other oxygenated species. In fact, pattern identified in Figure 5c confirms that the particle is the bare FeP/CF electrode before etching in acid is completely composed of FeP PNMA crystal phase, according to the covered by a phosphate and/or oxide layer based on the previous analysis before HER (Figure 1a and d). The EELS spectrum in the P 2p region (Figure S12) in which the peak analysis in Figure 5d shows a homogenous distribution of the related to the metal phosphide is absent. These superficial Fe and P elements through the particles. From Figure S9, the oxygenated species are at least partly removed after the acid concentration of Fe and P results in 55(� 2)% of Fe and 45(� etching, as was previously observed after immersing the 2)% of P (Fe : P = 1.22 : 1), which is near the stoichiometric ratio electrodes in HCl solution.[18] Then, this pretreatment enhances with a slight excess of Fe. the direct presence of metal phosphide at the surface. By To gain knowledge about the surface composition of the analyzing Figure 6, it is evident a higher surface concentration FeP/CF electrodes during HER, they were analyzed by XPS after of phosphates in the electrode measured in the alkaline 30 min at a cathodic current of around 100 mA · cm 2. During electrolyte. The formation of iron phosphate may be favored ChemElectroChem 2023, 10, e202201152 (5 of 8) © 2023 The Authors. ChemElectroChem published by Wiley-VCH GmbH 21960216, 2023, 17, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/celc.202201152 by Readcube (Labtiva Inc.), Wiley Online Library on [25/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Research Article ChemElectroChem doi.org/10.1002/celc.202201152 Electrochemical performance of the FeP/CF cathodes in a PEM electrolyzer prototype The previous results in three-electrode configuration are useful to investigate the electrochemical properties of the electrodes toward the desired reaction in controlled conditions. However, testing the electrochemical performance in a flow cell is the next step for the evaluation of the industrial application of the fabricated electrodes. Also, the use of polymeric membranes is advisable for more efficient separation of the produced gases, allowing the hydrogen gas to be obtained with higher purity. For this goal, FeP/CF electrodes were tested in the PEM flow cell prototype depicted in Figure 7a. A commercial membrane electrode assembly (MEA) composed of IrRuOx anode and a Nafion 115 membrane is arranged in zero-gap with the FeP/CF cathode in contact with the graphite current collector. This configuration allows for minimizing the electrolyte resistance through the cell, which is critical for achieving a low applied voltage, i. e., high cell efficiency. Figure 7b shows the resulting cyclic voltammograms in two-electrode configuration using this small prototype with a geometric area in the cathode of 1.8 cm2 and a deposited area of IrRuOx of 10 cm2. At room temperature, Figure 6. XPS spectra in the P 2p region for FeP/CF electrodes after HER at the applied potentials to achieve 100 and 800 mA · cm 2 are 100 mA · cm 2 in (a) 0.5 M H2SO4 and (b) 1 M KOH electrolyte. 1.85 V and 2.75 V, respectively, while after IR correction they are 1.77 V and 2.17 V, respectively. After increasing the temperature to 50 °C the potentials are reduced to 1.75 V and 1.68 V (IR- under these conditions when the cathodic bias is removed. Nevertheless, the XPS analysis demonstrates that FeP remains at the electrode surface after the HER at such high current densities of 100 mA · cm 2 in both pH conditions. The P/Fe ratio found by XPS is 2.7 and 1.7 in the electrodes measured in acid and alkaline electrolyte, respectively, which indicates that the electrode surface is enriched with phosphorous. Commercial PEM electrolyzers typically work at 500– 1000 mA · cm 2, reaching higher hydrogen production rates than alkaline-based electrolyzers. For evaluating the potential of the FeP/CF cathodes in such conditions, the stability of the FeP/ CF electrodes was measured at 1 A · cm 2 and 50 °C in both acidic and alkaline electrolytes (Figure S13). In the acidic electrolyte, the cyclic voltammetries before and after the stability test (Figure S14a) present the same j-V curve that is indicative of good stability. Additionally, the capacitive currents in the region without faradaic processes (above 0 V vs RHE) are unaltered (Figure S14b), which may indicate that there is any significant change in the electrochemical surface area after the chronoamperometry. In both electrolytes, after 2 h at 1 A · cm 2, the peaks of FeP are present in the XRD patterns (Figure S15) and the presence of iron and phosphorous was further corroborated by EDX. However, for 1 M KOH, two new peaks appear in the pattern that are in agreement with the presence of metallic iron. This indicates that despite the good stability of the current with time, the electrode undergoes chemical changes under these conditions. Figure 7. (a) Sketch of the PEM cell configuration, using an FeP/CF electrode of 1.8 cm2 and anodic area of 10 cm2 (b) Cyclic voltammograms at 50 mV · s 1 in the positive-going scan in the PEM cell in (a) with and without IR correction at room temperature (RT) and 50 °C. ChemElectroChem 2023, 10, e202201152 (6 of 8) © 2023 The Authors. ChemElectroChem published by Wiley-VCH GmbH 21960216, 2023, 17, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/celc.202201152 by Readcube (Labtiva Inc.), Wiley Online Library on [25/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Research Article ChemElectroChem doi.org/10.1002/celc.202201152 corrected) for 100 mA · cm 2 and 2.62 V and 2.07 V (IR-corrected) 20 min under vigorous stirring.[14] Round pieces of CF (GFD 2.5 EA, for 800 mA · cm 2. This excellent performance is competitive SGL Carbon) of 1 cm in diameter (geometric area of 0.785 cm2) were with the values of the literature for metal phosphides,[30] dip-coated in this precursor solution, and subsequently, the ink excess was removed. Afterward, they were dried at 150 °C and then corroborating the suitability of the selected FeP/CF cathodes in heated at 400 °C for 1 h in air leading to the Fe2O3/CF electrodes. In PEM cells. Furthermore, the stability of the cell was tested at an the second step, they were placed in a porcelain boat with applied voltage of 1.69 V. The cyclic voltammograms before NaH2PO2 · H2O (Alfa Aesar) in separate positions and then introduced and after 1 h of HER present similar j-E characteristic curves in a tubular oven. Before heating, Ar gas was flowing for 1 h at room (Figure S16), which indicates good stability of the components temperature to remove the oxygen inside the oven tube. Then, the temperature was raised to 350 °C and kept for 1 h using a low Ar flux of the cell. These results are promising for a potential industrial that was estimated to be around 5 mL · min 1 leading to the FeP/CF application of the developed cathodes for the HER and will electrodes (caution: toxic PH3 gas is generated during the decom- motivate further studies in the optimization of the configuration position of NaH2PO2. PH3 self-ignites in air, so the reaction should be and the scaling up of the device. In addition, the combination performed under air-free conditions). with a MEA containing an Earth-abundant-based anode is Physicochemical characterization: Structural characterization was advisable for reducing the device fabrication costs. performed by XRD using a D8 Advance Bruker diffractometer equipped with a Cu Kα (1.5406 Å) radiation source, LYNXEYE super speed detector and Ni filter. A Bragg Brentano (θ–2θ) configuration Conclusion was used with a step size of 0.025°. The morphology of the electrodes was analyzed by an XHRSEM Magallan 400 L equipment. Both the HRTEM and scanning TEM (STEM)-EELS were obtained in FEI In this work, we develop a 3D cathode consisting of FeP and a F20 STEM microscope operated at 200 kV. The HRTEM was acquired carbonaceous support (carbon felt) that shows outstanding with a condenser aperture of 100 μm, no objective aperture, spot water electrolysis performance in a PEM cell. Remarkably, the size 3 and using a BM-UltraScan CCD camera. The STEM-EELS was fabricated electrode is only composed of Earth-abundant acquired with a condenser aperture of 70 μm, no objective aperture, elements. It displays low Tafel slopes and ultralow overpoten- a nominal camera length of 30 mm, spot size 6, and using a Gatan QUANTUM GIF spectrometer. The same conditions apply for STEM- tials for the HER of only 41 and 63 mV in acidic and alkaline EDX, with a detector EDAX super ultra-thin window (SUTW) X-ray electrolytes, respectively, to generate 10 mA · cm 2. Such high detector, 136 eV resolution (Z > Z(Be)), detector area of 30 mm2. For performance is ascribed to the good electron transport through these measurements, part of the FeP/CF electrode was scratched off the electrode, and the homogenous deposition of FeP nano- and mixed with hexane. Before the analysis, the dispersion was particles on the carbon support with a high coverage leading to sonicated in an ultrasonic bath, then deposited on a grid and dried a high number of exposed active sites. This was achieved by at 50 °C. Core level XPS photoemission spectra were registered in normal emission mode at room temperature by using a NEXSA X-ray devising a careful synthesis procedure. The favorable conduc- Photoelectron Spectrometer (Thermo-Scientific) using a monochro- tive interface was obtained by using Fe2O3 nanoparticles matic Al Kα line as an X-ray source. directly grown on the carbon fibers as a sacrificial intermediate Electrochemical characterization: The study of the electrochemical material for the generation of FeP without the use of any activity of the FeP/CF electrodes was carried out in a three- binder. The FeP/CF electrode retains impressive electrochemi- electrode cell. All the potentials were measured against an Ag/ cally active areas of more than 4000 cm2 while the TOF was AgCl/KCl reference electrode. The potentials against the reversible ~ 0.4 s 1 at an overpotential of 100 mV. Additionally, the FeP/CF hydrogen electrode (RHE) in acidic electrolyte were estimated in electrode exhibits excellent stability and the FeP phase was three-electrode cell using 0.5 M H2SO4 electrolyte saturated with preserved after HER at 100 mA · cm 2 in both acid and alkaline high purity H2 and a Pt wire as a working electrode, leading to E(vs RHE) = E(vs Ag/AgCl) + 0.230 V. In alkaline media, the potentials vs media. Reaching higher current densities of 1 A · cm 2 is RHE were calculated as E(vs RHE) = E(vs Ag/AgCl) + 0.205 + 0.059 facilitated by the 3D structure of the employed carbon scaffold, pH. The electric contact with the FeP/CF electrodes was performed with adequate porous size for gas bubbles release. Due to the with a Ta wire clip. To evaluate HER, linear sweep and cyclic potential of these electrodes for practical energy conversion voltammetries were recorded at a scan rate of 2 mV · s 1 in 0.5 M devices, they were combined with a membrane electrode H2SO4 or 1 M KOH under stirring at 300 rpm. All the potentials were corrected with the IR drop, and the corresponding resistance, R, assembly in zero-gap configuration in a small electrolyzer was determined by electrochemical impedance spectroscopy (EIS). prototype. The device reached high hydrogen productivity of Before all the electrochemical measurements, the electrodes were 14.9 mmol H2 · h 1 · cm 2 with a current density of 800 mA · cm 2 etched in 0.5 M H2SO4 for 30–60 min to remove impurities. For the at only 2.1 V in acidic electrolyte. This work demonstrates that analysis in 1 M KOH solution, after the etching, the electrode was the developed low-cost electrode is promising for industrial washed with water and then kept for 1 h in 1 M KOH solution. PEM electrolyzers for high hydrogen productivity by a feasible Measurements for ECSA and TOF were performed after a cyclic voltammetry in the HER potential region up to cathodic current scaling up toward higher electrodic areas. density of 150 mA · cm 2 in 0.5 M H2SO4. Full flow cell measure- ments were performed in a two-electrode configuration, with the FeP/CF electrode (1.8 cm2) in zero-gap with a MEA composed of Experimental Section Nafion 115 and 4 mg cm 2 IrRuOx catalyst (10 cm2, The Fuel Cell Store). 0.5 M H2SO4 electrolyte was recirculated in the cathode and Electrode preparation: FeP/CF electrodes were prepared by a two- anode in two independent bottles at about 60 mL · min 1. In this step synthesis. In the first step, an ink was prepared by dissolving case, linear sweep voltammograms were recorded at 50 mV · s 1. 20 wt.% Fe(NO3)3 · 9H2O and 2.5 wt.% polyvinylpyrrolidone (molecular weight of 40000) in N,N-Dimethylformamide and heating at 70 °C for ChemElectroChem 2023, 10, e202201152 (7 of 8) © 2023 The Authors. ChemElectroChem published by Wiley-VCH GmbH 21960216, 2023, 17, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/celc.202201152 by Readcube (Labtiva Inc.), Wiley Online Library on [25/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Research Article ChemElectroChem doi.org/10.1002/celc.202201152 Acknowledgements [9] Y. Li, Z. Dong, L. Jiao, Adv. Energy Mater. 2020, 10, 1902104. [10] R. Koutavarapu, C. Venkata Reddy, B. Babu, K. R. Reddy, M. Cho, J. Shim, Int. J. Hydrogen Energy 2020, 45, 7716. 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