Rague-Like FeP Nanocrystal Assembly on Carbon Cloth: An Exceptionally Efficient and Stable Cathode for Hydrogen Evolution

There is a strong demand to replace expensive Pt catalysts with cheap metal sulfides or phosphides for hydrogen generation in water electrolysis. The earth-abundant Fe can be electroplated on carbon cloth (CC) to form high surface area rague-like FeOOH assembly. Subsequent gas phase phosphidation converts the FeOOH to FeP or FeP 2 and the morphology of the crystal assembly is controlled by the phosphidation temperature. The FeP prepared at 250 o C presents lower crystallinity and those prepared at higher temperatures 400 o C and 500 o C possess higher crystallinity but lower surface area. The phosphidation at 300 o C produces nanocrystalline FeP and preserves the high-surface area morphology; thus it exhibits the highest HER efficiency in 0.5 M H 2 SO 4 ; i.e

indicating that the hydrogen evolution for our best FeP is limited by Tafel reaction (same as Pt).
Importantly, the FeP/CC catalyst exhibits much better stability in a wide range working current density (up to 1 V/cm 2 ), suggesting that it is a promising replacement of Pt for HER.
Hence, the possibility of using iron phosphide as an HER catalyst in an acidic solution has been extensively studied. [29][30][31][32][33][34] FeP 2 was obtained by the pyrolysis of ferrocene and red phosphorous. 28 Nanoporous FeP nanosheets were prepared by ion-exchange synthesis using Fe 18 S 25 and trioctyl-phosphine as sources. 34 Alternatively, FeP nanoparticles were synthesized by gas phase phosphidation of Fe 3 O 4 nanoparticles at 350 o C. 33 These reports have demonstrated a Tafel slope ranging from 59 to 67 mV/dec. Very recently, dense FeP nanowire array has been achieved by chemical phosphidation of the hydrothermally grown FeOOH nano array, 30 where the Tafel slope of the system has been further lowered to 38 mV/dec. Various Tafel slopes explored from these results indicate that the synthetic method significantly affect the HER mechanisms for the FeP catalysts. In addition to the active FeP materials, the selection of acid-resistant conducting substrates for HER are limited to novel metals, Ti, and carbon materials. Carbon cloth (CC) is cheap, highly conductive and flexible. It has become one of the popular substrates used for electrocatalysis. In this contribution, we perform the direct electroplating of Fe on carbon cloth and convert it to FeOOH or Fe 2 O 3 at various oxidation temperatures. Controlled gas phase phosphidation reveals that nanocrystalline FeP obtained with the phosphidation temperature at 300 o C exhibits better HER performance than the highly crystalline FeP and FeP 2 phosphidated at a higher temperature 400 or 500 o C. For the best FeP catalyst, the required overpotential to reach 10 and 20 mA/cm 2 (ƞ 10 and ƞ 20 ) is 34 and 43 mV respectively. These values are lowest among the reported non-precious metal phosphides on CC.
Although the values are slightly higher than the 29 and 36 mV for the Pt coated CC (Pt/CC), the FeP nanocrystal based HER catalysts exhibit much better stability than Pt/CC in a wide range of working current density (up to 1 A/cm 2 in our test), demonstrating that FeP/CC catayst is an attractive Pt alternative for the practically used water electrolysis systems. Fig. 1a shows the scanning electron microscopy (SEM) images for the surafces of the carbon cloth used in this study, where the surface of the fibers in CC is smooth and its diameter is around 9 m. The electroplating of Fe on carbon cloth is performed in a 0.1 M FeSO 4 solution and the subsequent oxidation in air forms a layer of uniform coating on the fiberous structure of CC as shown in the SEM images of Fig. 1b. The electroplating time is optimized at 20 min as described in Methods. The magnified SEM image reveals that the layer of coating is composed of many randomly oriented micron-sized sheets perpendicularly grown on the fiber surfaces. These sheets assemble to form rague-like morphologies, which are beneficial for catalytic reaction owing to its high surface area. Fig. 1c displays the X-ray diffraction (XRD) pattern for these electrochemically deposited layer. The peaks can be well indexed to the polycrystalline and -FeOOH, indicating that the deposited layer is a mixture of and -FeOOH crystallites.

Vapor Phase Phosphidation:
The vapor phase phosphidation is carried on in a two-zone chemical vapor deposition (CVD) furnace, where the NaH 2 PO 2 is dissociated at 300 o C to form PH 3 gases. 41 The produced PH 3 vapors are brought to the FeOOH on CC at the downstream site which can be set at a desired reaction temperature such as 250, 300, 400, and 500 o C. Fig. 2a schematically illustrates the set-up for the gas phase phosphidation. We observe that the FeOOH structures start to be phosphidated with the vapor phase reaction at the temperature >250 o C. Note that the phosphidation of FeOOH does not occur when the temprature is lower than 200 o C. The XRD results in Fig. 2b show that the orthorombic FeP crystals form after phosphidation at 400 o C and some FeP 2 also appears after 500 o C phosphidation. All peaks are well indexed to orthorombic FeP or FeP 2.  rather poor, where the required overpotential ƞ 10 and ƞ 20 are 46 and 59 mV respectively (Fig. S3). .
Hence, the control of the chemical structure of the electroplated Fe is critical for achieving better phosphidation. We will focus our discussions on the catalysts prepared from FeOOH only. Catalyst). 6 The Tafel slope for the FeP/CC prepared at 400 o C (highly crystalline orthorombic FeP) is 39.7 mV/dec, indicating that the HER proceeds with a Volmer-Heyrovsky mechanism and the rate-determing step is the Heyrovsky reaction (Catalyst-H + H + + e -→ H 2 + Catalyst). 15 The Tafel slope further increases to 51.4 mV/dec for the FeP/CC preapred at 500 o C (a mixture of FeP and FeP 2 ). It is noted that low Tafel slope is desirable for the practical high working current density HER since a lower applied voltage to drive the water electrolysis is beneficial. The HER characteristics for the four FeP catalysts are also summarized in Table 1.

HER Characteristics for FeP/CC
We perform the scanning rate measurement for these samples to extract their capacitance of the double layer at the solid-liquid interface (see Fig. S4 for details). Fe and P in FeP nanocrystals, 30 and the peaks at 720.2 eV and 133.3 eV can be attributed to the oxidized Fe and P species which could be partly due to the oxidation in air. 29 The XPS results confirm the presence of FeP and some oxidized FeP and Fe. In addition, some residual elementary Fe which remains un-oxidized or un-phosphidated is also found in the structure but it is considered as an inert impurity since Fe metal is much less active in HER.

Estimation of Active Site Number:
The underpotential deposition (UPD) of copper has been used to extract the density of active sites for Pt 42 and WS 2 , 43 where they observe that the coverage and surface density of the copper is the same as that for adsorbed hydrogen on the HER catalyst surface.

HER Stability of the FeP/CC:
In addition to the high HER efficiency, the catalyst also presents excellent stability in acidic solutions. Fig. 6a displays the time dependent measurement for FeP/CC and Pt/CC at a low working current density (ca. 100 mA/cm 2 ). It is observed that the decay of HER current for FeP/CC is 20% after 24 h but the decaly is 85% for Pt/CC likely due to the well-known aggregation of Pt and/or poisoning by carbon monoxide. 44 Actually the surface of CC is relatively smooth as shown in Figure 1(a). Thus, the Pt tends to aggregate during the electrocatalytic reaction.
Another commonly known reason is the poisoning of Pt surface by carbon monoxide and other impurities. By contrast, the active FeP is embedded in a three dimensional structure which can effectively inhibit the aggregation. Hence, FeP/CC is more stable than Pt/CC. Fig. 6b

CONCLUSIONS
In summary, iron phosphide was synthesized by vapor phase phosphidation of the FeOOH obtained by electroplating. The rague-like morphology formed during the electroplating provides a large surface area. The subsequent phosphidation at 300 o C converts the FeOOH to nanocrystalline FeP while maintaining the rague morphology. The Tafel slop reaches 29.2 mV/dec, close to that of Pt/CC. The low Tafel slop, the high HER performance (ƞ 10 = 34 mV/dec and ƞ 20 = 43 mV/dec) and excellent stability of FeP/CC suggest that it is a promising replacement of Pt for HER. The morphology and the crystal structures of FeP seem to strongly affect the HER characteristics. It is anticipated that more research efforts in this area may bring these cheap catalysts to practical applications in the near future.
The water used throughout all experiments was purified through a Millipore system.

ACKNOWLEDGMENTS:
This research was supported by KAUST.