Developing highly efficient and low-cost electrocatalysts with superior durability for hydrogen evolution reaction (HER) is a big challenge. Here, we design and fabricate a highly efficient electrocatalyst with unique three-dimensional (3D) porous peony-like micro-flower Mn-CoP nano-structure on flexible carbon cloth (Mn-CoP PMFs/CC), which exhibits high electrocatalytic activity toward the HER in both acid and alkaline circumstances. Remarkably, benefiting from the unique 3D porous structure with large surface areas, good electron conductivity for fast electron transport, and effective channels for the release of gas, the resultant catalyst exhibits an ultra-low overpotential of 28 and 90 mV to arrive the current density of 10 mA/cm(2) in 0.5 M H2SO4 and 1.0 M KOH solutions, respectively. Meanwhile, the Mn-CoP PMFs/CC material shows small Tafel slopes and good long-term stability in acid and alkaline media. Density Functional Theory (DFT) calculations illustrate that Mn doping indeed improve electron transfer, and makes the thermo-neutral hydrogen adsorption free energy (Delta G(H*)) of CoP on the surface of (0 1 1) sharply close to zero, which is very conducive to the adsorption and desorption of hydrogen, thereby making Mn-CoP PMFs/CC with significant enhanced electrocatalytic HER performance. Our 3D porous electrocatalyst has greatly promoted the efficient electrolysis of water to produce hydrogen. (C) 2020 Elsevier Inc. All rights reserved.