How to create a personalized chapter heading style

Here's a solution built around your example, made with titlesec. It also uses the microtype package to add space between the letters of "CHAPTER".

\documentclass[a4paper,12pt,twoside]{book}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[english]{babel} % If you write in French
\usepackage{lipsum}
\usepackage{times}
\usepackage{xcolor}
\usepackage{amsthm}
\usepackage{amssymb,amsmath,bbm}
\usepackage{mathtools}
\usepackage[a4paper]{geometry}
\usepackage[letterspace=200]{microtype}

\definecolor{MyGrey}{RGB}{160,160,160}
\definecolor{OtherGrey}{RGB}{200,200,200}
\newcommand{\greytriangleright}{{\color{MyGrey}$\blacktriangleright$}}
\newcommand{\greytriangleleft}{{\color{MyGrey}$\blacktriangleleft$}}
\newcommand{\greytriangledown}{{\color{OtherGrey}$\blacktriangledown$}}

\newcommand{\HUGE}{\fontsize{40}{36}\selectfont}

%%% HEADERS and FOOTERS %%%
\renewcommand{\baselinestretch}{1.05}
\usepackage{fancyhdr}
\pagestyle{fancy}
\fancyhead{}
\fancyfoot{}
\fancyhead[LE]{\textbf{\thepage} \:\greytriangleright\: \nouppercase{\leftmark}}
\fancyhead[RO]{\nouppercase{\rightmark} \:\greytriangleleft\: \textbf{\thepage}}
\setlength{\headheight}{20pt}

\renewcommand{\chaptermark}[1]{\markboth{\chaptername\ \thechapter.\emph{ #1}}{}}
\renewcommand{\sectionmark}[1]{\markright{\thesection \emph{ #1}}{}}

\let\headruleORIG\headrule
\renewcommand{\headrule}{\color{black} \headruleORIG}
%\renewcommand{\headrulewidth}{1pt}
\renewcommand{\footrulewidth}{0pt}

\fancypagestyle{plain}{
  \fancyhead{}
  \fancyfoot[C]{\textbf{\thepage}}
  \renewcommand{\headrulewidth}{0pt}
  \renewcommand{\footrulewidth}{0pt}
}

%%% CUSTOM CHAPTER FORMAT %%%
\newlength{\trianglewidth}
\newlength{\triangleheight}
\settowidth{\trianglewidth}{\greytriangledown}
\settoheight{\triangleheight}{\greytriangledown}
\usepackage{titlesec}
\titleformat{\chapter}[display]
    {\normalfont}
    {\filcenter\resizebox{6\trianglewidth}{6\triangleheight}{\greytriangledown}\\[-6ex]%
     \filcenter\MakeUppercase{\lsstyle\bfseries\chaptertitlename}\\[4ex]%
     \HUGE\thechapter}
    {4ex}
    {\filcenter\Huge\scshape\uppercase}
\titlespacing*{\chapter}{0pt}{-50pt}{30pt}[0pt]
\titleformat{\section}{\large\uppercase}{\hspace{-.5em}\llap{\greytriangleright\enspace\thesection}}{.5em}{}

\begin{document}

\chapter[Introduction and overview of electrode processes]{Introduction\\and overview\\of electrode\\processes}
%
\section{Introduction}
Electrochemistry is the branch of chemistry concerned with the interrelation of electrical and chemical effects. A large part of this field deals with the study of chemical changes caused by the passage of an electric current and the production of electrical energy by chemical reactions. In fact, the field of electrochemistry encompasses a huge array of different phenomena (e.g., electrophoresis and corrosion), devices (electrochromic displays, electro analytical sensors, batteries, and fuel cells), and technologies (the electroplating of metals and the large-scale production of aluminum and chlorine). While the basic principles of electrochemistry discussed in this text apply to all of these, the main emphasis here is on the application of electrochemical methods to the study of chemical systems.

\lipsum[1-10]
\end{document}